electric motor

Abstract

To provide a motor capable of reducing common mode current.SOLUTION: The motor comprises: an electric motor; an inverter that drives the electric motor; a noise reduction unit that reduces common mode noise; a control unit; and a metal housing. The electric motor has a three-phase coil. The inverter has a switching element. The noise reduction unit is provided closer to an input side than the inverter. The control unit controls the switching element by using a PWM signal generated on the basis of a voltage command value and a carrier frequency. The housing stores the electric motor, the inverter, the noise reduction unit, and the control unit. The housing is grounded to a body of a vehicle. The control unit includes a calculation unit that calculates a modulation factor of the voltage command value, and a setting unit that sets the carrier frequency in accordance with the modulation factor calculated by the calculation part. The setting part sets, if the modulation factor is lower than a prescribed modulation factor, a carrier frequency higher than that when the modulation factor is equal to or higher than the prescribed modulation factor.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an electric motor.

Background Art

[0002] Patent Document 1 discloses an air conditioner including an electric motor, an inverter unit, a noise reduction unit, a control unit, and a metal housing. The electric motor has a three-phase coil. The inverter unit has a switching element. The inverter unit drives the electric motor. The noise reduction unit is provided on the input side of the inverter unit. The noise reduction unit reduces common mode noise. The control unit controls the switching element using a PWM signal generated based on a voltage command value and a carrier frequency. The housing houses the electric motor, the inverter unit, the noise reduction unit, and the control unit. The housing is grounded to the vehicle body.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] One of the common mode noises is the common mode current caused by the neutral point voltage of the electric motor. Reduction of this common mode current is desired.

Means for Solving the Problems

[0005] The electric motor for solving the above problems comprises an electric motor having three-phase coils, an inverter unit having switching elements and driving the electric motor, a noise reduction unit provided on the input side of the inverter unit and reducing common-mode noise, a control unit that controls the switching elements using a PWM signal generated based on a voltage command value and a carrier frequency, and a metal housing that houses the electric motor, the inverter unit, the noise reduction unit, and the control unit and is grounded to the vehicle body, wherein the control unit comprises a calculation unit that calculates the modulation rate of the voltage command value, and a setting unit that sets the carrier frequency according to the modulation rate calculated by the calculation unit, and the setting unit sets the carrier frequency to a higher value when the modulation rate is lower than a predetermined modulation rate than when the modulation rate is equal to or greater than the predetermined modulation rate.

[0006] In the above configuration, the setting unit sets the carrier frequency to a higher value when the modulation rate is lower than a predetermined modulation rate than when the modulation rate is equal to or greater than the predetermined modulation rate. As a result, the pulse width of the neutral point voltage becomes shorter when the carrier frequency when the modulation rate is lower than the predetermined modulation rate is the same as when the modulation rate is equal to or greater than the predetermined modulation rate, and therefore the amplitude of the common-mode current becomes smaller. Thus, the common-mode current can be reduced.

[0007] In the above-described electric motor, the noise reduction unit comprises a capacitor and a choke coil, and the setting unit may set the carrier frequency to a frequency higher than the resonant frequency band of the noise reduction unit when the modulation rate is lower than the predetermined modulation rate.

[0008] In the above configuration, the carrier frequency when the modulation rate is lower than a predetermined modulation rate is not included within the resonant frequency band of the noise reduction unit, thus suppressing the generation of noise due to resonance phenomena. [Effects of the Invention]

[0009] According to the present invention, common-mode current can be reduced. [Brief explanation of the drawing]

[0010] [Figure 1] This is a block diagram showing the configuration of a vehicle in an embodiment. [Figure 2] This is a block diagram showing the configuration of the inverter unit, noise reduction unit, and inverter control device in the embodiment. [Figure 3] This diagram shows the relationship between the current flowing through the choke coil and the inductance of the choke coil. [Figure 4] This figure shows the relationship between the carrier frequency and the resonant frequency band. [Figure 5] This is a flowchart showing the operation of the control unit in the embodiment. [Figure 6] (a) is a diagram showing the three-phase voltage command values ​​and carrier signals, (b) is a diagram showing the PWM signals for each phase, and (c) is a diagram showing the neutral point voltage and common-mode current. [Figure 7] (a) is a diagram showing the three-phase voltage command values ​​and carrier signals, (b) is a diagram showing the PWM signals for each phase, and (c) is a diagram showing the neutral point voltage and common-mode current. [Figure 8] (a) is a diagram showing the three-phase voltage command values ​​and carrier signals, (b) is a diagram showing the PWM signals for each phase, and (c) is a diagram showing the neutral point voltage and common-mode current. [Modes for carrying out the invention]

[0011] The following describes one embodiment of the electric motor with reference to Figures 1 to 8. The electric motor of this embodiment is mounted on a vehicle. The electric motor of this embodiment is used in an electric compressor for an air conditioning system.

