Control unit and control method for rotating electric machine

Abstract

Control unit for a rotating electrical machine, wherein the control unit (40) is applied to a rotating electrical machine (10) comprising a rotor (12) with an excitation winding (11) and a stator (13) with an armature winding (10a, 10b) to control an excitation current passing through the excitation winding (11), wherein: a prescribed voltage (Va) is applied to the armature winding (10a, 10b); and the excitation current is controlled to be a first excitation current with which an amplitude of an induced voltage (Vb) generated in the armature winding (10a, 10b) by a rotation of the rotor (12) corresponds to an amplitude of the applied voltage (Va) of the armature winding (10a, 10b) with an error that is within a prescribed range.

Description

Technical FieldThe present disclosure relates to control techniques for controlling a field winding type rotating electric machine.BACKGROUND OF THE INVENTIONRecently, ISG (Integrated Starter Generators) having functions of both starters and electric generators have been used for starters and electric generators of automobiles. For the ISG, exciting-winding type AC rotating electric machines are used. Such an in-vehicle rotary electric machine requires an improvement in the power efficiency. For example, JP 4 662 119 B2 proposes techniques for improving the power efficiency of exciting-winding type AC rotary electric machines.Summary of the InventionTechnical ProblemAccording to the techniques described in JP 4 662 119 B2, when the rotation speed of a motor is low, a stator current and an excitation current are increased, thereby ensuring generated power. In the techniques described in JP 4 662 119 B2, as the rotational speed of the motor becomes higher, the stator current is decreased to reduce copper loss and maintain a large excitation current, thereby ensuring generated power. Then, according to the techniques described in JP 4 662 119 B2, as the rotational speed of the motor becomes much higher, the excitation current is decreased to reduce iron loss and increase the stator current, thereby ensuring generated power.The techniques described in JP 4 662 119 B2 require map data in which the value of either output torque or generated power and the value of rotational speed are used as input values and a stator current and a field current are used as output values. Accordingly, the techniques described in JP 4 662 119 B2 involve a considerable number of tests and require a storage capacity to store a large amount of data. In addition, in the techniques described in JP 4 662 119 B2, the stator current and the excitation current are simultaneously controlled. Accordingly, in the techniques described in JP 4 662 119 B2, control over the stator current and the excitation current is complicated.The document DE 690 23 955 T2 describes a charging generator which charges a battery by an output voltage from a generator unit which has an armature coil and an excitation coil. The generator includes a voltage step-up / step-down circuit connected between the output of the generating unit and the battery, a first control circuit connected between the exciting coil of the generating unit and the battery to control an exciting current flowing through the exciting coil to regulate a voltage generated in the armature coil of the generating unit in response to a voltage across the battery, and a second control circuit connected to the voltage step-up / step-down circuit to control the step-up / step-down ratio of the voltage step-up / step-down circuit.The document DE 34 06 269 C2 describes a converter synchronous motor. A control device for a thyristor motor comprises a voltage detector, a rotation speed detector circuit and a correction circuit. The voltage detector detects a DC voltage which is supplied to an inverter circuit. The speed detecting circuit detects the angular speed of a synchronous motor and outputs a speed voltage proportional thereto. The correction circuit detects the difference between the DC voltage and the rotational speed voltage, and outputs a correction signal to an excitation circuit when a difference is detected. The excitation circuit corrects the field current in response to the correction signal such that the DC voltage and the rotational speed voltage are equal.The publication DE 103 17 207 A1 describes a method for controlling an electric machine, in particular a synchronous machine in motor operation, by means of an associated converter bridge which is arranged between the electric machine and a battery. The converter bridge is controlled by block clocking in such a way that the switching times are determined by the rotor position and by the rotational speed. By means of speed-dependent shift angle tracking and / or speed-dependent excitation current reduction, speed-torque characteristic curves are obtained which are increased and thus improved in particular in the upper speed range compared to conventional speed-torque characteristic curves. The relationships required for the control or regulation are stored, for example, in table form in the memory of a control device, so that no further actual value detection is required in addition to the angular position and rotational speed detection of the armature of the electric machine.U.S. Pat. No. 8,410,737 B2 describes an apparatus and a method for generating initial operating points for controlling an internal permanent magnet machine (IPM). The method includes charging an inductance lookup table, first calculating a maximum torque per ampere (MTPA) trajectory for a first threshold speed based on engine parameters of the IPM machine, second calculating a cut voltage limit ellipse with monotonically increasing torque for a first speed based on the engine parameters when the first speed is higher than the first threshold speed, determining an operating trajectory at the first speed based on the calculated MTPA trajectory and / or the calculated cut voltage limit ellipse, and generating an Id,Iq map, associating an Id value and an Iq value with each torque command of a plurality of torque commands for the first speed based on the determined operating trajectory.EP 1 787 385 B1 describes a method of controlling a wound rotor synchronous motor comprising measuring a plurality of electromagnetic parameters indicative of the operation of the motor, generating a voltage across a stator of the motor to obtain a stator current, and generating a further voltage across the rotor to obtain a rotor current, wherein prior to the generating and the further generating the parameters are processed to calculate references for the stator current and for the rotor current, the references corresponding to the generation of the maximum achievable torque by the motor with a given supplied stator current.JP 4 662 119 B2 describes a control method for a rotating electric machine in which an excitation coil current and a stator current are controlled by a controller. The stator current is vector controlled and controlled so that the phase maintains an efficient motor zone. In the case of rated power generation, at a low engine speed, the phase current and the exciting coil current are increased to reserve the generated power. When the rotational speed is increased, the phase current is decreased to reduce the copper loss, while instead of decreasing the phase current, the exciting coil current is kept high to reserve the generated power. When the speed is further increased, the exciting coil current is decreased to reduce the iron loss, while instead of decreasing the exciting coil current, the phase current is increased to reserve the generated power.The present disclosure has an object of providing control techniques for an alternating current rotating electric machine of an excitation winding type with which power efficiency is improved with simple control.Solution of the ProblemThe object is achieved by a control unit according to claim 1 and a control method according to claim 18. Advantageous further developments are specified in the dependent claims.The present disclosure relates to a control unit applied to a rotary electric machine including a rotor having an excitation winding and a stator having an armature winding to control an excitation current passing through the excitation winding. A prescribed voltage is applied to the armature winding. The excitation current is controlled to be a first excitation current with which an amplitude of an induced voltage generated in the armature winding by rotation of the rotor coincides with an amplitude of the applied voltage of the armature winding with an error that is within a prescribed range.The applicant of the present invention has found that when the value of the induced voltage and the value of the applied voltage approach each other, the amplitude of a phase current passing through the armature winding of the stator becomes small. Based on this finding, the control unit of the present disclosure controls the excitation current passing through the excitation winding such that a difference between a value of the induced voltage and a value of the applied voltage becomes equal to or less than a prescribed value. Thus, the control unit of the present disclosure is capable of reducing the amplitude of a phase current passing through the armature winding of the stator and reducing a power loss caused by the current passing through the armature winding. In this way, the control unit of the present disclosure can improve power efficiency of an excitation winding type AC rotating electric machine with simple control.Brief Description of the DrawingsFIG. 