System of a rotating electric machine

Abstract

Rotating electric machine system (MU) comprising a rotating electric machine (10) and an electrical power conversion circuit between which electrical power is transferred, wherein the rotating electric machine (10) has a stator core (15a) and N coils (12U, 12V, 12W) wound on the stator core (15a) and each belonging to N phases, where N is an integer greater than or equal to 2, wherein the coils (12U, 12V, 12W) are electrically connected to each other to define a neutral point (N) between them, the power conversion circuit comprising N high-side switches (SUp, SVp, SWp), N low-side switches (SUn, SVn, SWn), a selector switch (40), a full-wave driver, a half-wave driver and a drive control device, the N high-side switches (SUp, SVp, SWp) each corresponding to the N coils (12U, 12V, 12W) and a have a high-potential connection and a low-potential connection,wherein all high-potential terminals of the high-side switches (SUp, SVp, SWp) are electrically connected to a positive terminal of a DC power source (30), the N low-side switches (SUn, SVn, SWn) each correspond to the N coils (12U, 12V, 12W) and each have one high-potential terminal and one low-potential terminal, wherein all low-potential terminals of the low-side switches (SUn, SVn, SWn) are electrically connected to a negative terminal of the DC power source (30), the high-side switches (SUp, SVp, SWp) are configured either as permanently connected switches, wherein the low-side switches (SUn, SVn, SWn) are configured as selectively connected switches, or as selectively connected switches, wherein the low-side switches (SUn, SVn, SWn) are configured as permanently connected switches.wherein each of the permanently connected switches is permanently electrically connected to a corresponding coil (12U, 12V, 12W) of the rotating electric machine (10), each of the selectively connected switches is selectively electrically connected to either a corresponding permanently connected switch or the neutral point (N) between the coils (12U, 12V, 12W), for each of the permanently connected switches, that of the high-potential and low-potential terminals of the permanently connected switch which is electrically neither connected to the positive terminal nor to the negative terminal of the DC power source (30) forms a permanently connected terminal, each of the permanently connected terminals of the permanently connected switches is permanently connected to a corresponding non-neutral-point end of the coils (12U, 12V, 12W) of the rotating electric machine (10),For each of the selectively connected switches, the one of the high-potential and low-potential terminals of the selectively connected switch that is electrically neither connected to the positive terminal nor to the negative terminal of the DC power source (30) forms a selectively connected terminal; the selector switch (40) electrically selectively connects each of the selectively connected terminals of the selectively connected switches either to a corresponding permanently connected terminal of the permanently connected switches or to the neutral point (N) between the coils (12U, 12V, 12W); the full-wave driver is configured to perform a full-wave drive process for the full-wave drive of the rotating electric machine (10), wherein in the full-wave drive process the full-wave driver controls the on-off switching of the permanently connected switches and the selectively connected switches.wherein the selectively connected terminals of the selectively connected switches are electrically connected to the corresponding permanently connected terminals of the permanently connected switches by the selector switch (40), the half-wave driver is configured to perform a half-wave drive process for the half-wave drive of the rotating electric machine (10), wherein in the half-wave drive process the half-wave driver controls the on-off switching of the permanently connected switches, wherein the selectively connected terminals of the selectively connected switches are electrically connected to the neutral point (N) by the selector switch (40) and the selectively connected switches are held on, and the drive control device is configured to control both the full-wave driver and the half-wave driver,to selectively cause either the full-shaft driver to carry out the full-shaft drive process or the half-shaft driver to carry out the half-shaft drive process.

Description

BACKGROUND1. Technical FieldThe present invention relates to rotary electric machine systems (rotary electric machine systems) installed in, for example, motor vehicles, and having a rotary electric machine and an electric power conversion circuit between which electric power is transmitted.2. Description of the Prior ArtJP H04-168 994 A discloses a motor driving apparatus in which a shift controller detects a rotation speed of a motor at a predetermined timing and compares it with that at a current timing. When the speed detected at the present time is larger, driving by a bipolar system is continued, while when the speed detected at the present time is equal to the speed detected at the previous time, driving by a unipolar system is switched to.JP S59-153 486 A discloses a driving apparatus for a motor which uses an inverter type driving system at a low speed and a unidirectional feed driving system at a high speed. The inverter type drive system and the unidirectional energization drive system are switched by switches.KR 10 2006 0 008 372 A discloses an inverter circuit in which it is possible to switch between a bipolar drive and a unipolar drive. A controller is provided to remove back torque due to unexpected reflux current generated during unipolar driving.US 2016 / 0 173 020 A1 discloses a power conversion device in which measures are provided for carrying out a fast protective operation in which a phase in which a fault has occurred is disconnected in the event of a fault, and a driver circuit is connected to a motor connection to be rubbed.There are known rotary electric machine systems installed in, for example, motor vehicles, and include a rotary electric machine and an electric power conversion circuit between which electric power is transmitted.When the rotating electric machine is used as an electric motor in a wide rotation speed range, it is necessary to realize required performance of the rotating electric machine in a low rotation speed region and a high rotation speed region, respectively.For this, it can be considered to employ a DC-DC converter and drive the rotating electric machine with a voltage varied by the DC-DC converter applied to the machine, thereby realizing the required performance. However, in this case, the system cost would be significantly increased.Alternatively, during operation of the rotating electric machine, the electrical connection of a stator coil of the rotating electric machine may be considered to switch between a Δ-circuit suitable for high-speed operation and a Y-circuit (or Y-connection) suitable for high-torque operation, thereby realizing the required performance. However, in this case, the system configuration would become complicated. Further, it would be necessary to stop the operation of the rotating electric machine each time the electrical connection of the stator coil is switched.Japanese Patent Application Publication No. JP H11-332 288 A discloses a motor drive circuit that performs, for a motor, a full-shaft drive process in a low-speed region and a half-shaft drive process in a high-speed region. Specifically, the motor drive circuit includes a three-phase inverter configured with high-side switches and low-side switches. Further, high-potential-side terminals of the high-side switches are electrically connected to a neutral point between three-phase coils of the motor via a semiconductor switching element such as a transistor. In the half-wave driving process, the on-off switching of the low-side switches is controlled in which the semiconductor element is turned on and the high-side switches are turned off. In addition, such a half-wave driving process is also disclosed in Japanese Patent Application Publication No. JP H11 356 082 A.However, with the above-described configuration of the motor drive circuit, it is necessary to use the semiconductor element to perform the half-wave drive process, thereby increasing the size of the motor drive circuit. Further, during the half-wave driving process, the high-side switches are kept off, that is, the high-side switches become redundant.SUMMARYThe present invention has been made in view of the above-described circumstances. It is therefore a primary object of the present invention to provide a rotary electric machine system (rotary electric machine system) having a configuration with which it is possible to efficiently use components of the system and minimize the size of the system.According to exemplary embodiments, a rotary electric machine system is provided that includes a rotary electric machine and an electric power conversion circuit between which electric power is transmitted. The rotating electric machine includes a stator core and N coils wound on the stator core and each belonging to N phases, where N is an integer greater than or equal to 2. The coils are electrically connected to each other to define a neutral point therebetween. The power conversion circuit includes N high-side switches, N low-side switches, a selection switch, a full-wave driver, a half-wave driver, and a drive controller. The N high-side switches respectively correspond to the N coils and have a high-potential-side terminal and a low-potential-side terminal. All high potential side terminals of the high side switches are electrically connected to a positive terminal of a DC power source. The N low-side switches respectively correspond to the N coils and respectively have a high-potential-side terminal and a low-potential-side terminal. All low-potential-side terminals of the low-side switches are electrically connected to a negative terminal of the DC power source. The high-side switches are either configured as permanently connected switches, the low-side switches being configured as selectively connected switches, or are configured as selectively connected switches, the low-side switches being configured as permanently connected switches. Each of the permanently connected switches is permanently electrically connected to a corresponding one of the coils of the rotating electrical machine. Each of the selectively connected switches is selectively electrically connected to either a corresponding one of the permanently connected switches or the neutral point between the switches. For each of the permanently connected switches, the one of the high-potential-side and low-potential-side terminals of the permanently connected switch that is electrically connected neither to the positive terminal nor to the negative terminal of the DC power source constitutes a permanently connected terminal. Each of the permanently connected terminals of the permanently connected switches is permanently connected to a corresponding one of non-neutral point side ends of the coils of the rotating electric machine. For each of the selectively connected switches, the one of the high-potential-side and low-potential-side terminals of the selectively connected switch that is electrically connected neither to the positive terminal nor to the negative terminal of the DC power source constitutes a selectively connected terminal. The selector switch electrically selectively connects each of the selectively connected terminals of the selectively connected switches to either a corresponding one of the permanently connected terminals of the permanently connected switches or the neutral point between the coils. The full-wave driver is configured to perform a full-wave driving process for full-wave driving of the rotating electric machine. In the full-wave driving process, the full-wave driver controls the on-off switching of the permanently connected switches and the selectively connected switches, the selectively connected terminals of the selectively connected switches being electrically connected to the corresponding permanently connected terminals of the permanently connected switches through the selection switch. The half-wave driver is configured to perform a half-wave driving process for half-wave driving the rotating electric machine. In the half-wave driving process, the half-wave driver controls the on-off switching of the permanently connected switches, the selectively connected terminals of the selectively connected switches are electrically connected to the neutral point through the selection switch, and the selectively connected switches are kept on. The drive controller is configured to control both the full-wave driver and the half-wave driver to selectively cause either the full-wave driver to perform the full-wave driving process or the half-wave driver to perform the half-wave driving process.With the configuration described above, it becomes possible to perform both the half-wave driving process and the full-wave driving process without using a semiconductor element, as disclosed in Japanese Patent Application Publication No. JP H11-332 288 A; in the patent document, the semiconductor element is electrically connected between the high-side switches and the neutral point.Further, with the above-described configuration, all of the high-side switches and the low-side switches are used during the half-wave driving process. That is, there is no redundant switch in the rotary electric machine system. Consequently, it is possible to effectively use all of the switches constituting the electric power conversion circuit, thereby minimizing the size of the rotary electric machine system.Further, with the above-described configuration, during the half-wave driving process, the electric current from the DC power source is distributed to the N switches selectively connected. Consequently, it becomes possible to prevent concentration of load on any of the selectively connected switches, thereby effectively releasing heat generated by these switches. As a result, it becomes possible to improve the reliability of the rotary electric machine system.In comparison, in the motor drive circuit disclosed in Japanese Patent Application Publication No. H11-332 288A, all phase currents converge on the semiconductor element electrically connected to the neutral point. Therefore, in order to ensure the reliability of the motor drive circuit, it is necessary to set the rated current of the semiconductor element to be N times (for example, 3 times in the case that the number of phases N is 3) the rated current of the high-side and low-side switches. Consequently, the size and manufacturing cost of the semiconductor element would be increased.According to a first exemplary embodiment, in the half-wave driving process, the half-wave driver cyclically powers the permanently connected switches such that start timings of the power of the permanently connected switches are offset from each other and each of the permanently connected switches