JP2020171135A - Power converter - Google Patents

Abstract

To suppress generation of torque impact in switching of a modulation method.SOLUTION: In a power converter 10 having a switching circuit 2 which converts direct current into alternating current and a PWM control part 3 which has PWM control of the switching circuit, the PWM control part includes: a current control part 31 which calculates a voltage command based on a current command and current supplied to a load from the switching circuit; a scheduler 35 which calculates, based on the current command, a ratio of a correction amount for N phase modulation of the voltage command and a correction amount for (N-1) phase modulation of the voltage command, in which N is an integer of 3 or more; and a PWM duty generation part 32 which calculates, based on the ratio, a voltage command with correction by adding the correction amount for N phase modulation and the correction amount for (N-1) phase modulation to the voltage command.SELECTED DRAWING: Figure 3

Description

The present invention relates to a power conversion device that performs PWM control and supplies power to a load such as a motor.

Patent Document 1 describes a power conversion device including a two-phase / three-phase modulation selection means. This two-phase / three-phase modulation selection means selects a voltage command value for two-phase modulation or for three-phase modulation depending on whether the accuracy of current control is emphasized or the suppression of heat generation is emphasized. Instructs whether to select the voltage command value of. Further, the two-phase / three-phase modulation selection means includes a current value determining means for selecting and instructing a voltage command value for two-phase modulation when the absolute value of the torque current command value is larger than a predetermined reference value. It is described in the same document.

Patent Document 2 describes a control device for a PWM inverter. In this control device, a frequency command is input to the three-phase modulation wave forming unit to generate a three-phase modulation wave signal, and the three-phase modulation wave signal is output to the two-phase modulation calculation unit and the addition unit, respectively. When a two-phase modulation command is input from the outside, the output of the two-phase modulation calculation unit that calculates the difference from the two-phase modulation wave based on the three-phase modulation wave is output to the addition unit, and when the three-phase modulation command is input, the addition unit A two-phase modulation wave or a three-phase modulation wave is generated by blocking the output of the two-phase modulation calculation unit to. A switching state signal generation unit is provided on the output side of the two-phase modulation calculation unit, and an intermediate state signal is generated from the switching state signal generation unit when a mutual switching command signal is generated between the three-phase modulation state and the two-phase modulation state from the outside. Is generated for a predetermined period and is output to the addition unit. It is also described in the same document that the intermediate state signal output by the switching state signal generation unit is an output having a stepped or inclined shape.

Japanese Unexamined Patent Publication No. 2004-48885 Japanese Unexamined Patent Publication No. 2009-100613

When the modulation method is switched from one of the two-phase modulation and the three-phase modulation to the other, torque impact may occur in the load of the motor or the like connected to the power conversion device. An object of the present invention is to suppress the generation of torque impact when switching the modulation method.

In order to achieve the above object, the PWM control unit of the power conversion device having a switching circuit for converting DC to AC according to the present invention and a PWM control unit for PWM control of the switching circuit includes a current command and the above-mentioned A current control unit that calculates a voltage command based on the current supplied from the switching circuit to the load, an N-phase modulation correction amount of the voltage command, and a (N-1) phase modulation correction of the voltage command. A scheduler that calculates the ratio with the amount based on the current command, and the N is an integer of 3 or more. The scheduler, the correction amount for N-phase modulation and the (N-) based on the ratio. 1) A PWM duty generation unit that calculates a corrected voltage command by adding a correction amount for phase modulation to the voltage command is provided.

According to the present invention, it is possible to suppress the generation of torque impact when switching the modulation method.

It is explanatory drawing which shows the voltage command with bias of the three-phase modulation system in three-phase PWM and the pulse waveform of the output voltage. It is explanatory drawing which shows the voltage command with bias of the two-phase modulation system in three-phase PWM and the pulse waveform of the output voltage. It is explanatory drawing which shows the power conversion apparatus in one Embodiment. It is explanatory drawing which shows the relationship between a torque load factor and a coefficient K. It is explanatory drawing which shows the pulse waveform of the voltage command with bias and the output voltage at the time of shifting from three-phase modulation to two-phase modulation through the modulation of an intermediate state in three-phase PWM. It is explanatory drawing which shows the voltage command with bias of the two-phase modulation system (6 steps) in three-phase PWM and the pulse waveform of the output voltage. An explanatory diagram showing a biased voltage command and a pulse waveform of an output voltage when shifting from 3-phase modulation (6 steps) to 2-phase modulation (6 steps) through intermediate state modulation (6 steps) in 3-phase PWM. is there.