1 is an electrical configuration diagram of a present embodiment. FIG. 2 is diagrams each illustrating exciting current characteristics of dq-axis current. FIG. 3 is diagrams each showing a relationship between dq-axis current amplitude and excitation current characteristics in the case of a change in an applied voltage. FIG. 4 is diagrams each showing a relationship between dq-axis current amplitude and output characteristics in the case of a change in an excitation current. FIG. 5 is a graph showing a relationship between exciting current and torque characteristics. FIG. 6 is diagrams each illustrating a vector location with an applied voltage that is constant and a vector location with a torque that is constant in a dq axis coordinate system. FIG. 7 is a functional block diagram illustrating functions of a control unit. FIG. 8 is graphs each illustrating changes in responsiveness and power efficiency depending on setting of a d-axis current command value. FIG. 9 is a flowchart illustrating selection processing of an excitation current target value. FIG. 10 is diagrams each illustrating an operation of the present embodiment. FIG. 11 is a diagram illustrating settings of a d-axis current command value and an excitation current reference value in a modification.DESCRIPTION OF THE EMBODIMENTSReferring to the drawings, a first embodiment will be described below. In the first embodiment, a control unit of the present disclosure is applied to a vehicle provided with an engine.As shown in FIG. 1, a motor 10 is a field winding type rotating electric machine including a polyphase multi-winding. In the present embodiment, the motor 10 is an excitation winding type synchronous motor having a three-phase double winding. In the present embodiment, an ISG (integrated starter generator) in which the functions of a starter and an alternator (an electric generator) are integrated is used as the engine 10. In the present embodiment, specifically, the motor 10 functions as a starter when an engine 20 is initially started. Furthermore, in the present embodiment, the motor 10 also functions as a starter when a function of automatically stopping and restarting the engine 20 is executed in the case where respective prescribed automatic stop and restart conditions are satisfied (for example, an "idle stop function" or the like).A rotor 12 configuring the motor 10 includes a field winding 11. the rotor 12 is configured to enable drive power transmission to / from a crankshaft 20 aof the engine 20. In the present embodiment, the rotor 12 is coupled to the crankshaft 20 avia a belt 21. Specifically, the rotor 12 is directly coupled to the crankshaft 20 avia the belt 21.Two armature winding groups (hereinafter referred to as a "first winding group 10 a" and a "second winding group 10 b") are wound around a stator 13 of the motor 10. The first and second winding groups 10 aand 10 bare configured to share the rotor 12. The first and second winding groups 10 aand 10 bare configured with respective three-phase windings, the three-phase windings having different neutral points. In the present embodiment, a winding number N 1 of the wiring configuring the first winding group 10 ais set to be equal to a winding number N 2 of the wiring configuring the second winding group 10 b.Two inverters (hereinafter referred to as a "first inverter INV 1" and a "second inverter INV 2") corresponding to the respective first and second winding groups 10 aand 10 bare electrically connected to the motor 10. Specifically, the first inverter INV 1 is connected to the first winding group 10 a, while the second inverter INV 2 is connected to the second winding group 10 b. A high voltage battery 22 as a DC power supply is connected in parallel and is shared between the first and second inverters INV 1 and INV 2. The high voltage battery 22 is configured such that an output voltage of a low voltage battery 24 boosted by a boost DCDC converter 23 can be applied thereto. The output voltage of the low voltage battery 24 (for example, a "lead battery" or the like) is set to be lower than the output voltage of the high voltage battery 22 (for example, a "lithium ion battery" or the like).The first inverter INV 1 includes a first U-phase high-potential side switch SUp 1, a first V-phase high-potential side switch SVp 1, a first W-phase high-potential side switch SWp 1, a first U-phase low-potential side switch SUn 1, a first V-phase low-potential side switch SVn 1, and a first W-phase low-potential side switch SWn 1. In addition, the first inverter INV 1 includes three sets of serial connections in which the U, V, and W-phase high and low potential side switches are connected in series on a phase basis. Connection points of the respective serial connections of the U, V, and W phases are connected to U, V, and W phase terminals of the first winding group 10 a, respectively. In the present embodiment, an N-channel MOSFET is used for each of the high-potential side switches SUp 1 to SWp 1 and each of the low-potential side switches SUn 1 to SWn 1. Diodes DUp 1, DVp 1, DWp 1, DUn 1, DVn 1, and DWn 1 are connected in reverse parallel to the high-potential side switches SUp 1 to SWp 1 and the low-potential side switches SUn 1 to SWn 1, respectively. The diodes DUp 1 to DWp 1 and DUn 1 to DWn 1 may be body diodes of the high potential side switches SUp 1 to SWp 1 and the low potential side switches SUn 1 to SWn 1, respectively. In addition, the high potential side switches SUp 1 to SWp 1 and the low potential side switches SUn 1 to SWn 1 are not limited to the N-channel MOSFETs, and IGBTs may be used for this, for example.As in the first inverter INV 1, the second inverter INV 2 includes a second U-phase high-potential side switch SUp 2, a second V-phase high-potential side switch SVp 2, a second W-phase high-potential side switch SWp 2, a second U-phase low-potential side switch SUn 2, a second V-phase low-potential side switch SVn 2, and a second W-phase low-potential side switch SWn 2. The second inverter INV 2 includes three sets of serial connections in which the U, V, and W-phase high and low potential side switches are connected in series on a phase basis. Connection points of the respective serial connections in the U, V, and W phases are connected to U, V, and W phase terminals of the second winding group 10 b, respectively. In the present embodiment, an N-channel MOSFET is used for each of the high-potential side switches SUp 2 to SWp 2 and each of the low-potential side switches SUn 2 to SWn 2. Diodes DUp2, DVp2, DWp2, DUn2, DVn2 and DWn2 are connected in reverse parallel to the high-potential side switches SUp2 to SWp2 and the low-potential side switches SUn2 to SWn2, respectively. The diodes DUp 2 to DWp 1 and DUn 1 to DWn 2 may be body diodes of the high potential side switches SUp 2 to SWp 2 and the low potential side switches SUn 2 to SWn 2, respectively. In addition, the high potential side switches SUp2 to SWp2 and the low potential side switches SUn2 to SWn2 are not limited to the N-channel MOSFETs, and for example, IGBTs may be used therefor.A positive electrode terminal of the high voltage battery 22 is connected to each of the high potential side terminals of the first and second inverters INV 1 and INV 2 (a drain side terminal of each of the high potential side switches SUp 1 to SWp 2). On the other hand, a negative electrode terminal of the high voltage battery 22 is connected to each of the low potential side terminals (a source side terminal of each of the low potential side switches SUn 1 to SWn 2).The field winding 11 is configured such that a DC voltage can be applied thereto through a field circuit 36. The excitation circuit 36 adjusts the DC voltage to be applied to the excitation winding 11 to thereby control an excitation current If passing through the excitation winding 11.The present embodiment includes a control system provided with various sensors such as a rotation angle sensor 30, a voltage sensor 31, an excitation current sensor 32, and a phase current detector 33. The voltage sensor 31 detects power supply voltages VIN of the first and second inverters INV 1 and INV 2. The excitation current sensor 32 detects the excitation current If passing through the excitation winding 11. The phase current detector 33 detects phase currents of the first winding group 10 a(currents passing through the first winding group 10 ain the fixed coordinate system) and phase currents of the second winding group 10 b(currents passing through the second winding group 10 bin the fixed coordinate system). Note that, as the rotation angle sensor 30, for example, a resolver may be used. Further, as the excitation current sensor 32 and the phase current detector 33, those including, for example, a current transformer or a resistor may be used.Detected values of the various sensors are input to a control unit 40. The control unit 40 is a software processing device that includes a central processing unit (CPU), a memory, or the like, and allows the CPU to execute a program stored in the memory. The control unit 40 executes the program to realize prescribed control functions. The control unit 40 performs control such that a controlled variable of the motor 10 is approximated to a command value instructing control over the motor 10. Accordingly, the control unit 40 generates a manipulation signal to manipulate the first and second inverters INV 1 and INV 2 based on the detected values of the various sensors, and outputs the manipulation signal. A controlled variable of the engine 10 during power running is an output torque T that is output (transmitted) to the crankshaft 20 a, and a command value thereof is a command torque T*. A controlled variable of the motor 10 during regeneration is an output power P output from the motor 10 by power generation (a generated power during regeneration), and a command value thereof is a command power P*. The controlled variable of the engine 10 during power running may be an input power P input to the engine 10 (power consumption during power running). The controlled variable of the engine 10 during regeneration may be an input torque T input from the crankshaft 20 a(a loss torque associated with power generation).The control unit 40 adjusts the excitation current If passing through the excitation winding 11 and the phase current passing through the first and second winding groups 10 aand 10 bto thereby execute control to cause the output power P (the controlled variable) of the motor 10 to approach the command power P* (the command value). The excitation winding 11 of the rotor 12 has a larger winding number, a