JP2697201B2 - Simple calculation method of three-phase sine wave PWM in inverter - Google Patents

Description

The present invention relates to a method for calculating a three-phase sine wave PWM in an inverter that drives a three-phase AC motor at a variable speed.

[Conventional technology]

Conventional methods for calculating this type of three-phase sine wave PWM include:
One that performs the magnitude comparison operation between the sine wave control signal and the carrier triangular wave signal as described above for all phases of the three-phase output AC of the inverter using the three-phase sine wave control signal having a mutual phase difference of 120 electrical degrees. Are known.

[Problems to be solved by the invention]

The operation of the three-phase sine wave PWM in the inverter needs to be processed within a half cycle of the carrier triangular wave signal, and therefore the upper limit of the frequency of the carrier triangular wave signal, that is, the carrier frequency is limited by the time required for the PWM operation. Will be.

Even in the conventional PWM calculation method as described above, the upper limit of the carrier frequency is restricted by setting the time required to perform the required calculation for all phases of the inverter three-phase output AC as a value corresponding to a half cycle. In order to increase the carrier frequency and improve the inverter performance, the margin when selecting the optimal carrier frequency for the inverter system becomes small. Had to be expensive.

In view of the above, an object of the present invention is to provide a simplified calculation method of a three-phase sine wave PWM in an inverter for reducing a required calculation time by reducing a calculation step itself of the PWM calculation.

[Means for solving the problem]

In order to achieve the above object, a simplified calculation method of a three-phase sine wave PWM in an inverter according to the present invention relates to a calculation of a conduction or cutoff command period for a main circuit switching element of the inverter, and has a frequency equal to the inverter output frequency. The calculation of the command period, in which the magnitude of the sine wave control signal whose amplitude is proportional to the required output voltage and the carrier triangular wave signal having a constant peak value is performed for each cycle of the carrier triangular signal, is calculated by the three-phase output AC of the inverter. Any of 2
For the remaining one phase in the output alternating current, from the 3/2 times the period of the carrier triangular wave signal.
The command period calculation value is obtained by subtracting the sum of the command period calculation values for the phase.

[Action]

The calculation of the three-phase sine wave PWM in the inverter determines the conduction or cutoff command period for the main circuit switching element of the inverter that operates corresponding to each phase of the three-phase output AC. has its Fuhaba V _m equal frequency is proportional to the desired output voltage of the sine wave control signal and the peak value V _c is constant the carrier triangular wave signal comparison between a carrier triangular wave signal to the frequency and f _c This is performed for each cycle (period _Tc ).

Now, each phase of the three-phase output AC of the inverter is U-phase, V-phase, and W-phase, and the sine-wave control signal is greater than the carrier triangular-wave signal. if the conduction command period, is approximated by the sine wave control signal is changed stepwise to each period of the carrier triangular wave signal pseudo sine wave and forming a peak value and the Fuhaba V _m and the carrier triangular wave The phase angle of the sine wave control signal corresponding to the center of a certain cycle in each cycle of the signal,
For example, assuming that θ is for the U phase, the conduction command period for the U phase is a period of width 2 · T _nu around the phase angle θ, and the one-width period T _nu is given by the following equation (1).

However _{λ = V m / V c,} T c = 1 / f c the U, V, since the W phase has a phase difference of 120 electrical degrees each in accordance with phase sequence, the conductive Directive on the V, W phases The periods T _nv and T _nw are as shown in Expression (2) according to Expression (1).

According to the above equations (1) and (2), the interruption periods t _nu , t _nv and t _nw in each cycle of the carrier triangular wave signal are as shown in the following equation (3).

The operation according to each of the formulas (1), (2) or (3) can be performed by specifying a numerical value from a data table, and the conventional PWM operation method is applied to each of the T _nu , T _nv , and T _nw . The calculation according to the equations (1) and (2) is performed.

On the other hand, each of the single-width periods according to the above-described equations (1) and (2)
T _nu , T _nv , T _nw The sum of the three becomes a constant value (3/4) T _c , and therefore, U, V, W phase conduction command periods 2T _nu , 2T _nv , 2T _nw in each cycle of the carrier triangular wave signal. The sum of the three is also constant (3/2) T _c
And the following equation (4) is obtained.

