Motor drive device and cooling circuit device

Abstract

This motor drive device includes a first inverter, a second inverter, a plurality of opening and closing connections, a plurality of semiconductor switching elements, and a control unit. The control unit performs a pseudo-neutral point operation when opening or closing the opening and closing connections and turns on the semiconductor switching elements in advance. First wirings each have a first inductance, second wirings each have a second inductance, and third wirings each have a third inductance. The value of the third inductance is smaller than the sum of the value of the first inductance and the value of the second inductance.

Description

Technical FieldThe present invention relates generally to a motor drive device for a motor including a plurality of phase wires that are not connected to each other, and a cooling circuit device equipped with the motor drive device.BackgroundAs drive motors for a compressor installed in a refrigeration cycle device such as an air conditioner, a permanent magnet synchronous motor having a plurality of phase wires and an open winding motor (open winding motor) having a plurality of, for example, three phase wires separated from each other are known.A motor driving device that drives an open winding motor (abbreviated as a motor) includes a first inverter that controls energization of one end of each phase wire of the motor, a second inverter that controls energization of the other end of each phase wire of the motor, and one or more switches for connecting the other ends of the respective phase wires, and selectively sets a star connection mode in which the first inverter is independently switched to drive the motor by establishing a connection or a so-called star connection (also referred to as a star connection) of the other ends of the respective phase wires by closing the switches, and an idle mode in which the first and second inverters are connected to each other to drive the motor in a disconnected state of disconnection of the other ends of the respective phase wires by opening the switches.A voltage applied to each phase wire can be increased to overcome the counter electromotive force generated in a permanent magnet synchronous motor and drive the motor at high speeds by setting the open winding mode, and the motor can be driven with high efficiency by setting the star-switching mode in a low speed range. In other words, the engine can be operated as efficiently as possible over a wide operating range from high speeds to low speeds. Therefore, it is possible to both extend the operating range of the engine and improve the efficiency of the engine driving apparatus.In a star motor drive, the current (motor current) flows between the first inverter and each phase line through the switch. By using a mechanical switching contact with a small resistance value, for example a relay contact, as a switch, the power loss in the switch can be reduced and the efficiency of the motor can be improved.During motor driving, however, a potential difference is generated between the both ends of the switching contacts, i.e., between the other ends of the respective phase wires. When the switching contact is opened and closed in a state where such a potential difference occurs, an overvoltage or an arc is generated between both ends of the switching contact, which adversely affects the life of the switching contact. In addition, the switching elements of each inverter may be destroyed by these overvoltage and arcing.For this reason, as a countermeasure, a pseudo neutral point operation that does not cause a potential difference between the both ends of the switching contact is performed by switching the second inverter, and during its operation, control for opening and closing the relay contact is performed.List of Citer ListsPatent LiteraturePatent Document 1: JP 4804381 BPatent Document 2: JP 2019-62726 ASUMMARY OF THE INVENTIONTechnical ProblemHowever, in various tests, it has been found that even during the pseudo-neutral operation, a potential difference may occur between the opening and closing contacts, depending on the relationship between the switching time of the second inverter and the operating times of the relay contacts.Therefore, the embodiments described herein aim to provide a motor drive device and a cooling circuit device with excellent safety and reliability, which are capable of keeping the potential difference between the two ends of the opening and closing contact as small as possible.Solution of the ProblemA motor drive device according to an embodiment is a motor drive device of a motor having a plurality of phase wires separated from each other, and the motor drive device includes: a first inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to a DC power supply, a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to one end of each of the phase wires; a second inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to the DC power supply, a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to the other end of each of the phase wires through each of the first wires; a plurality of switching contacts connected between the other ends of each of the phase wires by each of the second wires; a plurality of semiconductor switching elements connected in parallel to each of the switching contacts by each of the third wires; and a controller that controls the driving of the first inverter, the driving of the second inverter, and the opening and closing of each of the switching contacts. During opening and closing of each of the switching contacts, the controller performs a pseudo neutral point operation in which all the upper switching elements and all the lower switching elements in the second inverter are alternately turned on and off, and each of the semiconductor switching elements is turned on in advance. Each of the first wires has a first inductance, each of the second wires has a second inductance, and each of the third wires has a third inductance. A value of the third inductance is smaller than a total value of the value of the first inductance and the value of the second inductance.The refrigeration cycle apparatus of this embodiment includes a compressor driven by the motor drive device.Brief Description of the DrawingsFIG. 1 is a block diagram showing a configuration of a first embodiment. FIG. 2 is a flowchart showing the control of the first embodiment. FIG. 3 is a time chart showing the operation of the pseudo neutral point and the closing operation of each switching contact performed when switching from an open winding mode to the star connection mode in the first embodiment. FIG. 4 is a time chart showing the operation of the pseudo neutral point and the opening operation of each switching contact performed in the switching from the star connection mode to the open winding mode in the first embodiment. FIG. 5 is a timing chart showing the turning on and off of each switching element in the pseudo neutral point operation of FIGS. 3 and 4 in a time-enlarged manner. FIG. 6 is a diagram showing the current flow during a dead time in the first embodiment. FIG. 7 is a graph showing changes in voltage and potential difference at various parts when the current shown in FIG. 6 flows. FIG. 8 is a diagram showing other changes in voltage and potential difference at various parts when the current shown in FIG. 6 flows. FIG. 9 is a block diagram showing a configuration of a second embodiment. FIG. 10 is a block diagram showing a configuration of a third embodiment. FIG. 11 is a block diagram showing a configuration of a fourth embodiment.Embodiments of the InventionFirst EmbodimentThe first embodiment will be described below with reference to the accompanying drawings.As shown in FIG. 1, a motor drive circuit 2 is connected to a three-phase AC power source 1, and a motor 3 and a control unit 4 are connected to an output end of the motor drive circuit 2. In this embodiment, the motor 3 is a compressor drive motor that drives a compressor of an air conditioner which is a refrigeration cycle device.The motor 3 is a three-phase permanent magnet synchronous motor for driving a compressor having three phase lines Lu, Lv, and Lw separated from each other, more specifically, a so-called open winding motor having three terminals 31 u, 31 v, and 31 wthat are the ends of the respective phase lines Lu, Lv, and Lw, and three terminals 32 u, 32 