JP4153760B2 - Motor circuit and motor control method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a motor circuit for driving two motors by supplying driving currents from two inverters to two motors, and more particularly, to a DC power source arranged between neutral points of stator windings of two motors. .
[0002]
[Prior art]
Conventionally, in electric vehicles, hybrid electric vehicles, etc., there are a wide variety of systems that obtain direct current power from a mounted battery into a predetermined alternating current by an inverter and supply it to a motor to obtain a motor output corresponding to the amount of accelerator depression. It has been adopted.
[0003]
Here, the width of the output torque required for the motor in traveling of the vehicle is very large. In order to obtain a large output from the motor, a large amount of current must be passed if the power supply voltage is low, and the loss tends to increase. To reduce the loss, the motor must be enlarged. On the other hand, if the power supply voltage is high, the loss can be reduced accordingly. However, there is a problem that a large number of battery cells need to be connected in series.
[0004]
Furthermore, the battery voltage is likely to fluctuate in accordance with the driving state of the motor, which causes a problem that the inverter input voltage changes and stable motor driving control cannot be performed.
[0005]
Therefore, a system has been proposed in which the battery voltage is boosted by a DC-DC converter and the obtained high voltage is input to the inverter.
[0006]
That is, as shown in FIG. 1, the output of the battery B is boosted by the DC-DC converter CONV and supplied to the inverter INV. Then, a predetermined switching operation is performed on the DC power boosted by the inverter INV, and a predetermined AC current is supplied to the motor M. That is, by controlling the switching of the inverter INV based on the motor output torque command, the output current of the inverter INV is controlled so that the motor output torque matches the command.
[0007]
Thus, by providing the DC-DC converter CONV, the voltage of the battery B can be made relatively low and the motor drive voltage (inverter INV input voltage) can be made high, and the inverter input can be made by the drive control of the DCDC converter CONV. The voltage can be stabilized.
[0008]
In addition, the DC-DC converter requires an inductance such as a coil, and the circuit becomes relatively large. Therefore, the use of a DC voltage variable type inverter utilizing the inductance of the motor coil in the motor has been studied. That is, as shown in FIG. 2, the positive electrode of the battery B is connected to the neutral point of the motor M, and is connected to the negative electrode bus of the negative inverter INV of the battery B. Further, the output side of the inverter INV is connected to each phase coil end of the motor M in the same manner as described above. A capacitor C is connected between the positive and negative buses of the inverter INV.
[0009]
In this DC voltage variable inverter, DC power from the capacitor C is supplied to the motor M via the inverter INV, but the motor neutral point voltage is fixed to the voltage Vb of the battery B. Then, considering the total of each phase current of the motor by the inverter INV, the equivalent circuit of the circuit of FIG. 2 is as shown in FIG. If the on-duty of the upper switching element of the inverter INV is equal to the on-duty of the lower switching element, the voltage of the capacitor C should be 2 Vb, which is twice the voltage Vb of the battery B. Therefore, according to this system, the voltage of the battery B can be reduced to half of the input voltage of the inverter INV. Such a system is described in, for example, Japanese Patent Laid-Open No. 2001-37247 (Patent Document 1).
[0010]
[Patent Document 1]
JP 2001-37247 A
[0011]
[Problems to be solved by the invention]
Here, the system as shown in FIG. 2 has a problem that a DC-DC converter including a coil is required separately as described above.
[0012]
Further, in the DC voltage variable type inverter using the coil inductance in the motor, the fluctuation of the current of the battery B is large, it is difficult to ensure a stable operation, and the input voltage (capacitor voltage) of the inverter is 2 of the battery B. There was a problem of being limited to twice.
[0013]
Furthermore, when a motor and another inverter are installed separately, a wiring for connecting the neutral point to the battery is required, so that there is a problem that the power system wiring requires the number of drive phases plus one. there were.
[0014]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a motor circuit that can perform stable operation or simplify wiring.
[0015]
[Means for Solving the Problems]
The present invention includes two inverters each having a plurality of arms each composed of two switching elements connected in series between a positive bus and a negative bus, each having an output at the midpoint of the arms. Two motors having a plurality of stator windings connected to each of the outputs, each connected in common at a neutral point, and a DC connected between the neutral points of the stator windings of the two motors A motor having a power source and two capacitors respectively connected between a positive bus and a negative bus of the two inverters, and a neutral point of the stator winding is connected to the positive side of the battery A negative bus of the inverter to be driven and a positive bus of the other inverter are connected.
