Motor drive system and heat source device

Abstract

A motor drive system according to an embodiment of the present invention comprises: a rectifier circuit that rectifies an AC voltage supplied from an AC power supply; a noise filter that is positioned between the AC power supply and the rectifier circuit; a first DC / AC conversion circuit that is connected to the rectifier circuit output side and is composed of a plurality of semiconductor switches; a first motor in which windings in which each phase is independent each have one end connected to an AC output terminal of the first DC / AC conversion circuit; a second DC / AC conversion circuit that is composed of a plurality of semiconductor switches, the other ends of said windings being connected to an AC output terminal of said second DC / AC conversion circuit; a capacitor that is connected in parallel with the second DC / AC conversion circuit; a third DC / AC conversion circuit that is connected in parallel with the capacitor and is composed of a plurality of semiconductor switches; and a second motor that is connected to an AC output terminal of the third DC / AC conversion circuit.

Description

Motor drive system and heat source device

[0001] An embodiment of the present invention relates to a system for simultaneously driving two motors, and a heat source device including the system.

[0002] Conventionally, there have been systems that use a commercial AC power source as a power source and simultaneously drive two or more motors with inverters. For example, in the outdoor unit of an air conditioner described in Patent Document 1, a motor that drives a compressor in a refrigeration cycle and an outdoor fan motor that blows air to an outdoor heat exchanger are each driven at variable speeds by inverters. In this configuration, the AC power source is rectified by a single full-wave rectifier circuit, and the rectified DC voltage is supplied to the inverter for the compressor motor and the inverter for the outdoor fan motor.

[0003] In circuits like this that use inverters to drive motors, harmonic noise, or common mode noise, is generated. When the compressor and outdoor fan are driven simultaneously, the common mode noise generated in the above system is a combination of noise generated by the inverter for the compressor motor and noise generated by the inverter for the outdoor fan motor. Within this noise frequency band, the noise from the inverter with the highest level flows out to the power supply.

[0004] Patent No. 6957756

[0005] Generally, the carrier frequency in PWM control, i.e., the switching frequency of the inverter's switching elements, is higher in the inverter for the outdoor fan motor. Therefore, the common-mode noise generated by the fan drive inverter is generally greater than that generated by the inverter for the compressor motor. In other words, the harmonic noise of the fan drive inverter is dominant. Therefore, to suppress this common-mode noise, a noise filter is provided upstream of the rectifier circuit. The higher the common-mode noise, the larger the coils and capacitors required to reduce the common-mode noise to an acceptable level, which leads to increased size and cost.

[0006] Therefore, the present invention provides a motor drive system that can drive two motors simultaneously while suppressing increases in the size and cost of noise filters, and a heat source device that includes the system.

[0007] a first DC / AC conversion circuit configured with a plurality of semiconductor switches and connected to the output side of the rectifier circuit; a first motor having one end of a winding, each phase of which is independent, connected to an AC output terminal of the first DC / AC conversion circuit; a second DC / AC conversion circuit configured with a plurality of semiconductor switches and having the other end of the winding connected to an AC output terminal of the first DC / AC conversion circuit; a capacitor connected in parallel to the second DC / AC conversion circuit; a third DC / AC conversion circuit configured with a plurality of semiconductor switches and connected in parallel to the capacitor; and a second motor connected to the AC output terminal of the third DC / AC conversion circuit.

[0008] In addition, the heat source device of the embodiment includes the motor drive system of the embodiment, a compressor, a heat exchanger, and a fan that blows air to the heat exchanger, and the compressor is driven by the first motor and the fan is driven by the second motor.

[0009] FIG. 1 is a diagram showing the configuration of a motor drive system according to one embodiment. FIG. 2 is a diagram showing the configuration of an air conditioner to which the motor drive system is applied, focusing on the outdoor unit side. FIG. 3 is a diagram of the circuit shown in FIG. 1 converted into a common mode equivalent circuit. FIG. 4 is a diagram (part 1) showing a specific example of the configuration of a noise filter. FIG. 5 is a diagram (part 2) showing a specific example of the configuration of a noise filter. FIG. 6 is a diagram showing noise terminal voltage characteristics for a conventional configuration and the configuration of this embodiment. FIG. 7 is a diagram showing the ground impedance characteristics of a 4-pole, 6-slot motor and an 8-pole, 12-slot motor according to a second embodiment.

