JP7202244B2 - power converter - Google Patents

Description

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

Patent Literature 1 describes a power conversion device having 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 a Select whether to select the voltage command value of Further, the two-phase/three-phase modulation selection means includes current value determination means for selecting and instructing a voltage command value for two-phase modulation when the absolute value of the torque current command value is greater than a predetermined reference value. It is described in the same document.

Patent Literature 2 describes a control device for a PWM inverter. In this control device, a frequency command is input to a three-phase modulated wave generator to generate a three-phase modulated wave signal, which is output to a two-phase modulated wave calculator and an adder, respectively. When a two-phase modulation command is input from the outside, the output of the two-phase modulation calculation section for calculating the difference from the two-phase modulation wave based on the three-phase modulation wave is output to the addition section, and when the three-phase modulation command is input, the addition section. A two-phase modulated wave or a three-phase modulated wave is generated by blocking the output of the two-phase modulation computing section to . A switching state signal generating section is provided on the output side of the two-phase modulation calculating section, and an intermediate state signal is generated from the switching state signal generating section when a mutual switching command signal between the three-phase modulation state and the two-phase modulation state is generated from the outside. is generated for a predetermined period and output to the adder. The document also describes that the intermediate state signal output by the switching state signal generator is a stepped or sloping output.

JP 2004-48885 A JP 2009-100613 A

When switching the modulation method from one of the two-phase modulation and the three-phase modulation to the other, there is a possibility that a torque impact will occur in a load such as a motor connected to the power converter. SUMMARY OF THE INVENTION An object of the present invention is to suppress the occurrence of a torque impact when switching the modulation method.

In order to achieve the above object, the PWM control unit of a power conversion device having a switching circuit that converts direct current to alternating current according to the present invention and a PWM control unit that performs PWM control of the switching circuit includes a current command and the A current control unit that calculates a voltage command based on a current supplied from a switching circuit to a load, a correction amount for N-phase modulation of the voltage command, and a correction for (N−1)-phase modulation of the voltage command a scheduler for calculating a ratio between the N-phase modulation correction amount and the (N- 1) A PWM duty generation unit that calculates a voltage command with correction by adding a correction amount for phase modulation to the voltage command.

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

FIG. 4 is an explanatory diagram showing pulse waveforms of a biased voltage command and an output voltage of a three-phase modulation method in three-phase PWM; FIG. 4 is an explanatory diagram showing pulse waveforms of a biased voltage command and an output voltage of a two-phase modulation method in three-phase PWM; BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows the power converter device in one Embodiment. 4 is an explanatory diagram showing the relationship between a torque load factor and coefficient K; FIG. FIG. 4 is an explanatory diagram showing pulse waveforms of a voltage command with a bias and an output voltage when three-phase modulation shifts from three-phase modulation to two-phase modulation via modulation in an intermediate state in three-phase PWM; FIG. 4 is an explanatory diagram showing pulse waveforms of a biased voltage command and an output voltage of a two-phase modulation method (6 steps) in three-phase PWM; FIG. 10 is an explanatory diagram showing a voltage command with a bias and a pulse waveform of an output voltage when shifting from three-phase modulation (6 steps) to two-phase modulation (6 steps) via intermediate state modulation (6 steps) in three-phase PWM; be.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below based on the illustrated embodiments. However, the present invention is not limited by the embodiments described below.

First, 3-phase modulation and 2-phase modulation of 3-phase PWM (Pulse Width Modulation) will be described.

In the three-phase PWM duty distribution method, the duty of each phase is distributed with a bias of 50%, which corresponds to 0 [V], which is the center value of the AC voltage to be output. That is, 0 [V] is given a duty bias of 50%. This method is called "three-phase modulation".

Let _VU , _VV , and _VW be the voltage commands for each phase output from the current control unit that performs current control based on the current command and the detected value of the current supplied from the switching circuit to the load. Then, voltage commands with bias by three-phase modulation are set to _W3U , _W3V , and _W3W . Let A be the voltage bias corresponding to a 50% duty. Voltage commands for three-phase modulation with bias are as follows.
W _3U = _VU + A
_W3V = _VV +A
_W3W = _VW +A

The upper part of FIG. 1 shows temporal changes of the triangular wave as a carrier wave and the voltage commands ( _W3U , _W3V , _W3W ) of the U-phase, V-phase and W-phase in 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 (-) 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 with the other phases. In the case of three-phase PWM, this is called "two-phase modulation". Switching is halted for the phase with 100% low side. In two-phase modulation, the duty of the phase with the lowest voltage among the three phases is set to 0% by bias.