[0016]
With this configuration, the amount of current in the DC power supply and the voltage of the capacitor can be freely controlled by controlling the inverter. Therefore, stable motor drive can be performed by controlling the capacitor voltage without being affected by voltage fluctuations of the DC power supply. In other words, since the inverter function configuration has a transformation function with respect to fluctuations in the DC power supply voltage, the drive voltages of the two inverters for driving the two motors are not close to the optimum value without adding a DC-DC converter. It becomes possible to control and stabilize, and even when the battery voltage fluctuates, the maximum output can be secured, so there is no need to increase the size of the motor, and the specifications of the two motors can be optimized. In particular, since this configuration is a configuration in which both of the two motors are driven or regeneratively controlled, it is suitable for a two-motor drive that directly drives the drive wheels. It becomes possible to optimize and stabilize the driving state of the motor.
[0017]
Moreover, it is preferable that the withstand voltage of the switching element in each inverter is larger than the voltage of the DC power supply. As a result, a large voltage corresponding to the capacitor voltage is applied to the switching element of each inverter. However, measures such as disconnecting the capacitor voltage when the inverter stops can be achieved by setting the breakdown voltage of the switching element of the inverter higher than the DC power supply voltage. Even if not, the device will not be destroyed.
[0018]
Moreover, it is preferable that the switching element of each inverter is a MOSFET. As a result, when the DC power supply voltage is approximately 200 V or less, the occurrence of loss in the switching element can be reduced.
[0019]
In addition, it is preferable that the inverter and the motor have an integrated configuration installed in the same casing. Thus, since the inverter and the motor are integrated, there are few harmful effects due to an increase in the number of wirings with the external power source.
[0020]
In addition, the control method according to the present invention provides a high voltage state in which all the switching elements on the positive bus side of the inverter are turned on, all the switching elements on the negative bus side are turned off, and all the switching elements on the positive bus side of the inverter are turned off. By switching the low voltage state in which all the switching elements on the negative bus side are turned on by time control between the two inverters, the current flowing through the neutral point of the two motor stator windings and the negative bus of each inverter The voltage of the capacitor | condenser respectively connected between the side buses is each controlled.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0023]
FIG. 4 is a diagram illustrating an example of a motor circuit according to the embodiment. The positive and negative buses of the inverter INV1 are connected to both ends of the capacitor C1. The inverter INV1 has three arms (for three phases) formed of two transistors connected in series, and these are connected between the positive electrode bus and the negative electrode bus. The midpoint of each arm is the output end of each phase.
[0024]
Each phase coil end of the motor M1 is connected to each output end of the inverter INV1. The motor M1 is a three-phase PM motor, has a permanent magnet in the rotor, and has a three-phase star connection coil as a stator coil (armature coil).
[0025]
In this embodiment, a capacitor C2, an inverter INV2, and a motor M2 having the same configuration are provided.
[0026]
The positive electrode of the battery B is connected to a neutral point that is a common connection point of the three-phase coils of the motor M1, and the negative electrode of the battery B is connected to the neutral point of the three-phase coil of the motor M2. Further, the negative bus of the inverter INV1 and the positive bus of the inverter INV2 are connected. That is, the capacitors C1 and C2 are connected in series.
[0027]
In FIG. 4, the switching elements constituting the inverters INV1 and INV2 are MOSFETs, and the withstand voltage of each MOSFET is larger than the voltage of the battery B, and is approximately double.
[0028]
In such a configuration, based on the output torque command of the motors M1 and M2, the inverters INV1 and INV2 are controlled, and the three-phase motor drive currents to the motors M1 and M2 are controlled, whereby the outputs of the motors M1 and M2 are controlled. Is controlled according to the output torque command. In addition, regenerative power from the motors M1 and M2 is supplied to the capacitors C1 and C2 via the inverters INV1 and INV2, respectively.
[0029]
Further, assuming that the voltages of the capacitors C1 and C2 are Vc1 and Vc2, the battery voltage is Vb, and the on-duty of the upper switching element and the on-duty of the lower switching element in the inverters INV1 and INV2 are the same, the motors M1 and M2 The neutral point voltages are Vc1 / 2 + Vc2 and Vc2 / 2, respectively. Since the difference between the two becomes the battery voltage Vb, there is a relationship of Vc1 / 2 + Vc2-Vc2 / 2 = Vb, and therefore Vc1 + Vc2 = 2Vb.
