JP2024068733A - Motor control device and electric vehicle - Google Patents

Abstract

[Problem] To provide a motor control device that randomly selects a plurality of carrier frequency candidates to distribute harmonic components, and that can suppress electromagnetic noise by widening the distribution of harmonic components while suppressing the amount of heat generated by the switching elements of the inverter. [Solution] A motor control device that controls an inverter that supplies AC power to an AC motor, comprising: a carrier frequency generation unit that selects each of a plurality of carrier frequency candidates at a predetermined ratio based on a random number and outputs it as a carrier frequency, a carrier wave generation unit that generates a carrier wave based on the selected carrier frequency, and a PWM controller that outputs a PWM signal based on the carrier wave to the inverter, characterized in that the predetermined ratio is determined based on the frequencies of the plurality of carrier frequency candidates and a predetermined maximum average switching frequency. [Selected Figure] Figure 2

Description

The present invention relates to a motor control device that suppresses electromagnetic noise and harmonics generated by a motor drive inverter, and an electric vehicle equipped with the motor control device.

The PWM waveform generating device of Patent Document 1 is known as a conventional technique for suppressing electromagnetic noise and harmonics generated by a motor drive inverter by dispersing the carrier frequency. For example, the abstract of Patent Document 1 states that the objective is to "provide a PWM waveform generating device that can suppress particularly harsh carrier sounds (applicant's note: according to paragraph 0009 of the document, synonymous with "electromagnetic noise") in a PWM-controlled inverter device mounted on an air conditioner without excessively increasing the carrier frequency, thereby equivalently reducing the noise level of the air conditioner."

Furthermore, claim 5 of the same document states, "The PWM waveform generating device according to claim 2, characterized in that the second carrier frequency switching means comprises a random order setting means for randomly setting a selection order for the plurality of carrier frequencies of the carrier frequency setting means, and a fifth carrier frequency switching means for selecting and switching a carrier frequency at an appropriate timing from among the plurality of carrier frequencies of the carrier frequency setting means in accordance with the random selection order set by the random order setting means."

Thus, Patent Document 1 proposes a PWM waveform generation method that disperses harmonic components and reduces electromagnetic noise by sequentially selecting one of multiple carrier frequency candidates according to a selection frequency that is weighted in a random order at random timing.

Japanese Patent Application Laid-Open No. 9-47026

The amount of heat generated by the switching elements of the inverter is roughly determined according to the temporal average value f _ave of the carrier frequency, but Patent Document 1 makes no mention whatsoever of the technical idea of suppressing the amount of heat generated by the switching elements by controlling the magnitude of the average value f _ave . Therefore, as a result of the carrier frequency dispersion in Patent Document 1, it is conceivable that inappropriate motor control will result from the viewpoint of suppressing the amount of heat generated by the switching elements, and in such a case, it is not possible to eliminate the possibility that excessive heat generated by the switching elements will cause a breakdown of the inverter.

In addition, under the condition that there are a plurality of carrier frequency candidates, a method of uniformly reducing the frequency of each carrier frequency candidate can be considered as a method of suppressing the heat generation amount of the switching element. However, since it is necessary to set each carrier frequency candidate under the constraints shown in FIG. 1, it is difficult to uniformly reduce the frequency of each carrier frequency candidate. That is, since the upper limit of the maximum value f _max of the carrier frequency is determined according to the processing performance of the motor control device (especially the built-in microcomputer), and the lower limit of the minimum value f _min of the carrier frequency is determined according to the response speed required in the specifications of the motor control device, each carrier frequency candidate needs to be set within the range from the minimum value f _min to the maximum value f _max . Therefore, in order to reduce the average value f _ave of the carrier frequency that affects the heat generation amount of the switching element, it is effective to concentrate and arrange each carrier frequency candidate near the minimum value f _min of the carrier frequency, but in that case, the dispersion of the harmonic components becomes insufficient, and there is a possibility that the electromagnetic noise increases.

The present invention aims to provide a motor control device that randomly selects multiple carrier frequency candidates to distribute harmonic components, and that can suppress electromagnetic noise by widening the distribution of harmonic components while suppressing the amount of heat generated by the inverter switching elements.

A motor control device that controls an inverter that supplies AC power to an AC motor, comprising: a carrier frequency generation unit that selects each of a plurality of carrier frequency candidates at a predetermined ratio based on a random number and outputs the selected carrier frequency as the carrier frequency; a carrier wave generation unit that generates a carrier wave based on the selected carrier frequency; and a PWM controller that outputs a PWM signal based on the carrier wave to the inverter, the predetermined ratio being determined based on the frequencies of the plurality of carrier frequency candidates and a predetermined maximum average switching frequency.

