JPH01177872A - Current instantaneous value control type PWM inverter - Google Patents

Abstract

PURPOSE:To prevent the generation of a ripple component by calculating the mean value of three-phase ACs and subtracting only the mean value from the currents of each phase. CONSTITUTION:A current instantaneous-value control type PWM inverter is composed of a current regulator 1, a comparator 2, a three-phase triangular- wave generating circuit 3, a main circuit 4 constituting the inverter, a current detector 6, etc., and supplies load 5 with electric power. A mean value circuit 8 arithmetically operating a mean value and an addition circuit 9 are mounted, and the mean value circuit 8 prepares (-1/3) times a-s high as the sum of each output from the current regulators 1a-1c and outputs them. When the sum total of voltage commands at points U-W is brought to zero, the mean value circuit 8 outputs zero, but the mean value circuit 8 operates so that the sum total is brought to zero in voltage commands output from the addition circuits 9a-9c when the sum total of U-W are not brought to zero, and output voltage can be adjusted so as to be balanced at all times.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement of a current instantaneous value control type PWM inverter using a voltage type PWM inverter for current control.

[Conventional technology]

FIG. 2 is a circuit diagram showing a conventional example of a current instantaneous value controlled PWM inverter. In the figure, 1 is a current regulator (however, the subscript aFiU phase, b is V phase, and C is W phase. In other words, 1atiU phase current regulator, 1b is V phase current regulator, 1c is W phase current regulator, and so on 0
2 is a comparator, 3 is a three-phase triangular wave generation circuit, and 4 is the main circuit that constitutes an inverter (a three-phase parallel circuit of a transistor Tr and a diode D as switching elements) 5 is an expansion load, 6 is a current detector, and E is a DC power supply.

The circuit operation will be briefly explained. For example, explanation will be given by taking the U phase as an example. The current regulator 1a is inputted with the deviation (t a t a ) between the actual value of the load current detected by the current detector 6 a and its command value i, and calculates the proportional/integral (P. ) is calculated and output. The comparator 2a receives the adjustment output from the current regulator 1a and the triangular wave signal belonging to the U phase from the three-phase triangular wave generating circuit 3, compares the two, and makes the difference so that the deviation becomes zero. A predetermined switching element (transistor Tr
) outputs a pulse width modulated (PWM) firing command.

In this way, the load current supplied from the voltage source inverter to the load 5 is controlled.

[Problem to be solved by the invention]

Now, the conventional current instantaneous value control type PWM as shown in Fig. 2
Inverters have a problem in that even if the current command value is a clean sine wave, the actual value of the current supplied to the load is a waveform with ripple components superimposed thereon.

This is due to the fact that when the output of the operational amplifier (op-amp) constituting the comparator 2 becomes large, it becomes saturated. In other words, the number of operational amplifiers is the same as the number of comparators, that is, 3.
If it is a phase, there are 3 in total, one for each digit, and 3
Three-phase AC current for each digit flows through the operational amplifiers. 3
Phase alternating current consists of currents that are out of phase by 12σ, so when a current with the maximum amplitude is flowing through one of the three operational amplifiers, a current with a smaller amplitude is flowing through the other operational amplifiers. .

In an operational amplifier, output saturation occurs when this maximum amplitude current flows. Output saturation does not occur in operational amplifiers where the maximum amplitude current is not flowing.

As a result, an imbalance occurs in the three-phase output voltages output from the three operational amplifiers, which causes the actual value of the load current to be
This appears as a ripple component.

An object of the present invention is to prevent ripple components from occurring in the actual load current value even if an operational amplifier as a comparator constituting a current instantaneous value controlled PWM inverter has characteristics that cause output saturation. An object of the present invention is to provide a current instantaneous value controlled PWM inverter.

[Means to solve the problem]

In order to achieve the above object, the present invention includes a detector that detects the multiphase load current supplied from the PWM inverter to the multiphase load, and a difference between the multiphase load current detected by the detector and its command value. A multi-phase current regulator that generates a regulated output upon input, a multi-phase triangular wave generating circuit, and a regulated output from the multi-phase current regulator and a triangular wave from the multi-phase triangular wave generating circuit that are input and both and a comparator that compares the values between the two and creates and outputs a firing command for the switching elements of the main circuit constituting the inverter based on the comparison result. a first arithmetic circuit that calculates the average value by summing the adjustment outputs for each digit; a second arithmetic circuit for subtracting the digit-by-digit adjustment output before it is input to the comparator.

[Effect]

In the case of three-phase AC, the average value of the three-phase AC is taken, and the average value is subtracted from the current of each phase. Therefore, since the average value is subtracted from the maximum amplitude current value, the amplitude becomes correspondingly smaller, and the output of the operational amplifier to which it is input will not be saturated.

On the other hand, the relative value for each digit is the same as before subtraction because the same average value is subtracted for all phases.

In this way, it is possible to ensure that no ripple component occurs in the actual value of the load current.

〔Example〕

Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In this figure, the same parts as in FIG. 2 are given the same reference numerals. In addition, 8 is an average value circuit for calculating an average value, and 9 is an adder circuit.

The average value circuit 8 is a circuit that generates and outputs (-1/3) times the total of each output of the current regulators 13 to 1c.

Originally, the current command values ia, 1b, and 1c should add up to zero, which is the actual current value ia.

Since 1 br I c is also a current flowing into a load (for example, a motor), the sum should be zero.

In other words, if the load input current is ia * ib y ic, then (1) should hold, and it seems that the average value circuit 8 does not operate due to this, but due to the circumstances already mentioned Therefore, on an actual circuit, the sum of the outputs at points U, V, and W does not become zero. Therefore, when the sum of the voltage commands in the average value circuit 8 is zero, this circuit 8 outputs zero, but when the sum of U, V, and W is not zero, the voltage output from the adder circuit 9a t 9b + 9C The average value circuit 8 operates so that the sum of the commands becomes zero, and can always adjust the output voltage to be balanced. This has the advantage that unbalance of load input current can be suppressed.

〔Effect of the invention〕

According to the present invention, in a current instantaneous value controlled PWM inverter, even if the operational amplifier constituting the comparator has characteristics that cause output saturation, it is possible to prevent ripple components from occurring in the actual value of the load current. It has the advantage of being able to

4. Simple ff of the drawing! i! 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional current instantaneous value control type PWM inverter.

Sign li! 1...Current regulator, 2...Comparator, 3...Three-phase triangular wave generation circuit, 4...
・Main circuit, 5...Load, 6...Current detector, 8...Average value circuit, 9...Addition circuit

Claims (1)

[Claims] 1) A detector that detects a polyphase load current supplied from a PWM inverter to a polyphase load, and a difference between the polyphase load current detected by the detector and its command value is inputted. a multi-phase current regulator that generates a regulated output, a multi-phase triangular wave generating circuit; A current instantaneous value control type PWM inverter comprising a comparator that compares and generates and outputs a firing command for the switching elements of the main circuit constituting the inverter based on the comparison result, a first arithmetic circuit that calculates the sum of the adjustment outputs for each phase and obtains an average value; A current instantaneous value controlled PWM inverter, comprising: a second arithmetic circuit for subtracting the control output from the control output before inputting it to the comparator.