[0030]
In the circuit of FIG. 4, all the upper switching elements of the inverters INV1 and INV2 are turned on and all the lower switching elements are turned off. Conversely, all the upper switching elements of the inverters INV1 and INV2 are turned off and all the lower switching elements are turned off. Can be on. An equivalent circuit in this case is shown in FIG.
[0031]
By controlling the switching elements of the inverters INV1 and INV2, the current I in the battery B and the voltages Vc1 and VC2 of the capacitors C1 and C2 can be arbitrarily controlled.
[0032]
For example, in FIG. 5A, all the upper switching elements of the inverters INV1 and INV2 are turned on and all the lower switching elements are turned off (1, 1, 1) and (1, 1, 1). Accordingly, both ends of the battery B are connected to both ends of the capacitor C1 via the motor M1 and the inverter INV1. Accordingly, the voltage Vc1 of the capacitor C1 = Vb.
[0033]
In FIG. 5B, all the upper switching elements of the inverters INV1, INV2 are turned off and all the lower switching elements are turned on (0, 0, 0), (0, 0, 0). Thereby, both ends of the battery B are connected to both ends of the capacitor C2 via the motor M2 and the inverter INV2. Therefore, the voltage Vc1 = Vb of the capacitor C2. Thereby, the fluctuation of the battery current I can be basically zero.
[0034]
In the figure, all upper switching elements of inverters INV1 and INV2 are turned on (all lower switching elements are turned off) (1, 1, 1), and all upper switching elements are turned off (all lower switching elements are turned on). It is shown as (0, 0, 0).
[0035]
Here, if the phase currents of the motors M1 and M2 are Iu1, Iv1, Iw1, Iu2, Iv2, and Iw2, the battery current I that charges and discharges the capacitors C1 and C2 connected in series is the upper three-phase current Iu1. , Iv1, Iw1, that is, I = Iu1 + Iv1 + Iw1, and −I = Iu2 + Iv2 + Iw2 in relation to the lower three-phase currents Iu2, Iv2, Iw2.
[0036]
Charging (driving) or discharging (during regeneration) of the capacitors C1 and C2 connected in series with the current I is performed.
[0037]
This battery current I has all the upper switching elements of the inverter INV1 turned on (all the lower switching elements are turned off) (1, 1, 1) so that the connection relationship of FIG. This shows a state in which all the switching elements are turned off (all lower switching elements are turned on) (0, 0, 0). Thus, when all the switching elements of the inverter INV1 are connected to the high voltage side and all the switching elements of the inverter INV2 are connected to the low voltage side, the battery current I decreases.
[0038]
On the other hand, all the upper side switching elements of the inverter INV1 are turned off (all the lower side switching elements are turned on) (0, 0, 0) so that the connection relationship of FIG. When it is turned on (all lower switching elements are turned off) (1, 1, 1), the battery current I increases.
[0039]
Therefore, the battery current I can be adjusted to a desired value by appropriately using the connection states shown in FIGS. 6 (a) and 6 (b). In FIG. 6, the capacitor voltage is Vb.
[0040]
Also, the control time of FIG. 6 is Tj, the voltages between the neutral point of the three-phase coil and the midpoint potential of the series capacitor are V1 and V2, respectively, and the average voltage of V1 and V2 within the control period Tj of FIG. , V2j.
[0041]
Next, voltage adjustment between the capacitors C1 and C2 will be described. All the upper switching elements of the inverter INV1 are turned on (all the lower switching elements are turned off) (1, 1, 1), and all the upper switching elements of the inverter INV2 are turned on so that the connection relationship of FIG. When all the lower switching elements are turned off (1, 1, 1), the upper capacitor C1 is charged (during driving) and discharged (during regeneration).
[0042]
Further, all the upper side switching elements of the inverter INV1 are turned off (all the lower side switching elements are turned on) (0, 0, 0) so that the connection relation of FIG. When turned off (all lower switching elements are turned on) (0, 0, 0), the lower capacitor C2 is charged (during driving) and discharged (during regeneration).