v, and 32 wthat are the other ends of the respective phase lines Lu, Lv, and Lw.The motor drive circuit 2 includes a DC power source, e.g., an inverter 10 connected to the three-phase AC power source 1, a positive power line C 1 and a negative power line C 2 connected to the output of the inverter 10, and an inverter (first inverter) 20 and an inverter (second inverter) 30 connected between the positive power line C 1 and the negative power line C 2.The inverter 10 is, for example, a full-wave rectifier or a PWM converter, and converts the AC voltage of the three-phase AC power source 1 into a DC voltage. The inverter 20 controls the energization of the terminals 31 u, 31 v, and 31 wthat are the ends of the respective phase wires Lu, Lv, and Lw of the open winding motor 3. The inverter 30 controls the energization of the terminals 32 u, 32 v, and 32 wthat are the other ends of the respective phase wires Lu, Lv, and Lw from the open winding motor 3. The inverter 10 adopts a common DC link system configuration that is a common DC power source for the first inverter 20 and the second inverter 30.The first inverter 20 is a so-called three-phase inverter including a U-phase series circuit formed by series-connecting an upper switching element Tu and a lower switching element Tx, a V-phase series circuit formed by series-connecting an upper switching element Tv and a lower switching element Ty, and a W-phase series circuit formed by series-connecting an upper switching element Tw and a lower switching element Tz. One end of each of the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit is connected to the positive power line C 1, and the other end of each of the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit is connected to the negative power line C 2.A connection point Au of the upper switching element Tu and the lower switching element Tx is connected to a terminal 31 u, which is one end of the phase wire Lu, via a wire 51 u, e.g., a lead wire or a conductive pattern. A connection point Av of the upper switching element Tv and the lower switching element Ty is connected to a terminal 31 v, which is one end of the phase wire Lv, via a wire 51 v, for example, a lead wire or a conductive pattern. A connection point Az of the upper switching element Tw and the lower switching element Tz is connected to a terminal 31 w, which is one end of the phase wire Lz, by a wire 51 wsuch as a lead wire or a conductive pattern.The second inverter 30 is a so-called three-phase inverter having the same circuit configuration as the first inverter 20 and including a U-phase series circuit formed by series connection of an upper switching element Tu and a lower switching element Tx, a V-phase series circuit formed by series connection of an upper switching element Tv and a lower switching element Ty, and a W-phase series circuit formed by series connection of an upper switching element Tw and a lower switching element Tz. One end of each of the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit is connected to the positive power line C 1, and the other end of each of the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit is connected to the negative power line C 2.A connection point Bu of the upper switching element Tu and the lower switching element Tx is connected to a terminal 32 u, which is the other end of the phase wire Lu, by a wire (first wire) 52 usuch as a lead wire or a conductive pattern. A connection point Bv of the upper switching element Tv and the lower switching element Ty is connected to a terminal 32 v, which is the other end of the phase wire Lv, by a wire (first wire) 52 vsuch as a lead wire or a conductive pattern. A connection point Bw of the upper switching element Tw and the lower switching element Tz is connected to a terminal 32 w, which is the other end of the phase wire Lz, by a wire (first wire) 52 wsuch as a lead wire or a conductive pattern.All the switching elements Tu to Tz of the inverters 20 and 30 are IGBTs in which the free wheeling diodes (also referred to as free wheeling diodes) D are connected in antiparallel to the main bodies of the switching elements. In addition to the IGBTs, MOS-FETs or the like may also be used as each of the switching elements Tu to Tz.The first inverter 20 is a module in which a main circuit formed by bridge-connecting the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit and peripheral circuits such as the driving circuit for driving the individual switching elements of this main circuit are housed in a single package or a so-called intelligent power module (IPM). The second inverter 30 is also an IPM. Besides the IPM, inverters 20 and 30 in which all switching elements Tu to Tz and the drive circuits are configured as discrete components may also be used. The inverters are not limited to the three-phase inverters, but the two three-phase inverters 20 and 30 may be configured with three single-phase inverters because only the circuit of six phases needs to be formed.A switch comprising a mechanical switch contact, for example, a normally open first switch contact (referred to as relay contact) 12a of a relay 12, is connected between the other end (terminal 32u) of the phase wire Lu and the other end (terminal 32v) of the phase wire Lv in a motor 1M by wires (second wires) 53u and 53v such as lead wires and conductive patterns. A switch having a mechanical switching contact, for example, a normally open second switching contact (referred to as relay contact) 13 aof a relay 13, is connected between the other end (terminal 32 v) of the phase wire Lv and the other end (terminal 32 w) of the phase wire Lw in the motor 1M by wires (second wires) 53 vand 53 wsuch as lead wires and line patterns. The relays 12 and 13 are controlled to be turned on (energized) by supplying an energizing current and turned off (de-energized) by turning off the energizing current in synchronism with each other by the controller 4, and for this reason, a relay having two relay contacts may be used instead of two relays 12 and 13.By turning on (energizing) the relays 12 and 13, the relay contacts 12 aand 13 aare closed, the other end of the phase wire Lu and the other end of the phase wire Lv are connected to each other via the relay contact 12 a, and the other end of the phase wire Lv and the other end of the phase wire Lw are connected to each other via the relay contact 13 a. In other words, the phase wires Lu, Lv, and Lw are in a neutral state (also referred to as a neutral state). By turning off (turning off) the relays 12 and 13, the relay contacts 12 aand 13 aare opened, and the phase wires Lu, Lv, and Lw are in a disconnected state, i.e., in an open winding state, in which they are electrically disconnected.In addition, a series circuit of auxiliary switches SW 1 and SW 2 is connected in parallel to the relay contact 12 avia wires (third wires) 54 uand 54 v, e.g., lead wires and conductive patterns. A series circuit of auxiliary switches SW 3 and SW 4 is connected in parallel to the relay contact 13 avia wires (third wires) 54 vand 54 wsuch as lead wires and line patterns.More specifically, one end of the series circuit of the auxiliary switches SW 1 and SW 2 is connected to a connection point N 1 between the wire 53 uand one end of the relay contact 12 avia the wire 54 u. The other end of the series circuit of the auxiliary switches SW 1 and SW 2 is connected to a connection point N 2 between the wire 53 vand the other end of the relay contact 12 a(and one end of the relay contact 12 b), and one end of the series circuit of the auxiliary switches SW 3 and SW 4 is connected to a connection point N 2 via the same wire 54 v. The other end of the series circuit of the auxiliary switches SW 3 and SW 4 is connected via the line 54 wto a connection point N 3 between the line 53 wand the other end of the relay contact 12 b. The connection points N 1, N 2, and N 3 are branching points from the wires 53 u, 53 v, and 53 wto the wires 54 u, 54 v, and 54 w. The connection points N 1, N 2, and N 3 will be referred to as branching points N 1, N 2, and N 3 hereinafter.In other words, the wires 53 u, 53 v, and 53 wbegin at the terminals 32 u, 32 v, and 32 wthat are the other ends of the motor wires Lu, Lv, and Lw, and end at the branching points N 1, N 2, and N 3. The first and second wires 54 uand 54 vbegin at the branching points N 1 and N 2 and end at both ends of the series circuit of the auxiliary switches