The motor control device of the present invention can suppress the amount of heat generated by the inverter switching elements while widening the distribution of harmonic components to suppress electromagnetic noise.

1 is a graph illustrating restrictions on carrier frequencies. FIG. 1 is a functional block diagram showing an overall configuration of a motor drive system according to a first embodiment. FIG. 2 is a functional block diagram illustrating a configuration of a carrier frequency generating unit according to the first embodiment. FIG. 11 is a configuration diagram of a drive system of an electric vehicle according to a second embodiment.

The following describes an embodiment of the motor drive device of the present invention using the drawings.

First, the motor control device 1 according to the first embodiment of the present invention will be described with reference to Figures 2 and 3.

Figure 2 is a functional block diagram showing the overall configuration of the motor drive system 100 of this embodiment. As shown here, the motor drive system 100 includes a motor control device 1, a motor 2, an inverter 3, a DC power supply 4, a DC voltage detector 5, a phase current detector 6, and a magnetic pole position detector 7. Below, the configuration of the motor drive system 100 other than the motor control device 1 will be outlined one by one, and then the motor control device 1 will be described in detail.

Motor 2 is a permanent magnet synchronous motor (PMSM) driven by AC power. Note that the effects of the present invention described below can also be obtained when using an AC machine such as a synchronous reluctance motor, a permanent magnet synchronous generator, a wound-type synchronous machine, an induction motor, or an induction generator instead of a permanent magnet synchronous motor.

The inverter 3 is a power conversion device that converts DC power to AC power in accordance with a gate signal (PWM signal) from the motor control device 1 and supplies it to the motor 2. The inverter 3 uses multiple semiconductor switching elements (IGBT, MOSFET, etc.) to convert DC power to AC power, and the temperature of each semiconductor switching element rises during the conversion. Although not shown in the figure, each semiconductor switching element is provided with a temperature sensor, and the element temperature detected by each temperature sensor is also input to the motor control device 1.

The DC power supply 4 is a power supply that supplies DC power to the inverter 3. This DC power supply 4 may be a battery, or a power supply that outputs DC power that has been rectified and smoothed from commercial AC power.

The DC voltage detector 5 is a sensor that detects the output voltage of the DC power supply 4 and outputs it to the motor control device 1 as DC voltage information Vdc.

The phase current detector 6 is a sensor that consists of a Hall CT (Current Transformer) and detects the three-phase currents Iu, Iv, and Iw supplied from the inverter 3 to the motor 2, that is, U-phase, V-phase, and W-phase, and outputs the current waveforms Iuc, Ivc, and Iwc to the motor control device 1.

The magnetic pole position detector 7 is a sensor that consists of a resolver or the like, detects the magnetic pole position of the motor 2, and outputs the magnetic pole position information θ to the motor control device 1.

<Details of Motor Control Device 1>
As shown in Fig. 2, the motor control device 1 of this embodiment includes a frequency calculation unit 11, a coordinate conversion unit 12, a current control unit 13, a coordinate conversion unit 14, a carrier frequency generation unit 15, a carrier wave generation unit 16, and a PWM controller 17. Each unit will be described in order below. Specifically, the motor control device 1 is a computer including a calculation unit such as a CPU, a storage device such as a semiconductor memory, and hardware such as a communication device. The calculation unit executes a desired program to realize each of the above-mentioned functional units, but the following description will omit such well-known techniques as appropriate.

The frequency calculation unit 11 calculates and outputs the speed information ω1 from the magnetic pole position information θ detected by the magnetic pole position detector 7. Note that, for example, a differential calculation can be used for this calculation.

The coordinate conversion unit 12 converts the current waveforms Iuc, Ivc, and Iwc detected by the phase current detector 6 into coordinates using the magnetic pole position information θ detected by the magnetic pole position detector 7, and outputs the d- and q-axis current detection values Idc and Iqc.

The current control unit 13 calculates and outputs appropriate dq-axis voltage command values Vd*, Vq* so that the dq-axis current command values Id ^* , Iq ^* input from a higher-level device or the like match the dq-axis current detection values ^Idc , ^Iqc input from the coordinate conversion unit 12.

