ENGINE DRIVE SYSTEM AND HEAT SOURCE DEVICE

Abstract

A motor drive system of one embodiment comprises a rectifier circuit that rectifies alternating voltage supplied by an AC power supply, a noise filter arranged between the AC power supply and the rectifier circuit, a first DC-AC conversion circuit comprising a plurality of semiconductor switches and connected to an output side of the rectifier circuit, a first motor in which one end of a winding, in which each phase is independent, is connected to an AC output terminal of the first DC-AC conversion circuit, a second DC-AC conversion circuit comprising a plurality of semiconductor switches and having an AC output to which the other end of the winding is connected, a capacitor connected in parallel to the second DC-AC conversion circuit, and a third DC-AC conversion circuit.which includes a large number of semiconductor switches connected in parallel to the capacitor, and a second motor connected to an AC output of the third DC-to-AC conversion circuit.

Description

Technical field

[0001] Embodiments of the present invention relate to a system that drives two motors simultaneously and a heat source device equipped with this system. State of the art

[0002] Previously, a system existed that drove two or more motors simultaneously via an inverter using a standard AC power supply as the power source. For example, in an outdoor air conditioning system described in patent literature 1, inverters provide variable speed control for each motor, driving a compressor in a cooling circuit and an outdoor fan motor that directs air to an outdoor heat exchanger. In this configuration, the AC power supply is rectified by a 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 such a circuit, where a motor is driven by an inverter, a higher level of harmonic noise, i.e., common-mode noise, is generated. When the compressor and the outdoor fan are driven simultaneously, the common-mode noise present in the system described above is a combination of the noise present in the inverter for the compressor motor and the noise present in the inverter for the outdoor fan motor. In a given noise frequency band, the inverter noise on the side with the higher signal level flows to the power supply side. Citation list for patent literature

[0004] Patent literature 1: Japanese patent no. 6957756 Summary of the invention: Technical problem

[0005] In general, inverters for outdoor fan motors have a higher carrier frequency, i.e., a higher switching frequency of an inverter switching element, in PWM control. Therefore, the common-mode noise emanating from fan-driven inverters is generally greater than the common-mode noise from inverters used for compressor motors. In other words, the higher harmonic noise of fan-driven inverters is dominant. Therefore, a noise filter is placed before the rectifier circuit to suppress such common-mode interference. To reduce common-mode interference to an acceptable level, the noise filter requires a larger inductor and capacitor, as the increased common-mode interference necessitates larger dimensions and costs.

[0006] Thus, a motor drive system capable of driving two motors simultaneously, while suppressing the enlargement and cost increase of a noise filter, and a heat source device containing the system are provided. Solution to the problem

[0007] A motor drive system of one embodiment comprises a rectifier circuit that rectifies alternating voltage supplied by an AC power supply, a noise filter arranged between the AC power supply and the rectifier circuit, a first DC-AC conversion circuit comprising a plurality of semiconductor switches and connected to an output side of the rectifier circuit, a first motor in which one end of windings, in which each phase is independent, is connected to an AC output terminal of the first DC-AC conversion circuit, a second DC-AC conversion circuit comprising a plurality of semiconductor switches and having an AC output to which the other end of the windings is connected, and a capacitor connected in parallel to the second DC-AC conversion circuit.a third DC-AC conversion circuit comprising a plurality of semiconductor switches connected in parallel to the capacitor, and a second motor connected to an AC output of the third DC-AC conversion circuit.

[0008] Furthermore, a heat source of an embodiment comprises the motor drive system of the embodiment, a compression machine, a heat exchanger and a fan that directs air to the heat exchanger, wherein the compression machine is driven by the first motor and the fan is driven by the second motor. Brief description of the drawings [ Fig. 1] Fig. Figure 1 is a diagram of a first embodiment showing a configuration of a motor drive system. [ Fig. 2] Fig. Figure 2 is a diagram showing a configuration of an air conditioner to which the motor drive system is applied, with the focus on the side of an outdoor unit. [ Fig. ] Fig. is a diagram in which a Fig. The circuit shown has been converted into a common-mode equivalent circuit. [ Fig. ] Fig. is a diagram (No. 1) that shows a specific configuration example of a noise filter. [ Fig. ] Fig. is a diagram (No. 2) that shows a specific configuration example of the noise filter. [ Fig. ] Fig. is a diagram showing the noise terminal voltage characteristics of a conventional configuration and a configuration of the present embodiment. [ Fig. ] Fig. Figure 1 is a diagram of a second embodiment showing the mass impedance characteristics of a motor with 4 poles and 6 slots and a motor with 8 poles and 12 slots. Description of the embodiments (First embodiment)

