JPH0564457A - Dead time compensating method in inverter control - Google Patents

Abstract

PURPOSE:To suppress torque ripple due to dead time in low frequency region by deciding the polarity of sine wave at the stage for combining two-phase sine wave output voltage command signals into a three-phase sine wave output voltage command signal and combining a required compensation component into the sine wave. CONSTITUTION:A voltage command V and a square voltage at the joint of resistors RA, RB are combined at a ratio of the resistances RB, RA to produce a compensated output voltage command V' which is then fed to a PWM comparator 4. The compensated output voltage command V' has such waveform as the sine wave of the output voltage command is superposed on a square wave obtained by eliminating the waveform distortion. Consequently, the compensation ratio of the resistor RB with respect to the output voltage command V has high effect on the output voltage command V close to zero whereas the effect is low for a high output voltage command V. According to the invention, torque ripple due to dead time can be suppressed in low frequency region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dead time compensation method in inverter control for suppressing the influence of a short circuit prevention period on the output side in a PWM inverter control device.

[0002]

2. Description of the Related Art A PWM inverter is often used to drive an AC motor at a variable speed. In this case, when trying to drive the electric motor without the speed feedback loop, current hunting is likely to occur in a specific frequency region at light load, which may cause uncontrollability, and a large torque ripple is likely to occur. Have a big problem.

In particular, in low speed driving, the effect of torque ripple becomes large, and the effective variable speed range becomes a rotation speed range of about 20 to 1, which cannot be applied to applications where vibration at low speed is disliked.

The cause of this is that the power switching elements T _{U to} T _Z forming the inverter main circuit 8 shown in FIG.
There is a delay in the OFF operation, and when the switching elements connected in series for each phase u, v, w are turned on at the same time for a short time, a DC power supply short circuit is caused and the switching elements are destroyed. Therefore, it is necessary to delay the switching command for turning the OFF state of each switching element into the ON state. The delay time of the command from the OFF state to the ON state is called dead time, and this influence causes waveform distortion in the output voltage as shown in FIG.

The voltage reduction amount ΔV of the waveform distortion due to the dead time when the PWM method is the triangular comparison method is ΔV = Ed * Td * Fc, where Ed: inverter main circuit DC voltage (V) Td: dead time ( S) Fc: can be represented by triangular wave frequency (Hz).

In the recent PWM inverter, the triangular wave frequency Fc is in the audible range (about 10 KHz) from the viewpoint of noise reduction.
The number set higher is increasing, and this increase in switching frequency also makes the problem of waveform distortion a big problem.

When an AC drive is driven by such an inverter device, there are many vibrations, especially in the low frequency region, and it cannot be applied to applications where vibration is disliked. The following method is used as a method for solving this.

(1) A method of reducing the torque ripple at low speed by providing a current control loop and controlling the waveform of the current flowing through the AC motor.

(2) A voltage control system in which the output voltage is detected and compared with the voltage command to determine the control signal for the switching element.

(3) Depending on the polarity of the output voltage command signal,
A method of outputting a compensation signal of a voltage ΔV corresponding to a voltage reduction amount of waveform distortion in a superimposed manner, that is, a method of using a waveform shown in FIG.

[0011]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Since both methods (2) perform feedback control, by selecting an appropriate detection method and control gain, the characteristics seen from the vibrating surface are good, but the cost is increased.

In the method (3), when control is performed using a general-purpose CPU, even the gate control signal of the inverter is C
Since the calculation capacity of the program in the PU is large (the higher the switching frequency, the larger)
In the PU, the output voltage commands v _u *, v _V * for the two phases obtained by calculating the phase of the two-phase output of the sine wave and performing D / A conversion are external hardware, as shown in FIG. The three-phase voltage commands v _u *, v _V *, v _W * are obtained by combining in the adder circuit 1 and compared with the triangular wave v _C by the PWM modulation comparators 4 _U , 4 _V , 4 _W. You are getting a signal.

In this case, the two-phase output voltage commands v _U *, v _V
* Voltage reduction of waveform distortion due to dead time
Even if the waveform of V is superimposed and synthesized, the waveform of the synthesized phase does not become the original waveform as shown in FIG. 7. Therefore, when the CPU performs this voltage ΔV operation, the D / A conversion hardware is used. Need to be added, which leads to an increase in the amount of calculation processing in the CPU.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to reduce the torque ripple in the low frequency region due to the dead time without increasing the cost. It is to provide a dead time compensation method in inverter control that can be performed.

[0015]

In order to achieve the above object, a dead time compensating method in an inverter control according to the present invention uses an output voltage command signal in which the output voltage decrease due to the dead time has the same polarity as the command signal. Superimpose,
In the inverter control method for compensating for the waveform distortion of the dead time by using this composite signal as the output voltage command,
At the stage where the three-phase sine wave output voltage command signals are combined from the two-phase sine wave output voltage command signals, the polarity of this sine wave is determined and the necessary compensation component is combined with this sine wave. ..