larger reactance value, and a larger circuit time constant compared with those of the first and second winding groups 10 aand 10 b(the two armature winding groups) of the stator 13. Consequently, in the case of adjusting the excitation current If to execute control for causing the output power P of the motor 10 to approach the command power P*, responsiveness is deteriorated. Also during power running, in the case of adjusting the exciting current If to execute control for causing the output torque T of the motor 10 to approach the command torque T*, responsiveness is deteriorated.To cope with this, the control unit 40 according to the present embodiment sets an amplitude Va of the voltage applied to each of the first and second winding groups 10 aand 10 b(hereinafter referred to as "applied voltage") to a constant value and adjusts a phase δ of the applied voltage. The control unit 40 thereby performs control during regeneration to cause the output power P of the motor 10 to approach the command power P*, and performs control during power running to cause the output torque T of the motor 10 to approach the command torque T*.Then, in addition to adjusting the applied voltage, the control unit 40 adjusts the excitation current If output from the excitation circuit 36. Thus, the control unit 40 performs control for reducing a copper loss caused by the current passing through the first and second winding groups 10 aand 10 b. Note that in FIG. 1, the signals that manipulate the high-potential side switches SUp 1 to SWp 1 of the first inverter INV 1 are respectively indicated as first manipulation signals gUp 1, gVp 1, and gWp 1. In addition, in FIG. 1, the signals that manipulate the low-potential side switches SUn 1 to SWn 1 of the first inverter INV 1 are respectively indicated as first manipulation signals gUn 1, gV 1, and gWn 1. Further, in FIG. 1, the signals that manipulate the high-potential side switches SUp 2 to SWp 2 of the second inverter INV 2 are respectively indicated as second manipulation signals gUp 2, gVp 2, and gWp 2. In addition, in FIG. 1, the signals that manipulate the low-potential side switches SUn 2 to SWn 2 of the second inverter INV 2 are respectively indicated as second manipulation signals gUn 2, gV 2, and gWn 2. The excitation circuit 36 may be incorporated in the control unit 40 or may be externally added to the control unit 40.Referring to FIGS. 2 to 6, the relationship among the output power P, the applied voltage amplitude Va, the applied voltage phase δ, the excitation current If, the input torque T, and a dq axis current Idq will be described. The following description deals with the case of regeneration. Note that by replacing the output power P with an input power P and replacing the input torque T with an output torque T, the following description also serves as the description during power running, thus omitting the description during power running.FIG. 2 shows a relationship between the excitation current If and the dq-axis current Idq when the amplitude Va of the applied voltage is set to a constant value (48V) on condition that the output power P is set to a prescribed command power P* (a constant value). FIGS. 2( a) and 2( b) differ in a rotational speed of the motor 10. the output power P is determined here by multiplying the input torque T by an angular speed ω of the motor 10 (P=T×ω). The dq axis current Idq is a current value obtained by coordinate conversion of each phase current that passes through the first and second winding groups 10 aand 10 band is detected by the phase current detector 33.FIG. 2( a) shows a relationship between the excitation current If and a current amplitude Ia of the dq axis current Idq at a prescribed first rotation speed RS 1. FIG. 2( b) shows a relationship between a q-axis current Iq and a d-axis current Id in the dq-axis current Idq. The current amplitude Ia of the dq-axis current Idq is an absolute value of a vector represented by a set of a value of the q-axis current Iq and a value of the d-axis current Id. The current amplitude Ia of the dq-axis current Idq is minimized when the excitation current If is 2A. As the excitation current If decreases or increases from 2A, the current amplitude Ia of the dq-axis current Idq increases. In other words, when the excitation current If becomes 2A, power loss caused by the phase currents passing through the first and second winding groups 10 aand 10 bis minimized.FIG. 2( c) shows a relationship between the field current If and the current amplitude Ia of the dq axis current Idq at a prescribed second rotational speed RS 2 (RS2=2·RS1). FIG. 2( d) shows a relationship between the q-axis current Iq and the d-axis current Id in the dq-axis current Idq. Under the condition that the output power P becomes a constant value, the rotation speed of the motor 10 shifts from the first rotation speed RS 1 to the second rotation speed RS 2 (the rotation speed is doubled), and the input torque T decreases to half. The current amplitude Ia of the dq-axis current Idq is minimized when the excitation current If is 1A. As the excitation current If decreases or increases from 1A, the current amplitude Ia of the dq axis current Idq increases. In other words, when the excitation current If becomes 1A, power loss caused by the current passing through the first and second winding groups 10 aand 10 bis minimized.FIG. 3 shows a relationship among the amplitude Va of the applied voltage, the excitation current If, and the current amplitude Ia under the condition that the output power P is set to a prescribed command power P* (a constant value). FIGS. 3( a) and 3( b) differ in a rotational speed of the engine 10.FIG. 3( a) shows a relationship among the applied voltage amplitude Va, the field current If, and the current amplitude Ia at the prescribed first rotation speed RS 1. When the amplitude Va of the applied voltage is 48V, the current amplitude Ia of the dq-axis current Idq is minimized at an excitation current If of 2A. When the amplitude Va of the applied voltage is 36V, the current amplitude Ia of the dq-axis current Idq is minimized at an excitation current If of 1.5A. When the amplitude Va of the applied voltage is 24V, the current amplitude Ia of the dq-axis current Idq is minimized at an excitation current If of 1A. Note that when the amplitude Va of the applied voltage is 12V and the excitation current If becomes less than or equal to about 1.2A, the condition for the output power P to assume a constant value can no longer be satisfied.FIG. 3( b) shows a relationship among the applied voltage amplitude Va, the excitation current If, and the current amplitude Ia at the second rotation speed RS 2 (RS2=2·RS1). When the amplitude Va of the applied voltage is 48V, the current amplitude Ia of the dq-axis current Idq is minimized at an excitation current If of 1A. When the amplitude Va of the applied voltage is 36V, the current amplitude Ia of the dq-axis current Idq is minimized at an excitation current If of 0.75A. When the amplitude Va of the applied voltage is 24V, the current amplitude Ia is minimized at an excitation current If of 0.5A. Note that when the amplitude Va of the applied voltage is 12V and the excitation current If becomes less than or equal to about 1.2A, the condition for the output power P to assume a constant value can no longer be satisfied.In the present embodiment, the larger amplitude Va of the applied voltage causes the smaller minimum value of the current amplitude Ia. In other words, in the control unit 40 according to the present embodiment, the amplitude Va of the applied voltage is increased and the optimum excitation current If is allowed to flow, thereby minimizing the current amplitude Ia of the dq axis current Idq. Thus, the control unit 40 can minimize the power loss caused by the current passing through the first and second winding groups 10 aand 10 b.FIG. 4 shows a relationship among the excitation current If, the output power P, and the current amplitude Ia in the case where the excitation current If and the applied voltage phase δ are changed under the condition that the applied voltage amplitude Va is set to a constant value (48V). FIGS. 4( a) and 4( b) differ in a rotation speed of the motor 10.FIG. 4( a) shows a relationship among the excitation current If, the output power P, and the current amplitude Ia at the prescribed first rotation speed RS 1. In a range where the output power P is less than or equal to about 5000W, the current amplitude Ia is minimized when the excitation current If is 2A. In a range where the output power P is greater than about 5000W, the current amplitude Ia is minimized when the excitation current If is 3A. In a range where the output power P is greater than about 8000 W, the current amplitude Ia is minimized when the excitation current If is 4A.FIG. 4( b) shows a relationship among the excitation current If, the output power P, and the current amplitude Ia at the second rotation speed RS 2 (RS2=2·RS1). In a range where the output power P is less than or equal to about 2500W, the current amplitude Ia is minimized when the excitation current If is 1A. In a range where the output power P is greater than about 2500W, the current amplitude Ia is minimized when the excitation current If is 1.5A. In a range where the output power P is greater than about 4000W, the current amplitude Ia is minimized when the excitation current If is 2A. In a range where the output power P is greater than about 6000W, the current amplitude Ia is minimized when the excitation current If is 3A. In a range where the output power P is greater than about 9000W, the current amplitude Ia is minimized when the excitation current If is 4A.For example, in the case where the rotational speed of the motor 10 is the first rotational speed RS 1 (in the case of FIG. 4( a) ), the excitation current If is set to 2A in the range where the output power P is less than about 5000W. In the range where the output power P is greater than or equal to about 5000 W and is less than about 8000 W, the excitation current If is set to 3A. In the range where the output line P is larger than about 8000 W, the