The coordinate conversion unit 14 performs coordinate conversion on the dq-axis voltage command values Vd ^* , Vq ^* input from the current control unit 13 using the magnetic pole position information θ detected by the magnetic pole position detector 7, and outputs three-phase voltage command values Vu ^* , Vv ^* , Vw ^* .

The carrier frequency generating unit 15 outputs a carrier frequency fc that is effective in suppressing heat and electromagnetic noise generated by the inverter 3. Details of the carrier frequency fc generated here will be described later.

The carrier wave generating unit 16 generates a carrier wave such as a triangular wave or a sawtooth wave based on the carrier frequency fc output by the carrier frequency generating unit 15.

The PWM controller 17 calculates a duty signal from the three-phase voltage command values Vu ^* , Vv ^* , Vw ^* and the DC voltage information Vdc, compares it with a carrier wave, and outputs a gate signal (PWM signal) for each phase. The switching elements of the inverter 3 are PWM-controlled in response to this gate signal (PWM signal), so that the motor 2 is controlled to rotate at a desired rotation speed in response to a command from a higher-level device or the like.

<<Details of Carrier Frequency Generation Unit 15>>
The following describes the details of the carrier frequency generating unit 15, which is the essence of the present invention, and the principles and effects of the present invention.

Figure 3 shows an example of a functional block of the carrier frequency generation unit 15 in this embodiment. As shown here, the carrier frequency generation unit 15 includes a first random number generation unit 15a, a call period determination unit 15b, a maximum average switching frequency determination unit 15c, a selection ratio determination unit 15d, and a carrier frequency determination unit 15e. Each unit will be described in turn below.

The first random number generation unit 15a generates a random number _RN1 , which is a natural number. This random number _RN1 is, for example, any natural number from 1 to 8, and may be a pseudo-random number generated by using a map reference or a linear congruential algorithm.

The calling period determination unit 15b randomly determines the timing (hereinafter referred to as the "calling period") for calling the carrier frequency determination unit 15e described later based on the random number _RN1 generated by the first random number generation unit 15a. This calling period is, for example, a period obtained by multiplying a half period (fc/2) of the current carrier frequency fc by n (n is a natural number). If the random number _RN1 generated by the first random number generation unit 15a is 3, 3fc/2 is determined as the calling period, and if the random number _RN1 is 8, 4fc is determined as the calling period. Note that the carrier frequency determination unit 15e described later is also allowed to continuously select the same carrier frequency fc as the previous time, so the carrier frequency fc is not always updated in synchronization with the calling period determined by the calling period determination unit 15b.

The maximum average switching frequency determiner 15c determines the maximum value of the average switching frequency based on the motor speed ω, the motor current I = √( ^Id2 + ^Iq2 ), the element temperature, etc., so as to suppress the amount of heat generated by the switching elements to a level that does not cause a failure of the inverter 3.

Here, the maximum average switching frequency f _{ave_max} is determined as follows. For example, in a region where the motor speed ω is low and the motor current I is high, the temperature rise of the switching elements tends to be large, so the maximum average switching frequency f _{ave_max} is set lower than that in normal operation. Alternatively, when the temperature of the switching elements exceeds a predetermined value, the maximum average switching frequency f _{ave_max} is set lower than that in normal operation.

The selection ratio determination unit 15d determines the selection ratio of each of the multiple prepared carrier frequencies fc based on the maximum average switching frequency f _{ave_max} . For example, if two types of frequencies, a first frequency f ₁ and a second frequency f ₂ (where f ₁ > f ₂ ), are prepared as candidates for the carrier frequency fc, the selection ratio determination unit 15d limits the selection ratio r ₁ of the first frequency f ₁ , which generates a larger amount of heat in the switching element, according to formula 1. The effect of limiting the selection ratio r ₁ of the first frequency f ₁ according to formula 1 will be described later.

The carrier frequency determination unit 15e is called when the call cycle determined by the call cycle determination unit 15b is reached, and selects one of the multiple prepared carrier frequency candidates within the allowable range of the selection ratio determined by the selection ratio determination unit 15d, and outputs it as the carrier frequency fc. To achieve this function, the carrier frequency determination unit 15e includes a second random number generation unit 15e1 and a frequency selection unit 15e2.

The second random number generation unit 15e1 generates a random number _RN2 , which is a natural number. This random number _RN2 is, for example, any natural number from 1 to 100, and may be a pseudo-random number generated by using a map reference or a linear congruential algorithm.