[0012] As shown in Figure 1, the vehicle 100 is equipped with an air conditioning unit 110 and a power storage unit 120. The air conditioning system 110 includes an electric compressor 111, an external refrigerant circuit 112, and an air conditioning ECU 113. The electric compressor 111 compresses the refrigerant. The external refrigerant circuit 112 includes, for example, a heat exchanger and an expansion valve. The air conditioning system 110 provides heating and cooling to the vehicle interior by compressing the refrigerant with the electric compressor 111 and by performing heat exchange and expansion of the refrigerant with the external refrigerant circuit 112. The air conditioning ECU 113 controls the entire air conditioning system 110. The air conditioning ECU 113 is configured to grasp parameters such as the interior temperature and the set temperature of the car air conditioner. Based on the grasped parameters, the air conditioning ECU 113 transmits various commands, such as the command rotation speed Nc, to the electric compressor 111.

[0013] The energy storage device 120 can be any device capable of charging and discharging DC power. Examples of the energy storage device 120 include secondary batteries and electric double-layer capacitors. The energy storage device 120 is mounted on the body of the vehicle 100. The energy storage device 120 is used as the DC power source for the electric compressor 111. The negative electrode of the energy storage device 120 is grounded to the body of the vehicle 100.

[0014] The electric compressor 111 comprises an electric motor 10 and a compression unit 11. The electric motor 10 includes an electric motor 12, an inverter unit 13, a noise reduction unit 14, and an inverter control device 15. The electric motor 12 drives the compression unit 11. The inverter unit 13 drives the electric motor 12. The noise reduction unit 14 reduces common-mode noise. The inverter control device 15 is used to control the inverter unit 13.

[0015] As shown in Figure 2, the electric motor 10 has a housing 16. The housing 16 houses the electric motor 12, the inverter unit 13, the noise reduction unit 14, and the inverter control device 15. The housing 16 is made of metal. Therefore, the housing 16 is conductive. The housing 16 is grounded to the body of the vehicle 100.

[0016] <Electric motor> As shown in FIG. 1, the electric motor 12 has a rotating shaft 21, a rotor 22, and a stator 23. The electric motor 12 is a three-phase motor. The rotor 22 is fixed to the rotating shaft 21. The rotor 22 includes a permanent magnet 22a. The stator 23 is disposed opposite to the rotor 22. The stator 23 has a stator core (not shown) and three-phase coils 24u, 24v, 24w. The three-phase coils 24u, 24v, 24w are the u-phase coil 24u, the v-phase coil 24v, and the w-phase coil 24w. The three-phase coils 24u, 24v, 24w are wound around the stator core. The rotor 22 rotates when the three-phase coils 24u, 24v, 24w are energized in a predetermined pattern. The rotating shaft 21 rotates integrally with the rotor 22.

[0017] As shown in FIG. 2, the three-phase coils 24u, 24v, 24w are Y-connected. The first ends of the three-phase coils 24u, 24v, 24w are electrically connected to each other to form a neutral point 24n. The second ends, which are the opposite ends of the first ends of the three-phase coils 24u, 24v, 24w, are connected to the inverter unit 13.

[0018] <Compression section> The compression section 11 compresses the refrigerant when the electric motor 12 is driven. Specifically, the compression section 11 compresses the refrigerant supplied from the external refrigerant circuit 112 when the rotating shaft 21 rotates. The refrigerant compressed by the compression section 11 is discharged into the external refrigerant circuit 112. The compression section 11 can be of any type such as a scroll type, a piston type, a vane type, etc.

[0019] <Inverter unit> The inverter unit 13 converts the DC power output from the power storage device 120 into AC power. The electric motor 12 is driven by the AC power output from the inverter unit 13.

[0020] As shown in Figure 2, the inverter unit 13 has switching elements Qu1, Qu2, Qv1, Qv2, Qw1, and Qw2. Hereafter, the switching elements Qu1, Qu2, Qv1, Qv2, Qw1, and Qw2 will be referred to as "switching elements Qu1 to Qw2".

[0021] The switching elements Qu1 to Qw2 in this embodiment are, for example, power switching elements such as IGBTs. Each of the switching elements Qu1 to Qw2 has a freewheeling diode. Specifically, the u-phase switching element Qu1 has a freewheeling diode Du1. The u-phase switching element Qu2 has a freewheeling diode Du2. The v-phase switching element Qv1 has a freewheeling diode Dv1. The v-phase switching element Qv2 has a freewheeling diode Dv2. The w-phase switching element Qw1 has a freewheeling diode Dw1. The w-phase switching element Qw2 has a freewheeling diode Dw2. Hereinafter, the freewheeling diodes Du1, Du2, Dv1, Dv2, Dw1, and Dw2 will be referred to as "freewheeling diodes Du1 to Dw2". The cathodes of the freewheeling diodes Du1 to Dw2 are connected to the collectors of the corresponding switching elements Qu1 to Qw2. The anodes of the freewheeling diodes Du1 to Dw2 are connected to the emitters of the corresponding switching elements Qu1 to Qw2.