[0010] (First Embodiment) FIG. 2 shows the configuration of an air conditioner, focusing on the outdoor unit 100 installed outdoors, as an example of the application of the motor drive system of this embodiment. The compressor constituting the heat pump system 1 is, for example, a rotary type and is represented by the symbol of the compressor motor CM. This is referred to as the first motor 2. The first motor 2 is, for example, a concentrated-winding, 4-pole, 6-slot, embedded-magnet DC brushless motor, with independent windings for each phase and an open-winding motor with all winding ends open. The first motor 2 (compressor), a four-way valve 3, an indoor heat exchanger 4, a pressure reducing device PMV 5, and an outdoor heat exchanger 6 are sequentially connected by pipes serving as refrigerant flow paths to form a closed-loop heat pump refrigeration cycle. The air conditioner is configured with the heat pump system 1 described above.

[0011] During heating, high-temperature refrigerant compressed by the compressor using the first motor 2 is supplied through the four-way valve 3 to the indoor heat exchanger 4 where it condenses, then it is decompressed by the PMV 5, becomes colder, and flows to the outdoor heat exchanger 6 where it evaporates and returns to the compressor. On the other hand, during cooling, high-temperature refrigerant compressed by the compressor is supplied through the four-way valve 3 to the outdoor heat exchanger 6 where it condenses, then it is decompressed by the PMV 5, becomes colder, and flows to the indoor heat exchanger 4 where it evaporates and returns to the compressor.

[0012] The indoor and outdoor heat exchangers 4, 6 are respectively supplied with air by an indoor fan 7 and an outdoor fan 8, and this air ensures efficient heat exchange between the indoor air and the outdoor air of each heat exchanger 4, 6. For this reason, the compressor, indoor fan 7, and outdoor fan 8 are generally operated simultaneously. The indoor heat exchanger 4 and indoor fan 7 are components of the indoor unit 200 installed indoors, and the rest are components of the outdoor unit 100, which corresponds to the heat source device. Note that the heat source device is not limited to the outdoor unit 100 of an air conditioner, but may also be a chiller device that generates hot water or cold water using a heat pump refrigeration cycle.

[0013] The first motor 2 is driven by a first inverter 11 and a second inverter 12. The outdoor fan 8 is driven by a second motor 14, which is a fan motor (FM), via a third inverter 13. For example, the second motor 14, which is a fan motor (FM), is a concentrated winding 4-pole 6-slot motor, similar to the first motor 2 that drives the compressor. A rectifier circuit 17 is connected to a three-phase AC power source 15 via a noise filter 16. A capacitor 18 and the first inverter 11 are connected in parallel to the DC output terminal of the rectifier circuit 17. A reactor 19 is inserted between the positive output terminal of the rectifier circuit 17 and the capacitor 18. The reactor 19 may be disposed upstream of the rectifier circuit 17. A capacitor 20 is connected in parallel to the third inverter 13, and DC power is supplied to the third inverter 13 via the first inverter 11 and the second inverter 12.

[0014] 1 shows in more detail the connection state between the first to third inverters 11 to 13, which correspond to the first to third AC / DC conversion circuits, and the first motor 2 and second motor 14. The rectifier circuit 17 is configured with six diodes 21 connected in a three-phase bridge. This rectifier circuit 17 rectifies the AC voltage of the three-phase AC power supply 15. Each of the inverters 11 to 13 is configured with IGBTs 22, which are switching elements, connected in a three-phase bridge. A freewheel diode 23 is connected in anti-parallel to each IGBT 22.

[0015] The first motor 2 may be a three-phase permanent magnet synchronous motor or an induction motor, but in this embodiment, a permanent magnet synchronous motor is used. The three-phase windings of the first motor 2 are not connected to each other, with both terminals open. That is, the first motor 2 has six winding terminals Ua, Va, Wa, Ub, Vb, and Wb. The phase output terminals of the first inverter 11 are connected to the winding terminals Ua, Va, and Wa of the first motor 2, respectively, and the phase output terminals of the second inverter 12 are connected to the winding terminals Ub, Vb, and Wb of the first motor 2, respectively. A capacitor 20 and a third inverter 13 are connected in parallel to the second inverter 12. The high-voltage side terminal and the low-voltage side terminal of the first inverter 11, i.e., both ends of the capacitor 18, are not connected to the second inverter 12, and the second inverter 12 is independent. That is, the output terminals of the first inverter 11 and the second inverter 12 are connected only via the independent windings of the first motor 2.