[0009] Fig. Figure 1 shows an air conditioning system configuration as an example of the application of a motor drive system of the present embodiment, with the center of gravity on the side of an outdoor unit 100 located outdoors. A compressor machine forming a heat pump system 1 is, for example, of the rotary type and is represented only by a symbol for the compressor motor CM. This is considered the first motor 2. The first motor 2 is, for example, a brushless DC motor with an embedded magnet and a concentrated winding with 4 poles and 6 slots, and is an open-winding motor in which the winding of each phase is independent and all winding ends are open.The first motor 2, representing the compression machine, a four-way valve 3, an internal heat exchanger 4, a PMV 5, representing a decompression device, and an external heat exchanger 6 are connected sequentially by a pipe, representing a refrigerant flow path, to form a heat pump cooling circuit, which is a closed loop. The air conditioning system is formed by the heat pump system 1 described above.

[0010] During heating, a high-temperature refrigerant, compressed by the compression engine using the first engine 2, is routed from the four-way valve 3 to the internal heat exchanger 4 to be condensed, then decompressed by the PMV 5 and cooled to a low temperature before flowing to the external heat exchanger 6, where it evaporates and returns to the compression engine. During cooling, the high-temperature refrigerant, compressed by the compression engine, is routed from the four-way valve 3 to the external heat exchanger 6, where it condenses, then decompressed by the PMV 5 and cooled to a low temperature before flowing to the internal heat exchanger 4, where it evaporates and returns to the compression engine.

[0011] Air is supplied to the indoor and outdoor heat exchangers 4 and 6, respectively, by an indoor fan 7 and an outdoor fan 8, thus enabling efficient heat exchange between each heat exchanger 4 and 6 and the indoor and outdoor air. The compressor, indoor fan 7, and outdoor fan 8 are therefore operated simultaneously. The indoor heat exchanger 4 and the indoor fan 7 are configurations on the side of an indoor unit 200, which is located indoors, while the others are configurations on the side of the outdoor unit 100, which corresponds to a heat source. It should be noted that the heat source is not necessarily limited to the outdoor unit 100 of the air conditioner, but can also be a cooling device that generates hot or cold water using a heat pump cooling circuit.

[0012] The first motor 2 is driven by a first inverter 11 and a second inverter 12. The external fan 8 is driven by a second motor 14, a fan motor (FM), via a third inverter 13. As in the case of the first motor 2, which drives the compressor, the second motor 14, the fan motor (FM), is also a centroidal winding motor with 4 poles and 6 slots. A rectifier circuit 17 is connected to a three-phase AC power supply 15 via a noise filter 16. A capacitor 18 and the first inverter 11 are connected in parallel to a DC output terminal of the rectifier circuit 17. An inductor 19 is inserted between a positive output terminal of the rectifier circuit 17 and the capacitor 18. Note that the inductor 19 can be located between the noise filter 16 and the rectifier circuit 17.A capacitor 20 is connected in parallel to the third inverter 13, and direct current is supplied via the first inverter 11 and the second inverter 12.

[0013] Fig. Figure 1 shows in more detail a connection state of the first to third inverters 11-13 corresponding to the first to third DC-AC conversion circuit with the first motor 2 and the second motor 14. The rectifier circuit 17 is formed by a three-phase bridge circuit of six diodes 21. This rectifier circuit 17 rectifies the AC voltage of the three-phase AC power supply 15. Each of the inverters 11-13 is formed by a three-phase bridge circuit of IGBTs 22, which are switching elements. A freewheeling diode 23 is connected antiparallel to each IGBT 22.

[0014] The first motor 2 is assumed to be a three-phase permanent magnet synchronous motor, an induction machine, or the like, with the permanent magnet synchronous motor being used in the present embodiment. The three-phase windings of the first motor 2 are not connected to each other, and both terminals are in an open state. That is, the first motor 2 comprises six winding terminals Ua, Va, Wa, Ub, Vb, Wb. The output terminals of each phase of the first inverter 11 are each connected to the winding terminals Ua, Va, Wa of the first motor 2, and the output terminals of each phase of the second inverter 12 are each connected to the winding terminals Ub, Vb, Wb of the first motor 2. The capacitor 20 and the third inverter 13 are connected in parallel to the second inverter 12. A high-pressure side terminal and a low-pressure side terminal of the first inverter 11, i.e.,Both ends of 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 each connected only via the windings in which each phase is independent of the first motor 2.