As described above, the voltage commands _VU , _VV and _VW for each phase are output from the current control section. Let W _2U , W _2V , and W _2W be voltage commands with bias by two-phase modulation, and let B be the lowest voltage among V _U , V _V , and V _W whose sign is inverted. In addition, in the case of three-phase alternating current, the minimum voltage becomes a negative value. The voltage command for two-phase modulation with bias is as follows.
_W2U = _VU + B
_W2V = _VV +B
_W2W = _VW +B

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

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 distributed twice with respect to the PWM period, and the crest value is small, and the noise is also small.
・When the carrier wave frequency is several kHz or more, the higher the frequency of the main component of the PWM noise, the lower the human hearing sensitivity (the PWM cycle has one high and one low interval, Since the pulse voltage is generated twice per PWM cycle in sections other than the above, the ripple frequency is doubled). For example, if the carrier frequency is 8 kHz, the ripple frequency will be 16 kHz. Assuming that the maximum value of the audible range is 15 kHz, the ripple frequency of 16 kHz is outside the audible range, so the noise is small.

On the other hand, two-phase modulation has the following features.
• Switching of one of the phases is always paused, so the switching loss of the output element is reduced compared to three-phase modulation.
・The current ripple has a component that is the same as the PWM cycle, and the noise is louder than that of three-phase modulation.

As described above, when three-phase modulation and two-phase modulation are compared, switching loss is large in three-phase modulation but noise is small, while switching loss is small in two-phase modulation but noise is large. Therefore, in the case of high loads, it is desirable to select 2-phase modulation with low switching loss in order to minimize loss, while in the case of low load, it is desirable to select 3-phase modulation with low noise, leaving the loss aside. .

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

[First embodiment]
FIG. 3 shows a motor 1 as a load and a power conversion device 10 that supplies AC power to the motor 1 . The power conversion device 10 includes a switching circuit 2 that converts direct current into alternating current to be supplied to the motor 1 and a PWM control unit 3 that performs PWM control of the switching circuit 2 . The PWM controller 3 includes a current controller 31 , a PWM duty generator 32 , a low-pass filter 33 , an absolute value calculator 34 and a scheduler 35 . A current command (q-axis current command) is input to the power converter 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 _VU , _VV and _VW to control the current so that the detected value of the current supplied to the motor 1 matches the current command. A voltage command calculated by the current controller 31 is sent to the PWM duty generator 32 .

The current command is also input to the low-pass filter 33 . The low-pass filter 33 is a filter that attenuates frequency components higher than a predetermined frequency in the input current command, and allows passage of frequency components lower than the predetermined frequency without substantially attenuating them. The output signal of the low-pass filter 33 is input to the absolute value calculator 34 .

The absolute value calculator 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 commands (torque commands) are made positive. An output signal from the absolute value calculator 34 is input to the scheduler 35 .

The scheduler 35 calculates the coefficient K based on the current command (torque command) input from the absolute value calculator 34 and having the positive sign.

The coefficient K is 0.0 or more and 1.0 or less, and is the bias A (voltage correction value) for calculating the voltage command for three-phase modulation and the bias B for calculating the voltage command for two-phase modulation. (voltage correction value). Three-phase modulation if K=1.0, two-phase modulation if K=0.0, intermediate state between two-phase modulation and three-phase modulation if 0.0<K<1.0 modulation method.

FIG. 4 shows an example of the relationship between the torque load factor and the coefficient K when the rated torque is 100%. 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 represented by a linear function with a negative slope.

In this manner, the scheduler 35 calculates the torque load factor based on the current command (torque command) with positive sign input from the absolute value calculation unit 34, and the torque load factor and the torque load factor shown in FIG. The coefficient K is calculated from the relationship The calculated coefficient K is input to the PWM duty generator 32 .

The PWM duty generation unit 32 generates a _{voltage command} with a _corrected bias (corrected Voltage commands) W _U , W _V , and 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, K=0.0 is two-phase modulation and K=1.0 is three-phase modulation. Further, when 0.0<K<1.0, the modulation is in an intermediate state between two-phase modulation and three-phase modulation. The factor K defines the ratio between 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 _VU , _VV and _VW to calculate the biased voltage commands _WU , _WV and _WW .