excitation current If is set to 4A. In this way, in the control unit 40 according to the present embodiment, the excitation current If is adjusted according to the output line P, thereby minimizing the current amplitude Ia and increasing the output power P. Note that the excitation current If may be continuously changed according to the change in the output power P.The applicant of the present invention has carried out experiments on the condition that the amplitude Va of the applied voltage has been set to a constant value (37.5 V), the rotational speed of the motor 10 has been set to a constant value (3000 Rpm), and the output power P has been set to a constant value (5000 W). The applicant of the present invention has obtained experimental results showing a relationship between the excitation current If and the input torque T under this condition. FIG. 5 shows the relationship between the exciting current If and the input torque T obtained in the experiments. When the excitation current If becomes about 0.75A, the input torque T is minimized, and when the excitation current If becomes greater or less than about 0.75A, the ineffective input torque T that is not toward the power generation becomes greater.When the excitation current If was 0.25A, the induced voltage had an amplitude Vb of 16.8V. When the excitation current If has been 0.5A, the amplitude Vb of the induced voltage has been 27.9V. When the excitation current If has been 0.75A, the amplitude Vb of the induced voltage has been 36.7V. When the excitation current If has been 1A, the amplitude Vb of the induced voltage has been 42.1V. When the excitation current If has been 1.25A, the amplitude Vb of the induced voltage has been 52.1V. In other words, the present experiments have revealed that when the amplitude Vb of the induced voltage and the amplitude Va of the applied voltage have matched with each other with an error in a prescribed range (for example, "when Va=37.5V and Vb=336.7V"), the input torque T satisfying the command power P* has assumed a minimum value.The control unit 40 according to the present embodiment uses the above-described characteristics as a basis to set the amplitude Va of the applied voltage to a maximum value. In other words, the control unit 40 sets the amplitude Va of the applied voltage to a maximum value of the output voltage of the boost DC-DC converter 23 (48V). In this way, in the control unit 40, the amplitude Va of the applied voltage is set to a maximum value, thereby reducing copper loss caused by the phase currents passing through the first and second winding groups 10 aand 10 b. In addition, the control unit 40 adjusts the phase δ of the applied voltage, thereby controlling the output power P and the output torque T of the motor 10. Specifically, during regeneration, the control unit 40 performs control to cause the output power P of the motor 10 to approach the command power P*, and during power running, control to cause the output torque T of the motor 10 to approach the command torque T*. Further, the control unit 40 adjusts the excitation current If such that the amplitude Vb of the induced voltage coincides with the amplitude Va of the applied voltage.FIG. 6 shows a relationship between the q-axis current Iq and the d-axis current Id under the condition that the applied voltage amplitude Va is set to a maximum value (48V) and the excitation current If is set such that the induced voltage amplitude Vb coincides with the applied voltage amplitude Va. Specifically, FIG. 6 shows arc-shaped locations of current vectors each represented by a combination of the q-axis current Iq and the d-axis current Id in the dq-axis coordinate system (hereinafter, each referred to as a "voltage limiting circle line"). Also, FIG. 6 shows linear locations of current vectors each represented by a combination of the q-axis current Iq and the d-axis current Id in the dq-axis coordinate in the case where the command power P* is constant (hereinafter, each referred to as "linear current vector location"). In FIG. 6, each of the intersections of the linear current vector location when the command power P* is constant and the voltage limiting circle line represents a combination of the q-axis current Iq and the d-axis current Id that satisfies the applied voltage amplitude Va and the command power P*.FIG. 6( a) shows a relationship between the linear current vector location when the command power P* is constant and the voltage limiting circle line in the case where the rotational speed of the motor 10 is the first rotational speed RS 1. Under the condition shown in FIG. 6( a), the excitation current If is set to 2A. FIG. 6( b) shows a relationship between the linear current vector location when the command power P* is constant and the voltage limiting circle line in the case where the rotational speed of the motor 10 is the second rotational speed RS 2 (RS2=2·RS1). Under the condition shown in FIG. 6( b), the excitation current If is set to 1A.When comparing FIG. 6( b) with FIG. 6( a), the rotation speed of the motor 10 is doubled, with the excitation current If being decreased to half. Accordingly, the q-axis currents Iq having the same value with an error in a prescribed range can output an equivalent power. In addition, when the rotational speed is doubled, the radius of the tension limiting circle line shown in FIG. 6( b) is decreased to half with respect to the radius of the tension limiting circle line shown in FIG. 6( a).The d-axis current Id is determined by an equation (1).In the equation, φ(If) is a link magnetic flux caused by the excitation current If, ω is an angular velocity of the motor 10, Lq is a q-axis inductance, and Ld is a d-axis inductance. When the d-axis current and the q-axis current Iq are set to 0 (Id=Iq=0), the equation (1) can be expressed by the following equation (2).As expressed by the following equation (3), the amplitude Vb of the induced voltage is equal to the applied voltage Va.In the equation, Lf is an inductance of the excitation winding 11. in other words, when the amplitude Vb of the induced voltage is equal to the amplitude Va of the applied voltage, the right end of the voltage limiting circle line is superimposed on the origin of the dq axis coordinates.When the right end of the voltage limiting circle line is superimposed on the origin of the dq axis coordinates ((Id, Iq)=(0, 0)), the intersections of the linear current vector location approach when the command power P* is constant and the voltage limiting circle line approaches the origin. In addition, the d-axis current Id at each of the intersections is approximated to 0. Thus, in the present embodiment, the excitation current If can be controlled so as to cause the deviation between the induced voltage amplitude Vb and the applied voltage amplitude Va to approach a current value that becomes equal to or less than a prescribed value. As a result, the current amplitude Ia of the dq-axis current Idq can be decreased. In the present embodiment, the excitation current If is controlled so as to cause the current value to be a minimum excitation current value If_min (a first excitation current) at which the amplitude Vb of the induced voltage coincides with the amplitude Va of the applied voltage, and an error is within a prescribed range.However, when the excitation current If becomes the minimum excitation current value If_min and then the value of the excitation current If becomes constant, the maximum value of the output power P is limited by a value of the q-axis current Iq corresponding to a peak value of the voltage limiting circle line. In this regard, the control unit 40 according to the present embodiment controls the excitation current If so as to cause the current value to be a value larger than the minimum excitation current value If_min when the command power P* becomes larger than a prescribed value or when the command torque T* becomes larger than a prescribed value. In this way, by controlling the excitation current If to a value greater than or equal to the minimum excitation current value If_min, the d-axis current Id at the intersection (at the right end of the voltage limiting circle line) of the voltage limiting circle line and the d-axis of the dq-axis coordinate system becomes 0 or less. Specifically, in the control unit 40, the d-axis current Id is used as a basis for calculating an excitation current reference value If_c (a second excitation current). Further, in the control unit 40, a prescribed d-axis current target value Id* (a target current) is set. In the control unit 40, it is determined whether or not an absolute value of the d-axis current Id whose value is increased by an increase in the command power P* or an increase in the command torque T* exceeds an absolute value of the d-axis current set value Id*. Accordingly, in the control unit 40, when the absolute value of the d-axis current Id exceeds the absolute value of the d-axis current command value Id*, the excitation current If is controlled so as to cause the current value to be an excitation current reference value If_c.FIG. 7 is a functional block diagram illustrating a function of the control unit 40 according to the present embodiment. The command torque T* is input to the control unit 40. The control unit 40 performs control to cause the torque T of the motor 10 to approach the input command torque T*.The command torque T* satisfying the command power P* and an estimated value of the input torque T are input to a deviation calculator 41 during regeneration. The deviation calculator 41 calculates a deviation between the input command torque T* and the estimated value of the input torque T. In addition, the command torque T* satisfying the command power P* and an estimated value of the output torque T are input to the deviation calculator 41 during power running. The deviation calculator 41 calculates a deviation between the input command torque T* and the estimated value of the output torque T. The estimated value of the torque T is calculated by a torque estimated value calculator 42. The torque estimation value calculator 42 calculates an estimation value