is powered for half or less of each cycle. Furthermore, the starting times of the feeding of the permanently connected switches are offset from one another by 360° / N at an electrical angle.The electric power conversion circuit further includes a selection switch controller. The selection switch control device controls the selection switch to perform, for each of the N coils, switching between electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the permanently connected terminal of the corresponding permanently connected switch and electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the neutral point only when the absolute value of the electric current flowing through the coil is less than or equal to a threshold current value.For each of the N coils, during switching between electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the permanently connected terminal of the corresponding permanently connected switch and electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the neutral point, a closed circuit including the DC power source and the coil is temporarily opened, which generates a surge voltage. The larger the absolute value of the electric current flowing in the coil when the closed circuit is opened, the higher the surge voltage generated becomes. Further, the electric current flowing in the coil is an alternating current; therefore, there are zero-crossing points in the electric current, and the absolute value of the electric current is small in the vicinity of the zero-crossing points. Thus, by performing the switching only when the absolute value of the electric current is less than or equal to the threshold current value, it is possible to suppress the surge voltage generated during the switching.The rotating electric machine further includes a rotor having a plurality of magnetic poles. The electric power conversion circuit further includes an acquisition unit that acquires magnetic pole position information regarding a magnetic pole position of the rotor. The selection switch control device determines, for each of the N coils, whether the absolute value of the electric current flowing in the coil is less than or equal to the threshold current value on the basis of the magnetic pole position information acquired by the acquisition unit.The electrical phase of the electric current flowing in the coil refers to the magnetic pole position of the rotor. Therefore, it is possible for the selection switch control device to determine whether the absolute value of the electric current is less than or equal to the threshold current value based on the magnetic pole position information.The selection switch includes N pairs of first and second connection switches corresponding to the N coils, respectively. For each of the N coils, the first connection switch is configured to electrically connect the selectively connected terminal of the corresponding selectively connected switch to the permanently connected terminal of the corresponding permanently connected switch; the second connection switch is configured to electrically connect the selectively connected terminal of the corresponding selectively connected switch to the neutral point. The selection switch controller performs first and second switching processes for each of the N coils. In the first switching process, the selection switch controller switches the corresponding first connection switch from on to off and switches the corresponding second connection switch from off to on. In the second switching process, the selection switch controller switches the corresponding first connection switch from off to on and switches the corresponding second connection switch from on to off. In each of the first and second switching processes, a period of time is provided during which both of the corresponding first and second connection switches are held on by the selection switch controller.During the first and second switching processes, if there were a period of time for which both of the corresponding first and second connection switches are in an off state, an electric current flow path including the coil would be interrupted and thus a surge voltage could be generated. However, by providing a time period during which both of the corresponding first and second connection switches are held on in each of the first and second switching processes, it is possible to prevent the electric current flow path from being interrupted, thereby suppressing generation of surge voltage.According to a second exemplary embodiment, the coils are first coils, the neutral point between the coils is a first neutral point, the high-side switches are first high-side switches, the low-side switches are first low-side switches, and the selection switch is a first selection switch. The permanently connected switches are each configured with the first high-side switches. The selectively connected switches are configured with the first low-side switches, respectively. The permanently connected terminals each consist of the low-potential-side terminals of the high-side switches. The selectively connected switches are respectively composed of the high-potential-side terminals of the first low-side switches. The rotating electric machine further includes N second coils wound on the stator core so as to be electrically insulated from the first coils and respectively belong to N phases different from the N phases of the first coils. The second coils are electrically connected to each other to define a second neutral point therebetween. The electric power conversion circuit further includes N second high-side switches, N second low-side switches, and a second selection switch. The N second high-side switches respectively correspond to the N second coils and each have a high-potential-side terminal and a low-potential-side terminal. All of the high potential side terminals of the second high side switches are electrically connected to the positive terminal of the DC power source. The N second low-side switches respectively correspond to the N second coils and respectively have a high-potential-side terminal and a low-potential-side terminal. All of the low-potential-side terminals of the second low-side switches are electrically connected to the negative terminal of the DC power source. Each of the high-potential-side terminals of the second low-side switches is permanently electrically connected to a corresponding one of ends, which are not on the second neutral point side, of the second coils of the rotating electric machine. The second selection switch selectively electrically connects each of the low-potential-side terminals of the second high-side switches to either a corresponding one of the high-potential-side terminals of the second low-side switches or the second neutral point between the second coils. In the full-wave driving process, the full-wave driver controls the on-off switching of the first high-side switches, the first low-side switches, the second high-side switches, and the second low-side switches, wherein the high-potential-side terminals of the first low-side switches are electrically connected to the corresponding low-potential-side terminals of the first high-side switches through the first selection switch, and the low-potential-side terminals of the second high-side switches are electrically connected to the corresponding high-potential-side terminals of the second low-side switches through the second selection switch. In the half-wave connection process, the half-wave driver controls the on-off switching of the first high-side switches and the second low-side switches, the high-potential-side terminals of the first low-side switches are electrically connected to the first neutral point through the first selection switch, and the low-potential-side terminals of the second high-side switches are electrically connected to the second neutral point through the second selection switch, and the first low-side switches and the second high-side switches are kept on.With the configuration described above, during the half-wave driving process, an electric current flows in the direction from the corresponding first high-side switch to the first neutral point in each of the first coils. On the other hand, in each of the second coils, an electric current flows in the direction from the second neutral point to the corresponding second low-side switch. That is, the directions of the electric currents flowing in the first coils with respect to the first neutral point are opposite to the directions of the electric currents flowing in the second coils with respect to the second neutral point. Consequently, it is possible to reduce the maximum instantaneous torque while increasing the minimum instantaneous torque of the rotating electric machine. As a result, it becomes possible to reduce the torque ripple of the rotating electric machine.The rotary electric machine system further includes a positive conductor connected to the positive terminal of the DC power source, a negative conductor electrically connected to the negative terminal of the DC power source, and a capacitor electrically connected between the positive and negative conductors. Each of the high-potential-side terminals of the first and second high-side switches is electrically connected to the positive conductor. Each of the low-potential-side terminals of the first and second low-side switches is electrically connected to the negative conductor.With the configuration described above, during the half-wave driving process, the electric current ripple generated in the positive and negative conductors due to the on-off switching of the first high-side switches can be cancelled by the electric current ripple generated in the positive and negative conductors due to the on-off switching of the second low-side switches. Consequently, it becomes possible to reduce the total electric current ripple generated in the positive and negative conductors, thereby making it possible to decrease the capacitance of the capacitor. As a result, it is possible to minimize the size of the capacitor.Further, in the half-wave driving process, the half-wave driver cyclically powers the first high-side switches such that start timings of the power of the first high-side switches are offset from each other and each of the first high-side switches is powered for a predetermined first period in each cycle; the first predetermined period is shorter than or equal to half of each cycle. The half-wave driver also cyclically powers the second low-side switches such that the start timings of the power of the second low-side switches are offset from each other and each of the second low-side switches is powered for a second predetermined period in each cycle; the second predetermined period is shorter than or equal to half of each cycle. More specifically, the starting timings of feeding the first high-side switches are offset from each other by 360° / N in electrical angle; the starting timings of feeding the second low-side switches are also offset from each other by 360° / N in electrical angle.Further, each of the first and second predetermined periods is set to be longer than or equal to (90°+180° / N) and shorter than or equal to 180° in electrical angle.If the energization periods of the first and second coils were short, the torque ripple of the rotating electric machine would be increased. However, by setting the first and second predetermined periods corresponding to the number of phases N as described above, sufficiently long energization periods of the first and second coils can be ensured, thereby effectively suppressing the torque ripple of the rotating electric machine.The first coils and the second coils are wound on the stator core such that each of the first coils has the same spatial phase as a corresponding one of the second coils. In the half-wave driving process, the half-wave driver cyclically powers the first high-side switches and the second low-side switches such that, for each pair of the first and second equi-spatial-phase coils, the start timing of the power of one of the first high-side switches corresponding to the first coil in the equi-spatial-phase pair and the start timing of the power of one of the second low-side switches corresponding to the second coil of the equi-spatial-phase pair are offset from each other by 180° in electrical angle.With the above-described configuration, for each pair of the first and second coils having the same spatial phase, the first coil is energized during the shutdown periods of the second coil and the second coil is energized during the shutdown periods of the first coil. Consequently, it is possible to increase the minimum value of the cyclically changing torque of the rotating electric machine, thereby increasing the average torque of the rotating electric machine.In addition, in the case where each of the first and second predetermined periods is respectively set to be 180° in electrical angle, for each pair of the first and second coils having the same spatial phase, the end timing of feeding the corresponding first high-side switch coincides with the start timing of feeding the corresponding second low-side switch.The electric power conversion circuit further includes first and second selector switch controllers. The first selection switch control device controls the first selection switch to perform, for each of the N first coils, switching between electrically connecting the high-potential-side terminal of the corresponding first low-side switch to the low-potential-side terminal of the corresponding first high-side switch and electrically connecting the high-potential-side terminal of the corresponding first low-side switch to the first neutral point only when the absolute value of the electric current flowing in the first coil is less than or equal to a first threshold current value. The second selection switch control device controls the second selection switch to perform, for each of the N second coils, switching between electrically connecting the low-potential-side terminal of the corresponding second high-side switch to the high-potential-side terminal of the corresponding second low-side switch and electrically connecting the low-potential-side terminal of the corresponding second high-side switch to the second neutral point only when the absolute value of the electric current flowing in the second coil is less than or equal to a second threshold current value.With the above-described configuration, it becomes possible to suppress the surge voltages generated during switching by the first and second selection switches, thereby improving the reliability of the rotary electric machine system.The first selection switch includes N pairs of first and second connection switches corresponding to the N first coils, respectively. For each of the N first coils, the first connection switch is configured to electrically connect the high-potential-side terminal of the corresponding first low-side switch to the low-potential-side terminal of the corresponding first high-side switch, and the second connection switch is configured to