[0043]
Therefore, adjustment of the voltages of the capacitors C1 and C2 can be achieved by appropriately using the connection states shown in FIGS. 7A and 7B. The control time in this case is Tk, and the average of V1 and V2 is V1k and V2k.
[0044]
As an example of drive control, vector point control other than the zero vector is performed based on a conventional triangular wave comparison control method. That is, as described above, each phase current of the motors M1, M2 is controlled based on the output torque command for the motors M1, M2. The control time at this time is Ti, and the average of V1 and V2 is V1i and V2i.
[0045]
Next, overall control will be described. First, the battery current I is controlled. That is, assuming that the battery voltage is Vb, the driving power of the motor (stator coil) M1 driven by the upper inverter INV1 is W1, and the driving power of the lower motor (stator coil) M2 is W2, so that I × Vb = W1 + W2. Take control.
[0046]
By this control, the values of Tj, V1j, and V2j are determined. In the case of regeneration, the values of W1 and W2 are negative, so the value of the battery current I is also negative.
[0047]
Next, w1 and W2 are determined and Ti, V1i, and V2i are determined based on the control instruction values of the drive current (output torque command values of the motors M1 and M2).
[0048]
Further, since it is necessary to control the voltages of the capacitors C1 and C2 such that W1 = V1 × I and W2 = V2 × I, V1 = Vb × W1 / (W1 + W2), V2 = Vb × W2 / (W1 + W2) ) To control the upper and lower capacitor voltages Vc1 and VC2.
[0049]
Specifically, Tc = Tj + Tk + Ti,
W1 = {(Tj × V1j + Tk × V1k + Ti × V1i) / Tc} × I (1)
W2 = {(Tj × V2j + Tk × V2k + Ti × V2i) / Tc} × I (2)
By adjusting Tk, V1k, and V2k so as to satisfy, the electric power supplied (regenerated) to the motor M1 and the motor M2 and the driving (regenerative) electric power are balanced. That is, since W1 = V1 × I and W2 = V2 × I, the power flowing in and out of the capacitors C1 and C2 becomes zero on average, and the capacitor voltage is stably controlled. The capacitor voltages Vc1 and Vc2 are approximately twice as large as V1 and V2.
[0050]
By the control according to the above procedure, the drive (regeneration) control of the circuit of FIG. 4 can be performed.
[0051]
The input voltages Vc1 and Vc2 of the inverters INV1 and INV2 are transformed to the optimum voltages of the motors M1 and M2, and the optimum voltages (Vc1 and Vc2) are optimum even when the battery voltage (Vb) fluctuates. The voltage is controlled to a constant value. For this reason, the stator coils of the motor M1 and the motor M2 are always controlled at the same optimum voltage regardless of the fluctuation of the battery power supply voltage Vb, and can always be used with the motor specifications optimized. . This configuration can be applied even when the optimum drive voltage varies depending on the use conditions.
[0052]
In particular, the battery B is connected to the neutral point of the two stator coils (motor coils), and the capacitors C1 and C2 are connected to the high-voltage side and the low-voltage side of the inverters INV1 and INV2, respectively. The coil acts as a simple coil when all the switching elements of one inverter INV1, INV2 are switched to the high voltage side or the low voltage side in conjunction with each other. Therefore, when viewed from the capacitors connected to the high-voltage side and the low-voltage side of the two inverters INV1 and INV2, it operates as two DC-DC converters. Therefore, the two capacitors C1 and C2, that is, the input voltages of the inverters INV1 and INV2, can be optimally driven according to the driving conditions.
[0053]
In addition, since the time required for the operation of such a DC-DC converter can be reduced, normal driving can be performed in substantially the same manner as normal driving by switching the switches of the respective inverters in combination.
[0054]
Further, the switching elements of the inverters INV1 and INV2 are constituted by MOSFETs. Thereby, when the battery B voltage is approximately 200 V or less, the occurrence of loss in the switching element can be reduced.
[0055]
Next, in the case of a single phase, the configuration is as shown in FIG. In this configuration, inverters INV1 and INV2 each have only two arms, and motors M1 and M2 have only two-phase stator coils.
[0056]
Even in such a configuration, all the upper switching elements of the inverters INV1 and INV2 can be turned on (or off) and all the lower switching elements can be turned on (or off) in the same manner, and basically the same control can be performed. .