That is, for each of the phase conduction command periods 2T _nu , 2T _nv , and 2T _nw , it can be obtained by subtracting the sum of the other two from the constant value (3/2) T _c . The following equation (5) is used, for example, to determine the conduction command period 2T _nw for the W phase.

Similarly, the following equation (6) holds for the shut-off periods t _nu , t _nv , and t _nw .

FIG. 2 is an operation waveform diagram of each of the PWM conduction command signal pulses PWM- _u , PWM- _v , and PWM- _{w for} each of the U, V, and W phases defined by the above-described PWM calculation. Is divided into two at the center of the pulse due to the necessity of creating a pulse waveform for identifying the rising and falling points of each pulse.

According to the present invention, in the above-described three-phase sine wave PWM operation, multivariable addition / subtraction multiplication including a sine wave function is performed in accordance with the above formula (1) and (2) or formula (3) for any two of the three phases. By performing simple addition and subtraction according to the above equation (5) or (6) for the other phase, the time required for the PWM operation can be shortened, and the carrier signal frequency can be increased. Things.

〔Example〕

FIG. 1 shows a three-phase sine wave PWM according to an embodiment of the present invention.
The operation will be described with reference to a flowchart.

As shown in FIG. 1, in the present PWM calculation, an interrupt is generated every half cycle of the carrier triangular wave signal, and in step 1, the output setting is made by changing the left and right of the center of the carrier cycle as shown in FIG. A decision is made as to which one. The right and left output settings are usually repeated alternately. If it is now on the right side, the process proceeds to steps 2 to 4 sequentially to calculate each of the above periods T _nu , T _nv , and T _nw, and the output time of the right waveform is set in step 5. If it is on the left side, dead time calculation for preventing a short circuit between the upper and lower arms of the inverter main circuit switching element is performed in step 6, and output time of the left waveform is set in step 7.

Further, when the frequency of the carrier triangular wave signal is about 3 kHz which is optimized for a transistor type inverter,
The half cycle T _c / 2 of the carrier signal is set to, for example, 164 μs in consideration of the resolution of the CPU that performs the various operations. However, the operation of the step 4 is not performed as in the step 2 or 3, and the steps 2 and 3 are not performed. By performing simple addition and subtraction using the result of (3), it is possible to complete the required three-phase operation within the aforementioned 164 μs.

〔The invention's effect〕

According to the present invention, a three-phase sine wave PWM in an inverter
In the calculation of the above, the operation of comparing the magnitude of the sine wave control signal and the carrier signal for each cycle of the carrier signal is performed for two phases out of three phases, and for the other one phase, the frequency of the carrier signal is By subtracting the sum of the operation values of the two phases from the 3/2 times value to obtain the required operation result, it is possible to reduce the required operation step and the associated operation period, and thus further reduce the carrier signal frequency. The allowable upper limit is expanded, and the combination of the optimum carrier frequencies for the inverters becomes easy. One-chip CPU1 when applying a carrier frequency of about 3kHz, which is optimal for transistor inverters
The required PWM calculation can be performed with this unit.

[Brief description of the drawings]

FIG. 1 is a flowchart of a three-phase sine wave PWM calculation showing an embodiment of the present invention, and FIG. 2 is an operation waveform diagram of a PWM conduction command signal pulse.

Claims (1)

(57) [Claims] 1. A method of calculating a three-phase sine wave PWM (pulse width modulation) in an inverter that drives a three-phase AC motor at a variable speed, the method comprising calculating a conduction or cutoff command period for a main circuit switching element of the inverter. In the command period, a magnitude comparison between a sine wave control signal having a frequency equal to the inverter output frequency and a magnitude of which is proportional to a required output voltage and a carrier triangular signal having a constant peak value is performed for each cycle of the carrier triangular signal. The calculation is performed for any two phases of the three-phase output AC of the inverter, and for the remaining one phase of the output AC, a command period calculation for the two phases is performed based on 3/2 times the cycle of the carrier triangular wave signal. A simplified calculation method for a three-phase sine wave PWM in an inverter, wherein the command period calculation value is obtained by subtracting the sum of the values.