Sw 1 and Sw 2. The second and third wires 54 vand 54 wbegin at the branching points N 2 and N 3 and end at both ends of the series connection of the auxiliary switches Sw 3 and Sw 4.The auxiliary switches SW 1 to SW 4 are semiconductor switching elements in which a free wheeling diode D is connected in antiparallel to the main body of each element. The series circuit of the auxiliary switches SW 1 and SW 2 is connected such that the auxiliary switches SW 1 and SW 2 are connected in the opposite direction. In other words, the outputs (current outflow sides) of the two auxiliary switches SW 1 and SW 2 are connected to each other. Similarly, the series connection of the auxiliary switches SW 3 and SW 4 is connected such that the auxiliary switches SW 3 and SW 4 are connected in opposite directions. For this reason, in the series connection of the auxiliary switches SW 1 and SW 2, a current flows in both directions via the free wheeling diode D of one of the auxiliary switches when the auxiliary switches SW 1 and SW 2 are turned on, and no current flows in both directions when the auxiliary switches SW 1 and SW 2 are turned off. Similarly, in the series connection of the auxiliary switches SW 3 and SW 4, a current flows in both directions via the free wheeling diode D of one of the auxiliary switches when the auxiliary switches SW 3 and SW 4 are turned on, and no current flows in either direction when the auxiliary switches SW 3 and SW 4 are turned off.The wire 52 ubetween the connection point Bu of the second inverter 30 and the other end (terminal 32 u) of the phase wire Lu has a first inductance (parasitic inductance) Lsu 1. The wire 53 ubetween the other end (terminal 32 u) of the phase wire Lu and the branch point N 1 has a second inductance (parasitic inductance) Lsu 2. The wire 52 vbetween the connection point Bvof the second inverter 30 and the other end (terminal 32 v) of the phase wire Lvhas a first inductance (parasitic inductance) Lsv 1. The wire 53 vbetween the other end (terminal 32 v) of the phase wire Lv and the branch point N 2 has a second inductance Lsv 2. The line 52 wbetween the connection point Bw of the inverter 30 and the other end (terminal 32 w) of the phase line Lw has a first inductance (parasitic inductance) Lsw 1. The wire 52 wbetween the other end (terminal 32 w) of the phase wire Lw and the branching point N 3 has a second inductance Lsw 2. The first inductances Lsu 1, Lsv 1, and Lsw 1 have substantially the same value, but may be slightly different in size depending on the routing conditions of the individual wires 52 u, 52 v, and 52 w. Similarly, the second inductances Lsu 2, Lsv 2, and Lsw 2 have substantially the same value, but may be slightly different in size depending on the conditions for laying the individual wires 53 u, 53 v, and 53 w.The wire 54 ubetween the branch point N 1 and one end of the series circuit of the auxiliary switches SW 1 and SW 2 has a third inductance (parasitic inductance) Lsu 3. The wire 54 vhas a third inductance Lsv 3 between the branch point N 2 and the other end of the auxiliary switches SW 1 and SW 2, and also has the same third inductance Lsv 3 between the branch point N 2 and one end of the auxiliary switches SW 3 and SW 4. Incidentally, since the wire at the connection point between the collector of the auxiliary switch SW 2 and the collector of the auxiliary switch SW 3 may be extremely short, the inductance of the wire from the branching point N 2 to the connection point between the auxiliary switch SW 2 and the auxiliary switch SW 3 is substantially dominant as the third inductance Lsv 3 of the wire 54 v. The wire 54 wbetween the branch point N 3 and the other end of the series connection of the auxiliary switches SW 3 and SW 4 has a third inductance Lsw 3.In summary, it can be said that the relay contact 12a is connected between the branch points N1 and N2, and a series circuit of the auxiliary switches SW1 and SW2 is connected between the branch points N1 and N2. The relay contact 13a is connected between the branch points N2 and N3, and the series circuit of the auxiliary switches SW3 and SW4 is connected between the branch points N2 and N3.Current sensors 11 u, 11 v, and 11 ware provided on the lines 51, 51 v, and 51 zbetween the connection points Au, Av, and Az of the inverter 20 and the ends (terminals 31 u, 31 v, and 31 z) of the respective phase lines Lu, Lv, and Lw, and the output signals of these current sensors are sent to the controller 4. The current sensors 11 u, 11 v, and 11 wdetect the currents (referred to as motor currents) Iu, Iv, and Iw flowing through the phase wires Lu, Lv, and Lw.The control unit 4 includes a main control section 40, a current detection section 41, a relay drive section 42, and an auxiliary SW drive section 43, and controls the opening / closing of the relay contacts 12 aand 13 aand the drive (switching) of the inverters 20 and 30 so that the rotation speed N of the motor 3 becomes a target rotation speed Nt set by a higher-order external device (e.g., a control device of an air conditioner) and that high-efficiency operation is achieved.The current detection section 41 detects the instantaneous values of the motor currents Iu, Iv, and Iw detected by the current sensors 11 u, 11 v, and 11 w, respectively. The relay driving section 42 controls the relays 12 and 13 in response to commands from the main control section 40. the auxiliary SW driving section 43 controls the auxiliary switches SW 1 to SW 4 in accordance with the commands from the main control section 40.The main control section 40 is composed of a microcomputer and its peripheral circuits, and selectively sets a star connection mode in which the other ends of the phase wires Lu, Lv, and Lw are connected to each other by closing the relay contacts 12 aand 13 ato independently control the inverter 20, and an open winding mode in which the other ends of the phase wires Lu, Lv, and Lw are separated from each other by opening the relay contacts 12 aand 13 ato drive the inverters 20 and 30 in connection to each other, in accordance with the values of the motor currents Iu, Iv, and Iw corresponding to the magnitude of the load, and the like. For example, the star connection mode is set at a low load time when the engine speed N is low and the motor currents Iu, Iv, and Iw are below a predetermined value, and the idle mode is set at a high load time when the engine speed N rises and the motor currents Iu, Iv, and Iw become equal to and higher than a predetermined value. The high efficiency can thus be achieved over the entire operating range of the engine. Incidentally, the selection of the star connection mode and the open winding mode may be changed by determining various motor related parameters such as a combination of motor speed and field weakening degree in addition to the above elements. Incidentally, under abnormal conditions, when the motor currents Iu, Iv, and Iw become overcurrent, either one of the star connection and the open circuit modes may be preferably changed.When changing from the open winding mode to the neutral connection mode and changing from the neutral connection mode to the open winding mode, the main control section 40 performs the pseudo neutral point operation in which all the upper switching elements Tu, Tv, and Tw and all the lower switching elements Tx, Ty, and Tz in the inverter 30 are alternately turned on and off at an duty ratio of 50% so that the potential difference between the both ends of the relay contact 12 aand the potential difference between the both ends of the relay contact 13 abecomes zero.Specifically, during the execution of the pseudo neutral point operation, at the time of switching from the open winding mode to the star connection mode, the main control section 40 turns on the relays 12 and 13 in advance in a state of turning on the auxiliary switches SW 1 to SW 4, and turns off the auxiliary switches SW 1 to SW 4 after a certain time t 1 longer than the time required for closing the relay contacts 12 aand 13 a indeed elapses. Similarly, during the execution of the pseudo neutral point operation at the time of switching from the star connection to the open winding mode, the main control section 40 turns off the relays 12 and 13 in a state of previously turning on the auxiliary switches SW 1 to SW 4 and turns off the auxiliary switches SW 1 to SW 4 after a certain time t 2 longer than the time required for opening the relay contacts 