[0057]
Further, FIG. 9 shows a configuration in which motors M3 and M4 are connected to capacitors C1 and C2 via other inverters INV3 and INV4. According to such a configuration, the other motors M3 and M4 can be driven using the capacitors C1 and C2 as power sources. In this case, it is necessary to add the outputs W3 and W4 of the motors M3 and M4 to the output and perform overall control.
[0058]
A stator coil (armature coil) in a single motor is composed of a plurality of pole pairs in the case of a motor of several tens of kW class. For this reason, it can be roughly handled as two motors by driving in two parts.
[0059]
In this case, in order to reduce the number of wires between the motor and the inverter, a structure in which the inverter is installed in the motor housing is advantageous.
[0060]
FIG. 10 shows an example in which motor circuits such as a motor and an inverter are accommodated in one housing.
[0061]
A pair of bearings 32 is provided at the center of the front surface and the rear surface (left side and right side in the drawing) of the housing 30, and a shaft 34 penetrating the housing 30 is rotatably supported by the bearings 32. A rotor 36 is fixed to an intermediate portion of the shaft 34 in the housing 30. The rotor 36 is provided with a predetermined number of permanent magnets corresponding to the number of poles.
[0062]
A stator 38 made of a silicon steel plate is arranged so as to surround the rotor 36. A plurality of stator coils (armature coils) 40 are fixed to the stator 38. In this way, the PM motor is configured.
[0063]
An inverter 42 is accommodated in the housing 30. In the drawing, an end portion is fixed to the peripheral portion of the stator 38, and a mounting plate 44 bulging so as to cover the rear side of the stator 38 and the rotor 36 is provided, and an inverter 42 is attached thereto. Yes. The inverter 42 is composed of a switch element mounting board on which a plurality of switching elements 42a made of MOSFETs are mounted. A water cooling mechanism 46 for cooling the switching element on the front side (left side in the figure) of the inverter 42 is disposed.
[0064]
Although only two wires are shown in the figure, the inverter 42 and the stator coil 40 are connected by three connecting wires 48. Further, two connection ends 50 to the battery are provided, the connection end 50 to the battery positive electrode is connected to the neutral point of the stator coil 40, and the negative electrode is connected to the negative electrode bus of the inverter circuit 42.
[0065]
Further, one capacitor C is built in and connected to the inverter 42 by a connection line 52. That is, both ends of the capacitor C are connected to the positive and negative buses of the inverter circuit 42. Thus, by incorporating the capacitor C, the number of connection terminals with the outside is only two for connection with the battery.
[0066]
In this way, the circuit as shown in FIG. 2 is configured. When all of the stator coils (armature coils) 40 are switched in the same phase, the circuit of FIG. 2 can be handled as one coil mainly having a parasitic inductance component as shown in FIG. Described with two arms.
[0067]
If the circuit as shown in FIG. 4 is configured, one motor is provided with two terminals, a neutral point and a terminal connected to the negative bus of the inverter circuit 40, respectively. The neutral point and the positive electrode bus of the inverter circuit 40 may be taken out as terminals. If a capacitor is externally attached, both motors may be provided with terminals connected to the positive and negative buses of the inverter circuit 40, and three terminals may be prepared for each.
[0068]
Thus, by integrating the motor with the inverter while having a DC voltage variable function, it is possible to configure a compact motor drive system with a small number of wires.
[0069]
As described above, since the number of wires in the electric power system is only two connections with the battery by adopting the integrated type, the overall cost can be reduced. In particular, it has a variable voltage (step-up) function against fluctuations in battery power supply voltage, and the inverter drive voltage can be constantly optimized and stabilized, so that the size of the motor can be minimized. Further, there is no need to attach a DC-DC converter externally, and the capacity of the inverter element can be increased slightly, so that the overall configuration can be reduced in size. Also, the number of wires in the power system is only two even in the case of a multiphase drive motor. In particular, it is preferable that the capacitor is also incorporated in the housing.
[0070]
In the above description, the battery current and the like are adjusted by turning on and off all of the upper switching element and the lower switching element. However, it is not always necessary to perform control of all on and all off.
[0071]
That is, by changing the ratio of the on-duty of the upper switching element and the on-duty of the lower switching element in the inverters INV1 and INV2, control equivalent to all on and all off of the upper and lower switching elements can be performed. it can.