[0015] The first motor 2 and the second motor 14 each have a ground capacitance Ce1, Ce2. The common-mode current flowing through each of the ground capacitances Ce1, Ce2 is in Fig. represented by an arrow made of a thick line.

[0016] The first motor 2 is integrated into a path that runs from the second motor, via the AC power supply 15, back to the third inverter 13. Each phase has an independent winding in the first motor 2, and thus the inductance of the first motor 2 is inserted into each phase. Since the inductance is inserted into each phase, it also acts as an inductance or impedance in common-mode operation. Therefore, the impedance of a common-mode path from the second motor 14 to the AC power supply 15 and from the first inverter 11 to the third inverter 13 becomes extremely high, and the common-mode current emanating from the second motor 14 is largely suppressed.

[0017] Fig. is a diagram in which the in Fig. The circuit shown has been converted into a common-mode equivalent circuit. Fig. shows an example of the formation of the noise filter 16 with one stage of an LC filter, and Fig. Figure 16 shows an example of the construction of the noise filter 16 with two stages of LC filters. The noise filter 16 is generally formed with a common-mode choke, which is an inductance component inserted into each phase of the alternating current, and a capacitor to ground, which is a capacitance component connected from each phase of the alternating current to ground.

[0018] In this context, the impedance of the common-mode choke coil contained in the noise filter 16 is given by Z CMC denoted as Z the impedance of the capacitor to ground YCAPA , the voltage at both ends of capacitor 18 with V DC1 , the carrier frequency, i.e., the switching frequency of the first inverter 11, with f INV1 , the mass impedance of the first motor 2 with Z MOTOR1 , the impedance between the terminals of the first motor 2 with Z MOTOR1L , the voltage at both ends of the capacitor 20 with V DC2, the switching frequency of the third inverter 13 with f INV3 and the mass impedance of the second motor 14 with Z MOTOR2 In this context, the switching frequency of the second inverter 12 is the same as the switching frequency of the first inverter 11, since its switching process is carried out in cooperation with the first inverter 11. (ZYCAPA+ZMOTOR1)×VDC1×fINV1 / ZCMC>(ZYCAPA+ZMOTOR2)×VDC2×fINV2 / (ZCMC+ZMOTOR1L)

[0019] At the time of measuring EMI noise, such as the noise gate voltage, which represents the magnitude of the common-mode noise, the amount of noise current flowing to the power supply is observed, and the amount of observed noise is determined by an amplitude and a frequency contained in the noise current. That is, both sides of formula (1) simply calculate the amount of noise exiting to the power supply side. The left side of the inequality sign in this formula (1) corresponds to the noise amount of the first motor 2, and the right side of the inequality sign in formula (1) corresponds to the noise amount of the second motor 14.The left side of the inequality sign in formula (1) is a formula in which 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, is multiplied by a switching frequency of the first inverter 11, which is a repetition frequency. The right side of the inequality sign in formula (1), on the other hand, is a formula in which the noise current obtained by dividing the voltage of the third inverter 13, which is also the noise source, by the combined impedance of the noise propagation path, is multiplied by a switching frequency of the third inverter 13, which is a repetition frequency. By satisfying formula (1), the common-mode noise caused by the third inverter 13 becomes smaller than the common-mode noise caused by the first inverter 11.

[0020] In this context, a representative formula for impedance is shown. Regarding inductance (an inductive component): Z=2πfL Regarding capacity (a capacitive component): Z=1 / (2πfC) f is a frequency, Z is the impedance at frequency f, L is the inductance at frequency f, and C is the capacitance at frequency f.

[0021] The impedance of the aforementioned common-mode choke coil, or between the terminals of the first motor 2, is primarily an inductance component and can be estimated using formula (2). Furthermore, the impedance of the capacitor to ground, or the ground impedance of the first and second motors 2 and 14, is primarily a capacitance component and can be estimated using formula (3).

[0022] Common-mode noise is simply calculated as the product of the propagation path impedance (i.e., the ground impedance) and the noise source. In equation (1), with respect to the impedance term, the common-mode noise decreases as the numerator becomes smaller and the denominator larger. Furthermore, the voltage generated in the inverters is applied to both ends of capacitors 18 and 20, and the common-mode noise is proportional to the voltage amplitude and frequency. By configuring the system according to equation (1), the common-mode noise from the third inverter 13 and the second motor 14 can be suppressed to a level smaller than the common-mode noise from the first inverter 11 and the first motor 2.