[0022] The u-phase switching elements Qu1 and Qu2 are connected in series via a u-phase connecting line. The u-phase connecting line branches off and connects to the second end of the u-phase coil 24u. The collector of u-phase switching element Qu1 is connected to the positive bus L1. The emitter of u-phase switching element Qu2 is connected to the negative bus L2. The v-phase switching elements Qv1 and Qv2 are connected in series via a v-phase connecting line. The v-phase connecting line branches off and connects to the second end of the v-phase coil 24v. The collector of v-phase switching element Qv1 is connected to the positive bus L1. The emitter of v-phase switching element Qv2 is connected to the negative bus L2. The w-phase switching elements Qw1 and Qw2 are connected in series via a w-phase connecting line. The w-phase connecting line branches off and connects to the second end of the w-phase coil 24w. The collector of w-phase switching element Qw1 is connected to the positive bus L1. The emitter of the w-phase switching element Qw2 is connected to the negative busbar L2.

[0023] <Noise Reduction Section> The noise reduction unit 14 is located between the energy storage device 120 and the inverter unit 13. The noise reduction unit 14 is located on the input side of the inverter unit 13. The noise reduction unit 14 reduces the common-mode noise contained in the DC power before it is input from the energy storage device 120 to the inverter unit 13. The common-mode noise reduced by the noise reduction unit 14 includes the common-mode current Ic caused by the neutral point voltage Vn, which is the voltage at the neutral point 24n of the electric motor 12. This common-mode current Ic flows out to the body of the vehicle 100 via the stray capacitance Cs, and then returns to the positive busbar L1 and negative busbar L2 via the body.

[0024] The noise reduction unit 14 has a choke coil 41. The choke coil 41 in this embodiment is a common-mode choke coil. The choke coil 41 has a core 41a and a coil 41b wound around the core 41a. The choke coil 41 is provided on the positive busbar L1 and the negative busbar L2. The noise reduction unit 14 also has two capacitors 42. The capacitors 42 are Y capacitors. The two capacitors 42 are connected in series. The connecting wire between the two capacitors 42 branches off and is connected to the housing 16, thereby grounding to the body of the vehicle 100. The two capacitors 42 are connected in parallel to the choke coil 41. The two capacitors 42 are provided on the inverter unit 13 side relative to the choke coil 41. The noise reduction unit 14 is an LC filter composed of the choke coil 41 and the capacitors 42.

[0025] Figure 3 shows the relationship between the current I flowing through coil 41b of the choke coil 41 and the inductance L of the choke coil 41. When the current I flowing through coil 41b of the choke coil 41 increases, the inductance L decreases due to magnetic saturation of the core 41a.

[0026] As shown in Figure 4, the noise reduction unit 14 has a resonant frequency band A. When the inductance L decreases due to an increase in the current I flowing through the coil 41b of the choke coil 41, the resonant frequency band A shifts to the higher frequency side.

[0027] <Inverter control device> As shown in Figure 2, the inverter control device 15 includes a voltage sensor 51, a current sensor 52, and a control unit 53.

[0028] The voltage sensor 51 detects the DC voltage Vdc, which is the voltage of the energy storage device 120. The current sensor 52 detects three-phase currents Iu, Iv, and Iw. The three-phase currents Iu, Iv, and Iw are the u-phase current Iu, the v-phase current Iv, and the w-phase current Iw. The u-phase current Iu is the current flowing through the u-phase coil 24u. The v-phase current Iv is the current flowing through the v-phase coil 24v. The w-phase current Iw is the current flowing through the w-phase coil 24w.

[0029] The control unit 53 is implemented, for example, by a hardware processor such as a CPU (Central Processing Unit) executing a program (software). Furthermore, some or all of these components may be implemented by hardware (including circuitry) such as an LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), or GPU (Graphics Processing Unit), or by the cooperation of software and hardware. The program may be stored in advance in a storage device (not shown) equipped with a non-transient storage medium such as an HDD (Hard Disk Drive) or flash memory provided by the inverter control device 15. The storage device may be implemented by, for example, the above-mentioned storage media, or by EEPROM (Electrically Erasable Programmable Read Only Memory), ROM (Read Only Memory), RAM (Random Access Memory), etc.

[0030] The control unit 53 drives the electric motor 12 by controlling the inverter unit 13, specifically the switching elements Qu1 to Qw2. The control unit 53 includes a 3-phase / 2-phase conversion unit 53a, a position estimation unit 53b, a 3-phase voltage command value derivation unit 53c, a calculation unit 53d, a setting unit 53e, a generation unit 53f, and a rotation control unit 53g. The control unit 53 is connected to the air conditioning ECU 113.