[0072]
For example, if the on-duty of the upper switching element is 1/3 (the on-duty of the lower switching element is 2/3), the neutral point potential becomes 1/3 of the input voltage of the inverter.
[0073]
Therefore, if the on-duty of the upper switching element of the inverter INV2 is 1/3 and the on-duty of the upper switching element of the inverter INV1 is 1/2, the neutral point potential of the motor M2 is Vc2 / 3, and the neutral point of the motor M1. The point potential is (Vc2) / 3 + Vb. The neutral point voltage of the motor M1 is (Vc1) / 2 + (Vc2). Therefore, (Vc2) / 3 + Vb = (Vc1) / 2 + Vc2 → Vb = (Vc1) / 2 + (Vc2) × 2/3.
[0074]
Also by such control, the same control as the above-mentioned case can be performed.
[0075]
【The invention's effect】
As described above, according to the present invention, the amount of current in the DC power source and the voltage of the capacitor can be freely controlled by controlling the inverter. Therefore, stable motor drive can be performed by controlling the capacitor voltage without being affected by voltage fluctuations of the DC power supply. In other words, since the inverter function configuration has a transformation function with respect to fluctuations in the DC power supply voltage, the drive voltage of the two inverters for driving the two motors without adding a DC-DC converter is close to the optimum value. Therefore, the maximum output can be secured even when the battery voltage fluctuates, so there is no need to increase the size of the motor, and the specifications of the two motors can be optimized. In particular, since this configuration is a configuration in which both of the two motors are driven or regeneratively controlled, it is suitable for a two-motor drive that directly drives the drive wheels. It becomes possible to optimize and stabilize the driving state of the motor.
[0076]
In addition, since the withstand voltage of the switching element in each inverter is larger than the voltage of the DC power supply, the switching element of each inverter has a large voltage corresponding to the capacitor voltage. By making it large, the device will not be destroyed without taking countermeasures such as disconnecting the capacitor voltage when stopped.
[0077]
In addition, since the switching element of each inverter is a MOSFET, the occurrence of loss in the switching element can be reduced when the DC power supply voltage is approximately 200 V or less.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a conventional example.
FIG. 2 is a diagram showing a configuration of another conventional example.
FIG. 3 is a diagram showing an equivalent circuit of another conventional example.
FIG. 4 is a diagram illustrating a configuration of an example of an embodiment.
FIG. 5 is a diagram illustrating an example of connection of inverters.
FIG. 6 is a diagram illustrating control of battery current.
FIG. 7 is a diagram illustrating voltage control of a capacitor.
FIG. 8 is a diagram showing a configuration of single-phase driving.
FIG. 9 is a diagram illustrating a configuration of an application example.
FIG. 10 is a diagram showing a configuration when housed in a housing.
[Explanation of symbols]
B Battery, C1, C2 capacitor, INV1, INV2 inverter, M1, M2 motor.

Claims (5)

Two inverters each having a plurality of arms formed by connecting two switching elements in series between the positive bus and the negative bus, and outputting the midpoint of the arms;
Two motors each having a plurality of stator windings connected to the outputs of the two inverters, each connected in common at a neutral point;
A DC power source connected between the neutral points of the stator windings of the two motors;
Two capacitors respectively connected between the positive bus and the negative bus of the two inverters;
Have
The motor circuit which connected the negative side bus of the inverter which drives the motor by which the neutral point of the stator winding was connected to the positive electrode side of the said battery, and the positive side bus of the other inverter. The circuit of claim 1, wherein
A motor circuit in which the withstand voltage of the switching element in each inverter is larger than the voltage of the DC power supply. The circuit according to claim 1 or 2,
A motor circuit in which the switching element of each inverter is a MOSFET. The circuit according to any one of claims 1 to 3,
A motor circuit having an integrated configuration in which the inverter and the motor are installed in the same casing.   5. The circuit according to claim 1, wherein all the switching elements on the positive bus side of the inverter are turned on, all the switching elements on the negative bus side are turned off, and the positive bus side of the inverter The current flowing through the neutral point of the two motor stator windings and the inverters are switched by switching the low-voltage state in which all the switch elements of the inverter are turned off and all the switch elements on the negative bus side are turned on by time control between the two inverters. Motor control method for controlling the voltages of capacitors respectively connected between the negative bus and the positive bus.

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