electrically connect the high-potential-side terminal of the corresponding first low-side switch to the first neutral point. The second selection switch includes N pairs of third and fourth connection switches corresponding to the N second coils, respectively. For each of the N second coils, the third connection switch is configured to connect the low-potential-side terminal of the corresponding second high-side switch to the high-potential-side terminal of the corresponding second low-side switch, and the fourth connection switch is configured to connect the low-potential-side terminal of the corresponding second high-side switch to the second neutral point. The electric power conversion circuit further includes a first selection switch controller that controls the first selection switch and a second selection switch controller that controls the second selection switch. The first selection switch control device performs first and second switching processes for each of the N first coils. In the first switching process, the first selection switch controller switches the corresponding first connection switch from on to off and the corresponding second connection switch from off to on. In the second switching process, the first selection switch control device switches the corresponding first selection switch from off to on and the corresponding second connection switch from on to off. In each of the first and second switching processes, there is a period during which both of the corresponding first and second connection switches are held on by the first selection switch controller. The second selection switch control device performs third and fourth switching processes for each of the N second coils. In the third switching process, the second selection switch controller switches the corresponding third connection switch from on to off and the corresponding fourth connection switch from off to on. In the fourth switching process, the second selection switch controller switches the corresponding third connection switch from off to on and the corresponding fourth connection switch from on to off. In each of the third and fourth switching processes, there is a period during which both of the corresponding third and fourth connection switches are held on by the second selection switch controller.With the above-described configuration, it becomes possible to prevent electric current flow paths in the rotating electric machine from being interrupted during the first to fourth switching processes, thereby suppressing generation of surge voltage in the rotating electric machine.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will be more fully understood from the detailed description given below and from the accompanying drawings of exemplary embodiments, which, however, should not be taken to limit the invention to the specific embodiments, but are for the purpose of explanation and understanding only.In the accompanying drawings, there are shown: FIG. 1 is a schematic diagram illustrating the overall configuration of an in-vehicle rotary electric machine system according to a first embodiment, FIG. 2 is an electrical connection diagram of a three-phase stator coil of a rotating electric machine included in the system according to the first embodiment, FIG. 3 is a schematic diagram of the three-phase stator coil shown in FIG. 2, FIG. 4 is a circuit diagram of a U-phase part of a selection switch included in the system according to the first embodiment, FIG. 5 is a schematic diagram illustrating switching between a full-wave driving process and a half-wave driving process according to the first embodiment, FIG. 6 is a schematic diagram illustrating a energization pattern of the high-side U-phase, V-phase, and W-phase switches in the half-wave driving process according to the first embodiment, FIG. 7 is a waveform diagram illustrating the waveforms of phase currents flowing in the three-phase stator coil during the half-wave driving process according to the first embodiment, FIG. 8 is a waveform diagram illustrating a switching timing of the selection switch according to the first embodiment, FIG. 9 is a schematic diagram illustrating first and second switching processes of the selection switch according to the first embodiment, FIG. 10 is a schematic diagram illustrating the overall configuration of an in-vehicle rotary electric machine system according to a second embodiment, FIG. 11 is a cross-sectional view of a rotary electric machine included in the system according to the second embodiment, FIG. 12 is a circuit diagram showing an X-phase part of a second bypass switch included in the system according to the second embodiment, FIG. 13 is a schematic diagram illustrating control of turning on-off the high-side U-phase, V-phase, and W-phase switches and the low-side X-phase, Y-phase, and W-phase switches in a half-wave driving process according to the second embodiment, FIG. 14 is a waveform diagram illustrating the waveforms of phase currents flowing in the first and second three-phase stator coils during the half-wave driving process according to the second embodiment, FIG. 15 is a schematic diagram illustrating the electric current flow paths in the rotating electric machine during the half-wave driving process according to the second embodiment, FIG. 16 is a schematic diagram illustrating advantageous effects achievable by the half-wave driving process according to the second embodiment; and FIG. 17 is a cross-sectional view of a rotary electric machine according to a modification of the second embodiment.DESCRIPTION OF EMBODIMENTSExemplary embodiments are described below with reference to FIGS. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 to 17. It should be noted that, for clarity and understanding purposes, identical components having identical functions are denoted by the same reference numerals throughout the specification where possible in each of the figures, and that for redundancy avoidance, descriptions of identical components are not repeated.[First Embodiment]FIG. 1 shows the overall configuration of a rotary electric machine system MU according to a first embodiment.According to the present embodiment, the rotary electric machine system MU is installed in a motor vehicle having an engine 20.As shown in FIG. 1, the rotary electric machine system MU includes a rotary electric machine 10. According to the present embodiment, the rotating electric machine 10 is configured as a permanent magnet type synchronous machine. More specifically, the rotating electric machine is realized by an ISG (integrated starter-generator) that can selectively function as either an electric motor or an electric generator.The rotating electric machine 10 includes a rotor 11 mechanically connected to an output shaft of the engine 20 to provide / receive mechanical power to / from the output power. More specifically, according to the present embodiment, the rotor 11 is mechanically connected to the output shaft of the engine 20 via a belt (not shown). Further, to the output shaft of the engine 20, drive wheels 21 of the vehicle are mechanically connected via a power transmission path including a transmission (not shown) and an axle (not shown) of the vehicle.Further, the rotor 11 includes a plurality of permanent magnets forming a plurality of magnetic poles; the polarities of the magnetic poles alternate between N (north) and S (south) in a circumferential direction of the rotor 11.The rotating electric machine 10 further includes a stator core (not shown) on which a three-phase stator coil is wound. The three-phase stator coil includes a U-phase coil 12U, a V-phase coil 12V, and a W-phase coil 12W. The U-phase, V-phase, and W-phase coils 12U, 12V, and 12W are star-connected (or Y-connected) to define a neutral point N therebetween. Further, the U-phase, V-phase and W-phase coils 12U, 12V and 12W are wound on the stator core in such a manner as to be offset from each other by 120° in electrical angle.In addition, in FIG. 2, the U-phase, V-phase, and W-phases are represented by the coils 12U, 12V, and 12W such that the phase offset of 120° is represented in the electrical angle therebetween. On the other hand, in FIG. 1, for convenience, the U-phases, V-phases, and W-phase coils 12U, 12V, and 12W are illustrated as shown by enlargement in FIG. 3.Referring back to FIG. 1, the rotary electric machine system MU includes a pair of a high-side U-phase switch SUp and a low-side U-phase switch SUn, a pair of a high-side V-phase switch SVp and a low-side V-phase switch SVn, and a pair of a high-side W-phase switch SWp and a low-side W-phase switch SWn. Further, high-potential-side terminals of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp are electrically connected to a positive terminal of a DC power source 30 via a positive conductor Lp such as a bus bar. On the other hand, the low potential side terminals of the low side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn are electrically connected to a negative terminal of the DC power source 30 via respective shunt resistors 13U, 13V, and 13W and a negative conductor Ln such as a bus bar.The rotary electric machine system MU also includes a capacitor 31. A high potential side terminal of the capacitor 31 is electrically connected to the positive conductor Lp, and a low potential side terminal thereof is electrically connected to the negative conductor Ln.According to the present embodiment, each of the high-side U-phase, V-phase and W-phase switches SUp, SVp and SWp and the low-side U-phase, V-phase and W-phase switches SUn, SVn and SWn is realized by a voltage control type semiconductor switching element, more specifically, an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Accordingly, for each of the switches SUp-SWp and SUn-SWn, the high potential side terminal is the drain and the low potential side terminal is the source. In addition, each of the switches SUp-SWp and SUn-SWn includes a body diode (not shown) connected in antiparallel (or inversely in parallel) thereto.The negative conductor Ln is electrically connected to ground. According to the present embodiment, the DC power source 30 is realized by a battery. Further, the rated voltage of the DC power source 30 is, for example, 12 volts.First ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W are electrically connected to the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, respectively, via an electric conductor such as a bus bar. On the other hand, second ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W are electrically connected to each other at the neutral point N.In addition, according to the present embodiment, each of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp is configured as a "permanent connected switch" that is permanently (or always) electrically connected between the positive terminal of the DC power source 30 and a corresponding one of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W. Further, each of the sources (or low potential side terminals) of the high side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp constitutes a "permanently connected terminal" that is permanently electrically connected to a corresponding one of first ends (or non-neutral point side ends) of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W.The rotary electric machine system MU further includes a selection switch 40. The selection switch 40 selectively electrically connects each of the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to either a corresponding one of the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp or the neutral point N between the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W.In addition, according to the present embodiment, each of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn is configured as a "selectively connected switch" that is selectively electrically connected to either a corresponding one of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp or the neutral point N. Further, each of the drains (or high-potential-side terminals) of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn constitutes a "selectively connected terminal" that is selectively electrically connected to either a corresponding one of the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp or the neutral point N.FIG. 4 illustrates the configuration of the selection switch 40 according to the present embodiment. The selection switch 40 includes a U-phase part, a V-phase part, and a W-phase part corresponding to the U, V, and W phases, respectively. The U-phase, V-phase and W-phase parts of the selection switch 40 are identical in configuration. Therefore, for simplicity, only the U-phase portion of the selector switch 40 is shown in FIG. 4. In addition, the U-phase shunt resistor 13U is also omitted from FIG. 4 for simplicity.As shown in FIG. 4, the U-phase part of the selection switch 40 includes a first connection switch 40A and a second connection switch 40B. According to the present embodiment, each of the first and second connection switches 40A and 40B is realized by an N-channel MOSFET. The drain of the first connection switch 40A is electrically connected to the source of the high-side U-phase switch SUp, and the source thereof is electrically connected to the drain of the low-side U-phase switch SUn. On the other hand, the drain of the second connection switch 40B is electrically connected to the neutral point N via an electric conductor such as a bus bar, and the source thereof is electrically connected to the drain of the low-side U-phase switch SUn.Referring back to FIG. 1, the rotary electric machine system MU further includes a control device 50. The control device 50 is supplied with magnetic pole position information Sig of the rotor 11. The control apparatus 50 includes a first control device 50A and a second control device 50B.The second controller 50B is supplied with electric potential differences via the U-phase, V-phase, and W-phase shunt resistors 13U, 13V, and 13W. The second controller 50B functions as "selection switch controller" for controlling the selection switch 40.Specifically, referring again to FIG. 4, the second controller 50B performs a first switching process in which the first connection switch 40A is turned off and the second connection switch 40B is turned on, thereby electrically connecting the drain of the low-side U-phase switch SUn to the neutral point N. Further, the second controller 50B also performs a second switching process in which the first connection switch 40A is turned on and the second connection switch 40B is turned off, thereby electrically connecting the drain of the low-side U-phase switch SUn to the source of the high-side U-phase switch SUp.In addition, as the magnetic pole position information Sig, electrical angle information obtained by an electrical angle sensor that detects the electrical angle of the rotor 11 may be used. In this case, the electric angle sensor may be realized by, for example, a resolver, a Hall IC (Integrated Circuit), an MR sensor (Magneto-Resistive Sensor), or an optical sensor. Alternatively, as the magnetic pole position