of the torque T based on a detected value of the d-axis current Id and the q-axis current Iq passing through the stator 13 (a detected value of the phase current detector 33), a detected value of the excitation current If (a detected value of the excitation current sensor 32), and the rotation speed of the motor 10.The deviation between the command torque T* and the estimated value of the torque T is input to a PID controller 43 ("DIP" means "proportional-integral derivative"). The PID controller 43 performs PDI control over the input deviation and outputs the result. The output value of the PID controller 43 is equivalent to a voltage phase command value δ*. The voltage phase command value δ* is input to a phase limiter (a limiter) 44. The phase limiter 44 limits an upper limit and a lower limit of the input voltage phase command value δ*. The voltage phase command value δ* whose upper and lower limits have been limited is input to a modulator 45. The modulation device 45 controls the first and second inverters INV 1 and INV 2 to cause the voltage phase δ output from the first and second inverters INV 1 and INV 2 (the phase difference between the applied voltage and the phase current) to serve as the voltage phase command value δ*. Specifically, the modulator 45 outputs the first manipulation signals gUp 1 to gWp 1 respectively manipulating the high-potential side switches SUp 1 to SWp 1 of the first inverter INV 1 to cause the voltage phase δ to serve as the voltage phase command value δ*. Also, the modulation device 45 outputs the first manipulation signals gUn 1 to gWn 1 that manipulate the low-potential side switches SUn 1 to SWn 1 of the first inverter INV 1, respectively. In addition, the modulator 45 outputs the second manipulation signals gUp 2 to gWp 2 that manipulate the high-potential side switches SUp 2 to SWp 2 of the second inverter INV 2, respectively. The modulation device 45 outputs the second manipulation signals gUn 2 to gWn 2 that manipulate the low-potential side switches SUn 2 to SWn 2 of the second inverter INV 2, respectively. In other words, in the control unit 40 according to the present embodiment, the deviation between the command torque T* and the torque T is provided as an input value using the voltage phase δ as a manipulated variable of the first and second inverters INV 1 and INV 2, thereby executing the PID control for decreasing the deviation between the command torque T* and the torque T. In the control unit 40, a gain of the PID controller 43 is set according to the output torque T. Specifically, in the control unit 40, the gain of the PID controller 43 is set large in a range of the output torque T in which variation in a value is large or in a range of the output power P in which variation in a value is large. Thus, the control unit 40 is capable of improving responsiveness to the variation in a value of the output torque T or the output power P.The detected d-axis current value Id and the d-axis current target value Id* are input to a deviation calculator 46. The deviation calculator 46 calculates a deviation between the input detected value Id of the d-axis current and the d-axis current target value Id*. A command value setter 47 sets the d-axis current command value Id*. The method for setting the d-axis current command value Id* executed by the command value setter 47 will be described below. The deviation between the detected value Id of the d-axis current and the d-axis current target value Id* is input to a PID controller 48. The PID controller 48 performs PDI control over the input deviation and outputs the result. The output value of the PID controller 48 is equivalent to the excitation current reference value If_c. The exciting current reference value If_c and the minimum exciting current value If_min are input to a target value selector 49. The command value selection means 49 selects one of the input values as an excitation current command value If*. The method of selecting the excitation current target value If* executed by the target value selection means 49 will be described below. A minimum value setter 50 uses the map data in which the rotation speed of the motor 10 and the torque T are used as input values to set a minimum excitation current value If_min such that the amplitude Vb of the induced voltage coincides with the amplitude Va of the applied voltage.The excitation current target value If* and the detected value If of the excitation current are input to a deviation calculator 51. The deviation calculator 51 calculates a deviation between the input excitation current target value If* and the detected value If of the excitation current. The deviation between the excitation current target value If* and the detected value If of the excitation current is input to a PID controller 52. The PID controller 52 performs PDI control over the input deviation and outputs the result. The output value of the PID controller 52 is equivalent to an excitation voltage command value Vf*. The excitation voltage command value Vf* is input to the field circuit 36.With reference to FIG. 8, the method for setting the d-axis current command value Id* executed by the command value setter 47 will be described. FIG. 8( a) shows a voltage limiting circle line in the case where the d-axis current command value Id* is set near the center of the voltage limiting circle line (on the peak side). FIG. 8( b) shows a voltage limiting circle line in the case where the d-axis current command value Id* is set near the right end of the voltage limiting circle line (on the q-axis side). In FIG. 8, the d-axis current command value Id* is set in the voltage limiting circle line such that the current vector is located on the q-axis side of the dq-axis coordinate system with respect to the peak value side at which the q-axis current Iq is maximized.The excitation current If in FIG. 8( a) is set to be smaller than the excitation current If in FIG. 8( b). Setting a larger excitation current If causes the voltage limiting circle line to be further separated from the q-axis of the dq-axis coordinate system in the negative direction.The torque T in FIG. 8( a) is identical to the torque T in FIG. 8( b). On the other hand, the exciting current If in FIG. 8(a) is smaller than the exciting current If in FIG. 8(b). The torque T is proportional to the q-axis current Iq and the excitation current If. Accordingly, in order to realize the identical torque T, the q-axis current Iq in FIG. 8( a) is made larger than the q-axis current Iq in FIG. 8( b).In the example shown in FIG. 8( a), the d-axis current command value Id* is set near the center of the voltage limiting circle line (on the peak side), thereby increasing the q-axis current Iq and realizing a prescribed torque T with the small excitation current If. In the example shown in FIG. 8( a), the small excitation current If is adjusted to thereby minimize the d-axis current Id having a low contribution rate to the torque T and improve power efficiency. On the other hand, there is a small difference between the present value of the torque T and the maximum value of the torque T (the maximum torque) (a small difference between the present value of the q-axis current Iq and the maximum value of the q-axis current Iq). Accordingly, in order to bring the torque T closer to the command torque T*, the exciting current If needs to be increased. In addition, the excitation current If has a lower responsiveness to the change in the torque T than the phase current does. Accordingly, the responsiveness of the torque T is deteriorated.In the example shown in FIG. 8( b), the d-axis current command value Id* is set near the right end of the voltage limiting circle line (on the q-axis side), thereby decreasing the q-axis current Iq and setting the excitation current If to be large to realize a prescribed torque T. In the example shown in FIG. 8( b), the large excitation current If is set to thereby make a change in the torque T due to the change in the q-axis current Iq large and to increase the difference between the present value of the torque T and the maximum value of the torque T (the maximum torque). Thus, in the example shown in FIG. 8( b), the responsiveness to the change in the torque T is improved. On the other hand, the d-axis current Id having a low contribution rate to the torque T is increased. Accordingly, the power efficiency is lowered.In this regard, in the setting method according to the present embodiment, the d-axis current command value Id* is set to set the current vector to the peak side of the voltage limiting circle line in the case where higher importance is given to the improvement in the power efficiency, as compared with the case where higher importance is given to the responsiveness to the variation in the command torque T*. The d-axis current Id at the right end of the voltage limiting circle line has a magnitude Idr represented by an equation (4).The d-axis current Id at the center of the voltage limiting circle line has a magnitude Idc represented by an equation (5).In other words, in the present embodiment, when obtaining various parameter values used for calculations in Equations (4) and (5), the magnitudes Idr and Idc of the d-axis current Id at the right end and at the center of the voltage limiting circle line can be calculated, respectively. Specifically, in the present embodiment, when the d-axis inductance Ld, the inductance Lf of the excitation winding 11, the excitation current If, the amplitude Va of the applied voltage, and the angular velocity ω of the motor 10 are obtained, the magnitudes Idr and Idc of the d-axis current Id at the right end and the center of the voltage limiting circle line, respectively, can be calculated.The target value setter 47 uses an equation (6) based on the magnitudes Idr and Idc of the d-axis current Id at the right end and the center of the voltage limiting circle line, respectively, and sets the d-axis current target value Id*.For example, when a is set to 0, the d-axis current command value Id* is superimposed on the right end of the voltage limiting circle line. When a is set to 1, the d-axis current