[0031] The 3-phase / 2-phase conversion unit 53a converts the 3-phase currents Iu, Iv, and Iw detected by the current sensor 52 into 2-phase currents Id and Iq. The 2-phase currents Id and Iq are the d-axis current Id and the q-axis current Iq. The d-axis current Id and the q-axis current Iq are orthogonal to each other. The d-axis current Id is the current in the magnetic flux axis direction of the rotor 22, i.e., the excitation component current. The q-axis current Iq is the torque component current that contributes to the torque of the electric motor 12.

[0032] The position estimation unit 53b estimates the rotational position and rotational speed of the rotor 22. The position estimation unit 53b estimates the rotational position of the rotor 22 and the actual rotational speed Nr, which is the actual rotational speed, based on, for example, at least one of the two-phase currents Id, Iq and the two-phase voltage command values ​​Vdr, Vqr, which will be described later. The unit of the actual rotational speed Nr is, for example, rpm.

[0033] The three-phase voltage command value derivation unit 53c obtains an external command value from the air conditioning ECU 113. The external command value is, for example, the command rotational speed Nc. The unit of the command rotational speed Nc is, for example, rpm. The air conditioning ECU 113 calculates the required refrigerant flow rate from the operating status of the air conditioning unit 110. The air conditioning ECU 113 calculates the command rotational speed Nc that can achieve the required refrigerant flow rate.

[0034] The 3-phase voltage command value derivation unit 53c derives the 3-phase voltage command values ​​Vur, Vvr, and Vwr based on the command rotational speed Nc obtained from the air conditioning ECU 113 and the actual rotational speed Nr estimated by the position estimation unit 53b. The 3-phase voltage command values ​​Vur, Vvr, and Vwr are the u-phase voltage command value Vur, the v-phase voltage command value Vvr, and the w-phase voltage command value Vwr. The u-phase voltage command value Vur is the target voltage applied to the u-phase coil 24u. The v-phase voltage command value Vvr is the target voltage applied to the v-phase coil 24v. The w-phase voltage command value Vwr is the target voltage applied to the w-phase coil 24w.

[0035] More specifically, the three-phase voltage command value derivation unit 53c derives two-phase current command values ​​Idr and Iqr based on the command rotational speed Nc obtained from the air conditioning ECU 113 and the actual rotational speed Nr estimated by the position estimation unit 53b. The two-phase current command values ​​Idr and Iqr are the d-axis current command value Idr and the q-axis current command value Iqr. The d-axis current command value Idr is the target value of the d-axis current Id. The q-axis current command value Iqr is the target value of the q-axis current Iq.

[0036] The 3-phase voltage command value derivation unit 53c derives 2-phase voltage command values ​​Vdr and Vqr based on the derived 2-phase current command values ​​Idr and Iqr and the 2-phase currents Id and Iq converted by the 3-phase / 2-phase conversion unit 53a. The 2-phase voltage command values ​​Vdr and Vqr are the d-axis voltage command value Vdr and the q-axis voltage command value Vqr. The d-axis voltage command value Vdr is the target voltage value applied to the d-axis of the electric motor 12. The q-axis voltage command value Vqr is the target voltage value applied to the q-axis of the electric motor 12.

[0037] The three-phase voltage command value derivation unit 53c derives the three-phase voltage command values ​​Vur, Vvr, and Vwr based on the two-phase voltage command values ​​Vdr and Vqr. The three-phase voltage command values ​​Vur, Vvr, and Vwr change according to the electrical angle. The three-phase voltage command values ​​Vur, Vvr, and Vwr are waveforms with a reference amplitude that has a period of, for example, 0° to 360° of electrical angle. The phases of the three-phase voltage command values ​​Vur, Vvr, and Vwr are different from each other. For example, the phases of the three-phase voltage command values ​​Vur, Vvr, and Vwr are shifted by 120° from each other. The waveforms of the three-phase voltage command values ​​Vur, Vvr, and Vwr can be arbitrary, such as a sine wave, a triangular wave, a square wave, or a modified version of these waveforms.

[0038] The calculation unit 53d calculates the modulation ratio M based on the DC voltage Vdc detected by the voltage sensor 51 and the two-phase voltage command values ​​Vdr and Vqr derived by the three-phase voltage command value derivation unit 53c. The modulation ratio M is the ratio of the motor voltage Vm to the DC voltage Vdc. The motor voltage Vm can be obtained using √(Vdr^2 + Vqr^2). Therefore, the modulation ratio M can be obtained as M = (√(Vdr^2 + Vqr^2)) / Vdc. The modulation ratio M is low when the electric motor 12 is starting up or rotating at low speeds, and high when it is rotating at high speeds. In this case, the modulation ratio M also depends on fluctuations in the DC voltage Vdc.

[0039] The setting unit 53e sets the carrier frequency f, which is the frequency of the carrier signal Pc, according to the modulation rate M calculated by the calculation unit 53d. If the modulation rate M is lower than a predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to a first frequency f1. If the modulation rate M is equal to or greater than the predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to a second frequency f2, which is lower than the first frequency f1. In other words, if the modulation rate M is lower than a predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to a higher value than when the modulation rate M is equal to or greater than the predetermined modulation rate Mth.