information Sig, electrical angle information obtained by position sensor-less control without using an electrical angle sensor may be used.Note that the components of the rotary electric machine system MU including the rotary electric machine 10, the switches SUp-SWp and SUn-SWn, the selection switch 40, and the controller 50 may be configured integrally or separately from each other.The first controller 50A controls the on-off switching of the switches SUp-SWp and SUn-SWn on the basis of the magnetic pole position information Sig of the rotor 11, thereby driving the rotating electric machine 10 to function as an electric motor. In addition, according to the present embodiment, the first controller 50A functions as a "half-wave driver", a "full-wave driver", and a "drive controller".Specifically, the controller 50A first calculates the rotational speed Nm of the rotor 11 based on the magnetic pole position information Sig. Then, the first controller 50A determines whether the calculated rotation speed Nm is higher than a first predetermined rotation speed Nth 1.Referring to FIG. 5, when it is determined that the calculated rotational speed Nm of the rotor 11 is not higher than (i.e., less than or equal to) the first predetermined rotational speed Nth 1, the first controller 50A performs a full-shaft drive process for full-shaft driving the rotating electric machine 10.In the full-wave driving process in which the selection switch 40 is controlled by the second controller 50B for electrically connecting the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, respectively, the first controller 50A controls the on-off switching of the switches SUp-SWp and SUn-SWn such that the high-side U-phase, V-phase, and W-phase switches SUp, SVp and SWp are alternately turned on with the low-side U-phase, V-phase and W-phase switches SUn, SVn and SWn, respectively. Consequently, a symmetrical three-phase alternating current is supplied from the connection points between the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W. In addition, a well-known method such as a vector control method for supplying the three-phase balanced alternating current may be used.On the other hand, when it is determined that the calculated rotation speed Nm is higher than the first predetermined rotation speed Nth 1, the first controller 50A performs a half-wave driving process for half-wave driving the rotating electric machine 10.In the half-wave driving process in which the selector switch 40 is controlled by the second controller 50B to electrically connect all drains of the low-side U-phase, V-phase and W-phase switches SUn, SVn and SWn to the neutral point N, the first controller 50A controls the on-off switching of the high-side U-phase, V-phase and W-phase switches SUp, SVp and SWp according to a energization pattern as shown in FIG. 6, while all the low-side U-phase, V-phase and W-phase switches SUn, SVn and SWn can be maintained in an on state.In addition, during the half-wave driving process, since the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn are maintained in the on state, no switching loss occurs in these switches SUn, SVn, and SWn.Further, referring back to FIG. 5, according to the present embodiment, when it is determined that the calculated rotational speed Nm of the rotor 11 is lower than a second predetermined rotational speed NTh 2 that is lower than the first predetermined rotational speed Nth 1 after being once determined to be higher than the first predetermined rotational speed Nth 1, the first controller 50A switches the driving process for the rotating electric machine 10 from the half-shaft driving process to the full-shaft driving process. Consequently, it is possible to avoid occurrence of frequent switching between the half-wave driving process and the full-wave driving process.Hereinafter, the half-wave driving process according to the present embodiment will be described in more detail with reference to FIGS. 6 and 7, each division on the horizontal axis representing 60° in electrical angle.As shown in FIG. 6, according to the present embodiment, the first controller 50A offsets the start timings of energizing the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp from each other by 120° in electrical angle. Further, the first controller 50A cyclically powers each of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp at 180° electrical angle intervals. Thus, for each of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, turn-off periods of 180° in electrical angle exist. Further, the energization periods of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp overlap each other by 60° in electrical angle.According to the present embodiment, the energization control of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp is performed by sine wave PWM control. Consequently, as shown in FIG. 7, the waveforms of the U-phase, V-phase, and W-phase currents flowing in the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, respectively, have the shape of a half-sine wave. More specifically, in each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, the phase current flows for only half in each sinusoidal cycle (i.e., 360° / 2=180° in electrical angle).In addition, as shown in FIG. 7, the directions of the U-phase, V-phase, and W-phase currents are defined to be positive when these currents flow from the first ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W to the neutral point N, respectively, and negative in the opposite directions.As described above, according to the present embodiment, with the selection switch 40, it is possible to perform the half-wave driving process as well as the full-wave driving process without using a semiconductor element, as disclosed in Japanese Patent Application Publication No. JP H11-332 288A; in the patent document, the semiconductor element is electrically connected between the high-side switches and the neutral point.Further, according to the present embodiment, during the half-wave driving process, all of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn are used. That is, there is no redundant switch in the rotary electric machine system MU. Consequently, it becomes possible to effectively use all of the SUp-SWp and SUn-SWn constituting the electric power conversion circuit, thereby minimizing the size of the rotary electric machine system MU.Further, according to the present embodiment, during the half-wave driving process, the electric current from the DC power source 30 is distributed to the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn. Consequently, it becomes possible to prevent concentration of load on any of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn, thereby effectively radiating heat generated by these switches. As a result, it becomes possible to improve the reliability of the rotary electric machine system MU.According to the present embodiment, the second controller 50B controls the selector switch 40 to perform, for each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, switching between electrically connecting the drain of the corresponding low-side switch to the source of the corresponding high-side switch and electrically connecting the drain of the corresponding low-side switch to the neutral point N only when the absolute value of the phase current flowing in the phase coil is less than or equal to a threshold current value Ith. Consequently, it is possible to suppress generation of a surge voltage during the first and second switching processes, thereby improving the reliability of the rotary electric machine system.More specifically, for each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, during the first and second switching processes (or during switching between electrically connecting the drain of the corresponding low-side switch to the source of the corresponding high-side switch and electrically connecting the drain of the corresponding low-side switch to the neutral point N), a closed circuit including the DC power source 30 and the phase coil is temporarily opened, which generates a surge voltage. The greater the absolute value of the phase current when the closed circuit is opened, the higher the surge voltage generated. Further, the phase current flowing in the phase coil is an alternating current; therefore, there are zero-cross points in the phase current, and the absolute value of the phase current is small in the vicinity of the zero-cross points. Thus, by performing the first and second switching processes only when the absolute value of the phase current is less than or equal to the threshold current value Ith, it is possible to suppress the surge voltage generated during the first two switching processes.FIG. 8 illustrates switching permission periods TRU, TRV, and TRW. During the switching permission periods TRU, the absolute value of the U-phase current flowing in the phase coil 12U is less than or equal to the threshold current value Ith; thus, the selection switch 40 is permitted to perform switching between electrically connecting the drain of the low-side U-phase switch SUn to the source of the high-side U-phase switch SUp and electrically connecting the drain of the low-side U-phase switch SUn to the neutral point N. Similarly, during the switching permission periods TRV, the absolute value of the V-phase current flowing in the V-phase coil 12V is less than or equal to the threshold current value Ith; thus, the selector switch 40 is permitted to perform switching between electrically connecting the drain of the low-side V-phase switch SVn to the source of the high-side V-phase switch SVp and electrically connecting the drain of the low-side V-phase switch SVn to the neutral point N. During the switching permission periods TRW, the absolute value of the W-phase current flowing in the W-phase coil 12W is less than or equal to the threshold current value Ith; thus, the selection switch 40 is permitted to perform switching between electrically connecting the drain of the low-side W-phase switch SWn to the source of the high-side W-phase switch SWp and electrically connecting the drain of the low-side W-phase switch SWn to the neutral point N.In addition, the threshold current value Ith may be set in consideration of both suppression of surge voltages and facilitation of determination of the switching permission periods TRU, TRV, and TRW. For example, the threshold current value Ith may be set to about 10% of the rated current value for the U-phase, V-phase, and W-phase currents flowing in the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, respectively.According to the present embodiment, during the full-wave driving process, the first controller 50A calculates the absolute values of the U-phase, V-phase, and W-phase currents based on the electric potential differences across the U-phase, V-phase, and W-phase shunt resistors 13U, 13V, and 13W and the energization pattern (or on-off switching pattern) of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn. This calculation method is based on the fact that the voltage vectors applied to the U-phase, V-phase and W-phase coils 12U, 12V and 12W corresponding to the energization pattern for the switches SUp-SWp and SUn-SWn are related to the U-phase, V-phase and W-phase currents detectable by the shunt resistors 13U, 13V and 13W. On the other hand, during the half-wave driving process, the first controller 50A calculates the absolute values of the U-phase, V-phase, and W-phase currents based on the electric potential differences across the U-phase, V-phase, and W-phase shunt resistors 13U, 13V, and 13W. This calculation method is based on the fact that the electric current flowing to the neutral point N is divided into three parts flowing through the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn, respectively.Further, the rotating electric machine 10 may be configured as a field winding type machine instead of a permanent magnet type machine. In this case, during the first and second switching processes for suppressing generation of a surge voltage, it may be possible to switch the driving states of the first and second connection switches 40A and 40B with one interruption of the supply of the excitation current (or field current) to the field winding. However, the time constant (for example, several hundred milliseconds) of the excitation current supplied to the field winding is at least an order of magnitude larger than the control constant of the switches SUp-SWp and SUn-SWn. Therefore, when the supply of the excitation current to the field winding is resumed after switching the driving states of the first and second connection switches 40A and 40B, it would take an excessively long time until the excitation current reaches a target value thereof. Consequently, with the execution of the first and second switching processes, the operation of the rotating electric machine 10 would be temporarily stopped. In contrast, if the first and second switching processes were performed only when the absolute value of the phase current is less than or equal to the threshold current value Ith, it would become unnecessary to interrupt the supply of the excitation current for the purpose of suppressing the generation of a surge voltage. Consequently, it would become possible to prevent the operation of the rotary electric machine 10 from being temporarily stopped.According to the present embodiment, the determination as to whether the absolute value of the phase current is less than or equal to the threshold current value Ith may alternatively be made based on the magnetic pole position information Sig. This determination method is based on the fact that the electrical phase of the phase current is related to the magnetic pole position of the rotor 11. More specifically, the second controller 50B may be modified to perform the first and second switching processes only when the magnetic pole position of the rotor 11 determined based on the magnetic pole position information Sig is within a range in which it is assumed that the absolute value of the phase current is less than or equal to the threshold current value Ith.According to the present embodiment, as shown in FIG. 9( a), in the first switching process, the second controller 50B turns on the second connection switch 40B before turning off the first connection switch 40A. Consequently, in the first switching process, there is a period during which both of the first and second connection switches 40A and 40B are held on by the second controller 50B. Further, as shown in FIG. 9( b), in the second switching process, the second controller 50B turns on the first connection switch 40A before turning off the second connection switch 40B. Consequently, in the second switching process, there is also a period during which both of the first and second connection switches 40A and 40B are held on by the second controller 50B. By performing the first and second switching processes as described above, it is possible to prevent an electric current flow path including the phase coil from being interrupted due to switching of the driving states of the first and second connection switches 40A and 40B, thereby suppressing generation of surge voltage.