[0016]

Since the three-phase output voltage command waveform is obtained from the two-phase output voltage command waveform and the compensation signal is superimposed and output according to the polarity of the output voltage command signal, the processing of the CPU is increased. Not accompanied.

The ratio of the compensation signal component to the output voltage command component before compensation in the output voltage command after compensation has a great influence when the command voltage is close to zero, and has a small influence when the command voltage is high. It is possible to compensate the waveform distortion due to.

[0018]

1 shows a general-purpose CPU-controlled PWM inverter device to which the method of the present invention is applied, and FIG. 2 shows one phase of an output voltage command generating circuit after compensation.

In FIG. 1, v _U *, v _V * is a CPU (not shown) that calculates the phases of U and V phase outputs of a three-phase sine wave, and D
U / V phase voltage command obtained by A / A conversion, v _W *
A voltage command for the W phase, which is obtained by synthesizing the data for the phases in the adding circuit 1,
2 _U , 2 _V , 2 _W is the output after compensation, which creates the output voltage command after compensation from the voltage commands v _U *, v _V *, v _W * and outputs it to the PWM modulation comparators 4 _U , 4 _V , 4 _W. It is a voltage command generation circuit. In the drawing, 5 is a triangular wave generating circuit 6 _U, 6 _V, 6
_W is a hysteresis comparator, 7 is a short circuit prevention time creation / base amplifier circuit, and 8 is an inverter main circuit. As shown in FIG. 2, the output voltage command generation circuit 2 after compensation of each phase receives the output voltage command v * (sine wave) of each phase, and a square wave voltage V _S having the same polarity as the waveform of this sine wave. And a resistor R _A , R _B circuit for comparing the output voltage command v * with the square wave voltage V _S from the zero-cross comparator 3, and a voltage command from the connection point of the resistors R _A and R _B. The corrected output voltage command v ′ * obtained by combining v * and the square wave voltage V _S with the ratio of the resistors R _B and R _A is output to the PWM modulation comparator 4. The resistances R _A and R _B are given by the following relationship.

[0020]

[Equation 1]

The waveform of the output voltage command v '* after compensation is shown in FIG.
The waveform is such that the sine wave of the output voltage command v and the square wave of the voltage ΔV are superimposed as shown in FIG. Therefore, the output voltage command v *
The ratio of the compensation value from the resistor R _B to ## EQU1 ## has a large effect when the output voltage command is close to zero, and has a small effect when the output voltage command is high, so the waveform distortion of the output voltage due to dead time is improved. ..

[0022]

Since the present invention is configured as described above, it has the following effects.

(1) With almost no increase in cost,
Further, it is possible to reduce the torque ripple in the low frequency region due to the dead time without increasing the processing of the CPU.

(2) Since the control stability in the low frequency region is high, a large rotational unevenness has conventionally been generated in the rotational speed range of about 1/20, but it is almost smooth even in the rotational speed range of about 1/200. It is possible to obtain a proper rotation.

(3) Since the feedback loop is not used, the detection circuit and the feedback control circuit are unnecessary, and the reliability can be improved.

(4) Application of Inverter Since a constant compensation component is superimposed on the sine wave command in the range of all frequencies, the compensation amount greatly affects when the inverter is operated at a low frequency (low voltage), When operated at high frequency (high voltage), the influence of the compensation amount is almost eliminated, it operates in the low frequency region such as reduction of torque ripple, and it does not cause malfunction at high frequency.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing an inverter device according to an embodiment.

FIG. 2 is a block circuit diagram showing an output voltage command generation circuit after compensation.

FIG. 3 is a waveform diagram showing an output voltage command after compensation.

FIG. 4 is a circuit diagram showing an inverter main circuit.

FIG. 5 is a waveform diagram showing an inverter output.

FIG. 6 is a block circuit diagram showing an inverter device according to a conventional two-phase three-phase combined output voltage command.

FIG. 7 is a waveform diagram of an output voltage command when correction is performed before three-phase synthesis.

[Explanation of symbols]

1 ... Adder circuit, 2, 2 _U , 2 _V , 2 _W ... Compensated output voltage command generation circuit, 3 ... Zero cross comparator, 4 _U ,
4 _V , 4 _W ... PWM comparator, 5 ... Triangular wave generation circuit, 6 _U , 6 _V , 6 _W ... Hysteresis comparator, 7 ...
Dead time creates based amplifier circuit, 8 ... inverter main circuit, T _U through T _Z ... power switching elements, M ... AC motor, R _A, R _B ... resistance.

Claims (1)

[Claims] 1. An inverter control for compensating for the waveform distortion of dead time by superimposing an output voltage decrease caused by dead time on the output voltage command signal with the same polarity as the command signal, and using the composite signal as an output voltage command. In the method 2
The present invention is characterized in that the polarity of this sine wave is determined at the stage where the three-phase sine wave output voltage command signal is synthesized from the phase sine wave output voltage command signal, and the necessary compensation component is synthesized into this sine wave. Dead time compensation method for inverter control.