[0040] For example, if the common-mode current Ic is greater than a predetermined current value Icth, the inductance L decreases due to magnetic saturation of the core 41a of the choke coil 41. In this embodiment, the predetermined modulation rate Mth and carrier frequency f are set so that the common-mode current Ic is less than or equal to the predetermined current value Icth.

[0041] The common-mode current Ic decreases as the modulation index M increases. When the carrier frequency f is the second frequency f2, the modulation index M at which the common-mode current Ic is greater than a predetermined current value Icth is defined as the modulation index M0. When the carrier frequency f is the second frequency f2, if the modulation index M is less than or equal to the modulation index M0, the common-mode current Ic is greater than the predetermined current value Icth. When the modulation index M is higher than the modulation index M0, the common-mode current Ic is less than or equal to the predetermined current value Icth. In this case, the predetermined modulation index Mth is set to be less than or equal to the modulation index M0.

[0042] The common-mode current Ic decreases as the carrier frequency f increases. The first frequency f1 is set so that the common-mode current Ic is less than or equal to a predetermined current value Icth, even when the modulation rate M is lower than a predetermined modulation rate Mth.

[0043] As shown in Figure 4, in this embodiment, the setting unit 53e sets the first frequency f1 to a frequency higher than the resonant frequency band A of the noise reduction unit 14. In other words, when the modulation rate M is lower than a predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to a frequency higher than the resonant frequency band A of the noise reduction unit 14. Also, the setting unit 53e sets the second frequency f2 to a frequency higher than the resonant frequency band A of the noise reduction unit 14. In other words, when the modulation rate M is equal to or greater than a predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to a frequency higher than the resonant frequency band A of the noise reduction unit 14.

[0044] The generation unit 53f generates PWM signals Pu, Pv, and Pw based on the three-phase voltage command values ​​Vur, Vvr, and Vwr derived by the three-phase voltage command value derivation unit 53c and the carrier signal Pc with a carrier frequency f set by the setting unit 53e. The PWM signals Pu, Pv, and Pw are the u-phase PWM signal Pu, the v-phase PWM signal Pv, and the w-phase PWM signal Pw. The u-phase PWM signal Pu is the voltage applied to the u-phase coil 24u. The v-phase PWM signal Pv is the voltage applied to the v-phase coil 24v. The w-phase PWM signal Pw is the voltage applied to the w-phase coil 24w. The PWM signals Pu, Pv, and Pw set the switching pattern of the switching elements Qu1 to Qw2, specifically the duty cycle.

[0045] In detail, the generation unit 53f generates a u-phase PWM signal Pu based on the u-phase voltage command value Vur derived by the three-phase voltage command value derivation unit 53c and the carrier signal Pc with a carrier frequency f set by the setting unit 53e. The generation unit 53f generates a v-phase PWM signal Pv based on the v-phase voltage command value Vvr derived by the three-phase voltage command value derivation unit 53c and the carrier signal Pc with a carrier frequency f set by the setting unit 53e. The generation unit 53f generates a w-phase PWM signal Pw based on the w-phase voltage command value Vwr derived by the three-phase voltage command value derivation unit 53c and the carrier signal Pc with a carrier frequency f set by the setting unit 53e.

[0046] The rotation control unit 53g controls the switching of switching elements Qu1 to Qw2 by outputting the PWM signals Pu, Pv, and Pw generated by the generation unit 53f to the switching elements Qu1 to Qw2. In other words, the control unit 53 controls the switching elements Qu1 to Qw2 using the PWM signals Pu, Pv, and Pw generated based on the three-phase voltage command values ​​Vur, Vvr, and Vwr and the carrier frequency f.

[0047] <Operation Flow> An example of the operation of the control unit 53 will be described below. As shown in Figure 5, in step S11, the three-phase voltage command value derivation unit 53c derives two-phase current command values ​​Idr and Iqr based on the command rotational speed Nc obtained from the air conditioning ECU 113 and the actual rotational speed Nr estimated by the position estimation unit 53b.

[0048] Next, in step S12, the three-phase voltage command value derivation unit 53c derives two-phase voltage command values ​​Vdr and Vqr based on the derived two-phase current command values ​​Idr and Iqr and the two-phase currents Id and Iq converted by the three-phase / two-phase conversion unit 53a.

[0049] Next, in step S13, the three-phase voltage command value derivation unit 53c derives the three-phase voltage command values ​​Vur, Vvr, and Vwr based on the two-phase voltage command values ​​Vdr and Vqr. Next, in step S14, the calculation unit 53d calculates the modulation rate M based on the two-phase voltage command values ​​Vdr and Vqr derived by the three-phase voltage command value derivation unit 53c and the DC voltage Vdc detected by the voltage sensor 51.