[Second Embodiment]A rotary electric machine system MU according to a second embodiment has a similar configuration to the rotary electric machine system MU according to the first embodiment. Accordingly, the differences therebetween will be mainly described below.FIG. 10 shows the overall configuration of the rotary electric machine system MU according to the present embodiment. Note that for simplicity, the engine 20 and the shunt resistors 13U, 13V, and 13W have been omitted from FIG. 10.According to the present embodiment, the selection switch 40 described in the first embodiment is referred to as a first selection switch 40. The neutral point N between the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W is referred to as a first neutral point N 1. All functions performed by the first and second controllers 50A and 50B of the controller 50 will be described as being performed by the controller 50, and for simplicity, the first and second controllers 50A and 50B are not shown in the figures.As shown in FIG. 10, according to the present embodiment, the rotating electric machine 10 includes two three-phase coils, i.e., a first three-phase stator coil composed of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W described according to the first embodiment, and a second three-phase stator coil composed of X-phase, Y-phase, and W-phase coils 14X, 14Y, and 14W.The first and second three-phase stator coils are wound on the same stator core 15 a(see FIG. 11 ) in such a manner that the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W are electrically insulated from the X-phase, Y-phase, and W-phase coils 14X, 14Y, and 14W.The X-phase, Y-phase, and W-phase coils 14X, 14Y, and 14W are star-connected to define a second neutral point N 2 therebetween. Further, the X-phase, Y-phase and W-phase coils 14X, 14Y and 14W are wound on the stator core 15 ain such a manner as to be offset from each other by 120° in electrical angle.In addition, according to the present embodiment, the number of turns of the X-phase, Y-phase, and W-phase coils 14X, 14Y, and 14W is set to be equal to the number of turns of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W.With reference to FIG. 11, the configuration of the rotary electric machine 10 according to the present embodiment will be described in detail below. In addition, FIG. 11 shows a cross section of the rotating electric machine 10 taken along a plane perpendicular to the axis O of rotation of the rotor 11.As shown in FIG. 11, according to the present embodiment, the rotating electric machine 10 includes the rotor 11 and a stator 15. The stator 15 includes the stator core 15 a, and the first and second three-phase stator coils wound on the stator core 15 a.The stator core 15 ahas an annular shape and is disposed coaxially with the rotor 11. That is, the axis of the annular stator core 15 acoincides with the rotation axis O of the rotor 11.According to the present embodiment, the rotor 11 is rotatably disposed radially inside the stator 15. That is, the rotating electric machine 10 is configured as an inner rotor type rotating electric machine.The rotor 11 includes a rotor core 11 afixed to a rotation shaft and a plurality (for example, 8 according to the present embodiment) of permanent magnets 11 bprovided on a radially outer periphery of the rotor core 11 a. The permanent magnets 11 bform a plurality (for example, 8 according to the present embodiment) of magnetic poles whose polarities alternate between north and south in the circumferential direction of the rotor core 11 a.The stator core 15 aincludes a plurality (for example, 48 according to the present embodiment) of teeth 15 bformed with a constant pitch in the circumferential direction thereof. The stator core 15 aalso has a plurality (for example, 48 according to the present embodiment) of slots each of which is formed between a circumferentially adjacent pair of the teeth 15 b.The first and second three-phase stator coils are wound on the teeth 15 bof the stator core 15 asuch that they are accommodated in the slots of the stator core 15 a.More specifically, as shown in FIG. 11, the first three-phase stator coil is wound on the teeth 15 bof the stator core 15 ain such a manner as to be accommodated in the slots of the stator core 15 ain the order of the U-phase coil 12U+, the W-phase coil 12W-, the V-phase coil 12V+, the U-phase coil 12U-, the W-phase coil 12W+, and the V-phase coil 12V- in the circumferential direction of the stator core 15 a. Further, each of the U-phase coil 12U+, the W-phase coil 12W-, the V-phase coil 12V+, the U-phase coil 12U-, the W-phase coil 12W+, and the V-phase coil 12V- is accommodated in pairs of circumferentially adjacent slots of the stator core 15 a. In addition, the characters "+" and "-" appended to each of the U-phase, V-phase and W-phase coils 12U, 12V and 12W respectively denote opposite polarities of the phase coil.Similarly, the second three-phase stator coil is wound on the teeth 15 bof the stator core 15 asuch that it is accommodated in the slots of the stator core 15 ain the order of the X-phase coil 14X+, the Z-phase coil 14Z-, the Y-phase coil 14Y+, the X-phase coil 14X-, the Z-phase coil 14Z+, and the Y-phase coil 14Y- in the circumferential direction of the stator core 15 a. Further, each of the X-phase coil 14X+, the Z-phase coil 14Z-, the Y-phase coil 14Y+, the X-phase coil 14X-, the Z-phase coil 14Z+, and the Y-phase coil 14Y- is accommodated in pairs of circumferentially adjacent slots of the stator core 15 a. In addition, the characters "+" and "-" appended to each of the X-phase, Y-phase and Z-phase coils 14X, 14Y and 14Z respectively denote opposite polarities of the phase coil.Further, according to the present embodiment, each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W of the first three-phase stator coil and a corresponding one of the X-phase Y-phase and Z-phase coils 14X, 14Y, and 14Z of the second three-phase stator coil are arranged in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W of the first three-phase stator coil are located radially outside the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z of the second three-phase stator coil.More specifically, as shown in FIG. 11, the U-phase coil 12U+ and the X-phase coil 14X+ are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the U-phase coil 12U+ is located radially outside the X-phase coil 14X+.Similarly, the W-phase coil 12W and the Z-phase coil 14Z- are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the W-phase coil 12W- is located radially outside the Z-phase coil 14Z-.The V-phase coil 12V+ and the Y-phase coil 14Y+ are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the V-phase coil 12V+ is located radially outside the Y-phase coil 14Y+.The U-phase coil 12U- and the X-phase coil 14X- are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the U-phase coil 12U- is located radially outside the X-phase coil 14X-.The W-phase coil 12W+ and the Z-phase coil 14Z+ are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the W-phase coil 12W+ is located radially outside the Z-phase coil 14Z+.The V-phase coil 12V- and the Y-phase coil 14Y- are disposed in the same slots of the stator core 15 ain such a manner as to be electrically insulated from each other. In addition, the V-phase coil 12V- is located radially outside the Y-phase coil 14Y-.Note that, for simplicity, the cross sections of the coils 12U to 12W and 14X to 14Z in FIG. 11 are not shaded.With the above-described arrangement of the first and second three-phase stator coils in the slots of the stator core 15 a, the spatial phase difference between each corresponding pair of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W and the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z is 0° in electrical angle.Further, since the first and second three-phase stator coils are wound on the same stator core 15 a, it becomes possible to suppress magnetic flux fluctuation in the air gap between the rotor 11 and the stator 15, thereby reducing the torque ripple of the rotary electric machine 10.Referring back to FIG. 10, according to the present embodiment, the rotary electric machine system MU further includes a pair of a high-side X-phase switch SXp and a low-side X-phase switch SXn, a pair of a high-side Y-phase switch SYp and a low-side Y-phase switch SYn, and a pair of a high-side Z-phase switch SZp and a low-side Z-phase switch SZn.According to the present embodiment, each of the high-side X-phase, Y-phase and Z-phase switches SXp, SYp and SZp and the low-side X-phase, Y-phase and Z-phase switches SXn, SYn and SZn is realized by a voltage control type semiconductor switching element, more specifically, by an N-channel MOSFET. Accordingly, for each of the switches SXp-SZp and SXn-SZn, the high potential side terminal is the drain and the low potential side terminal is the source. In addition, each of the switches SXp-SZp and SXn-SZn includes a body diode (not shown) connected in antiparallel thereto.The drains of the high-side X-phase, Y-phase and Z-phase switches SXp, SYp and SZp are electrically connected to the positive terminal of the DC power source 30 via the positive conductor LP. On the other hand, the sources of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn are electrically connected to the negative terminal of the DC power source 30 via the negative conductor Ln.First ends of the X-phase, Y-phase, and Z-phase coils 40X, 40Y, and 40Z are electrically connected to the drains of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn, respectively. On the other hand, second ends of the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z are electrically connected to each other at the second neutral point N 2.According to the present embodiment, the rotary electric machine system MU further includes a second selection switch 60. The second selection switch 60 selectively electrically connects each of the sources of the high-side X-phase, Y-phase, and Z-phase switches S XP, SYP, and SZP to either a corresponding one of the drains of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn or the second neutral point N 2 between the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z.FIG. 12 illustrates the configuration of the second selection switch 60 according to the present embodiment. The second selection switch 60 has an X-phase part, a Y-phase part and a Z-phase part corresponding to the X, Y and Z phases, respectively. The X-phase, Y-phase and Z-phase parts of the second selection switch 60 are identical in configuration. Therefore, for simplicity, only the X-phase part of the second selection switch 60 is shown in FIG. 12.As shown in FIG. 12, the X-phase part of the second selection switch 60 includes a third connection switch 60A and a fourth connection switch 60B. According to the present embodiment, each of the third and fourth connection switches 60A and 60B is realized by an N-channel MOSFET. The drain of the third connection switch 60A is electrically connected to the source of the high-side X-phase switch SXp, and the source thereof is electrically connected to the drain of the low-side X-phase switch SXn. On the other hand, the drain of the fourth connection switch 40B is electrically connected to the source of the high-side X-phase switch SXp, and the source thereof is electrically connected to the second neutral point N 2.According to the present embodiment, the control device 50 controls both the second selection switch 60 and the first selection switch 40.Specifically, as shown in FIG. 12, the control device 50 performs a third switching process in which the third connection switch 60A is turned off and the fourth connection switch 60B is turned on, thereby connecting the source of the high-side X-phase switch SXp to the second neutral point N 2. Further, the control device 50 also performs a fourth switching process in which the third connection switch 60A is turned on and the fourth connection switch 60B is turned off, thereby connecting the source of the high-side X-phase switch SXp to the drain of the low-side X-phase switch SXn.Further, according to the present embodiment, in the third switching process of the control device 50, the fourth connection switch 60B turns on before the third connection switch 60A turns off. Consequently, in the third switching process, there is a period during which both of the third and fourth connection switches 60A and 60B are held on by the controller 50. Further, in the fourth switching process, the control device 50 turns on the third connection switch 60A before turning off the fourth connection switch 60B. Consequently, in the fourth switching process, there is also a period during which both of the third and fourth connection switches 60A and 60B are kept on by the controller 50B. By performing the third and fourth switching processes as described above, it is possible to prevent an electric current flow path including the X-phase coil 14X from being interrupted due to switching of the driving states of the third and fourth connection switches 60A and 60B, thereby suppressing generation of a surge voltage.According to the present embodiment, the control device 50 controls the first selection switch 40 to perform, for each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, switching between electrically connecting the drain of the corresponding low-side switch to the source of the corresponding high-side switch and electrically connecting the drain of the corresponding low-side switch to the first neutral point N 1 only when the absolute value of the phase current flowing in the phase coil is less than or equal to a first threshold current value Ith 1. Further, the control device 50 controls the second selection switch 60 to perform, for each of the X-phase, Y-phase, and W-phase coils 14X, 14Y, and 14Z, switching between electrically connecting the source of the corresponding high-side switch to the drain of the corresponding low-side switch and electrically connecting the source of the corresponding high-side switch to the second neutral point N 2 only when the absolute value of the phase current flowing in the phase coil is less than or equal to a second threshold current value Ith 2.In addition, the first and second threshold current values Ith 1 and Ith 2 may be set equal to each other or different from each other. The absolute value of the phase current may be calculated based on either the phase current detectable by the corresponding shunt resistor or the magnetic pole position information Sig as described in the first embodiment.According to the present embodiment, the control device 50 controls the on-off switching of the switches SUp-SWp, SUn-SWn, SXp-SZp, and SXn-SZn on the basis of the magnetic pole position information Sig, thereby driving the rotating electric machine 10 to function as an electric motor.Specifically, when the rotation speed Nm of the rotor 11 calculated based on the magnetic pole position information Sig is not greater than the first predetermined rotation speed Nth 1, the