command value Id* is superimposed on the peak value of the voltage limiting circle line. When a is set to 0.5, the d-axis current command value Id* is superimposed on a midpoint between the right end and the peak value of the voltage limiting circle line. In other words, when the improvement in the power efficiency is given higher importance, a is set to a value larger than 0.5 to set the d-axis current command value Id* to the q-axis side with respect to the peak value of the voltage limiting circle line and to the peak value side with respect to the center of the range of the d-axis in the dq-axis coordinate system. When the responsiveness of the output torque T and the responsiveness of the output power P are given higher importance, a is set to a value less than or equal to 0.5 to set the d-axis current command value Id* to the q-axis side with respect to the peak value of the voltage limiting circle line and to the right end side with respect to the center of the range of the d-axis in the dq-axis coordinate system.Next, with reference to FIG. 9, the method for selecting the excitation current target value If* executed by the target value selection means 49 will be described.The target value selector 49 according to the present embodiment determines whether or not the minimum excitation current value If_min is selected as the excitation current target value If* (S 01). When the minimum excitation current value If_min is selected as the excitation current target value If* (S 01: YES), the target value selector 49 determines whether or not an absolute value |Id| of the d-axis current Id exceeds an absolute value |Id*| of the d-axis current target value Id* (S 02). When the absolute value |Id| of the d-axis current Id exceeds the absolute value |Id*| of the d-axis current target value Id* (S 02: YES), the target selector 49 calculates the excitation current reference value If_c based on the deviation between the d-axis current Id and the d-axis current target value Id* (S 03). Then, the target value selector 49 selects the exciting current reference value If_c as the exciting current target value If* (S04), ending the processing. When the absolute value |Id| of the d-axis current Id is less than or equal to the absolute value |Id*| of the d-axis current target value Id* in step S 02 (S 02: NO), the target selector 49 does not execute the processing in steps S 03 and S 04, and directly ends the processing.On the other hand, when the minimum excitation current value If_min is not selected as the excitation current target value If* (S 01: NO), the target selector 49 determines whether or not the magnitude Idr of the d-axis current Id at the right side of the voltage limiting circle line is greater than or equal to 0 (S 05). When the magnitude Idr of the d-axis current Id at the right end of the voltage limiting circle line is greater than or equal to 0 (S 05: YES), the target selector 49 calculates the minimum excitation current value If_min at which the amplitude Vb of the induced voltage matches the amplitude Va of the applied voltage (S 06). At this time, the target value selector 49 uses the map data that takes the rotation speed of the motor 10 and the output power P as input values or the map data that takes the rotation speed of the motor 10 and the output torque T as input values, thereby calculating the minimum excitation current value If_min. The target value selector 49 selects the minimum excitation current value If_min as the excitation current target value If* (S 07), ending the processing. When the magnitude Idr of the d-axis current Id at the right end of the voltage limiting circle line is less than 0 in step S 05 (S 05: NO), the target value selector 49 does not execute the processing in steps S 06 and S 07, and directly ends the processing.Although the target value selector 49 determines whether or not the absolute value |Id| of the d-axis current Id exceeds the absolute value |Id*| of the d-axis current target value Id* in step S 02, the determination method is not limited thereto. For example, the target value selector 49 may determine whether or not the absolute value |Id| of the d-axis current Id exceeds a value |Id*-ΔI| obtained by subtracting an allowable value ΔI from the absolute value |Id*| of the d-axis current target value Id* in step S 02. In this case, when the absolute value |Id| of the d-axis current Id exceeds |Id*-ΔI|, the target selector 49 selects the excitation current reference value If_c as the excitation current target value If*. When processing is executed in this manner, the control unit 40 according to the present embodiment is capable of improving the responsiveness of the output power P and the output torque T.FIG. 10 shows changes in dq axis current Idq and changes in excitation current target value If* in the case where command torque T* changes from a prescribed value T 1 to a prescribed value T 2.At a time point t 1 in FIG. 10( b), the command torque T* starts to increase from the prescribed value T 1. As shown in FIG. 10( a), the increase in the command torque T* causes the q-axis current Iq to start increasing, and also causes the absolute value |Id| of the d-axis current Id to start increasing (the state in FIG. 10( c)).At a time t 2 in FIG. 10( a), the d-axis current Id reaches the d-axis current target value Id*. Thus, as shown in FIG. 10( b), the excitation current reference value If_c becomes larger than the minimum excitation current value If_min, and the excitation current target value If* starts to increase (the state shown in FIG. 10( d)).At a time point t 3 in FIG. 10( a), after elapse of a time equivalent to a time constant of the excitation circuit 36, the increase in the excitation current target value If* causes the excitation current If to start increasing. Accordingly, the q-axis current Iq starts to decrease for the torque T to approach the command torque T*, and the absolute value |Id| of the d-axis current Id also starts to decrease (the state shown in FIG. 10( e)).At a time t 4 in FIG. 10( b), the command torque T* reaches the prescribed value T 2. Thus, the change stops in the q-axis current Iq, the d-axis current Id, and the excitation current If (the state shown in FIG. 10( f)).The advantageous effects of the present embodiment will be described.The applicant of the present invention has found that when the amplitude Vb of the induced voltage and the amplitude Va of the applied voltage are similar to each other, the current amplitude Ia of the phase current (the dq-axis current Idq) is minimized. Thus, in the control unit 40 according to the present embodiment, based on this finding, the excitation current If is controlled so as to cause a difference between the amplitude Vb of the induced voltage and the amplitude Va of the applied voltage to be equal to or less than a prescribed value. Accordingly, in the control unit 40 according to the present embodiment, the current amplitude Ia is decreased, and a power loss (a copper loss) caused by a flow of the phase current is decreased. In this way, in the control unit 40 according to the present embodiment, power efficiency in the engine 10 can be improved by simple control.Specifically, in the control unit 40 according to the present embodiment, control is executed to cause the value of the excitation current If to approach the minimum excitation current value If_min at which the value of the d-axis current Id becomes 0 at each of the intersections of the voltage limiting circle line and the d-axis of the dq-axis coordinate system. In addition, in the control unit 40, control is executed to cause the value of the excitation current If to approach the excitation current reference value If_c at which the value of the d-axis current Id becomes equal to or less than 0 at each of the intersections of the voltage limiting circular line and the d-axis of the dq-axis coordinate system.When the excitation current If is set to the minimum excitation current value If_min, then the value of the excitation current If is made constant, the magnitude of the connection magnetic flux (φ(If)) connected to the first and second winding groups 10 aand 10 bis fixed. In this case, the output torque T and the output power P of the motor 10 are determined by the current passing through the first and second winding groups 10 aand 10 band the connection magnetic flux. Consequently, when the magnitude of the connection magnetic flux is limited, the maximum value of both the output torque T and the output power P is limited. Thus, in the control unit 40 according to the present embodiment, it is ensured that when the command torque T* serving as a command value of the output torque T is greater than the prescribed value, or when the command power P* serving as a command value of the output power P is greater than the prescribed value, the exciting current reference value If_c that is greater than the minimum exciting current value If_min is set as the exciting current If. Thus, in the control unit 40 according to the present embodiment, the power loss can be minimized while preventing the maximum value of both the output torque T and the output power P from being limited.The power loss caused by a flow of the phase current is increased by an increase in the d-axis current Id. Thus, in the control unit 40 according to the present embodiment, it is ensured that the magnitude of the excitation current reference value If_c is set based on the d-axis current Id. Thus, in the control unit 40 according to the present embodiment, the power loss caused by an increase in the d-axis current Id can be minimized.The value of the d-axis current Id is increased by an increase in the output torque T or the output power P. Thus, in the control unit 40 according to the present embodiment, the d-axis current command value Id* is set, but when the detected value of the d-axis current Id exceeds the d-axis current command value Id* set, it is ensured that the excitation current reference value If_c is selected as the excitation current command value If*. Accordingly, in the control unit 40 according to the present embodiment, an increase in the current