12 aand 13 a indeed has elapsed.Incidentally, during the on / off driving of each upper switching element and each lower switching element of the inverters 20 and 30, the main control section 40 performs a complementary operation in which the lower switching element is turned off when the upper switching element in each series circuit is turned on, while the upper switching element is turned off, and when the lower switching element in each series circuit is turned on, during the pseudo neutral point operation. In this complementary operation, the main control section 40 ensures a dead time td in which both the upper switching element and the lower switching element are turned on / off in driving so that the upper switching element and the lower switching element of each series circuit are not simultaneously turned on and a short circuit is not formed. Incidentally, the dead time td is always provided not only during the pseudo neutral point operation but also during the PWM control in the normal operation to prevent a short circuit between the upper and lower switching elements.Next, the main controls executed by the main control section 40 of the controller 4 will be described with reference to the flowchart of FIG. 2. Steps S1, S2... are referred to simply as S1, S2... in the flowchartWhen the motor 3 is operated in the open winding mode (YES in S 1), the main control section 40 monitors whether or not it is necessary to change the mode to the star connection in response to a load drop (S 2). When a transition to the star connection mode is not required (NO in S 2), the main control section 40 repeats the above determination in S 1.When a transition to the star connection mode is required (YES in S 2), the main control section 40 performs the pseudo neutral point operation in which all the upper switching elements Tu, Tv, and Tw and all the lower switching elements Tx, Ty, and Tz in the inverter 30 are alternately turned on and off at an on / off ratio of 50% as shown in FIG. 3 so that the potential difference between both ends of each of the relay contacts 12 aand 13 abecomes zero (S 3).The relationship between the turning on and off of the upper switching elements Tu, Tv, and Tw and the turning on and off of the lower switching elements Tx, Ty, and Tz in this pseudo neutral point operation is shown in FIG. 5 in a time-enlarged manner to make the relationship easily understood. The main control section 40 ensures a dead time td in which both the upper switching elements Tu, Tv and Tw and the lower switching elements Tx, Ty and Tz are turned off to prevent the formation of a short circuit at the output terminal of the converter 10 when the upper switching elements Tu, Tv and Tw are turned on and the lower switching elements Tx, Ty and Tz are turned off. Also, the main control section 40 ensures a dead time td in which both the lower switching elements Tx, Ty, and Tz and the upper switching elements Tu, Tv, and Tw are turned to an off state to prevent the formation of a short circuit at the output terminal of the converter 10 when the lower switching elements Tx, Ty, and Tz are turned on and the upper switching elements Tu, Tv, and Tw are turned off.There are various methods for generating the dead time td, but the general method is to turn off the switching element to be turned off and then turn on the switching element to be turned on after the dead time td has elapsed. It is desirable to keep the dead time td as short as possible from the viewpoint of efficiency and waveform shaping, and in practice, the minimum time is set based on the on / off transient characteristic of the switching element.However, as described later, even when the pseudo neutral point operation is performed due to the presence of the dead time td, a potential difference may occur between the both ends of the relay contacts 12 aand 13 awhen the switching timing of the inverter 30 and the activation timing of the relay contacts 12 aand 13 acrosses with the dead time td.During the execution of the pseudo neutral point operation, the main control part 40 first turns on the auxiliary switches SW 1 to SW 4 (S 4), thereby short-circuiting both ends of each of the relay contacts 12 aand 13 a, and turns on the relays 12 and 13 after the short-circuiting (S 5). After a certain time t1 elapses, which is longer than the time required to actually close the relay contacts 12a and 13a (YES in S6), the main control part 40 turns off the auxiliary switches SW1 to SW4 (S7). Thereafter, the main control part 40 terminates the pseudo neutral point operation and shifts to motor drive in the star shift mode (S8).After the switching, the main control section 40 returns to the above determination in S 1. The on / off driving of turning on the auxiliary switches SW 1 to SW 4 in step S 4 and turning off the auxiliary switches SW 1 to SW 4 in step S 7 is desirably performed by synchronizing all the auxiliary switches from the viewpoint of simplification of the circuit and the like, but the auxiliary switches do not need to be turned on and off in complete synchronization. The point is that all the auxiliary switches SW1 to SW4 can be turned on before the relay contacts 12a and 13a are actually closed, and all the auxiliary switches SW1 to SW4 can be turned off after the relay contacts 12a and 13a are actually closed.The process shown in FIG. 3 is executed by the above processing. Since the auxiliary switches SW 1 to SW 4 are turned off by this operation, during stable operation in the star connection mode, the power consumption at the time when the auxiliary switches SW 1 to SW 4 are turned on is eliminated, power is saved, the heat generation of the auxiliary switches SW 1 to SW 4 does not occur, and measures against the temperature rise of these semiconductor switches are unnecessary.When the motor is operated in the star connection mode (NO in S 1), the main control section 40 monitors whether or not it is necessary to change the mode to the open winding mode in response to a load increase (S 9). When a transition to the open winding mode is not required (NO in S 9), the main control section 40 returns to the above determination in S 1.When a change to the open winding mode is required (YES in S 9), the main control section 40 performs the pseudo neutral point operation in which the upper switching elements Tu, Tv, and Tw and the lower switching elements Tx, Ty, and Tz in the inverter 30 are alternately turned on and off with an on / off duty ratio of 50%, as shown in FIG. 4, so that the potential difference between both ends of each of the relay contacts 12 aand 13 abecomes zero (S 10). This pseudo neutral point operation is the same as the pseudo neutral point operation at the time of switching from the open winding mode to the neutral connection mode. Incidentally, in this state, the relay contacts 12a and 13a are turned on because the operation is in the star connection mode.During the execution of the pseudo neutral point operation, the main control section 40 first turns on the auxiliary switches SW1 to SW4 (S11), thereby short-circuiting both ends of each of the relay contacts 12a and 13a, and turns off the relays 12 and 13 after the short-circuiting (S12). After elapse of a certain time t 2 longer than the time required for the actual opening of the relay contacts 12 aand 13 a(YES in S 13), the main control section 40 turns off the auxiliary switches SW 1 to SW 4 (S 14). Thereafter, the main control part 40 terminates the pseudo neutral point operation and enters the idle operation (S15).After the switching, the main control unit 40 returns to the above determination in S 1. The turning on / off of the auxiliary switches SW 1 to SW 4 in step S 11 and the turning off of the auxiliary switches SW 1 to SW 4 in step S 14 are preferably performed by synchronizing all the auxiliary switches SW 1 to SW 4, but the auxiliary switches need not be turned on and off in a completely synchronized manner. All the auxiliary switches SW 1 to SW 4 may be turned on before the relay contacts 12 aand 13 aare actually opened, and all the auxiliary switches SW 1 to SW 4 may be turned off after the relay contacts 12 aand 13 aare actually opened. The process shown in FIG. 4 is executed by the above processing.The times t1 and t2 may be the same and should be as short as possible from the viewpoint of efficiency. In the mechanical relays 12 and 13, between the energization (energization) and the