[0023] For example, if the inductance of the common-mode choke is 1 mH, the capacitance of the capacitor to ground is 0.01 µF, and the voltage V DCAt both ends of the capacitor 18 is 300 V, the switching frequency f INV1 of the first inverter 11 is 5 kHz, the mass capacitance of the first motor 2 is 1 nF, the inductance between the terminals of the first motor 2 is 1 mH, the voltage V DC2 at both ends of the capacitor 20 is 300 V, the switching frequency f INV3 of the third inverter 13 is 15 kHz, the mass capacitance of the second motor 14 is 2 nF and the specific frequency of the noise connection voltage is 150 kHz, then the values of the individual impedances are as follows. ZCMC = 942 Ω ZYCAPA = 106 Ω ZMOTOR1 = 1061 Ω ZMOTOR1L = 942 Ω ZMOTOR2 = 531 Ω

[0024] As a result, in formula (1) the left side becomes 1857555 and the right side becomes 1519818, and formula (1): <1857555> 1519818 is true.

[0025] Furthermore, if one calculates the case where the specific frequency is 1 MHz under the same conditions as in the case described above with a frequency f of 150 kHz, the values ​​of the individual impedances are as follows. ZCMC=6283 Ω ZYCAPA=16 Ω ZMOTOR1=159 Ω ZMOTOR1L=6283 Ω ZMOTOR2=80 Ω

[0026] In this context, in formula (1) the left side becomes 41795 and the right side becomes 34196, and formula (1): 41795 > 34196 also becomes true in the case of a specific frequency of 1 MHz.

[0027] It should be noted that in formula (2) the specific frequency f is multiplied in the formula for calculating the impedance Z, and in formula (3) the specific frequency f is in the denominator. Thus, the impedance Z CMC , Z MOTOR1L, which maps the inductance properties, is larger in the case of f = 1 MHz than in the case of f = 150 kHz, and conversely, the values ​​of the impedance Z YCAPA , Z MOTOR1 , Z MOTOR2 , which represent the capacity characteristics, smaller.

[0028] 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 flowing through the ground capacitance of the second motor 14 from the third inverter 13 is sufficiently suppressed. This means that even if the inductance between the terminals of the first motor 2, i.e., the impedance of the motor winding, is in series with the common-mode noise phasor, the common-mode noise flowing through the ground capacitance of the second motor 14 is significantly reduced. Consequently, on the AC supply side 15, the common-mode noise is predominantly due to the first inverter 11.This means that it is possible to design a noise filter by considering only the common-mode noise due to the first inverter 11 and not the common-mode noise due to the third inverter 13, and in particular the increase in size and cost of the common-mode choke can be suppressed.

[0029] It should be noted that, as in Fig. 5 shown that when the noise filter 16 is designed with two stages of LC filters, the inductance component of the common-mode choke used in formula (1) can be considered as the total value of the two stages, and likewise the capacitance of the capacitor to ground can be considered as the total value of the two stages.

[0030] Fig. Figure 1 is a diagram comparing the noise terminal voltage observed with LISN (pseudo-power supply network) for a drive configuration consisting of two pairs of inverters and motors connected in parallel with a rectifier circuit, as described in patent literature 1 as a conventional configuration, and the configuration of the present embodiment. The first and second motors are identical, and the switching frequency of the second inverter is twice that of the first. In the conventional configuration, common-mode noise dominates at the higher switching frequency.On the other hand, in the configuration of the present embodiment, the impedance of the first motor 2, being connected in series, acts as a common-mode choke coil, so that regardless of the fact that the switching frequency of the second inverter 12 is high, the interference voltage at the terminal is suppressed in a band of generally 400 kHz or less where the interference voltage at the terminal becomes larger.

[0031] Furthermore, in the present embodiment, as disclosed in Japanese patent application No. 6755845, the forward voltage current and the speed in the first motor 2 are controlled based on a duty cycle (Line Duty Ratio) of each of the first and second inverters 11, 12 in the PWM control and simultaneously the voltage V DC2The voltage W of the capacitor 20 connected in parallel to the second inverter 12 is separated from the voltage between the two ends of the capacitor 18, which represents the DC supply of the first inverter 11, and can be regulated to any constant value based on a common duty cycle for all phases of the first and second inverters 11, 12. For example, the voltage V DC2 regulated to a voltage value that is essentially that of V DC1 This corresponds to the operation of the second motor 14. In this way, the drive of the second motor 14 can be stably controlled.