control device 50 performs a full-shaft drive process for full-shaft driving of the rotating electric machine 10.In the full-wave driving process in which the first selection switch 40 is controlled to electrically connect the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, respectively, the controller 50 controls the on-off switching of the switches SUp-SWp and SUn-SWn such that the high-side U-phase, V-phase, and W-phase switches SUp, respectively, SVp and SWp are alternately turned on with the low-side U-phase, V-phase and W-phase switches SUn, SVn and SWn, respectively. Consequently, a symmetrical three-phase alternating current is supplied from the connection points between the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W. Further, when the second selection switch 60 is controlled to electrically connect the sources of the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp, and SZp to the drains of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn, respectively, the control device 50 controls the on-off switching of the switches SXp-SZp and SXn-SZn such that the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp and SZp are alternately turned on with the low-side X-phase, Y-phase and Z-phase switches SXn, SYn and SZn, respectively. Consequently, a symmetrical three-phase alternating current is supplied from the connection points between the high-side X-phase, Y-phase and Z-phase switches SXp, SYp and SZp and the low-side X-phase, Y-phase and Z-phase switches SXn, SYn and SZn to the X-phase, Y-phase and Z-phase coils 14X, 14Y and 14Z.On the other hand, when the calculated rotation speed Nm of the rotor 11 is greater than the first predetermined rotation speed Nth 1, the controller 50 performs a half-wave driving process for half-wave driving the rotating electric machine 10.In the half-wave driving process in which the first selection switch 40 is controlled to electrically connect all drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the first neutral point N 1, the controller 50 controls the on-off switching of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp according to a energization pattern as shown in FIG. 13, while all of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn are maintained in an on state. Further, when second selection switches 60 are controlled to electrically connect all sources of the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp, and SYp to the second neutral point N 2, the controller 50 controls the on-off switching of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn according to the energization pattern as shown in FIG. 13, while all of the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp, and SZp are maintained in an on state.Specifically, as shown in FIG. 13, the control device 50 offsets the start timings of energizing the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp from each other by 120° in electrical angle. Further, the controller 50 cyclically powers each of the high-side U-phase, V-phase and W-phase switches SUp, SVp and SWp at 180° electrical angle intervals. Thus, for each of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, there are turn-off periods of 180° in electrical angle. Further, the energization periods of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp overlap each other by 60° in electrical angle.Similarly, the controller 50 also offsets the start timings of energizing the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn from each other by 120° in electrical angle. Further, the controller 50 cyclically powers each of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn at 180° electrical angle intervals. Thus, for each of the low-side X-phase, Y-phase, and Z-phase switches SXp, SYn, and SZn, there are turn-off periods of 180° in electrical angle. Further, the energization periods of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn overlap each other by 60° in electrical angle.As described above, according to the present embodiment, the first and second three-phase stator coils are wound on the stator core 15 asuch that each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W has the same spatial phase as a corresponding one of the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z. Further, for each equal spatial phase pair of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W and the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z, the start timings of energizing a corresponding pair of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn are offset from each other by 180° in electrical angle, wherein the respective pair of switches respectively corresponds to the pair having the same spatial phase of the coils.More specifically, the U-phase coil 12U has the same spatial phase as the X-phase coil 14X; the start timings of energizing the high-side U-phase switch SUp and the low-side X-phase switch SXn are offset from each other by 180° in electrical angle. The V-phase coil 12V has the same spatial phase as the Y-phase coil 14Y; the start timings of feeding the high-side V-phase switch SVp and the low-side Y-phase switch SYn are offset from each other by 180° in electrical angle. The W-phase coil 12W has the same spatial phase as the Z-phase coil 14Z; the start timings of feeding the high-side W-phase switch SWp and the low-side Z-phase switch SZn are offset from each other by 180° in electrical angle.According to the present embodiment, the energization control of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn is performed by sine wave PWM control. Consequently, as shown in FIG. 14, the waveform of the U-phase, V-phase, and W-phase currents flowing in the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, respectively, and the X-phase, Y-phase, and Z-phase currents flowing in the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z, respectively, have the shape of a half-sine wave. More specifically, in each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W and the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z, the phase current flows for only half in each sinusoidal cycle (i.e., 360° / 2=180° in electrical angle).In addition, as shown in FIG. 14, the directions of the U-phase, V-phase, and W-phase currents are defined to be positive when these currents flow from the first ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W to the first neutral point N 1, respectively, and negative in the opposite directions. The directions of the X-phase, Y-phase, and Z-phase currents are defined to be positive when these currents flow from the first ends of the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z to the second neutral point N 2, respectively, and negative in the opposite directions.FIG. 15 illustrates the electric current paths in the rotating electric machine 10 when the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp and the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn are driven according to the energization pattern as shown in FIG. 13.In the first three-phase stator coil, as indicated by broken arrows in FIG. 15, electric current flows through a circuit path including the positive terminal of the DC power source 30, the positive conductor Lp, at least one of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp that is in an on state, the first neutral point N 1, the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn, the negative conductor Ln, and the negative terminal of the DC power source 30. That is, electric current flows from the DC power source 30 to the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W via the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, and returns to the DC power source 30 via the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn.In the second three-phase stator coils, as indicated by broken arrows in FIG. 15, electric current flows through a circuit path including the positive terminal of the DC power source 30, the positive conductor Lp, the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp, and SZp, the second neutral point N 2, at least one of the high-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn that is in an on state, the negative conductor Ln, and the negative terminal of the DC power source 30. That is, electric current flows from the DC power source 30 to the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z via the high-side X-phase, Y-phase, and Z-phase switches SXp, SYp, and SZp, and returns to the DC power source 30 via the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn.Next, with reference to FIG. 16, advantageous effects achievable by the half-wave driving process according to the present embodiment will be described.As shown in FIG. 16, with the half-wave driving process according to the present embodiment, it becomes possible to reduce the torque ripple Trqrip of the rotating electrical machine 10 as compared with the half-wave driving process according to the first embodiment.More specifically, in the case of performing the half-wave driving process according to the present embodiment, the torque ripple Trqrip is only 20% of the average torque Trqave of the rotating electrical machine 10.The reasons for the above-described reduction in the torque ripple Trqrip are as described below. According to the present embodiment, the directions of the U-phase, V-phase, and W-phase currents flowing in the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W with respect to the first neutral point N 1 are opposite to the directions of the X-phase, Y-phase, and Z-phase currents flowing in the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z with respect to the second neutral point N 2; thus, it becomes possible to reduce the maximum instantaneous torque, whereas the minimum instantaneous torque of the rotary electric machine 10 is increased.Specifically, the half-wave driving process according to the present embodiment has the following features (A), (B), and (C). Consequently, it becomes possible to cause the Z-phase current flowing in the Z-phase coil 14Z having the same spatial phase as the W-phase coil 12W to be maximum at an intermediate time point between the start time point of feeding the U-phase coil 12U and the end time point of feeding the V-phase coil 12V; at the intermediate time point, the W-phase current flowing in the W-phase coil 12W is zero (see FIGS. 13 to 14 ). As a result, it becomes possible to significantly reduce the torque ripple Trqrip of the rotating electric machine 10.(A) The start timings of energizing the high-side U-phase, V-phase and W-phase switches SUp, SVp and SWp are offset from each other by 120° (i.e., 360° / 3) in electrical angle. The start timings of energizing the low-side X-phase, Y-phase and Z-phase switches SXN, SYN and SZN are also offset from each other by 120° in electrical angle.(B) The U-phase coil 12U has the same spatial phase as the X-phase coil 14X; the start timings of energizing the high-side U-phase switch SUp and the low-side X-phase switch SXn are offset from each other by 180° in electrical angle. The V-phase coil 12V has the same spatial phase as the Y-phase coil 14Y; the start timings of energizing the high-side V-phase switch SVp and the low-side Y-phase switch SYn are offset from each other by 180° in electrical angle. The W-phase coil 12W has the same spatial phase as the Z-phase coil 14Z; the start timings of feeding the high-side W-phase switch SWp and the low-side Z-phase switch SZn are offset from each other by 180° in electrical angle.(C) Each of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W and the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z is energized for one half of each sinusoidal cycle (i.e., 360° / 2=180° in electrical angle). In other words, the energization periods of these coils 12U to 12W and 14X to 14Z are set to 180° in electrical angle.Referring back to FIG. 16, with the half-wave driving process according to the present embodiment, it also becomes possible to increase the average torque Trqave of the rotating electric machine 10 as compared with the half-wave driving process according to the first embodiment.More specifically, in the case of performing the half-wave driving process according to the present embodiment, the average torque Trqave of the rotating electric machine 10 is 35 Nm. In comparison, in the case of performing the half-wave driving process according to the first embodiment, the average torque Trqave of the rotating electric machine 10 is 32 Nm. In addition, the average torque Trqave is approximately equal to the average value of the maximum and minimum torques of the rotating electric machine 10.The reasons for the above-described increase in the average torque Trqave are as described below. According to the present embodiment, when the equal spatial phase pair of the U-phase coil 12U and the X-phase coil 14X is taken as an example, during the shut-off periods of the U-phase coil 12U, the X-phase coil 14X is energized such that the X-phase current whose direction is opposite to the direction of the U-phase current flows in the X-phase coil 14X (see FIGS. 13 to 14 ). Consequently, it becomes possible to increase the minimum value of the cyclically changing torque of the rotating electric machine 10, thereby increasing the average torque Trqave.In addition, with the half-wave driving process according to the present embodiment, it also becomes possible to reduce the electric current ripple generated in the positive and negative conductors Lp and Ln, thereby making it possible to decrease the capacitance of the capacitor 31. As a result, it becomes possible to minimize the size of the capacitor 31.Although the particular embodiments described above have been shown and described, it will be understood by those skilled in the art that various modifications, changes and improvements can be made without departing from the spirit of the present invention.For example, according to the second embodiment, the spatial phase difference between each corresponding pair of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W and the X-phase, Y-phase, and Z-phase coils 14X, 14Y, and 14Z is set to 0° in electrical angle.However, the spatial phase difference between each corresponding pair of the U-phase, V-phase and W-phase coils 12U, 12V and 12W and the X-phase, Y-phase and Z-phase coils 14X, 14Y and 14Z can also be set to other values such as 30° in electrical angle as shown in FIG. 17. More specifically, in this case, the first and second three-phase stator coils are wound on the teeth 15 bof the stator core 15 ain such a manner as to be wound in the slots of the stator core 15 ain the order of the U-phase coil 12U+, the X-phase coil 14X+, the W-phase coil 12W-, the Z-phase coil 14Z-, the V-phase coil 12V+, the Y-phase coil 14Y+, the U-phase coil 12U-, the X-phase coil 14X-, the W-phase coil 12W+, the Z-phase coil 14Z+, the V-phase coil 12V- and the Y-phase coil 14Y- are accommodated in the circumferential direction of the stator core 15 a. Further, six slot pitches in the circumferential direction of the stator core 15 aare 180° in electrical angle. Thus, circumferentially adjacent slots