Hereinafter, the present invention will be described based on the illustrated embodiment. However, the present invention is not limited to the embodiments described below.

First, three-phase modulation and two-phase modulation of three-phase PWM (Pulse Width Modulation) will be described.

In the three-phase PWM duty distribution method, the duty of each phase is distributed with the duty of 50% corresponding to 0 [V], which is the center value of the AC voltage required for output, as a bias. That is, 0 [V] is provided with a duty bias of 50%. This method is referred to as "three-phase modulation".

The voltage commands of each phase output from the current control unit that controls the current based on the current command and the detected value of the current supplied from the switching circuit to the load are V _U , V _V, and V _W. Then, the voltage commands with bias by three-phase modulation are W _3U , W _3V , and W _3W . Let A be the voltage bias corresponding to the 50% duty. The voltage command for biased three-phase modulation is as follows.
W _3U = V _U + A
W _3V = V _V + A
W _3W = V _W + A

The upper part of FIG. 1 shows the temporal change between the triangular wave which is a carrier wave and the voltage commands (W _3U , W _3V , W _3W ) of the U phase, V phase, and W phase in the three-phase modulation. The lower part of the figure shows the pulse waveform of the output voltage of each phase.

As another method, there is a method in which one phase that generates the maximum on the most (-) side is set to 0% on the high side (that is, 100% on the low side), and the voltage of each phase is generated by the duty difference from the other phase. In the case of 3-phase PWM, this is referred to as "two-phase modulation". Switching is stopped in the phase where the low side is 100%. In two-phase modulation, the duty of the lowest voltage phase among the three phases is set to 0% by bias.

As described above, the voltage commands V _U , V _V and V _{W of} each phase are output from the current control unit. Then, the voltage command with bias by two-phase modulation is W _2U , W _2V , W _2W, and the code-inverted lowest voltage among V _U , V _V, and V _W is B. In the case of three-phase alternating current, the minimum voltage is a negative value. The biased, two-phase modulation voltage command is:
W _2U = V _U + B
W _2V = V _V + B
W _2W = V _W + B

The upper part of FIG. 2 shows the temporal change between the triangular wave as a carrier wave and the biased voltage commands (W _2U , W _2V , W _2W ) of the U phase, V phase, and W phase in the two-phase modulation. The U-phase, V-phase, and W-phase biased voltage commands (W _3U , W _3V , W _3W ) in the three-phase modulation shown in FIG. 1 are also shown in FIG. 2 for reference. The lower part of FIG. 2 shows the pulse waveform of the output voltage of each phase.

The features of three-phase modulation are described below.
-Since three-phase switching is always performed, the switching loss of the output element is relatively large.
-The current ripple is dispersed twice with respect to the PWM cycle, the peak value is small, and the noise is also small.
-When the carrier frequency is several kHz or more, the higher the frequency of the main component of PWM noise, the less sensitive it is to human hearing (one full-phase high and one full-phase low section occurs in the PWM cycle, which is Since the pulse voltage is generated twice per PWM cycle in the sections other than the above, the ripple frequency is doubled). For example, when the carrier frequency is 8 kHz, the ripple frequency is 16 kHz. Assuming that the maximum value of the audible range is 15 kHz, the ripple frequency of 16 kHz is outside this audible range, so that the noise is small.

On the other hand, two-phase modulation has the following features.
-Since switching is always paused in any one phase, the switching loss of the output element is reduced as compared with the three-phase modulation.
-Current ripple has a component that is the same size as the PWM period, and noise is louder than that of three-phase modulation.

As described above, when the three-phase modulation and the two-phase modulation are compared, the three-phase modulation has a large switching loss but a small noise, and the two-phase modulation has a small switching loss but a large noise. Therefore, in the case of a high load, it is desirable to select two-phase modulation with a small switching loss in order to suppress the loss as much as possible, while in the case of a low load, it is desirable to set aside the loss and select a three-phase modulation with low noise. ..

As the load changes, the modulation method may be switched from one of the two-phase modulation and the three-phase modulation to the other. An embodiment for reducing the torque impact at the time of this switching will be described below.

[First Embodiment]
FIG. 3 shows a motor 1 as a load and a power conversion device 10 for supplying AC power to the motor 1. The power conversion device 10 includes a switching circuit 2 that converts direct current into alternating current for supplying to the motor 1, and a PWM control unit 3 that PWM-controls the switching circuit 2. The PWM control unit 3 includes a current control unit 31, a PWM duty generation unit 32, a low-pass filter 33, an absolute value calculation unit 34, and a scheduler 35. A current command (q-axis current command) is input to the power conversion device 10 from the outside. This current command corresponds to a torque command.