[0016] The first motor 2 and the second motor 14 have earth capacitances Ce1 and Ce2, respectively. The common-mode current flowing through these earth capacitances Ce1 and Ce2 is indicated by thick arrows in FIG. 1 . The first motor 2 is inserted in the path from the second motor 14 back to the third inverter 13 via the AC power supply 15. Because the first motor 2 has independent windings for each phase, the inductance of the first motor 2 is inserted in each phase. Because this inductance is inserted in each phase, it also acts as inductance or impedance in the common mode. Therefore, the impedance of the common-mode path from the second motor 14 to the AC power supply 15 and from the first inverter 2 to the third inverter 13 becomes very high, significantly suppressing the common-mode current leaking from the second motor 14.

[0017] Fig. 3 shows a common-mode equivalent circuit obtained by converting the circuit shown in Fig. 1. Fig. 4 shows an example in which the noise filter 16 is configured with a single stage of LC filters, and Fig. 5 shows an example in which the noise filter 16 is configured with two stages of LC filters. The noise filter 16 generally consists of a common-mode choke coil, which is an inductance component inserted in each AC phase, and a capacitor to ground, which is a capacitance component connected from each AC phase to ground.

[0018] Here, the impedance of the common mode choke coil included in the noise filter 16 is Z ＣＭＣ , the impedance of the capacitor to ground is Z ＹＣＡＰＡ , the voltage across the capacitor 18 is V ＤＣ１ , the carrier frequency of the first inverter 11, i.e., the switching frequency, is f ＩＮＶ１ , the ground impedance of the first motor 2 is Z ＭＯＴＯＲ１ , the terminal impedance of the first motor 2 is Z ＭＯＴＯＲ１Ｌ , the voltage across the capacitor 20 is V ＤＣ２ , the switching frequency of the third inverter 13 is f ＩＮＶ３ , the ground impedance of the second motor 14 is Z ＭＯＴＯＲ２ Incidentally, the switching frequency of the second inverter 12 is the same as the switching frequency of the first inverter 11 because the second inverter 12 performs switching operation in cooperation with the first inverter 11. (Z ＹＣＡＰＡ +Z ＭＯＴＯＲ１ ) x V ＤＣ１ ×f ＩＮＶ１ / Z ＣＭＣ >(Z ＹＣＡＰＡ +Z ＭＯＴＯＲ２ ) x V ＤＣ２ ×f ＩＮＶ２ / (Z ＣＭＣ +Z ＭＯＴＯＲ１Ｌ )…(1)

[0019] When measuring EMI noise, such as noise terminal voltage, which indicates the magnitude of common-mode noise, the amount of this noise current leaking into the power supply is observed, and the observed noise amount is determined by the amplitude and frequency of the noise current. In other words, both sides of equation (1) simply calculate the amount of noise leaking into the power supply. The left side of the inequality sign in equation (1) corresponds to the amount of noise from the first motor 2, and the right side of the inequality sign in equation (1) corresponds to the amount of noise from the second motor 2. The left side of the inequality sign in equation (1) is the noise current obtained by dividing the voltage of the first inverter 11, which is the noise source, by the combined impedance of the noise propagation path, multiplied by the switching frequency of the first inverter 11, which is the repetition frequency. Meanwhile, the right side of the inequality sign in equation (1) is the noise current obtained by dividing the voltage of the third inverter 13, which is the noise source, by the combined impedance of the noise propagation path, multiplied by the switching frequency of the third inverter 13, which is the repetition frequency. Therefore, if the formula (1) is satisfied, the common mode noise generated by the third inverter 13 is smaller than the common mode noise generated by the first inverter 11 .

[0020] Here are representative equations for impedance: For inductance (inductive component), Z = 2πfL (2) For capacitance (capacitive component), Z = 1 / (2πfC) (3) where f is frequency, Z is impedance at frequency f, L is inductance at frequency f, and C is capacitance at frequency f.

[0021] The above-mentioned common mode choke coil and inter-terminal impedance of the first motor 2 are mainly inductance components and can be estimated using equation (2). The impedance of the ground capacitor and the ground impedance of the first and second motors 2 and 14 are mainly capacitance components and can be estimated using equation (3).