of the stator core 15 aare spaced from each other by 30° (i.e., 180° / 6) in electrical angle. In this case, it is still possible to reduce the torque ripple of the rotating electric machine 10 as compared with the first embodiment.According to the first embodiment, it is also possible to electrically connect, instead of the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp, the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn to the non-neutral point-side ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W, respectively. In this case, each of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn constitutes a "permanent connected switch" which is permanently (or always) electrically connected to a corresponding one of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W. Further, each of the drains (or high-potential-side terminals) of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn constitutes a "permanently connected terminal" that is permanently electrically connected to a corresponding one of the non-neutral-point-side ends of the U-phase, V-phase, and W-phase coils 12U, 12V, and 12W. On the other hand, each of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp constitutes a "selectively connected switch" that is selectively electrically connected to either a corresponding one of the permanently connected switches (i.e., the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn) or the neutral point N. Further, each of the sources (or low-potential-side terminals) of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp constitutes a "selectively connected terminal" that is selectively electrically connected to either a corresponding one of the permanently connected terminals (i.e., the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn) or the neutral point N. The selection switch 40 electrically selectively connects each of the selectively connected terminals of the selectively connected switches (i.e., the sources of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp) to either a corresponding one of the permanently connected terminals of the permanently connected switches (i.e., the drains of the low-side U-phase, V-phase, and W-phase switches SUn, SVn, and SWn) or the neutral point N.According to the first embodiment, the energization control of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp is performed by sine wave PWM control. However, the energization control of the switches SUp, SVp, and SWp may also be performed by other methods. For example, each of the switches SUp, SVp, and SWp may be held on for half of each cycle. Alternatively, each of the switches SUp, SVp, and SWp may be turned on-off at a predetermined duty in the energization periods thereof. Here, the duty ratio denotes the ratio Ton / Tsw, where Ton is the on-time in each energization period and Tsw is the length of each energization period.Likewise, according to the second embodiment, the energization control of the low-potential side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn can also be performed by other methods as described above instead of the sine wave PWM control.According to the first and second embodiments, each of the energization periods of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp is set to half of each cycle (i.e., 360° / 2=180° in electrical angle). However, each of the energization periods of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SWp may be set to be longer than or equal to 120° and shorter than 180°, preferably set to be longer than or equal to 150° and shorter than 180° in electrical angle.Similarly, according to the second embodiment, each of the energization periods of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn may also be set to be longer than or equal to 120° and shorter than 180°, and preferably set to be longer than or equal to 150° and shorter than 180° in electrical angle.According to the second embodiment, the length of the energization periods of the high-side U-phase, V-phase, and W-phase switches SUp, SVp, and SW (which are hereinafter referred to as first predetermined periods) and the length of the energization periods of the low-side X-phase, Y-phase, and Z-phase switches SXn, SYn, and SZn (which are hereinafter referred to as second predetermined periods) are set to be equal to each other. However, the length of the first predetermined periods and the length of the second predetermined periods may be set to be different from each other, provided that it is still possible to effectively reduce the torque ripple compared to the first embodiment.According to the first and second embodiments, the number of phases of the stator coils is set to 3. However, the number of phases of the stator coils may be set to N, where N is an integer greater than or equal to 2, such as 2 or 4. Further, for each of the N-phase stator coils, the number of the respective high-side switches, the number of the respective low-side switches, and the number of pairs of the first and second connection switches included in the respective selection switch are also set to N.Further, in the half-wave driving process according to the first embodiment, the control device 50 may cyclically feed the N high-side switches such that the start timings of feeding the N high-side switches are offset from each other by 360° / N in electrical angle. Similarly, in the half-wave driving process according to the second embodiment, the controller 50 may cyclically feed the N high-side switches corresponding to the first N-phase stator coil such that the start timings of feeding the N high-side switches are offset from each other by 360° / N in electrical angle; the controller 50 may also cyclically feed the N low-side switches corresponding to the second N-phase stator coil such that the start timings of feeding the N low-side switches are offset from each other by 360° / N in electrical angle. In addition, in this case, each of the energization periods of the switches may be set to be longer than or equal to (90°+180° / N) and shorter than or equal to 180° in electrical angle.According to the first and second embodiments, it is possible to omit at least one of the processes illustrated in FIGS. 9 and 10.According to the second embodiment, in the half-wave driving process, the control device 50 can only drive the switches SUp-SWp and SUn-SWn corresponding to the U-phase, V-phase and W-phase coils 12U, 12V and 12W while the switches SXp-SZp and SXn-SZn corresponding to the X-phase, Y-phase and Z-phase coils 14X, 14Y and 14Z are maintained in an off state. Alternatively, the controller 50 may drive only the switches SXp-SZp and SXn-SZn while the switches SUp-SWp and SUn-SWn are maintained in an off state.According to the first and second embodiments, each of the switches is realized by an N-channel MOSFET. However, each of the switches may alternatively be realized by another semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor). In the case of an IGBT, the high-potential-side terminal is the collector and the low-potential-side terminal is the emitter. In addition, each of the switches may be formed of, for example, Si, SiC, or GaN.According to the first and second embodiments, the rotating electric machine 10 is configured as a permanent magnet type rotating electric machine. However, the rotating electric machine 10 may also be configured as a field winding type rotating electric machine. Alternatively, the rotating electric machine 10 may also be configured to include a field winding as well as a plurality of permanent magnets.According to the first and second embodiments, the DC power source 30 is realized by a battery whose rated voltage is 12V. However, the DC power source 30 may also be realized by a high voltage battery whose rated voltage is 100V or more.According to the first and second embodiments, each of the selection switches may also be realized by a relay.According to the first and second embodiments, the rotating electric machine 10 is mechanically connected to the output shaft of the engine 20 via a belt. However, the rotating electric machine 10 may be included in the power transmission path that connects the output shaft of the engine 20 and the drive wheels 21 to be mechanically connected between the transmission and the drive wheels 21. Alternatively, the rotating electric machine 10 may be included in the power transmission path to be mechanically connected between the engine 20 and the transmission.As described above, a rotary electric machine system includes a rotary electric machine and an electric power conversion circuit. The rotating electric machine has N coils connected to each other to define a neutral point therebetween. The electric power conversion circuit includes N high-side switches, N low-side switches, a selection switch, a full-wave driver, a half-wave driver, and a drive controller. The full-wave driver performs a full-wave driving process in which the high-potential-side terminals of the low-side switches are respectively connected to low-potential-side terminals of the high-side switches through the selection switch. The half-wave driver performs a half-wave driving process in which the high-potential-side terminals of the low-side switches are connected to the neutral point through the selector switch. The drive controller controls both the full-wave driver and the half-wave driver to selectively cause either the full-wave driver to perform the full-wave driving process or the half-wave driver to perform the half-wave driving process.

Claims

A rotary electric machine system (MU) comprising a rotary electric machine (10) and an electric power conversion circuit between which electric power is transmitted, the rotary electric machine (10) having a stator core (15a) and N coils (12U, 12V, 12W) wound on the stator core (15a) and each belonging to N phases, N being an integer greater than or equal to 2, the coils (12U, 12V, 12W) being electrically connected to each other to define a neutral point (N) therebetween, the power conversion circuit comprising N high-side switches (SUp, SVp, SWp), N low-side switches (SUn, SVn, SWn), a selector switch (40), a full-wave driver, a half-wave driver and a drive controller, comprising N high-side switches (SUp, SVp, SWp) respectively corresponding to the N coils (12U, 12V, 12W) and having a high-potential-side terminal and a low-potential-side terminal, all high-potential-side terminals of the high-side switches (SUp, SVp, SWp) being electrically connected to a positive terminal of a direct-current power source (30), the N low-side switches (SUn, SVn, SWn) respectively corresponding to the N coils (12U, 12V, 12W) and each having a high-potential-side terminal and a low-potential-side terminal, all low-potential-side terminals of the low-side switches (SUn, SVn, SWn) are electrically connected to a negative terminal of the DC power source (30), the high-side switches (SUp, SVp, SWp) are configured as either permanently connected switches, the low-side switches (SUn, SVn, SWn) are configured as selectively connected switches, or are configured as selectively connected switches, the low-side switches (SUn, SVn, SWn) are configured as permanently connected switches, each of the permanently connected switches being permanently electrically connected to a corresponding one of the coils (12U, 12V, 12W) of the rotating electric machine (10), each of the selectively connected switches being selectively electrically connected to either a corresponding one of the permanently connected switches or the neutral point (N) between the coils (12U, 12V, 12 w), for each of the permanently connected switches, that one of the high-potential-side and low-potential-side terminals of the permanently connected switch that is electrically connected neither to the positive terminal nor to the negative terminal of the DC power source (30) constitutes a permanently connected terminal, each of the permanently connected terminals of the permanently connected switches is permanently connected to a corresponding one of non-neutral-point-side ends of the coils (12U, 12V, 12W) of the rotating electric machine (10), for each of the selectively connected switches, that one of the high-potential-side and low-potential-side terminals of the selectively connected switch that is electrically connected neither to the positive terminal nor to the negative terminal of the DC power source (30) constitutes a selectively connected terminal, the selector switch (40) electrically selectively connects each of the selectively connected terminals of the selectively connected switches to either a corresponding one of the permanently connected terminals of the permanently connected switches or the neutral point (N) between the coils (12U, 12V, 12W), the full-wave driver is configured to perform a full-wave driving process for full-wave driving of the rotating electric machine (10), wherein in the full-wave driving process, the full-wave driver controls the on-off switching of the permanently connected switches and the selectively connected switches, wherein the selectively connected terminals of the selectively connected switches are electrically connected to the corresponding permanently connected terminals of the permanently connected switches through the selector switch (40), the half-wave driver is configured to perform a half-wave driving process for half-wave driving of the rotating electric machine (10), wherein, in the half-wave driving process, the half-wave driver controls the on-off switching of the permanently connected switches, the selectively connected terminals of the selectively connected switches are electrically connected to the neutral point (N) through the selection switch (40) and the selectively connected switches are kept on, and the drive controller is configured to control both the full-wave driver and the half-wave driver to selectively cause either the full-wave driver to perform the full-wave driving process or the half-wave driver to perform the half-wave driving process.The rotary electric machine system (MU) according to claim 1, wherein the half-wave driver cyclically powers the permanently connected switches in the half-wave driving process such that start timings of the powering of the permanently connected switches are offset from each other and each of the permanently connected switches is powered for half or less of each cycle.The rotary electric machine system (MU) according to claim 2, wherein the starting timings of feeding the permanently connected switches are offset from each other by 360° / N in electrical angle.The rotary electric machine system (MU) according to claim 1, wherein the electric power conversion circuit further includes a selection switch controller, and the selection switch controller controls the selection switch (40) to perform, for each of the N coils (12U, 12V, 12W), switching between electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the permanently connected terminal of the corresponding permanently connected switch and electrically connecting the selectively connected terminal of the corresponding selectively connected switch to the