[0050] If the modulation rate M is lower than a predetermined modulation rate Mth (YES in step S15), proceed to step S16. In step S16, the setting unit 53e sets the carrier frequency f to the first frequency f1. If the modulation rate M is greater than or equal to the predetermined modulation rate Mth (NO in step S15), proceed to step S17. In step S17, the setting unit 53e sets the carrier frequency f to the second frequency f2.

[0051] Next, in step S18, the generation unit 53f generates PWM signals Pu, Pv, and Pw based on the three-phase voltage command values ​​Vur, Vvr, and Vwr derived by the three-phase voltage command value derivation unit 53c and the carrier signal Pc, whose carrier frequency f is set by the setting unit 53e.

[0052] Then, in step S19, the rotation control unit 53g controls the switching elements Qu1 to Qw2 using the PWM signals Pu, Pv, and Pw generated by the generation unit 53f. [Operation of this embodiment] The operation of this embodiment will be explained using a comparative example.

[0053] In this embodiment, the setting unit 53e sets the carrier frequency f according to the modulation rate M calculated by the calculation unit 53d. If the modulation rate M is lower than a predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to the first frequency f1. If the modulation rate M is equal to or greater than the predetermined modulation rate Mth, the setting unit 53e sets the carrier frequency f to the second frequency f2. In contrast, in the comparative example, the carrier frequency f is set to a constant value regardless of the modulation rate M. The carrier frequency f is set to the second frequency f2.

[0054] First, the case where the modulation rate M is equal to or greater than a predetermined modulation rate Mth in the embodiment and comparative example will be explained using Figures 6(a), 6(b), and 6(c). Note that Figures 6(a), 6(b), and 6(c) are common to both the embodiment and the comparative example.

[0055] Figure 6(a) shows the three-phase voltage command values ​​Vur1, Vvr1, Vwr1 and the carrier signal Pc1 with a carrier frequency f of the second frequency f2. Figure 6(b) shows the PWM signals Pu1, Pv1, Pw1. The PWM signals Pu1, Pv1, Pw1 are generated based on the three-phase voltage command values ​​Vur1, Vvr1, Vwr1 and the carrier signal Pc1. Figure 6(c) shows the neutral point voltage Vn1 and the common-mode current Ic1. The neutral point voltage Vn is the sum of the u-phase PWM signal Pu, the v-phase PWM signal Pv, and the w-phase PWM signal Pw divided by 3. Therefore, the neutral point voltage Vn can be calculated as (Pu+Pv+Pw) / 3. The neutral point voltage Vn1 can be calculated as (Pu1+Pv1+Pw1) / 3.

[0056] When the modulation index M is greater than or equal to a predetermined modulation index Mth, and the carrier frequency f is the second frequency f2, the pulse widths of the PWM signals Pu1, Pv1, and Pw1 fluctuate. Furthermore, the rising and falling edge timings of the pulses of the PWM signals Pu1, Pv1, and Pw1 differ for each phase. Therefore, the pulse waveform of the neutral point voltage Vn1 is trapezoidal. In this case, the common-mode current Ic1 is less than or equal to a predetermined current value Icth.

[0057] Next, we will explain the case where the modulation rate M is lower than a predetermined modulation rate Mth in the comparative example, using Figures 7(a), 7(b), and 7(c). Figure 7(a) shows the three-phase voltage command values ​​Vur2, Vvr2, Vwr2 and the carrier signal Pc1 in the comparative example. The amplitudes of the three-phase voltage command values ​​Vur2, Vvr2, Vwr2 are smaller than the amplitudes of the three-phase voltage command values ​​Vur1, Vvr1, Vwr1. The carrier frequency f of the carrier signal Pc1 in the comparative example is the second frequency f2. Figure 7(b) shows the PWM signals Pu2, Pv2, Pw2 in the comparative example. The PWM signals Pu2, Pv2, Pw2 are generated based on the three-phase voltage command values ​​Vur2, Vvr2, Vwr2 and the carrier signal Pc1. Figure 7(c) shows the neutral point voltage Vn2 and common-mode current Ic2 in the comparative example. The neutral point voltage Vn2 is calculated as (Pu2 + Pv2 + Pw2) / 3.

[0058] When the modulation index M is lower than a predetermined modulation index Mth, and the carrier frequency f is the second frequency f2, the pulse widths of the PWM signals Pu2, Pv2, and Pw2 are approximately constant. Also, the rising and falling timings of the pulses of the PWM signals Pu2, Pv2, and Pw2 are approximately the same for each phase. Therefore, the waveform of the neutral point voltage Vn1 is a square wave. When the carrier frequency f is the second frequency f2, the amplitude of the common-mode current Ic2 when the modulation index M is lower than the predetermined modulation index Mth is larger than the amplitude of the common-mode current Ic1 when the modulation index M is equal to or greater than the predetermined modulation index Mth. In other words, when the carrier frequency f is constant, the common-mode current Ic increases as the modulation index M decreases.