[0032] The present embodiment is suitable for use in a heat source device, such as the outdoor unit 100 of an air conditioner. Generally, an air conditioner compressor motor is covered with a metal housing, and since it can also be wrapped with sound-absorbing material, etc., noise is less likely to become a problem. Because the power consumption is high, a switching frequency in the relatively low audio frequency range is often used to prevent a decrease in inverter efficiency. In contrast, it is difficult to encase fan motors and similar motors with sound-absorbing material because they are located directly in the outdoor air environment. Therefore, a relatively high switching frequency in the human audio frequency range or above is used to reduce noise.

[0033] Since the mass capacitance of a fan motor is large, the common-mode noise of the fan motor conventionally dominates over that of the compression engine, and since the switching frequency of the fan motor is high, the size and cost of countermeasure components such as noise filters increase. In contrast, by using the first motor 2 to drive the compression engine and the second motor 14 for the fan motor in a circuit configuration as in the present embodiment, it is possible to achieve noise reduction without increasing the size or cost of a countermeasure component, such as a noise filter. (Second embodiment)

[0034] In the following, as a second embodiment, parts identical to those in the first embodiment are designated with the same reference numerals, descriptions are omitted, and the differing parts are described. In the first embodiment, descriptions are given using an example of the use of a 4-pole, 6-slot concentrated winding DC brushless motor as the second motor 14, which is a fan motor. In the second embodiment, however, a 8-pole, 12-slot concentrated winding DC brushless motor is used for the second motor 14. Thus, in the second embodiment, the first motor 2 for driving the compression machine is the 4-pole, 6-slot motor, while the second motor 14 uses the 8-pole, 12-slot motor, which has a larger number of pole pairs and a larger number of slots.

[0035] Fig.The diagram shows the ground impedance (Ω) at the specific frequency (f) of the 4-pole, 6-slot motor used as the second motor in the first embodiment, and the 8-pole, 12-slot motor used in the second embodiment. As can be seen from this diagram, the ground impedance differs between these two motor types. The impedance, which exhibits a decreasing characteristic at higher frequencies, indicates that both are capacitance components of formula (3). An actual capacitance component is determined by the winding diameter, the thickness of the winding coating and its dielectric constant, the thickness of the insulating layer between the winding and the stator and its dielectric constant, etc., while the ground capacitance of a motor is generally proportional to the number of pole pairs of the motor and / or the number of slots in a stator, i.e.,The noise level increases with the number of teeth on which a winding is wound. From the points mentioned above, it follows that the amount of noise generated is greater when using a motor with 8 poles and 12 slots than when using a motor with 4 poles and 6 slots.

[0036] Furthermore, the switching frequency output by an inverter, which causes one revolution of a motor, increases proportionally to the number of pole pairs, while the number of switching operations in a cycle decreases. As the number of switching operations per cycle decreases, controllability deteriorates, the current waveform becomes distorted, and other issues arise. Therefore, the inverter's switching frequency must also be increased when the number of pole pairs is increased. This is another reason why the likelihood of noise deterioration is greater for an 8-pole, 12-slot motor, which requires an increased switching frequency, than for a 4-pole, 6-slot motor.

[0037] In the second embodiment, however, even when the 8-pole, 12-slot motor, which has a large number of pole pairs and a large number of stator slots, is used as the second motor 14 and this motor is driven at a high switching frequency by the third inverter 13, it is easy to deal with the noise, since the amount of generated noise itself can be kept low.

[0038] This means that in a motor drive system that drives two motors, a motor that, as a single-body characteristic, generates a substantially large amount of common-mode noise is used as the second motor 14, and a motor that has a substantially small amount of common-mode noise is used as the first motor 2. In this way, a reduction in the size and cost of the noise filter 16 can be achieved. In this context, "substantially large" means that the amount of noise generated at the time of evaluation is superior to the noise voltage at the terminal to be suppressed in a frequency range.

[0039] Since an external heat exchanger fan of a heat source device such as the outdoor unit 100 has a low rotational speed, and the degrees of freedom in the radial direction are often higher than in the stack thickness direction of the motor, a motor with a large number of poles is frequently used. With a large heat source, the capacitance of a fan motor used is high, and a metal frame is often employed to ensure heat dissipation characteristics. The mass capacitance in this case tends to be larger than in a compression machine, which uses a motor with a small number of pole pairs, and in the conventional configuration, the common-mode noise caused by the fan motor is more dominant. However, by using the configuration of the second embodiment, it is possible to reduce the volume and cost of the noise filter 16, which is inserted between the AC power supply 15 and the first inverter 11.In other words, by using a motor with a larger mass capacity as the second motor 14 and a motor with a smaller mass capacity as the first motor 2 in the motor drive system, it is possible to reduce the volume and cost of the noise filter 16. (Other embodiments)

[0040] The switching frequency of the third inverter does not necessarily have to be set higher than the switching frequencies of the first and second inverters.