amplitude Ia can be minimized while causing the output torque T to approach the command torque T*. Alternatively, in the control unit 40, an increase in the current amplitude Ia may be minimized while the output power P is made to approach the command power P*.In the control unit 40 according to the present embodiment, it is ensured that the d-axis current target value Id* is set on the q-axis side of the dq-axis coordinate system with respect to the peak value of the voltage limiting circle line. Thus, in the present embodiment, the intersections of the current vector location in which the output torque T or the output power P has a constant value and the voltage limiting circle line approach the origin of the dq axis coordinates (the distance between each of the intersections and the origin is decreased). In other words, in the control unit 40 according to the present embodiment, an increase in the current amplitude Ia of the phase current is minimized.The output torque T and the output power P change according to the q-axis current Iq. Thus, in the control unit 40 according to the present embodiment, the d-axis current target value Id* is set to cause the change in the q-axis current Iq with respect to the d-axis current Id in the voltage limiting circle line to become greater in the case where greater importance is placed on the improvement in the power efficiency than in the case where greater importance is placed on the responsiveness to the variation in the command torque T* or the variation in the command power P*.Specifically, in the control unit 40 according to the present embodiment, in the case where the improvement in the power efficiency is given greater importance, the d-axis current target value Id* is set to the peak value side with respect to the midpoint between the peak value and the right end of the voltage limiting circle line. Thus, in the present embodiment, the value of the q-axis current Iq is increased, and the value of the excitation current If is decreased. Accordingly, in the control unit 40 according to the present embodiment, the value of the excitation current If is decreased, thereby decreasing the current amplitude Ia of the dq axis current Idq (the phase current) and improving the power efficiency.In the control unit 40 according to the present embodiment, in the case where responsiveness to the variation in the command torque T* or the variation in the command power P* is given greater importance, the d-axis current command value Id* is set to the q-axis side of the dq-axis coordinate system with respect to the midpoint between the peak value and the right end of the voltage limiting circle line. Thus, in the present embodiment, the change in the q-axis current Iq with respect to the d-axis current Id becomes larger.In the control unit 40 according to the present embodiment, there is a configuration such that the amplitude Va of the applied voltage is made constant and the phase δ of the applied voltage is adjusted, thereby controlling the output torque T and the output power P. This configuration of the control unit 40 according to the present embodiment can simplify control over the phase current (the stator current) passing through the stator 13 and the excitation current If passing through the excitation winding 11. In addition, in the present embodiment, a value of the applied voltage (duty values of the boost DC-DC converter 23 and the first and second inverters INV 1 and INV 2) is set to a maximum value, for example, to reduce the current amplitude Ia of the phase current (the dq axis current Idq). As a result, the control unit 40 according to the present embodiment can improve the power efficiency.(Other Embodiments)In the above-described embodiment, there is a configuration such that the excitation current target value If* is set to cause the deviation between the amplitude Va of the applied voltage and the amplitude Vb of the induced voltage to be 0. In another embodiment, this configuration may be changed to one in which the excitation current target value If* is set so as to ensure that the deviation between the applied voltage amplitude Va and the induced voltage amplitude Vb is less than or equal to a prescribed value.In another embodiment, control may be performed with which the excitation current target value If* is constantly set to a minimum excitation current value If_min. When such control is executed, the power efficiency is improved while the maximum value of both the output power P and the output torque T is decreased.In another embodiment, the exciting current reference value If_c may be set based on the deviation between the command torque T* and the torque T.In the above-described embodiment, there is a configuration such that the amplitude Va of the applied voltage has a maximum value, and then the value of the amplitude Va is allowed to be constant. However, in another embodiment, this configuration may be changed.In the above-described embodiment, the control unit may be a device that inputs the command power P* and that performs control to bring the input-output power P of the motor 10 closer to the command power P*. In the case of such a control unit, the deviation between the command power P* and the input-output power P is used as an input value, the voltage phase δ is used as a manipulated variable of the first and second inverters INV 1 and INV 2, and the PID control is executed to decrease the deviation between the command power P* and the input-output power P.Further, the described embodiment may be changed as described below.In the control unit 40 according to the present modification, there is a configuration such that the d-axis current target value Id* is set so as not to cause the current amplitude Ia of the dq-axis current Idq (the phase current) to exceed a corresponding upper limit, namely, an upper current amplitude limit IaMAX. In this case, the upper current amplitude limit IaMAX is set to the maximum value of the amplitude of the current input to / output from the first and second inverter circuits INV 1 and INV 2, respectively, to the first and second winding groups 10 aand 10 b. The control unit 40 according to the present modification can minimize damage to the high potential side switches SUp 1 to SWp 1, the low potential side switches SUn 1 to SWn 1, and the like caused by excessive current flow between the first inverter INV 1 and the first winding group 10 a. Further, the control unit 40 according to the above-described embodiment can minimize damage to the high potential side switches SUp 2 to SWp 2, the low potential side switches SUn 2 to SWn 2, and the like caused by excessive current flow between the second inverter INV 2 and the second winding group 10 b.In the control unit 40 according to the present modification, the d-axis current target value Id* is set so as not to cause the current amplitude Ia of the dq-axis current Idq (the phase current) to exceed the upper amplitude limit IaMAX. Then, based on the d-axis current command value Id* set in this manner, the excitation current command value If* is set to maximize the torque T (or the power P). Note that, as shown in FIG. 10, the torque T is represented as an equation (7).The control unit 40 according to the present variation may control the current amplitude Ia of the dq axis current Idq (the phase current) so as not to exceed the upper amplitude limit IaMAX, and may maximize the torque T (or the power P).Specifically, in the control unit 40 according to the present modification, when the deviation between the upper amplitude limit IaMAXand the d-axis current target value Id* becomes equal to or less than a prescribed value (when the d-axis current target value Id* approaches the upper amplitude limit IaMAX), the current amplitude Iaof the dq-axis current Idq(the phase current) is set to the upper amplitude limit IaMAX. In this case, the control unit 40 of the above-described embodiment decreases the excitation current reference value If_c to decrease the phase of the current vector. Thus, the control unit 40 according to the above-described embodiment sets the excitation current reference value If_c to maximize the torque T (or the power P). Further, the control unit 40 according to the present modification decreases the excitation current reference value If_c until the current vector represented by a combination of the d-axis current Id and the q-axis current Iq reaches the peak value of the voltage limiting circle line. Thus, the control unit 40 according to the present modification may set the excitation current target value If* to maximize the torque T (or the power P).FIG. 11 shows an example of setting the d-axis current command value Id* and the excitation current reference value If_c according to the present modification. In the control unit 40 according to the present modification, when the d-axis current command value Id* is increased to approach the upper amplitude limit IaMAX, the d-axis current command value Id* is set to cause the current amplitude Ia of the dq-axis current Idq (the phase current) to be the upper amplitude limit IaMAX (the broken line in FIG. 11 ). Further, the control unit 40 according to the present modification decreases the excitation current reference value If_c to thereby move the voltage limiting circle line to the q-axis side of the dq-axis coordinate system. In the control unit 40 according to the present modification, the excitation current If is decreased while the q-axis current Iq is increased, to thereby increase the torque T. Then, the control unit 40 according to the present modification performs adjustment to decrease the excitation current If until the current vector represented by a combination of the d-axis current Id and the q-axis current Iq reaches the peak value of the voltage limiting circle line (the solid line in FIG. 11 ). In this way, the control unit 40 according to the present modification can maximize the torque T.List of reference characters10... motor, 10a... first winding group; 10b... second winding group; 11... exciter winding; 12... rotor; 13... stator; INV1, INV2... converter; 40... control unit.