The current command and the detected value of the supply current supplied from the switching circuit 2 to the motor 1 are input to the current control unit 31. The current control unit 31 calculates voltage commands V _U , V _V, and V _W in order to control the current so that the detected value of the supply current to the motor 1 matches the above current command. The voltage command calculated by the current control unit 31 is sent to the PWM duty generation unit 32.

The current command is also input to the low-pass filter 33. The low-pass filter 33 is a filter that attenuates a component having a frequency higher than a predetermined frequency among the input current commands and passes a component having a frequency lower than the predetermined frequency with almost no attenuation. The output signal of the low-pass filter 33 is input to the absolute value calculation unit 34.

The absolute value calculation unit 34 receives the input of the output signal of the low-pass filter 33 and calculates the absolute value of the output signal. As a result, the signs of the current command (torque command) are positively aligned. The output signal of the absolute value calculation unit 34 is input to the scheduler 35.

The scheduler 35 calculates the coefficient K based on the current command (torque command) in which the signs are positively aligned, which is input from the absolute value calculation unit 34.

The coefficient K is 0.0 or more and 1.0 or less, and bias A (voltage correction value) for calculating the voltage command for three-phase modulation and bias B for calculating the voltage command for two-phase modulation. It is a coefficient that determines the ratio with (voltage correction value). 3-phase modulation if K = 1.0, 2-phase modulation if K = 0.0, intermediate state between 2-phase modulation and 3-phase modulation if 0.0 <K <1.0 It becomes the modulation method of.

FIG. 4 shows the relationship between the torque load factor with the rated torque as 100% and the coefficient K as an example. When the torque load factor is 50% or less, K = 1.0. When the torque load factor is 60% or more, K = 0.0. When the torque load factor is greater than 50% and less than 60%, K is greater than 0.0 and less than 1.0. The relationship between the torque load factor of 50% or more and 60% or less and K is expressed by a linear function having a negative slope.

In this way, the scheduler 35 calculates the torque load factor based on the current command (torque command) in which the signs are positively aligned, which is input from the absolute value calculation unit 34, and the torque load factor is shown in FIG. The coefficient K is calculated from the relationship. The calculated coefficient K is input to the PWM duty generation unit 32.

The PWM duty generation unit 32 has a corrected biased voltage command (with correction) based on the voltage commands V _U , V _V , V _W input from the current control unit 31 and the coefficient K input from the scheduler 35. Voltage command) W _U , W _V , W _W are calculated as follows.
W _U = V _U + K ・ A + (1-K) ・ B
W _V = V _V + K ・ A + (1-K) ・ B
W _W = V _W + K ・ A + (1-K) ・ B

As described above, when K = 0.0, it is a two-phase modulation, and when K = 1.0, it is a three-phase modulation. Further, when 0.0 <K <1.0, it is an intermediate state modulation between the two-phase modulation and the three-phase modulation. The coefficient K determines the ratio of the bias A for three-phase modulation and the bias B for two-phase modulation. Then, the biases A and B according to this ratio are added to each of the voltage commands V _U , V _V and V _W , and the biased voltage commands W _U , W _V and W _W are calculated.

The PWM duty generator 32 further generates a pulse by comparing the calculated biased voltage commands W _U , W _V and _WW with the carrier wave. This pulse is sent to the switching circuit 2, and the switching circuit 2 is PWM-controlled.

In the upper part of FIG. 5, the triangular wave is a carrier wave, U-phase in the case of transition to over by 2-phase modulating the modulated intermediate state from the three-phase modulation voltage command with the bias of V-phase and W-phase (W _U, W _V, showing the time change of the _{W W).} The U-phase, V-phase, and W-phase biased voltage commands (W _3U , W _3V , W _3W ) in the three-phase modulation shown in FIG. 1 are also shown in FIG. 5 for reference. The lower part of FIG. 5 shows the pulse waveform of the output voltage of each phase.

[Second Embodiment]
In the above embodiment, the bias is calculated from the minimum voltage of the three-phase voltage command when generating the two-phase modulation. Alternatively, both the maximum voltage and the minimum voltage can be considered and biased toward the high side or the low side. In this case, there are 6 steps of phase states in which the high side is 100% or the low side is 100% with respect to the circumference of the electric angle.

In the upper part of FIG. 6, the triangular wave which is a carrier wave, the voltage command with bias of U-phase, V-phase and W-phase in three-phase modulation, and the bias of U-phase, V-phase and W-phase in two-phase modulation (6 steps). Shows the temporal change with the attached voltage command. The lower part of the figure shows the pulse waveform of the output voltage of each phase.