[0022] Common-mode noise can be simply obtained by the product of the propagation path impedance, that is, the impedance to ground, and the noise source. In equation (1), the smaller the numerator and the larger the denominator of the impedance term, the smaller the common-mode noise. Also, the voltage generated by the inverter is generated from both ends of capacitors 18 and 20, and the common-mode noise is proportional to the voltage amplitude and frequency thereof. By configuring the system to satisfy equation (1), the common-mode noise caused by the third inverter 13 and the second motor 14 can be suppressed to be smaller than the common-mode noise caused by the first inverter 11 and the first motor 2.

[0023] For example, when the inductance of the common-mode choke coil is 1 mH, the capacitance of the capacitor between the terminals and ground is 0.01 μF, the voltage V at both ends of capacitor 18 ＤＣ is 300 V, the switching frequency f ＩＮＶ１ of the first inverter 11 is 5 kHz, the capacitance between the terminals and ground of the first motor 2 is 1 nF, the inductance between the terminals of the first motor 2 is 1 mH, the voltage V at both ends of capacitor 20 ＤＣ２ is 300 V, the switching frequency f ＩＮＶ３ of the third inverter 13 is 15 kHz, the capacitance between the terminals and ground of the second motor 14 is 2 nF, and the specific frequency of the noise terminal voltage is 150 kHz, the values of each impedance are as follows. Z ＣＭＣ = 942 Ω Z ＹＣＡＰＡ = 106 Ω Z ＭＯＴＯＲ１ = 1061 Ω Z ＭＯＴＯＲ１Ｌ = 942 Ω Z ＭＯＴＯＲ２ = 531 Ω As a result, the left side of equation (1) is 1857555 and the right side is 1519818, and equation (1): <1857555> 1519818 holds.

[0024] Further, when calculating the case of a specific frequency of 1 MHz under the same conditions as the case where the frequency f is 150 kHz, the values of each impedance are as follows. Z ＣＭＣ = 6283 Ω Z ＹＣＡＰＡ = 16 Ω Z ＭＯＴＯＲ１ = 159 Ω Z ＭＯＴＯＲ１Ｌ = 6283 Ω Z ＭＯＴＯＲ２=80Ω Here, the left side of equation (1) is 41795 and the right side is 34196, and equation (1): 41795>34196 holds even at the specific frequency of 1 MHz.

[0025] In equation (2), the specific frequency f is multiplied in the calculation formula for impedance Z, and in equation (3), the specific frequency f is in the denominator. Therefore, the inductive impedance Z ＣＭＣ , Z ＭＯＴＯＲ１Ｌ is larger when f = 1 MHz than when f = 150 kHz, and conversely, the capacitive impedance Z ＹＣＡＰＡ , Z ＭＯＴＯＲ１ , Z ＭＯＴＯＲ２ becomes smaller in value.

[0026] This means the following: Even if the third inverter 13 has a switching frequency three times that of the first inverter 11 and the ground capacitance of the second motor 14 is twice that of the first motor 2, the common-mode noise that flows from the third inverter 13 through the ground capacitance of the second motor 14 is sufficiently suppressed because the inter-terminal inductance of the first motor 2, i.e., the impedance of the motor windings, is inserted in series into the common-mode noise path. Therefore, the common-mode noise due to the first inverter 11 is dominantly observed on the AC power supply 15 side. In other words, it becomes possible to design a noise filter that takes into account only the common-mode noise due to the first inverter 11, without considering the common-mode noise due to the third inverter 13, which in particular makes it possible to suppress increases in the size and cost of common-mode chokes.

[0027] As shown in FIG. 5, the noise filter 16 may be configured with two stages of LC filters. The inductance component of the common mode choke used in equation (1) can be considered as the total value of the two stages, and the capacitance of the ground capacitor can also be considered as the total value of the two stages.

[0028] FIG. 6 compares the noise terminal voltages observed by a LISN (Likely a pseudo-insulation network) between a conventional configuration, such as that described in Patent Document 1, in which two sets of inverters and motors are connected in parallel to a rectifier circuit and driven, and the configuration of this embodiment. The first and second motors are the same, and the switching frequency of the second inverter is twice that of the first inverter. In the conventional configuration, common mode noise from the higher switching frequency is dominant. In contrast, in the configuration of this embodiment, the impedance of the first motor 2 is inserted in series, which functions as a common mode choke coil. Therefore, despite the high switching frequency of the second inverter 14, the noise terminal voltage is kept low in the frequency band below approximately 400 kHz, where the noise terminal voltage is high.