neutral point (N) only when the absolute value of the electric current flowing through the coil is less than or equal to a threshold current value.The rotary electric machine system (MU) according to claim 4, wherein the rotary electric machine (10) further includes a rotor (11) having a plurality of magnetic poles, the electric power conversion circuit further includes an acquisition unit that acquires magnetic pole position information regarding a magnetic pole position of the rotor (11), and the selection switch control means determines, for each of the N coils (12U, 12V, 12W), whether the absolute value of the electric current flowing in the coil is less than or equal to the threshold current value based on the magnetic pole position information acquired by the acquisition unit.The rotary electric machine system (MU) according to claim 4, wherein the selection switch (40) includes N pairs of first and second connection switches (40A, 40B) respectively corresponding to the N coils (12U, 12V, 12W), for each of the N coils (12U, 12V, 12W), the first connection switch (40A) is configured to electrically connect the selectively connected terminal of the corresponding selectively connected switch to the permanently connected terminal of the corresponding permanently connected switch, and the second connection switch (40B) is configured to electrically connect the selectively connected terminal of the corresponding selectively connected switch to the neutral point (N), the selection switch control device performs first and second switching processes for each of the N coils (12U, 12V, 12W), in the first switching process, the selection switch control device switches the corresponding first connection switch (40A) from on to off and switches the corresponding second connection switch (40B) from off to on, in the second switching process, the selection switch control device switches the corresponding first connection switch (40A) from off to on and switches the corresponding second connection switch (40B) from on to off, and in each of the first and second switching processes, there is a period during which both of the corresponding first and second connection switches (40A, 40B) are held on by the selection switch control device.The rotary electric machine system (MU) according to claim 1, wherein the coils (12U, 12V, 12W) are first coils (12U, 12V, 12W), the neutral point (N) between the coils (12U, 12V, 12W) is a first neutral point (N1), the high-side switches (SUp, SVp, SWp) are first high-side switches (SUp, SVp, SWp), the low-side switches (SUn, SVn, SWn) are first low-side switches (SUn, SVn, SWn), and the selection switch (40) is a first selection switch (40), the permanently connected switches are configured with the first high-side switches (SUp, SVp, SWp), respectively, the selectively connected switches are configured with the first low-side switches (SUn, SVn, SWn), respectively, the permanently connected terminals are respectively composed of the low-potential side terminals of the high-side switches (SUp, SVp, SWp), and the selectively connected switches are respectively composed of the high-potential side terminals of the first low-side switches (SUn, SVn, SWn), the rotating electric machine (10) further includes N second coils (14X, 14Y, 14Z) wound on the stator core (15a) so as to be electrically wound from the first coils (12U, 12V, respectively, 12 w) and each belonging to N phases different from the N phases of the first coils (12U, 12V, 12W), the second coils (14X, 14Y, 14Z) being electrically connected to each other to define a second neutral point (N 2) therebetween, the electric power conversion circuit further includes N second high-side switches (SXp, SYp, SZp), N second low-side switches (SXn, SYn, SZn), and a second selection switch (60), the N second high-side switches (SXp, SYp, SZp) respectively belonging to the N second coils (14X, 14Y, 14z), and each having a high potential side terminal and a low potential side terminal, wherein all of the high potential side terminals of the second high side switches (SXp, SYp, SZp) are electrically connected to the positive terminal of the DC power source (30), the N second low side switches (SXn, SYn, SZn) each correspond to the N second coils (14X, 14Y, 14Z), and each having a high potential side terminal and a low potential side terminal, wherein all of the low potential side terminals of the second low side switches (SXn, SYn, SZn) are electrically connected to the negative terminal of the DC power source (30), each of the high potential side terminals of the second low side switches (SXn, SYn, SZn) is permanently electrically connected to a corresponding one of ends, which are not on the second neutral point (N 2) side, of the second coils (14X, 14Y, 14Z) of the rotating electric machine (10), the second selection switch (60) selectively electrically connects each of the low-potential side terminals of the second high-side switches (SXp, SYp, SZp) to either a corresponding one of the high-potential side terminals of the second low-side switches (SXn, SYn, SZn) or the second neutral point (N 2) between the second coils (14X, 14Y, 14Z), in the full-wave driving process, the full-wave driver turns on-off the first high-side switches (SUp, SVp, SWp), the first low-side switch (SUn, SVn, SWn), the second high-side switch (SXp, SYp, SZp), and the second low-side switches (SXn, SYn, SZn), wherein the high-potential-side terminals of the first low-side switches (SUn, SVn, SWn) are electrically connected to the corresponding low-potential-side terminals of the first high-side switches (SUp, SVp, SWp) through the first selection switch (40), and the low-potential-side terminals of the second high-side switches (SXp, SYp, SZp) are electrically connected to the corresponding high-potential-side terminals of the second low-side switches (SXn, SYn, SZn) through the second selection switch (60), in the half-wave connection process, the half-wave drivers control the on-off switching of the first high-side switches (SUp, SVp, SWp) and the second low-side switches (SXn, SYn, SZn), the high-potential-side terminals of the first low-side switches (SUn, SVn, SWn) are electrically connected to the first neutral point (N1) through the first selection switch (40), and the low-potential-side terminals of the second high-side switches (SXp, SYp, SZp) are electrically connected to the second neutral point (N 2) through the second selection switch ( 60), and the first low-side switches (SUn, SVn, SWn) and the second high-side switches are held on.The rotary electric machine system (MU) according to claim 7, further comprising: a positive conductor (Lp) connected to the positive terminal of the DC power source (30); a negative conductor (Ln) electrically connected to the negative terminal of the DC power source (30); and a capacitor (31) electrically connected between the positive and negative conductors, wherein each of the high-potential-side terminals of the first and second high-side switches is electrically connected to the positive conductor (Lp), and each of the low-potential-side terminals of the first and second low-side switches is electrically connected to the negative conductor (Ln).The rotary electric machine system (MU) according to claim 7, wherein in the half-wave driving process, the half-wave driver cyclically powers the first high-side switches (SUp, SVp, SWp) such that start timings of feeding the first high-side switches (SUp, SVp, SWp) are offset from each other and each of the first high-side switches (SUp, SVp, SWp) is fed for a predetermined first period in each cycle, the first predetermined period being shorter than or equal to half of each cycle, and the half-wave driver also cyclically powers the second low-side switches (SXn, SYn, SZn) such that the start timings of feeding the second low-side switches (SXn, SZn, SYn, SZn) are offset from each other and each of the second low-side switches (SXn, SYn, SZn) is supplied for a second predetermined period in each cycle, the second predetermined period being shorter than or equal to half of each cycle.The rotary electric machine system (MU) according to claim 9, wherein the start timings of feeding the first high-side switches (SUp, SVp, SWp) are offset from each other by 360° / N in electrical angle, and the start timings of feeding the second low-side switches (SXn, SYn, SZn) are also offset from each other by 360° / N in electrical angle.The rotary electric machine system (MU) according to claim 10, wherein the first coils (12U, 12V, 12W) and the second coils (14X, 14Y, 14Z) are wound on the stator core (15a) such that each of the first coils (12U, 12V, 12W) has the same spatial phase as a corresponding one of the second coils, and in the half-wave driving process, the half-wave driver cyclically feeds the first high-side switches (SUp, SVp, SWp) and the second low-side switches (SXn, SYn, SZn) such that, for each pair of the first and second coils having the same spatial phase, the start timing of feeding one of the first high-side switches (SUp, SVp, SWp) corresponding to the first coil in the same spatial phase pair and the start timing of feeding of one of the second low-side switches (SXn, SYn, SZn) corresponding to the second coil of the same spatial phase pair are offset from each other by 180° in electrical angle.The rotary electric machine system (MU) according to claim 9, wherein each of the first and second predetermined periods is longer than or equal to (90°+180° / N) and shorter than or equal to 180° in electrical angle.The rotary electric machine system (MU) according to claim 12, wherein the first coils (12U, 12V, 12W) and the second coils (14X, 14Y, 14Z) are wound on the stator core (15a) such that each of the first coils (12U, 12V, 12W) has the same spatial phase as a corresponding one of the second coils (14X, 14Y, 14Z), and in the half-wave driving process, the half-wave driver cyclically feeds the first high-side switches (SUp, SVp, SWp) and the second low-side switches (SXn, SYn, SZn) such that, wherein, for each pair of the first and second equi-spatial-phase coils, the start timing of feeding of one of the first high-side switches (SUp, SVp, SWp) corresponding to the first coil in the equi-spatial-phase pair and the start timing of feeding of one of the second low-side switches (SXn, SYn, SZn) corresponding to the second coil of the equi-spatial-phase pair are offset from each other by 180° in electrical angle.The rotary electric machine system (MU) according to claim 7, wherein the electric power conversion circuit further comprises first and second selection switch control means, the first selection switch control means controls the first selection switch (40) to perform, for each of the N first coils (12U, 12V, 12W), switching between electrically connecting the high-potential-side terminal of the corresponding first low-side switch (SUn, SVn, SWn) to the low-potential-side terminal of the corresponding first high-side switch (SUp, SVp, SWp) and electrically connecting the high-potential-side terminal of the corresponding first low-side switch (SUn, SVn, SWn) to the first neutral point (N1) only then, when the absolute value of the electric current flowing in the first coil is less than or equal to a first threshold current value, and the second selector control means controls the second selector (60) to perform, for each of the N second coils (14X, 14Y, 14Z), switching between electrically connecting the low-potential side terminal of the corresponding second high-side switch (SXp, SYp, SZp) to the high-potential side terminal of the corresponding second low-side switch (SXn, SYn, SZn) and electrically connecting the low-potential side terminal of the corresponding second high-side switch (SXp, SYp, SZp) to the second neutral point (N2) only then, when the absolute value of the electric current flowing in the second coil is less than or equal to a second threshold current value.The rotary electric machine system (MU) according to claim 7, wherein the first selection switch (40) includes N pairs of first and second connection switches (40A, 40B) respectively corresponding to the N first coils (12U, 12V, 12W), for each of the N first coils (12U, 12V, 12W), the first connection switch (40A) is configured to electrically connect the high-potential-side terminal of the corresponding first low-side switch (SUn, SVn, SWn) to the low-potential-side terminal of the corresponding first high-side switch (SUp, SVp, SWp), and the second connection switch (40B) is configured to electrically connect the high-potential-side terminal of the corresponding first low-side switch (SUn, SVn, SWn) to be electrically connected to the first neutral point (N 1), the second selection switch (60) includes N pairs of third and fourth connection switches (60A, 60B) respectively corresponding to the N second coils (14X, 14Y, 14Z), for each of the N second coils (14X, 14Y, 14Z), the third connection switch (60A) is configured to connect the low-potential side terminal of the corresponding second high-side switch (SXp, SYp, SZp) to the high-potential side terminal of the corresponding second low-side switch (SXn, SYn, SZn), and the fourth connection switch (60B) is configured to:, connecting the low potential side terminal of the corresponding second high side switch (SXp, SYp, SZp) to the second neutral point (N2), the electric power conversion circuit further includes a first selection switch controller that controls the first selection switch (40), and a second selection switch controller that controls the second selection switch (60), the first selection switch controller performs first and second switching processes for each of the N first coils (12U, 12V, 12W), in the first switching process, the first selection switch controller switches the corresponding first connection switch (40A) from on to off and the corresponding second connection switch (40B) from off to on, in the second switching process, the first selection switch controller switches the corresponding first selection switch (40) from on to off and the corresponding second connection switch (40B) from on to off, in each of the first and second switching processes, there is a period during which both of the corresponding first and second connection switches (40A, 40B) are held on by the first selection switch controller, the second selection switch controller performs third and fourth switching processes for each of the N second coils (14X, 14Y, 14Z), in the third switching process, the second selection switch controller switches the corresponding third connection switch (60A) from on to off and the corresponding fourth connection switch (60B) from off to on, in the fourth switching process, the second selection switch control device switches the corresponding third connection switch ( 60A) from off to on and the corresponding fourth connection switch ( 60B) from on to off, and in each of the third and fourth switching processes, there is a period during which both of the corresponding third and fourth connection switches ( 60A, 60B) are held on by the second selection switch control device.

Images