[0041] Furthermore, the number of pole pairs of the second motor does not need to be set higher than the number of pole pairs of the first motor.

[0042] The switching element is not limited to an IGBT (Insulated Gate Bipolar Transistor).

[0043] In each of the above-mentioned embodiments, the first motor 2 can be driven efficiently because the first inverter 11 and the second inverter 12 do not share a common power supply and the power supplies are separate, so that no unnecessary zero-axis current is generated when driving the first motor 2.

[0044] Although some embodiments of the present invention have been described, these embodiments serve only as examples, and the scope of the invention is not intended to be limited. These novel embodiments can be carried out in various other modes, and various omissions, substitutions, and modifications can be made without departing from the core of the invention. These embodiments and modifications are included in the scope and core of the invention and fall within the scope of the invention as described in the claims and their equivalents. List of reference symbols

[0045] In the drawings, 1 represents a heat pump system, 2 a first motor (motor for a compression machine), 6 an outdoor heat exchanger, 8 an outdoor fan, 11 to 13 a first to third inverter, 14 a second motor (fan motor), 16 a noise filter, 17 a rectifier circuit, 18 and 20 capacitors and 100 an outdoor unit (heat source). QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 6957756

[0004]

Claims

[1] A motor drive system comprising the following: a rectifier circuit that rectifies alternating voltage supplied by an AC power supply; a noise filter that is placed between the AC power supply and the rectifier circuit; a first DC-AC conversion circuit consisting of several semiconductor switches and connected to an output side of the rectifier circuit; a first motor in which one end of a winding in which each phase is independent is connected to an AC output terminal of the first DC-to-AC conversion circuit; a second DC-AC conversion circuit comprising a plurality of semiconductor switches and having an AC output terminal to which another end of the winding is connected; a capacitor connected in parallel to the second DC-AC conversion circuit; a third DC-AC conversion circuit comprising a plurality of semiconductor switches and connected in parallel to the capacitor; and a second motor connected to an AC output terminal of the third DC-to-AC conversion circuit. [2] The motor drive system according to claim 1, wherein the noise filter has a common-mode choke and several capacitors for grounding, and when the impedance of the common-mode choke coil is Z CMC is referred to as the impedance of the capacitors to grounding with Z YCAPA The voltage at both ends of the first capacitor is denoted by V. DC1 The switching frequency of the first DC-AC conversion circuit is denoted by f. INV1The earth impedance of the first motor is designated as Z. MOTOR1 The impedance between the terminals of the first motor is denoted by Z. MOTOR1L The voltage at both ends of the second capacitor is designated as V. DC2 , a switching frequency of the third DC-AC conversion circuit with f INV3 and the earth impedance of the second motor with Z MOTOR2 When denoted, the following formula applies: (ZYCAPA+ZMOTOR1)×VDC1×fINV1 / ZCMC>(ZYCAPA+ZMOTOR2)×VDC2×fINV2 / (ZCMC+ZMOTOR1L). [3] The motor drive system according to claim 1, wherein a switching frequency for controlling the multiple switching elements in the third DC-AC conversion circuit is higher than a switching frequency for controlling the multiple switching elements in the first and the second DC-AC conversion circuit. [4] The motor drive system according to claim 1, wherein the number of pole pairs of the second motor is greater than the number of pole pairs of the first motor. [5] The motor drive system according to claim 1, wherein a number of slots of the second motor is larger than a number of slots of the first motor. [6] The motor drive system according to claim 1, wherein the first and the second DC-AC conversion circuit regulate the voltage between the two ends of the capacitor to a predetermined value while driving the first motor. [7] The motor drive system according to claim 1, wherein the second motor has a substantially larger amount of common-mode noise generated as its single-body characteristic than the first motor. [8] The motor drive system according to claim 1, wherein the second motor has a larger earth capacity than the first motor. [9] A heat source comprising the following: a motor drive system according to any one of claims 1 to 8; a compression machine; a heat exchanger; and a fan that directs air to the heat exchanger, the compression machine is driven by the first motor and the fan is driven by the second motor.

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