[0029] In this embodiment, as disclosed in Japanese Patent No. 6755845, the current flowing through the first motor 2 and the rotation speed are controlled based on the line duty ratios of the first and second inverters 11 and 12 in PWM control, and at the same time, the voltage V of the capacitor 20 connected in parallel to the second inverter 12, which is separated from the voltage across the capacitor 18 that is the DC power source of the first inverter 11, is controlled. ＤＣ２ can be controlled to an arbitrary constant value based on a common duty ratio for all phases of the first and second inverters 11 and 12. ＤＣ２ V ＤＣ1 This allows the second motor 14 to be stably controlled and driven.

[0030] This embodiment is suitable for application to a heat source device such as an outdoor unit 100 of an air conditioner. Generally, compressor motors for air conditioners are enclosed in a metal case, which can be wrapped with soundproofing material, making noise less of a problem. Furthermore, because of their high power consumption, switching frequencies in the relatively low audible frequency range that do not reduce inverter efficiency are often applied. On the other hand, fan motors and the like are placed directly in an outdoor environment, making it difficult to wrap them with soundproofing material, so relatively high switching frequencies above the human audible frequency range are applied to reduce noise.

[0031] Therefore, in the past, because the ground capacitance of the fan motor was large, the common mode noise of the fan motor was more dominant than that of the compressor, and furthermore, the switching frequency of the fan motor was high, which increased the size and cost of countermeasure components such as noise filters.In contrast, with the circuit configuration of this embodiment, where the first motor 2 is used to drive the compressor and the second motor 14 is used as the fan motor, it is possible to achieve noise reduction without increasing the size and cost of countermeasure components such as noise filters.

[0032] Second Embodiment In the second embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and a description thereof will be omitted, and only differences will be described. In the first embodiment, an example was described in which a 4-pole, 6-slot concentrated winding DC brushless motor was used as the second motor 14, which is a fan motor. On the other hand, in the second embodiment, an 8-pole, 12-slot concentrated winding DC brushless slot motor is used for the second motor 14. For this reason, in this second embodiment, while the first motor 2 that drives the compressor is a 4-pole, 6-slot motor, an 8-pole, 12-slot motor with a larger number of pole pairs and slots is used for the second motor 14.

[0033] FIG. 7 shows the ground impedance (Ω) at a specific frequency (f) for the 4-pole, 6-slot motor used in the first embodiment and the 8-pole, 12-slot motor used in the second embodiment. As can be seen from this figure, the ground impedances of these two motors are different. The downward-sloping impedance indicates that both are capacitive components of Equation (3). The actual capacitance is determined by factors such as the winding diameter, the thickness and dielectric constant of the winding sheath, and the thickness and dielectric constant of the insulating layer between the winding and the stator. However, the ground capacitance of a motor generally increases in proportion to the number of pole pairs and the number of stator slots, i.e., the number of teeth on which the windings are wound. For these reasons, an 8-pole, 12-slot motor generates more noise than a 4-pole, 6-slot motor.

[0034] Furthermore, the switching frequency output by the inverter to rotate the motor once increases in proportion to the number of pole pairs, and the number of switching cycles per cycle decreases. A decrease in the number of switching cycles per cycle can lead to poorer controllability and distortion of the current waveform, so increasing the number of pole pairs also requires increasing the inverter's switching frequency. For this reason, an 8-pole, 12-slot motor, which requires a higher switching frequency, tends to produce even worse noise than a 4-pole, 6-slot motor.

[0035] However, in the second embodiment, an 8-pole, 12-slot motor having a large number of pole pairs and a large number of stator slots is used as the second motor 14, and even if this motor is driven at a high switching frequency by the third inverter 13, the amount of noise generated itself can be kept low, making it easier to deal with noise.

[0036] That is, in a motor drive system that drives two motors, a motor that generates a substantially large amount of common-mode noise as a standalone characteristic is used as the second motor 14, and a motor that generates a substantially small amount of common-mode noise is used as the first motor 2. This makes it possible to reduce the volume and cost of the noise filter 16. Here, "substantially large" means that the amount of noise generated is dominant when evaluated in the frequency range of the noise terminal voltage that is the target of suppression.