The frequency selection unit 15e2 selects one of the first frequency _f1 or the second frequency _f2 as the carrier frequency fc based on the random number _RN2 generated by the second random number generation unit 15e1 and the selection ratio _r1 determined by the selection ratio determination unit 15d. For example, the random number _RN2 is divided by 100 and compared with the selection ratio _r1 . If the calculated value is equal to or less than the selection ratio _r1 , the first frequency _f1 is selected as the carrier frequency fc, and if the calculated value is greater than the selection ratio _r1 , the second frequency _f2 is selected as the carrier frequency fc. Since the random number _RN2 is a random number, the selection ratio of the first frequency _f1 approaches the desired selection ratio _r1 by continuing the carrier frequency selection by this method.

<Effect of Formula 1>
The reason why the average switching frequency f _ave is equal to or lower than the desired maximum average switching frequency f _{ave_max} according to equation 1 will be described below.

The time it takes for a carrier frequency to be selected is proportional to the inverse of the carrier frequency, so Equation 2 holds true.

Solving Equation 2 for the selection ratio _r1 gives Equation 3.

Here, if the average switching frequency f _ave is set to be equal to or lower than the maximum average switching frequency f _{ave_max} , then formula 1 is obtained. Therefore, if the selection ratio r ₁ of the first frequency f ₁ is determined according to formula 1, it is possible to achieve a frequency equal to or lower than the maximum average switching frequency f _{ave_max} . This makes it possible to prevent the amount of heat generated by the switching elements from becoming greater than an expected value.

In addition, when changing the selection ratio, the selected carrier frequency does not change, so the selected carrier frequency itself is not changed to suppress the amount of heat generation, and only the selection ratio is changed. This eliminates the need to narrow the frequency selection range, so the degree of reduction in the harmonic dispersion effect is small. Therefore, when reducing the average switching frequency, it is possible to increase the harmonic dispersion effect compared to a measure of uniformly lowering the first frequency _f1 and the second frequency _f2 .

In this embodiment, the method of limiting the selection ratio _r1 using the formula 1 has been described, but the formula 4 may be used instead of the formula 1.

Equation 4 does not take into account that the time at which the carrier frequency is output varies depending on the frequency, and is based on the premise that the duration of the selected carrier frequency does not change regardless of the frequency. Since this simplifies the calculation, when the difference between the first frequency _f1 and the second frequency _f2 is small, calculation using Equation 4 will produce results that are almost equivalent to those of Equation 1.

In this embodiment, an example is shown in which the maximum average switching frequency varies depending on the speed, current, and temperature of the semiconductor switching elements of the inverter 3 (see the input signal in FIG. 3), but the maximum average switching frequency may be set to a fixed value assuming a worst case. Alternatively, the selection ratio itself may be calculated from the first frequency _f1 , the second frequency _f2 , and the maximum average switching frequency which are fixed, and set to a fixed value.

In addition, in this embodiment, an example in which the ringing cycle is random is shown, but even if the ringing cycle changes according to a predetermined rule, the same effect can be obtained, although there is a possibility of harmonic components being generated due to regular switching of the carrier frequency.

As described above, the motor control device of this embodiment can suppress the amount of heat generated by the inverter switching elements while widening the distribution of harmonic components to suppress electromagnetic noise.

<Modification>
Although the above describes a case where the candidates for the carrier frequency fc are two values, even if the candidates are three or four values, it is possible to similarly suppress the amount of heat generated in the inverter 3 by taking into consideration the selection ratio. Below, a method for selecting the frequency ratio will be described using an example where the candidates for the carrier frequency are three values.

If the selection ratio of the first frequency _f1, which is a candidate for the carrier frequency fc, is _r1 , the selection ratio of the second frequency _f2 (where _f1 > _f2 ) is _r2 , and the selection ratio of the third frequency _f3 (where _f2 > _f3 ) is _r3 (where _r3 = 1 - _r1 - _r2 ), then the time during which the carrier frequency fc is selected is proportional to the inverse of the carrier frequency, and therefore Equation 5 holds.

Solving Equation 5 for the frequency ratio _r1 results in Equation 6.

From the above, the selection ratio determination unit 15d limits the selection ratio _r1 of the first frequency _f1 according to Equation 7. Note that the relationship between Equation 6 and Equation 7 corresponds to the relationship between Equation 3 and Equation 1 in the case where the candidates for the carrier frequency fc are two values.