[0059] When the common-mode current Ic is greater than a predetermined current value Icth, magnetic saturation occurs in the core 41a of the choke coil 41, causing the inductance L of the choke coil 41 to decrease. When the inductance L of the choke coil 41 decreases, the resonant frequency band A of the noise reduction unit 14 shifts to the higher frequency side. Therefore, if the carrier frequency f is included in the resonant frequency band A of the noise reduction unit 14, noise due to the resonance phenomenon will be generated.

[0060] Next, in the embodiment, the case where the modulation rate M is lower than a predetermined modulation rate Mth will be explained using Figures 8(a), 8(b), and 8(c). Figure 8(a) shows the three-phase voltage command values ​​Vur2, Vvr2, Vwr2 and the carrier signal Pc2 in the embodiment. The carrier frequency f of the carrier signal Pc2 in the embodiment is a first frequency f1 which is higher than the second frequency f2. Figure 8(b) shows the PWM signals Pu3, Pv3, Pw3 in the embodiment. The PWM signals Pu3, Pv3, Pw3 are generated based on the three-phase voltage command values ​​Vur2, Vvr2, Vwr2 and the carrier signal Pc2. Figure 8(c) shows the neutral point voltage Vn3 and common-mode current Ic3 in the embodiment. The neutral point voltage Vn3 is calculated as (Pu3 + Pv3 + Pw3) / 3.

[0061] When the modulation index M is lower than a predetermined modulation index Mth, and the carrier frequency f is the first frequency f1, the pulse widths of the PWM signals Pu3, Pv3, and Pw3 are approximately constant. Also, the rising and falling timings of the pulses of the PWM signals Pu3, Pv3, and Pw3 are approximately the same for each phase. Therefore, the waveform of the neutral point voltage Vn3 is a square wave, similar to the comparative example. On the other hand, the carrier frequency f of the carrier signal Pc2 in the embodiment is higher than the carrier frequency f of the carrier signal Pc1 in the comparative example. In other words, the period of the carrier signal Pc2 in the embodiment is shorter than the period of the carrier signal Pc1 in the comparative example. Therefore, the pulse widths of the PWM signals Pu3, Pv3, and Pw3 in the embodiment are smaller than the pulse widths of the PWM signals Pu2, Pv2, and Pw2 in the comparative example. Consequently, the pulse width of the neutral point voltage Vn3 in the embodiment is smaller than the pulse width of the neutral point voltage Vn2 in the comparative example. As a result, the amplitude of the common-mode current Ic3 in the embodiment is smaller than the amplitude of the common-mode current Ic2 in the comparative example. In other words, in the embodiment, the common-mode current Ic is reduced compared to the comparative example. The common-mode current Ic3 in the embodiment is less than or equal to a predetermined current value Icth.

[0062] Thus, when the modulation index M is lower than a predetermined modulation index Mth, the common-mode current Ic is reduced by setting the carrier frequency f to a first frequency f1 that is higher than the second frequency f2. In this case, the decrease in inductance L is suppressed because magnetic saturation of the core 41a of the choke coil 41 is less likely to occur, so the resonant frequency band A of the noise reduction unit 14 is less likely to shift to the higher frequency side. Furthermore, in this embodiment, the first frequency f1 and the second frequency f2, which are carrier frequencies f, are set to frequencies higher than the resonant frequency band A. Therefore, the generation of noise due to resonance phenomena caused by the carrier frequency f being included in the resonant frequency band A is suppressed.

[0063] [Effects of this embodiment] The effects of this embodiment will now be explained. (1) The setting unit 53e sets the carrier frequency f to a higher value than when the modulation rate M is equal to or greater than the predetermined modulation rate Mth, when the modulation rate M is lower than the predetermined modulation rate Mth. As a result, the pulse width of the neutral point voltage Vn becomes shorter when the carrier frequency f when the modulation rate M is lower than the predetermined modulation rate Mth is the same as when the modulation rate M is equal to or greater than the predetermined modulation rate Mth, and the amplitude of the common mode current Ic becomes smaller. Therefore, the common mode current Ic can be reduced.

[0064] Furthermore, when the modulation index M is greater than or equal to a predetermined modulation index Mth, the common-mode current Ic is smaller than when the modulation index M is lower than the predetermined modulation index Mth. Therefore, when the modulation index M is greater than or equal to a predetermined modulation index Mth, the carrier frequency f can be set lower than when the modulation index M is lower than the predetermined modulation index Mth, thereby reducing the number of switching operations of the switching elements Qu1 to Qw2. Thus, switching losses can be reduced. In addition, component failure due to heat generation in the switching elements Qu1 to Qw2 can be suppressed.