energization (deenergization), there occurs a delay of 10 to 30 ms until the relay contacts 12 aand 13 aare actually opened and closed. It is desirable to set the predetermined times t1 and t2 to about 50 msec to 100 msec, which is achieved by adding an addition to the delay time for opening and closing the relay contacts 12a and 13a.As described above, when the relay contacts 12 aand 13 aare opened and closed, the pseudo neutral point operation is performed in advance, and the auxiliary switches SW 1 to SW 4 are turned on so that the potential difference between both ends of the relay contacts 12 aand 13 abecomes zero.However, even when the pseudo neutral point operation is performed, a current flows through a path passing through the free wheeling diode D of one of the upper switching elements Tu, Tv, and Tw and the lower switching elements Tx, Ty, and Tz only during the dead time td when the upper switching elements Tu, Tv, and Tw and the lower switching elements Tx, Ty, and Tz of the inverter 30 are both turned off. For example, as indicated by a solid line arrow in FIG. 6, the motor currents Iv and Iw flow through paths running from the phase wires Lv and Lw through the connection points Bv and Bw of the inverter 30 and the free wheeling diodes D of the respective upper switching elements Tv and Tw, and the motor current Iu flows through a path from the free wheeling diode D of the lower switching element Tx to the phase winding Lu through the connection point Bu.The relationship between a collector-emitter voltage Vcex of the lower switching element Tx, a collector-emitter voltage Vcey of the lower switching element Ty, a potential difference Vuv 1 between the connection points Bu and Bv, a potential difference Vuv 2 between both ends of the relay contact 12 a, and a potential difference Vuv 3 between both ends of the series connection of the auxiliary switches Sw 1 and Sw 2 in the current path of FIG. 6 is illustrated in FIG. 7. In other words, in a state where the collector-emitter voltage Vcex of the lower switching element Tx is zero, the collector-emitter voltage Vcey of the lower switching element Ty increases, and accordingly, the potential difference Vuv 1 of the connection points Bu and Bv does not become zero. When the potential difference Vuv 1 occurs, a current flows from the connection point Bv to the connection point Bu via the auxiliary switch SW 2 and the auxiliary switch Sw 1, a voltage is generated in the first inductances Lsu 1 and Lsv 1 and the second inductances Lsu 2 and Lsv 2, and the potential difference Vuv 2 between both ends of the relay contact 12 ais no longer zero as indicated by a broken line arrow in FIG. 6. Then, the collector-emitter voltage Vcex of the lower switching element Tx and the collector-emitter voltage Vcey of the lower switching element Ty become the same value, and accordingly, the potential difference Vuv 1 between the connection points Bu and Bv becomes zero. The same phenomenon also occurs at the relay contact 13a.Since the opening / closing timing of the relay contact 12 awhich is a mechanical opening / closing contact cannot be controlled strictly as described above, the relay contact 12 acan be opened or closed at a timing at which the potential difference Vuv 2 between the both ends of the relay contact 12 ais not zero. When the relay contact 12 ais opened or closed in a state where the potential difference Vuv 2 between both ends of the relay contact 12 ais not zero, overvoltage or arc may occur between both ends of the relay contact 12 a. Since the dead time td is extremely short compared to the regular on / off period of the inverter 30, it is highly unlikely that the relay contact 12 ais opened or closed in a state where the potential difference between the both ends of the relay contact 12 ais not truly zero. However, since the probability of occurrence is not 0, a kind of countermeasure is required.In this example, the potential difference Vuv 2 between the both ends of the relay contact 12 achanges depending on the relationship between the total value "Lsv1+Lsv 2" of the value of the first inductance Lsu 1 of the wire 52 ufrom the connection point Bu to the branch point N 1 and the value of the second inductance Lsu 2 of the wire 53 u, the total value "=Lsv1+Lsv 2" of the value of the first inductance Lsv 1 of the wire 52 vfrom the connection point Bv to the branch point N 2, and the value of the second inductance Lsv 2 of the wire 53 v, the value of the third inductance Lsu 3 of the wire 54 ubetween the branching point N 1 and one end of the series circuit of the auxiliary switches SW 1 and SW 2, and the third inductance Lsu 3 of the wire 54 vbetween the branching point N 2 and the other end of the series circuit of the auxiliary switches SW 1 and SW 2.For example, when the total value "Lsu1+Lsu2" of the value of the first inductance Lsu1 and the value of the second inductance Lsu2 is smaller than the value of the third inductance Lsu3("Lsu1+Lsu2"<Lsu3), and when the total value "Lsv1+Lsv2" of the value of the first inductance Lsv1 and the value of the second inductance Lsv2 is smaller than the value of the third inductance Lsv3("Lsv1+Lsv2"<Lsv3), the potential difference Vuv2 occurs in the magnitude shown in FIG. 7. On the other hand, when the value of the third inductance Lsu 3 is smaller than the above-mentioned total value "Lsu 1+Lsu 2" (Lsu 3<"Lsu 1+Lsu 2") and when the value of the third inductance Lsv 3 is smaller than the above-mentioned total value "Lsv 1+Lsv 2" (Lsv 3<"Lsv 1+Lsv 2"), the potential difference Vuv 2 can be suppressed to be smaller than the case of FIG. 7 as shown in FIG. 8.Also, the potential difference Vuw 2 between the both ends of the relay contact 13 achanges depending on the relationship between the total value "Lsv1+Lsv 2" of the value of the first inductance Lsu 1 of the wire 52 vfrom the connection point Bv to the branch point N 2 and the value of the second inductance Lsv 2 of the wire 53 v, the total value "=Lsw1+Lsw 2" of the value of the first inductance Lsw 1 of the wire 52 wfrom the connection point Bw to the branch point N 3, and the value of the second inductance Lsw 2 of the wire 53 w, the value of the third inductance Lsv3 of the wire 54v between the branching point N2 and one end of the series circuit of the auxiliary switches SW3 and SW4, and the third inductance Lsw3 of the wire 54w between the branching point N3 and the other end of the series circuit of the auxiliary switches SW3 and SW4.In other words, when the value of the third inductance Lsv 3 is smaller than the above-mentioned "total value" "Lsv 1+Lsv 2" (Lsv 3<"Lsv 1+Lsv 2") and when the value of the third inductance Lsw 3 is smaller than the above-mentioned "total value" "Lsw 1+Lsw 2" (Lsw 3<"Lsw 1+Lsw 2"), the potential difference Vvw 2 between both ends of the relay contact 13 acan be suppressed to a small value. By setting the inductance value in this manner, the deterioration of the relay contacts 12 aand 13 acan be reduced to a value that causes no problems.In view of these points, in this embodiment, the length of each of the wires (third wires) 54 u, 54 v, and 54 wis as short as or shorter than the total of the length of each of the wires (first wires) 52 u, 52 v, and 52 wand the length of each of the wires (second wires) 53 u, 53 v, and 53 wand as short as or shorter than the total of the length of each of the wires (first wires) 52 u, 52 v, and 52 wand the length of each of the wires (second wires) 53 u, 53 v, and 53 w, such that the value of the third inductance Lsu 2 is smaller than the total value "Lsu 1+Lsu 2" of the value of the first inductance Lsu 1 and the value of the second inductance Lsu 2 (Lsu 1<"Lsu 1+Lsu 2"), such that the value of the third inductance Lsv 2 is smaller than the total value "Lsv 1+Lsv 2" of the value of the first inductance Lsv 1 and the value of the second inductance Lsv 2 (Lsv 1<"Lsv 1+Lsv 2"), that the value of the third inductance Lsw 2 is smaller than the total value "Lsv 1+Lsv 2" of the value of the first inductance Lsw 1 and the value of the second inductance Lsw 2 (Lsv 1<"Lsw 1+Lsw 2"), and that the potential differences Vuv 2 and Vvw 2 become small. For example, by making the arrangement positions of the relay contacts 12 aand 13 aand the arrangement positions of the auxiliary switches SW 1 to SW 4 as close to each other as possible, the lengths of the wires 54 u, 54 v, and 54 wcan be shortened.The value of the parasitic inductance