Here, the selection ratio _r2 of the second frequency f2 _, which is the center of the three frequency candidates, is preferably a small value in consideration of the effect of dispersion, and is therefore fixed at, for example, 0.1 and used in the calculation of Equation 7. Alternatively, the selection ratio _r2 may be changed according to conditions with reference to the results of electromagnetic noise measurements. In any case, if the selection ratio _r1 is limited by Equation 7, the amount of heat generated can be suppressed even when there are three frequency candidates. Note that, even when there are three frequency candidates, Equation 8 may be used instead of Equation 7.

Equation 8 does not take into account that the time at which the carrier frequency is output varies depending on the frequency, but is premised on the fact that the duration of the selected carrier frequency does not change regardless of the frequency. This simplifies the calculation, so when the difference between _f1 , _f2 , and _f3 is small, calculation using equation 8 will produce results that are almost equivalent to those of equation 7.

Next, a second embodiment of the present invention will be described with reference to FIG. 4. Note that a duplicated description of the points common to the first embodiment will be omitted.

Figure 4 shows the configuration of the drive system of an electric vehicle according to the second embodiment of the present invention. As shown here, the electric vehicle of this embodiment is equipped with a transmission 21, a differential gear 22, a drive shaft 23, and wheels 24 in addition to the motor control device 1, motor 2, inverter 3, and DC power source 4 shown in the first embodiment.

The motor 2 is connected to a transmission 21. The transmission 21 is connected to a drive shaft 23 via a differential gear 22 to supply power to the wheels 24. Note that the transmission 21 may be omitted and the motor 2 may be directly connected to the differential gear 22, or the motor 2 and inverter 3 may be applied to each of the front and rear wheels.

Automobiles have strict requirements for motor noise, and because of response requirements, the carrier frequency cannot be set low, so it can be said that this is an application in which the effects of this invention are more pronounced than in other applications. Similarly, railways have strict requirements for motor noise, just like automobiles, so this is an application in which the effects of this invention are more likely to be seen.

By applying this invention, in automobiles and trains, it is possible to obtain the effect of dispersing harmonics by random PWM while restricting the heat generation limit of the semiconductor switching elements of the inverter 3 to a specified level. This leads to improved riding comfort for the driver and passengers.

100: Motor drive system 1: Motor control device 11: Frequency calculation unit 12: Coordinate conversion unit 13: Current control unit 14: Coordinate conversion unit 15: Carrier frequency generation unit 15a: First random number generation unit 15b: Call period determination unit 15c: Maximum average switching frequency determination unit 15d: Selection ratio determination unit 15e: Carrier frequency determination unit 15e1: Second random number generation unit 15e2: Frequency selection unit 16: Carrier wave generation unit 17: PWM controller 2: Motor 3: Inverter 4: DC power supply 5: DC voltage detector 6: Phase current detector 7: Magnetic pole position detector 21: Transmission 22: Differential gear 23: Drive shaft 24: Wheels

Claims (6)

A motor control device that controls an inverter that supplies AC power to an AC motor,
a carrier frequency generation unit that selects each of a plurality of carrier frequency candidates at a predetermined ratio based on a random number and outputs the selected carrier frequency as a carrier frequency;
A carrier wave generating unit that generates a carrier wave based on the selected carrier frequency;
a PWM controller that outputs a PWM signal based on the carrier wave to an inverter;
The motor control device according to claim 1, wherein the predetermined ratio is determined based on the frequencies of the plurality of carrier frequency candidates and a predetermined maximum average switching frequency. 2. The motor control device according to claim 1,
The motor control device, wherein the carrier frequency generation unit includes an update period determination unit that randomly determines the selected update period. 3. The motor control device according to claim 1,
The motor control device is characterized in that the carrier frequency candidates are two-valued. 4. The motor control device according to claim 3,
2. A motor control device according to claim 1, wherein the predetermined ratio is calculated by the following formula.

where _f1 is the first frequency, _f2 is the second frequency smaller than _f1 , _r1 is the selection ratio of the first frequency, and f _{ave_max} is the maximum average switching frequency. 3. The motor control device according to claim 1,
The motor control device according to claim 1, wherein the carrier frequency generating unit includes a maximum average switching frequency determining unit that determines the maximum average switching frequency based on a rotation speed, a current value, or an element temperature of the inverter of the AC motor. An electric vehicle equipped with an AC motor, an inverter, and a motor control device,
the AC motor drives wheels via a transmission, a differential gear, and a drive shaft;
3. An electric vehicle, wherein the motor control device is the motor control device according to claim 1 or 2.

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