The PWM duty generator 32 further generates pulses by comparing the calculated biased _voltage commands WU, _WV , and _WW with the carrier wave. This pulse is sent to the switching circuit 2, and the switching circuit 2 is PWM-controlled.

The upper part of FIG. 5 shows a triangular wave that is a carrier wave, and voltage commands (W _U , W _V , W _W ). For reference, FIG. 5 also shows the voltage commands (W _3U , W _3V , W _3W ) with biases for the U-phase, V-phase and W-phase in the three-phase modulation shown in FIG. 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 in generating the two-phase modulation. Alternatively, both the highest and lowest voltages can be considered and biased closer to the high side or the low side. In this case, there are 6 steps of 100% high-side or 100% low-side phase states for the revolution of the electrical angle.

In the upper part of FIG. 6, a triangular wave as a carrier wave, a voltage command with U-phase, V-phase and W-phase biases in 3-phase modulation, and U-phase, V-phase and W-phase biases in 2-phase modulation (6 steps) shows the time change with the voltage command with . The lower part of the figure shows the pulse waveform of the output voltage of each phase.

For the 3-phase modulation method and the 2-phase modulation method (6 steps) as well, by multiplying the bias of both modulation methods by the coefficient K, the modulation method (6 steps) in the intermediate state between the two modulation methods can be realized.

The upper part of FIG. 7 shows a triangular wave as a carrier wave, and U-phase, V-phase, and W-phase biases when shifting from three-phase modulation to intermediate state modulation (6 steps) to two-phase modulation (6 steps). shows the time change with the voltage command. For reference, FIG. 7 also shows the voltage commands with biases for the U-phase, V-phase and W-phase in the three-phase modulation shown in FIG. 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 following effects can be obtained.
・When the load is high and heat generation is high, two-phase modulation is selected to suppress loss, while when the load is low, three-phase modulation is selected to suppress noise.
• The modulation scheme is smoothly switched 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 through the intermediate state modulation scheme as the load changes. Therefore, it is possible to suppress the occurrence of a torque impact 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 of the intermediate state.
・When the load is in an intermediate state between high load and low load (for example, the torque load factor is 50% to 60% in FIG. 4), it operates stably in the state of modulation in the intermediate state. . If the intermediate load state continues, the intermediate state modulation continues regardless of how long it lasts.

Unlike the prior art, instead of choosing between two-phase modulation and three-phase modulation, an intermediate state modulation method can also be selected. Then, in the modulation of the intermediate state, the ratio of the correction value (voltage bias) for two-phase modulation and the correction value (voltage bias) for three-phase modulation is calculated using a coefficient K that continuously changes according to the load. is determined.

No specific operation is required to select between 2-phase modulation, 3-phase modulation, and intermediate state modulation. Selection of the modulation scheme is made according to the torque. There are no time restrictions. That is, torque-sensitive control can be realized.

Although the low-pass filter 33 and the absolute value calculator 34 shown in FIG. 3 are not essential, by providing them, it is possible to avoid control instability due to a sudden change in the coefficient K. FIG.

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

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

So far, 3-phase modulation and 2-phase modulation in 3-phase AC PWM, and modulation schemes in intermediate states thereof have been described. It can be extended and applied to phase modulation and its intermediate state modulation schemes. 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 changes are possible based on the technical idea of the present invention.

1 motor 2 switching circuit 3 PWM control unit 10 power converter 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 to alternating current and a PWM control unit that performs PWM control of 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 for calculating a ratio of the N-phase modulation correction amount of the voltage command and the (N−1)-phase modulation correction amount of the voltage command based on the current command, wherein the N is 3 or more. a scheduler, which is an integer;
a PWM duty generation unit that calculates 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 based on the ratio; A power conversion device 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 generator multiplies a value obtained by multiplying the correction amount for N-phase modulation by the coefficient K and a value obtained by subtracting the coefficient K from 1 to the correction amount for (N−1) phase modulation. 2. The power conversion apparatus according to claim 1, wherein the voltage command with correction is calculated by adding the value obtained by the above to the voltage command. a low-pass filter to which the current command is input;
and an absolute value calculator that calculates the absolute value of the output signal of the low-pass filter,
3. The power converter according to claim 1, wherein said scheduler calculates said ratio based on said absolute value.

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