As for the three-phase modulation method and the two-phase modulation method (6 steps), the modulation method (6 steps) in the intermediate state of both modulation methods can be realized by multiplying the bias of both modulation methods by the coefficient K.

In the upper part of FIG. 7, a triangular wave as a carrier wave and U-phase, V-phase, and W-phase biases when shifting from three-phase modulation to two-phase modulation (6 steps) via intermediate state modulation (6 steps) are provided. Shows the temporal change with the voltage command of. The biased voltage commands for the U-phase, V-phase, and W-phase in the three-phase modulation shown in FIG. 5 are also shown in FIG. 7 for reference. The lower part of FIG. 7 shows the pulse waveform of the output voltage of each phase.

According to the first embodiment and the second embodiment, the effects listed below can be obtained.
-While a two-phase modulation is selected and loss can be suppressed at a high load when heat generation is large, a three-phase modulation can be selected and noise can be suppressed at a low load.
-As the load changes, the modulation method smoothly switches from one of the two-phase modulation and the three-phase modulation to the other of the two-phase modulation and the three-phase modulation via the modulation method in the intermediate state. Therefore, the generation of torque impact can be suppressed as compared with the case of switching from one of the two-phase modulation and the three-phase modulation to the other without going through the modulation in the intermediate state.
-When the load is in an intermediate state between a high load and a low load (for example, the torque load factor is 50% to 60% in FIG. 4), stable operation is performed in the state of modulation in the intermediate state. .. When the intermediate state of the load continues, the modulation of the intermediate state is continuously performed regardless of the length of the duration.

Unlike the conventional method, the modulation method in the intermediate state can be selected instead of the alternative of two-phase modulation and three-phase modulation. Then, in the modulation in the intermediate state, the ratio of the correction value for two-phase modulation (voltage bias) and the correction value for three-phase modulation (voltage bias) is used by using the coefficient K that continuously changes according to the load. Is determined.

No specific operation is required to select two-phase modulation, three-phase modulation, or intermediate state modulation. The modulation method is selected according to the torque. There is no time constraint. That is, torque-sensitive control can be realized.

The low-pass filter 33 and the absolute value calculation unit 34 shown in FIG. 3 are not essential, but by providing them, control instability due to a sudden change in the coefficient K can be avoided.

As described above, by allocating the duty of the three-phase PWM, both noise reduction and loss reduction can be achieved.

In FIG. 4, as an example, it is decided that the modulation of the intermediate state is performed when the torque load factor is 50% to 60%. However, the torque load factor at which the intermediate state is modulated can be changed as appropriate.

So far, the three-phase modulation and the two-phase modulation in the three-phase AC PWM and the modulation method in the intermediate state have been described, but the above-described embodiment is the N-phase modulation in the N-phase AC PWM and (N-1). It can be extended to phase modulation and its intermediate state modulation method. However, N is an integer of 3 or more.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and modifications can be made based on the technical idea of the present invention.

1 Motor 2 Switching circuit 3 PWM control unit 10 Power conversion device 31 Current control unit 32 PWM duty generation unit 33 Low-pass filter 34 Absolute value calculation unit 35 Scheduler

Claims (3)

A power conversion device having a switching circuit that converts direct current into alternating current and a PWM control unit that PWM-controls the switching circuit.
The PWM control unit
A current control unit that calculates a voltage command based on the current command and the current supplied from the switching circuit to the load.
A scheduler that calculates the ratio of the N-phase modulation correction amount of the voltage command to the (N-1) phase modulation correction amount of the voltage command based on the current command, and the N is 3 or more. The scheduler, which is an integer,
A PWM duty generation unit that calculates a corrected voltage command by adding the N-phase modulation correction amount and the (N-1) phase modulation correction amount to the voltage command based on the ratio is provided. A power converter characterized by. The scheduler calculates a coefficient K of 0 or more and 1 or less that determines the ratio based on the current command.
The PWM duty generation unit multiplies the value obtained by multiplying the correction amount for N-phase modulation by the coefficient K, and the correction amount for (N-1) phase modulation by subtracting the coefficient K from 1. The power conversion device according to claim 1, wherein the corrected voltage command is calculated by adding the obtained value to the voltage command. A low-pass filter to which the current command is input and
It is further equipped with an absolute value calculation unit that calculates the absolute value of the output signal of the low-pass filter.
The power conversion device according to claim 1 or 2, wherein the scheduler calculates the ratio based on the absolute value.

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