[0065] (2) The noise reduction unit 14 includes a choke coil 41 and a capacitor 42. The setting unit 53e sets the carrier frequency f to a frequency higher than the resonant frequency band A of the noise reduction unit 14 when the modulation rate M is lower than a predetermined modulation rate Mth. As a result, the second frequency f2, which is the carrier frequency f when the modulation rate M is lower than a predetermined modulation rate Mth, is not included in the resonant frequency band A of the noise reduction unit 14. Therefore, the generation of noise due to resonance phenomena can be suppressed.

[0066] (3) As the common-mode current Ic is reduced, magnetic saturation of the core 41a of the choke coil 41 becomes less likely, and thus the decrease in the inductance L of the choke coil 41 is suppressed. As a result, the resonant frequency band A is less likely to shift to the higher frequency side, and thus the generation of noise due to resonance reduction caused by the carrier frequency f being included in the resonant frequency band A can be suppressed.

[0067] (4) For example, the common-mode current Ic can be reduced without changing the component configuration, such as changing the material of the core 41a or increasing the number of turns of the coil 41b. [Example of changes] The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0068] ○ The switching elements Qu1 to Qw2 are not limited to IGBTs. For example, the switching elements Qu1 to Qw2 may be MOSFETs. ○ The configuration of the noise reduction unit 14 may be changed as appropriate, provided that it is possible to reduce common-mode noise.

[0069] ○ The choke coil 41 may have a structure that reduces not only common-mode noise but also normal-mode noise due to the leakage flux of the core 41a. ○ The current sensor 52 does not need to detect all three phase currents Iu, Iv, and Iw. The current sensor 52 may detect two of the three phase currents Iu, Iv, and Iw, and the remaining current may be calculated.

[0070] ○ If the modulation rate M and time information are linked, the setting unit 53e may set the carrier frequency f based on the time information. For example, suppose that from time t0, when the air conditioning ECU 113 sends a start command for the electric motor 12 to the inverter control device 15, until a predetermined time t1, the modulation rate M is lower than a predetermined modulation rate Mth. Also, suppose that at the predetermined time t1, the modulation rate M becomes equal to or greater than the predetermined time Mth. In this case, the setting unit 53e sets the carrier frequency f to the first frequency f1 from time t0 to time t1. At time t1, the setting unit 53e switches the carrier frequency f from the first frequency f1 to the second frequency f2. In other words, the setting unit 53e sets the carrier frequency f without comparing the modulation rate M with the predetermined modulation rate Mth. Even in this case, if the modulation rate M is lower than the predetermined modulation rate Mth, the setting unit 53e will set the carrier frequency f to a higher value than if the modulation rate M were equal to or greater than the predetermined modulation rate Mth.

[0071] ○ The electric motor 10 is not limited to those used in the electric compressor 111. For example, the electric motor 10 may be used as a traction motor to move the vehicle 100. ○ The connecting wires that connect the two capacitors 42 may branch off and be grounded to the body of the vehicle 100, rather than branching off and being connected to the housing 16.

[0072] ○ The wiring that branches off from the connecting line between the two capacitors 42 and connects to the enclosure 16 is not necessary. ○ The common-mode current Ic may flow out to the housing 16 via the stray capacitance Cs, and then return to the positive bus L1 and negative bus L2 via the stray capacitance between the housing 16 and the positive bus L1, and the stray capacitance between the housing 16 and the negative bus L2. [Explanation of Symbols]

[0073] 10...Electric motor, 12...Electric motor, 13...Inverter unit, 14...Noise reduction unit, 16...Housing, 53...Control unit, 53d...Calculation unit, 53e...Setting unit, 24u, 24v, 24w...3-phase coil, 41...Choke coil, 42...Capacitor, 100...Vehicle, Qu1~Qw2...Switching element.

Claims

[Claim 1] An electric motor having three phase coils, wherein the first ends of the three phase coils are electrically connected to each other to form a neutral point, and the electric motor, An inverter unit having a switching element and driving the electric motor, A noise reduction unit provided on the input side of the inverter unit, between the DC power supply and the inverter unit, having a capacitor and a choke coil, wherein the noise reduction unit reduces common-mode noise, A control unit that controls the switching element using a PWM signal generated based on a voltage command value and a carrier frequency, A metal housing that houses the electric motor, the inverter unit, the noise reduction unit, and the control unit, and is grounded to the vehicle body, wherein a stray capacitance exists between the neutral point and the vehicle body, or between the neutral point and the metal housing, An electric motor equipped with, The control unit, A calculation unit that calculates the modulation rate of the voltage command value, A setting unit sets the carrier frequency according to the modulation rate calculated by the calculation unit, It has, The setting unit sets the carrier frequency to a first frequency when the modulation rate is lower than a predetermined modulation rate, and sets the carrier frequency to a second frequency lower than the first frequency when the modulation rate is equal to or greater than the predetermined modulation rate. The first frequency and the second frequency are set to frequencies higher than the resonant frequency band of the noise reduction unit. The motor is characterized in that the first frequency is set such that the common-mode current is less than or equal to a predetermined current value, even when the modulation rate is lower than the predetermined modulation rate.

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