Claims

A control unit for a rotating electric machine, wherein the control unit (40) is applied to a rotating electric machine (10) comprising a rotor (12) having an excitation winding (11) and a stator (13) having an armature winding (10a, 10b) to control an excitation current passing through the excitation winding (11), wherein: a prescribed voltage (Va) is applied to the armature winding (10a, 10b); and the excitation current is controlled to be a first excitation current with which an amplitude of an induced voltage (Vb) generated in the armature winding (10a, 10b) by rotation of the rotor (12) coincides with an amplitude of the applied voltage (Va) of the armature winding (10a, 10b) with an error that is within a prescribed range.The rotating electric machine control unit according to claim 1, wherein when a current passing through the armature winding (10a, 10b) is represented in a dq axis coordinate system by a current vector that is a combination of a d axis current and a q axis current, the excitation current is controlled to cause the d axis current to be less than or equal to 0 at each of intersections of a voltage limiting circle line and a d axis of the dq axis coordinate system, the voltage limiting circle line being an arc-shaped location of the current vector, the amplitude of the applied voltage having a constant value.The rotating electric machine control unit according to claim 1 or 2, wherein when a command value of an input-output torque or an input-output power of the rotating electric machine becomes larger than a prescribed value, the excitation current is controlled to cause a current value to be a second excitation current larger than the first excitation current.The rotary electric machine control unit according to claim 3, wherein the second exciting current is calculated based on the d-axis current passing through the armature winding (10a, 10b).The rotating electric machine control unit according to claim 4, wherein: when a current passing through the armature winding (10a, 10b) is represented in a dq-axis coordinate system by a current vector that is a combination of a d-axis current and a q-axis current, a target current serving as a target value of the d-axis current is set in a voltage limiting circle line that is an arc-shaped location of the current vector, the applied voltage having an amplitude of a constant value; and when an absolute value of a detected value of the d-axis current exceeds an absolute value of the target current, the excitation current is controlled to cause the current value to be the second excitation current.The rotating electric machine control unit according to claim 5, wherein the target current is set in the voltage limiting circle line to set the current vector on a q-axis side with respect to a peak value, wherein the q-axis current passing through the armature winding (10a, 10b) is maximized.The rotating electric machine control unit according to claim 6, wherein the target current is set to set the current vector to the peak value side of the voltage limiting circle line when power efficiency is given higher importance than responsiveness to variation of a command value of the input-output torque or the input-output power.The rotating electric machine control unit according to claim 7, wherein when the power efficiency is given higher importance, the target current is set to set the current vector more toward the q-axis side than the peak value of the voltage limiting circle line is, but toward the peak value side with respect to a center of a range of the d-axis.The rotating electric machine control unit according to claim 7 or 8, wherein when the responsiveness is given higher importance, the target current is set to set the current vector more to the q-axis side than is the peak value of the voltage limiting circle line but to the q-axis side with respect to the center of the range of the d-axis.The rotating electric machine control unit according to any one of claims 4 to 8, wherein: when a current passing through the armature winding (10a, 10b) is represented in a dq-axis coordinate system by a current vector that is a combination of a d-axis current and a q-axis current, the target current serving as a target value of the d-axis current is set in a voltage limiting circle line that is an arc-shaped location of the current vector, the applied voltage having an amplitude of a constant value; the rotating electric machine is connected to an inverter (INV1, INV2) that inputs / outputs a power to / from the armature winding (10a, 10b); a current amplitude that is an amplitude of a current passing through the armature winding (10a, 10b), has an upper amplitude limit as an upper limit set to a maximum value of an amplitude of a current that can be input / output from the inverter (INV1, INV2) to the armature winding (10a, 10b); and the target current is set to cause the current amplitude not to exceed the upper amplitude limit.The rotating electric machine control unit according to claim 10, wherein the second exciting current is set to maximize the input-output torque or the input-output power based on the target current set to cause the current amplitude not to exceed the upper amplitude limit.The rotating electric machine control unit according to claim 11, wherein when a deviation between the upper amplitude limit and the target current becomes equal to or less than a prescribed value, the current amplitude is set to the upper amplitude limit and the second excitation current is decreased to decrease a phase of the current vector, thereby setting the second excitation current to maximize the input-output torque or the input-output power.The rotating electric machine control unit according to claim 12, wherein the second excitation current is decreased until the current vector reaches a peak value of the voltage limiting circle line, the peak value maximizing the q-axis current passing through the armature winding (10a, 10b).The rotating electric machine control unit according to any one of claims 11 to 13, wherein: the applied voltage (Va) is allowed to have a constant amplitude; and a phase of the applied voltage (Va) is adjusted to thereby control the input-output torque or the input-output power of the rotating electric machine.The rotating electric machine control unit according to claim 14, wherein a deviation between a value of the input-output torque and the command value of the input-output torque is used as an input value, and a phase of the applied voltage is used as a manipulated variable of the inverter (INV1, INV2) to perform PID control such that the deviation is decreased.The rotating electric machine control unit according to claim 14, wherein a deviation between a value of the input-output power and the command value of the input-output power is used as an input value, and a phase of the applied voltage (Va) is used as a manipulated variable of the inverter (INV1, INV2) to perform PID control such that the deviation is decreased.The rotating electric machine control unit according to claim 15 or 16, wherein a gain in the PID control is set based on a command value of the input-output torque or the input-output power.A control method for a rotating electric machine, the method being applied to a rotating electric machine (10) comprising a rotor (12) having an excitation winding (11) and a stator (13) having an armature winding (10a, 10b) to control an excitation current passing through the excitation winding (11), wherein: a prescribed voltage (Va) is applied to the armature winding (10a, 10b); and the excitation current is controlled to be a first excitation current with which an amplitude of an induced voltage (Vb) generated in the armature winding (10a, 10b) by rotation of the rotor (12) coincides with an amplitude of the applied voltage (Va) of the armature winding (10a, 10b) with an error that is within a prescribed range.

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