occurring in wires such as the first inductances Lsu 1, Lsv 1, and Lsw 1, the second inductances Lsu 2, Lsv 2, and Lsw 2, and the third inductance Lsu 3, Lsv 3, and Lsw 3 is substantially proportional to the length of the wire. The shorter the lengths of the wires 54 u, 54 v, and 54 w, the smaller the values of the third inductances Lsu 3, Lsv 3, and Lsw 3 may be. Therefore, in the present embodiment, it is set that "length of the wire 54 u+length of the wire 52 u" >"length of the wire 53 u", "length of the wire 54 v+length of the wire 52 v" >"length of the wire 53 v", and "length of the wire 54 w+length of the wire 52 w" >"length of the wire 53 w".Incidentally, since the magnitudes of the potential differences Vuv2and Vvw2between the both ends of the respective relay contacts 12 aand 13 aare determined by the relative relationship between the above total values "Lsu1+Lsu 2", "Lsv1+Lsv 2", and "Lsw1+Lsw 2" and the values of the third inductances Lsu 3, Lsv 3, and Lsw 3, the potential differences Vuv2and Vvw2can be suppressed to small values even if the total values "Lsu1+Lsu 2", "Lsv1+Lsv 2", and "Lsw1+Lsw 2" are made larger than the values of the third inductances Lsu 3, Lsv 3, and Lsw 3. In order to make the total values "Lsu1+Lsu2", "Lsv1+Lsv2", and "Lsw1+Lsw2" larger than the values of the third inductance Lsu3, Lsv3, and Lsw3, the total values of the lengths of the wires 52 u, 52 v, and 52 wand the lengths of the wires 53 u, 53 v, and 53 wneed only to be elongated. In addition, in order to make the total values "Lsu1+Lsu 2", "Lsv1+Lsv 2", and "Lsw1+Lsw 2" larger than the values of the third inductances Lsu3, Lsv3, and Lsw 3, an inductance element such as a small coil may be inserted into a central part of each of the wires 52 u, 52 v, and 52 wand the wires 53 u, 53 v, and 53 w. However, since the resistance value increases accordingly and causes power loss as the wire length or the coil is added increases, it is desirable to keep the lengths of the wires 54 u, 54 v, and 54 was short as possible as described above.By suppressing the potential differences Vuv2 and Vvw2 occurring between the both ends of the respective relay contacts 12a and 13a, it is possible to eliminate the problem of large over-voltages and arcs that may cause problems between the both ends of the respective relay contacts 12a and 13a even when the relay contacts 12a and 13a are opened and closed with the potential differences Vuv2 and Vvw2. This makes it possible to avoid deterioration of the life of the relays 12 and 13 and destruction of the individual switching elements of the inverters 20 and 30 due to overvoltage and arc.Second EmbodimentFIG. 9 shows a configuration of the second embodiment. A series circuit of auxiliary switches SW 1 and SW 2 is connected to the other ends (terminals 32 uand 32 v) of the phase wires Lu and Lv of a motor 1M via wires (third wires) 54 uand 54 v 1 between the branch points N 1 and N 2 at the ends of the wires (second wires) 53 uand 53 v. A series circuit of auxiliary switches SW 3 and SW 4 is connected via wires (third wires) 54 v 2 and 54 wbetween the branching points N 2 and N 3 at the ends of the wires (second wires) 53 vand 53 wconnected to the other ends (terminals 32 vand 32 w) of the phase wires Lv and Lw of the motor 1M.Then, a relay contact 12 ais connected between the branch points N 1 and N 2 via the wires (fourth wires) 55 uand 55 v. A relay contact 13a is connected between the branch points N2 and N3 via the wires (fourth wires) 55v and 55w.In other words, the tip of the wire 53 ubranches into the wire 54 uand the wire 55 uat the branching point N 1, and the tip of the wire 53 vbranches into three wires, namely, the wires 54 v 1 and 54 v 2 and the wire 55 vat the branching point N 2. Similarly, the tip of the wire 53 wbranches into the wire 54 wand the wire 55 win the branching point N 3. The wire 54 uis connected to an auxiliary switch SW 1 in the series circuit of the auxiliary switches SW 1 and SW 2, and the wire 54 v 1 is connected to the auxiliary switch SW 2 in the series circuit of the auxiliary switches SW 1 and SW 2. The wire 54 v 2 is connected to the side of an auxiliary switch SW 3 in the series circuit of the auxiliary switches SW 3 and SW 4, and the wire 54 wis connected to the side of the auxiliary switch SW 4 in the series circuit of the auxiliary switches SW 3 and SW 4. The auxiliary switches SW 2 and SW 3 are connected in series to each other via the branching point N 2 and the wires 54 v 1 and 54 v 2.The wires 54 uand 54 v 1begin at the branching points N 1 and N 2 and end at both ends of the series circuit of the auxiliary switches SW 1 and SW 2. The wires 54 v 2 and 54 wbegin at the branching points N 2 and N 3 and end at both ends of the series circuit of the auxiliary switches SW 2 and SW 3.The relay contact 12a is connected in parallel with the series circuit of the auxiliary switches SW1 and SW2 via the wires 55u and 55v. The relay contact 13a is connected in parallel with the series circuit of the auxiliary switches SW3 and SW4 via the wires 55v and 55w. The wire 55 uis electrically connected to the wire 53 uvia the branching point N 1, the wire 55 vis electrically connected to the wire 53 vvia the branching point N 2, and the wire 55 wis electrically connected to the wire 53 wvia the branching point N 2. The other end of the relay contact 12 aand one end of the relay contact 13 aare electrically connected via a common connection point P 1 connected to the wire 55 v. The wires 55 uand 55 vbegin at the branching points N 1 and N 2 and end at both ends of the relay contact 12 a. The wires 55 vand 55 wbegin at the branching points N 2 and N 3 and end at both ends of the relay contact 12 a.Also in the second embodiment, similarly to the first embodiment, the relationship among the values of the first inductances Lsu 1, Lsv 1, and Lsw 1, the values of the second inductances Lsu 2, Lsv 2, and Lsw 2, and the values of the third inductances Lsu 3, Lsv 3, and Lsw 3 needs to satisfy the above-described conditions (Lsu 1<"Lsu 1+Lsu 2"), (Lsv 1<"Lsv 1+Lsv 2"), and (Lsw 1<"Lsw 1+Lsw 2"). Therefore, by using the circuit configuration of the second embodiment, the above conditions can be satisfied without making a cumbersome wiring design, because the length of the wires 54 uto 54 wcan be extremely shortened in view of the circuit configuration.The other constituent elements are the same as those in the first embodiment.Third EmbodimentFIG. 10 shows the principal parts of a configuration of the third embodiment.A relay contact 12a is connected between the branch points N1 and N2 at the tips of the wires 53u and 53v connected to the other ends (terminals 32u and 32v) of the phase wires Lu and Lv of a motor 1M. A relay contact 13 ais connected between the branch points N 2 and N 3 at the tips of the wires 53 vand 53 wconnected to the other ends (terminals 32 vand 32 w) of the phase wires Lv and Lw of the motor 1M.Then, a series circuit of auxiliary switches SW 1 and SW 2 is connected in parallel to the relay contact 12 aby connecting the wires 54 uand 54 vto the branch points N 1 and N 2. A series circuit of the auxiliary switches SW 2 and SW 3 is connected in parallel to a relay contact 13 avia the wires 54 vand 54 wconnected to the branch points N 2 and N 3. The auxiliary switches SW1 to SW3 are semiconductor switching elements, e.g., IGBT and MOS-FET, in which a free-wheeling diode D is connected in antiparallel with its element body. An emitter of each of the three auxiliary switches SW 1, SW 2, and SW 3 is usually connected to a common connection point (virtual neutral point) P 2 in the drawing.The first wire 54 uand the second wire 54 vbegin at the branching points N 1 and N 2, and end at both ends of the series circuit of the auxiliary switches SW 1 and SW 2. The second wire 54 vand the third wire 54 wbegin at the branching points N 2 and N 3 and end at both ends of the series circuit of the auxiliary switches SW 2 and SW 3.The other constituent elements are the same as those in the first embodiment, including the relationship among the values of the first inductances Lsu 1, Lsv 1, and Lsw 1, the values of the second inductances Lsu 2, Lsv 2, and Lsw 2, and the values of the third inductances Lsu 3, Lsv 3, and Lsw 3. The three auxiliary switches SW 1, SW 2, and SW 3 are turned on and off in the same manner as the four auxiliary switches SW 1 to SW 4 of the first embodiment.By turning off the auxiliary switches SW 1, SW 2, and SW 3 in a state in which the relay contacts 12 aand 13 aare opened, the phase wires Lu, Lv, and Lw of the motor 1M enter an open winding state and are disconnected from each other. By turning on the auxiliary switches SW 1, SW 2, and SW 3 in a state in which the relay contacts 12 aand 13 aare open, the other ends of the phase wires Lu, Lv, and Lw of the motor 1M are short-circuited via the auxiliary switches SW 1, SW 2, and SW 3 and the common terminal point P 2, and enter a star-switching mode. By closing the relay contacts 12 aand 13 ain a state where the auxiliary switches SW 1, SW 2, and SW 3 are turned off, the other ends of the phase windings Lu, Lv, and Lw of the motor 1M are short-circuited via the relay contacts 12 aand 13 aand the branch point N 2, and become a star connection mode.According to the configuration of the present embodiment, the number of the auxiliary switches SW 1, SW 2, and SW 3, i.e., the number of the semiconductor switching elements, is reduced to three, the number of the semiconductor switching elements is smaller than that of the first and second embodiments, and the circuit can be simplified.Fourth EmbodimentFIG. 