[0037] In particular, outdoor heat exchange fans for heat source devices such as the outdoor unit 100 often have low rotational speeds and greater radial freedom than thickness, so motors with a large number of poles are often used. Larger heat source devices often use larger fan motors and metal frames to ensure heat dissipation. In these cases, the ground capacitance tends to be larger than that of compressors using motors with fewer pole pairs. In conventional configurations, common-mode noise originating from the fan motor becomes more dominant. However, by adopting the configuration of the second embodiment, the volume and cost of the noise filter 16 inserted between the AC power supply 15 and the first inverter 11 can be reduced. In other words, by using the motor with the larger ground capacitance as the second motor 14 and the motor with the smaller ground capacitance as the first motor 2 in a motor drive system, the volume and cost of the noise filter 16 can be reduced.

[0038] (Other Embodiments) The switching frequency of the third inverter does not necessarily have to be higher than the switching frequencies of the first and second inverters. Furthermore, the number of pole pairs of the second motor does not necessarily have to be greater than the number of pole pairs of the first motor. The switching elements are not limited to IGBTs. In each of the above embodiments, the first inverter 11 and the second inverter 12 do not share a common power supply but are separated. This prevents unnecessary zero-axis current from being generated when driving the first motor 2, enabling the first motor 2 to be driven efficiently.

[0039] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0040] In the drawings, 1 indicates a heat pump system, 2 indicates a first motor (compressor motor), 6 indicates an outdoor heat exchanger (heat exchanger), 8 indicates an outdoor fan (fan), 11 to 13 indicate first to third inverters, 14 indicates a second motor (fan motor), 16 indicates a noise filter, 17 indicates a rectifier circuit, 18 and 20 indicate condensers, and 100 indicates an outdoor unit (heat source device).

Claims

1. A motor drive system comprising: a rectifier circuit which rectifies an AC voltage supplied from an AC power source; a noise filter arranged between the AC power source and the rectifier circuit; a first DC / AC conversion circuit composed of a plurality of semiconductor switches and connected to the output side of the rectifier circuit; a first motor having one end of a winding, each phase of which is independent, connected to an AC output terminal of the first DC / AC conversion circuit; a second DC / AC conversion circuit composed of a plurality of semiconductor switches and having the other end of the winding connected to an AC output terminal of the second DC / AC conversion circuit; a capacitor connected in parallel to the second DC / AC conversion circuit; 2. The noise filter has a common mode choke coil and a plurality of capacitors to ground, and the impedance of the common mode choke is set to Z ＣＭＣ , the impedance of the capacitor to ground is Z ＹＣＡＰＡ , the voltage across the first capacitor is V ＤＣ１ , the switching frequency of the first DC / AC conversion circuit is f ＩＮＶ１ , the impedance to ground of the first motor is Z ＭＯＴＯＲ１ , the terminal impedance of the first motor is Z ＭＯＴＯＲ１Ｌ , the voltage across the second capacitor is V ＤＣ２ , the switching frequency of the third DC / AC conversion circuit is f ＩＮＶ３ , the impedance to ground of the second motor is Z ＭＯＴＯＲ２ Then, the following equation (Z ＹＣＡＰＡ +Z ＭＯＴＯＲ１ ) x V ＤＣ１ ×f ＩＮＶ１ / Z ＣＭＣ >(Z ＹＣＡＰＡ +Z ＭＯＴＯＲ２ ) x V ＤＣ２ ×f ＩＮＶ２ / (Z ＣＭＣ +Z ＭＯＴＯＲ１Ｌ 2. The motor drive system according to claim 1, which satisfies the following:

3. A motor drive system as claimed in claim 1, wherein a switching frequency for driving said plurality of switch elements in said third DC / AC conversion circuit is higher than a switching frequency for driving said plurality of switch elements in said first and second DC / AC conversion circuits.

4. The motor drive system according to claim 1, wherein the number of pole pairs of said second motor is greater than the number of pole pairs of said first motor.

5. The motor drive system according to claim 1, wherein the number of slots of said second motor is greater than the number of slots of said first motor.

6. The motor drive system according to claim 1, wherein said first and second DC / AC conversion circuits control the voltage across said capacitor to a predetermined value while driving said first motor.

7. The motor drive system according to claim 1, wherein the second motor, as a standalone characteristic, generates a substantially larger amount of common mode noise than the first motor.

8. The motor drive system according to claim 1, wherein said second motor has a larger ground capacitance than said first motor.

9. A heat source device comprising: a motor drive system according to any one of claims 1 to 8; a compressor; a heat exchanger; and a fan for blowing air to the heat exchanger, wherein the compressor is driven by the first motor and the fan is driven by the second motor.

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