11 shows the principal parts of a configuration of the fourth embodiment.A series circuit of auxiliary switches SW 1 and SW 2 is connected via wires (third wires) 54 uand 54 vbetween the branching points N 1 and N 2 at the tips of the wires (second wires) 53 uand 53 vconnected to the other ends (terminals 32 uand 32 v) of the phase wires Lu and Lv of a motor 1M. A series circuit of auxiliary switches SW 3 and SW 4 is connected by wires (third wires) 54 vand 54 wbetween the branching points N 2 and N 3 at the ends of the wires (second wires) 53 vand 53 wconnected to the other ends (terminals 32 vand 32 w) of the phase wires Lv and Lw of the motor 1M.Then, a relay contact 12 ais connected in parallel to the series connection of the auxiliary switches SW 1 and SW 2 via wires (fourth wires) 55 uand 55 vconnected to the branch points N 1 and N 2. A relay contact 13 ais connected in parallel to the series circuit of the auxiliary switches SW 2 and SW 3 via wires (fourth wires) 55 vand 55 wconnected to the branch points N 2 and N 3. The other end of the relay 12 aconnected to the wire 55 vand the one end of the relay 13 aconnected to the wire 55 vare connected to a common connection point P 1.The first wire 54 uand the second wire 54 vbegin at the branching points N 1 and N 2, and end at the both ends of the series connection of the auxiliary switches SW 1 and SW 2. The second wire 54 vand the third wire 54 wbegin at the branching points N 2 and N 3 and end at both ends of the series circuit of the auxiliary switches SW 2 and SW 3.The first wire 55 uand the second wire 55 vbegin at the branch points N 1 and N 2 and end at both ends of the relay contact 12 a. The second wire 55 vand the third wire 55 wbegin at the branching points N 2 and N 3 and end at both ends of the relay contact 13 a.The other constituent elements are the same as those in the first embodiment, including the relationship among the values of the first inductances Lsu 1, Lsv 1, and Lsw 1, the values of the second inductances Lsu 2, Lsv 2, and Lsw 2, and the values of the third inductances Lsu 3, Lsv 3, and Lsw 3. The three auxiliary switches SW 1, SW 2, and SW 3 are turned on and off in the same manner as the four auxiliary switches SW 1 to SW 4 of the first embodiment.By turning off the auxiliary switches SW 1, SW 2, and SW 3 in a state in which the relay contacts 12 aand 13 aare opened, the phase wires Lu, Lv, and Lw of the motor 1M enter an open winding state and are disconnected from each other. By turning on the auxiliary switches SW 1, SW 2, and SW 3 in a state in which the relay contacts 12 aand 13 aare open, the other ends of the phase wires Lu, Lv, and Lw of the motor 1M are short-circuited via the auxiliary switches SW 1, SW 2, and SW 3 and the common terminal point P 2, and enter a star-switching mode. By closing the relay contacts 12 aand 13 ain a state where the auxiliary switches SW 1, SW 2, and SW 3 are turned off, the other ends of the phase windings Lu, Lv, and Lw of the motor 1M are short-circuited via the relay contacts 12 aand 13 aand the branch point N 2, and become a star connection mode.According to the configuration of the present embodiment, the number of the auxiliary switches SW 1, SW 2, and SW 3, i.e., the number of the semiconductor switching elements, is reduced to three, the number of the semiconductor switching elements is smaller than that of the first and second embodiments, and the circuit can be simplified.Modification ExamplesWhile certain embodiments have been described, they are presented by way of example only and are not intended to limit the scope of the inventions. Indeed, the novel embodiments and modification examples described herein may be embodied in a variety of other forms; moreover, various omissions, substitutions and changes in the form of the embodiments or examples described herein may be made without departing from the spirit of the inventions. These embodiments and modifications thereof are included within the scope and spirit of the invention and are also included within the scope of the invention described in the claims and the equivalent scope thereof.List of reference characters2 .. Drive Circuit, 3... Open winding motor, Lu, Lv, and Lw... Phase wires, 4... Control Unit 12 and 13... Relays (switches), 12a and 13a... Switch contacts (relay contacts), SW1 to SW4... Semiconductor switching elements, Lsu1, Lsv1 and Lsw1.. First inductances, Lsu 2, Lsv 2, and Lsw 2... Second inductances, 20,... Inverter (first inverter), 30... Inverters (second inverter) and 40... Main Control Part.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 4804381 B

[0007] JP 2019-62726 A

[0007]

Claims

A motor drive device of a motor including a plurality of phase wires separated from each other, the motor drive device including: a first inverter including a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to a DC power supply, and a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to an end of each of the phase wires; a second inverter including a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to the DC power supply, a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to the other end of each of the phase wires through each of the first wires; a plurality of switching contacts being connected between the other ends of each of the phase wires through each of the second wires; a plurality of semiconductor switching elements connected in parallel to each of the switching contacts via each of the third wires; and a control unit that controls driving of the first inverter, driving of the second inverter, and opening and closing of each of the switching contacts, wherein when each of the switching contacts is opened and closed, the control unit performs a pseudo neutral point operation in which all upper switching elements and all lower switching elements in the second inverter are alternately turned on and off and each of the semiconductor switching elements is turned on in advance, each of the first wires has a first inductance, each of the second wires has a second inductance, each of the third wires has a third inductance, and a value of the third inductance is smaller than a total value of the value of the first inductance and the value of the second inductance.The motor drive device according to claim 1, wherein the control unit selectively sets a star connection mode in which the other ends of the phase wires are connected to each other and on / off driving of each of the switching elements of the first inverter is performed by closing each of the switching contacts, and an idle mode in which the other end of each of the phase wires is placed in a disconnected state and on / off driving of each of the switching elements of the first inverter and each of the switching elements of the second inverter is performed in connection to each other by opening each of the switching contacts.The motor drive device according to claim 1, wherein the control unit ensures a dead time to turn off both the upper switching element and the lower switching element of each of the series circuits in the first inverter during the on / off driving of the upper switching element and the lower switching element in the first inverter, and to turn off both the upper switching element and the lower switching element of each of the series circuits in the second inverter during the on / off driving of the upper switching element and the lower switching element in the second inverter.A motor drive device of a motor having a plurality of phase wires separated from each other, the motor drive device comprising: a first inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to a DC power supply, and a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to one end of each of the phase lines; a second inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to the DC power supply, a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to the other end of each of the phase lines through each of the first lines; a plurality of semiconductor switching elements connected between the other ends of the respective phase wires by each of the second wires and each of the third wires; a plurality of switching contacts connected in parallel to each of the semiconductor switching elements via respective fourth wires; and a control unit that controls driving of the first inverter, driving of the second inverter, and opening and closing of each of the switching contacts, wherein when opening and closing each of the switching contacts, the controller performs a pseudo neutral point operation in which all upper switching elements and all lower switching elements in the second inverter are alternately turned on and off and each of the semiconductor switching elements is turned on in advance, each of the first wires has a first inductance, each of the second wires has a second inductance, each of the third wires has a third inductance, and a value of the third inductance is smaller than a total value of the value of the first inductance and the value of the second inductance.The motor drive device according to claim 4, wherein the control unit selectively sets a star connection mode in which the other ends of the phase wires are connected to each other and on / off driving of each of the switching elements of the first inverter is performed by closing each of the switching contacts, and an idle mode in which the other end of each of the phase wires is placed in a disconnected state and on / off driving of each of the switching elements of the first inverter and each of the switching elements of the second inverter is performed in connection to each other by opening each of the switching contacts.The motor drive device according to claim 4, wherein the control unit ensures a dead time to turn off both the upper switching element and the lower switching element of each of the series circuits in the first inverter during the on / off driving of the upper switching element and the lower switching element in the first inverter, and to turn off both the upper switching element and the lower switching element of each of the series circuits in the second inverter during the on / off driving of the upper switching element and the lower switching element in the second inverter.A motor driving apparatus of a motor having a plurality of phase wires separated from each other, the motor driving apparatus comprising: a first inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to a DC power supply, and a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to an end of each of the phase lines; a second inverter having a plurality of series circuits of upper switching elements and lower switching elements, both ends of the series circuits being connected to the DC power supply, a connection point of the upper switching element and the lower switching element of each of the series circuits being connected to the other end of each of the phase lines through each of the first lines; a plurality of switching contacts being connected between the other ends of each of the phase wires through each of the second wires; a plurality of semiconductor switching elements connected to each of the switching contacts in parallel via each of the third wires; and a control unit that controls driving of the first inverter, driving of the second inverter, and opening and closing of each of the switching contacts, wherein when each of the switching contacts is opened and closed, the control unit performs a pseudo neutral point operation in which all upper switching elements and all lower switching elements in the second inverter are alternately turned on and off and each of the semiconductor switching elements is turned on in advance, each of the first wires has a length, each of the second wires has a length, each of the third wires has a length, and in all of the third wires, a length of the third wire is shorter than a total of a length of the first wire connected to the third wire and a length of the second wire.The motor driving device according to any one of claims 1 and 7, wherein the phase wires are the three phase wires Lu, Lv, and Lw, the first wires are three first wires connected to the other ends of Lu, Lv, and Lw, the second wires are three second wires starting from the other ends of Lu, Lv, and Lw and ending at three branching points N1, N2, and N3, the switching contacts are a first switching contact connected between the branching points N1 and N2 and a second switching contact connected between the branching points N2 and N3, the semiconductor switching elements are three semiconductor switching elements Sw1, Sw2, and Sw3, and the third wires are three third wires, wherein a first third wire and a second third wire start at the branching points N 1 and N 2 and end at both ends of a series connection of the semiconductor switching elements Sw 1 and Sw 2, and wherein the second third wire and a third wire start at the branching points N 2 and N 3 and end at both ends of a series connection of the semiconductor switching elements Sw 2 and Sw 3.The motor driving device according to any one of claims 1 and 7, wherein the phase wires are the three phase wires Lu, Lv, and Lw, the first wires are three first wires connected to the other ends of Lu, Lv, and Lw, the second wires are three second wires starting from other ends of Lu, Lv, and Lw and ending at three branch points N1, N2, and N3, the switching contacts are a first switching contact connected between the branch points N1 and N2, and a second switching contact connected between the branch points N2 and N3, the semiconductor switching elements are three semiconductor switching elements Sw1, Sw2, and Sw3, the three semiconductor switching elements Sw1, Sw2, and Sw3 have common one-sided ends, and in the third wires, a first third wire starts at the branching point N 1 and ends at the other end of the semiconductor switching element Sw 1, the second third wire starts at the branching point N 2 and ends at the other end of the semiconductor switching element Sw 2, and the third third wire starts at the branching point N 3 and ends at the other end of the semiconductor switching element Sw 3.The motor drive device according to any one of claims 4 and 7, wherein the phase wires are the three phase wires Lu, Lv, and Lw, the first wires are three first wires connected to the other ends of Lu, Lv, and Lw, the second wires are three second wires starting at the other ends of Lu, Lv, and Lw and ending at three branching points N1, N2, and N3, the semiconductor switching elements are three semiconductor switching elements Sw1, Sw2, and Sw3, the third wires are three third wires in which a first third wire and a second third wire start at the branching points N1 and N2 and end at both ends of a series connection of the semiconductor switching elements Sw1 and Sw2, and wherein the second third wire and a third wire start at the branching points N 2 and N 3 and terminate at both ends of a series circuit of the semiconductor switching elements Sw 2 and Sw 3, and the fourth wires are three fourth wires, wherein a first fourth wire and a second fourth wire start at the branching points N 1 and N 2 and terminate at both ends of the first switching contact, and wherein the second fourth wire and a third fourth wire terminate at both ends of the second switching contact.The motor driving device according to any one of claims 1, 4 and 7, wherein each of the semiconductor switching elements has a free wheeling diode connected in antiparallel with its main body.A refrigeration cycle apparatus comprising a compressor driven by the motor driving apparatus according to any one of claims 1, 4 and 7.

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