JP2544196Y2 - AC motor control device - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for an AC motor.

[0002]

2. Description of the Related Art A configuration example of a conventional control device for controlling the speed of an AC motor will be described with reference to FIG. In the figure, 1 is an AC power supply, 2 is a rectifier circuit, 3 is a PWM (pulse width modulation) inverter, 4 is an AC motor, 5 is a speed sensor, 6 is a current detector, 7 is a speed control unit, and 8 is current control. An amplifier (ACR), 9 is a triangular wave oscillator (OSC), and 10 is a pulse width modulation (PWM) circuit.

In the control device shown in FIG. 3, a speed command ωr
^{For *,} the speed signal ωr from the speed sensor 5 is input to the speed control unit 7 to control the speed of the AC motor 4. The speed control unit 7 calculates a speed element and a current element,
As a result, the primary current command I ₁ of the AC motor 4 is output as an output.
^* (AC signal) is output. Since the speed controller 7 is not directly related to the present invention, a detailed description thereof will be omitted.

The above-mentioned primary current command I ₁ ^* of the alternating current is
An AC current detection signal I ₁ detected by a current detector 6 provided between the WM inverter 3 and the AC motor 4 is compared and amplified by a current control amplifier (ACR) 8. The output signal E ₁ ^* of the current control amplifier 8 is PWM-modulated (pulse width modulated) by the PWM circuit 10 by the triangular wave carrier outputted from the triangular wave oscillator 9, and is given to the PWM inverter 3 as a base signal, thereby providing a current control system. Is composed.

[0005]

FIG. 4 shows a circuit configuration of the current control amplifier 8 in such a current control system.

The current control amplifier 8 includes an operational amplifier Amp
And a proportional integration circuit including a feedback circuit connected in series with the input resistance Ri, the capacitor Cf and the resistance Rf, and a variable resistor VR connected to the output of the operational amplifier Amp and varying the gain of the feedback circuit. You. Thus, the current control amplifier 8 has the AC current command I ₁ ^* and the current detector 6
A deviation signal from the AC current detection signal I1 detected at ( ₁₎ is input, and the deviation signal is proportionally integrated.

When the current control amplifier 8 controls the current flowing through the AC motor to control the speed, the speed control unit 7 sets the gain of the feedback circuit to one point by the variable resistor VR. The current control amplifier 8 operates only at a constant gain with respect to the AC primary current command I ₁ ^* whose output frequency varies. As a result, when the gain of the current control amplifier 8 is set to a stable current control gain at a low frequency by the variable resistor VR, at a high frequency, the current according to the primary current command I ₁ ^{* is} reduced due to a decrease in the gain of the current control amplifier 8. No flow, stable current control is no longer possible,
Problems such as vibration due to torque ripple in the output load of the AC motor and abnormal heating of the AC motor occur. Further, the gain of the current control amplifier 8 is
If the current control gain is set to a stable value at a high frequency by R, the current control amplifier 8 becomes unstable at a low frequency due to a high gain, and as a result, there arises a problem that the speed control does not operate stably. Such a problem occurs remarkably when the AC motor is controlled over a wide range from a commercial frequency to a high frequency.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. An object of the present invention is to provide a current control system using a proportional-integral circuit even if the primary current command of an AC motor changes in frequency over a wide range. It is another object of the present invention to provide a control device for an AC motor capable of keeping the gain of a current control amplifier substantially constant and performing appropriate and stable current control.

[0009]

In order to achieve the above object, the present invention detects a current flowing in an AC motor with a current detector, and detects the AC current from the detection signal and a current command signal of the AC motor. A current control means for calculating an alternating current flowing through the motor;
In a control device for an AC motor that controls the AC motor via a circuit and a PWM inverter, the current control unit compares and amplifies a detection signal detected by the current detector with the current command signal, and sends the amplified signal to the PWM circuit. An operational amplifier that outputs a comparison amplification signal; a proportional integration circuit including an input resistor of the operational amplifier and a capacitor and a resistor that constitute a feedback circuit of the operational amplifier; and an output of the operational amplifier including a plurality of resistors connected in series. And a high-speed switching element that is connected in parallel with some of the resistors of the voltage-dividing feedback circuit and electrically turns on and off between the resistors. The high-speed switch element is controlled by a gate control circuit that performs on / off control according to the rotation speed of the AC motor.

[0010]

In the present invention, when the frequency of the primary current command changes, the primary frequency element of the AC motor is given to the high-speed switching element connected in parallel with the resistor in the voltage dividing feedback circuit, and the high-speed switching element is changed according to the primary frequency. By changing the resistance value of the resistor by the switch operation, the gain of the current control amplifier can be kept substantially constant even when the primary current command of the AC motor changes in frequency over a wide range.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an embodiment of the present invention. Elements having the same functions as those of the conventional configuration shown in FIG. 4 are denoted by the same reference numerals, and description thereof is omitted.

In FIG. 1, of the configuration of the current control amplifier 8A, an operational amplifier Amp, an input resistor Ri, a feedback circuit composed of a series connection of a capacitor Cf and a resistor Rf, and resistors R1 and R2 connected to the output of the operational amplifier Amp. Is a substitute for the conventional variable resistor VR, and determines the gain of the current control amplifier by a voltage dividing feedback circuit. SW is a high-speed switch element connected in parallel with the resistor R1 and the capacitor C1, and is connected in series with the resistor R2. The gate signal of the high-speed switch element SW is supplied from a gate control circuit 20 that receives the rotation speed ωr, which is the primary frequency element of the AC motor, and controls whether the high-speed switch element SW is fully on, all off, or on-duty controlled. It is configured to:

In the circuit configuration as described above, current control per phase will be described below. First, the resistance of the resistor is set to R1 = R2, and this resistance is selected to be sufficiently smaller than the impedance of the feedback circuit. When the primary frequency of the AC motor is low, the gate control circuit 20 outputs a signal for turning on all the high-speed switching elements SW. Therefore, the feedback circuit of the current control amplifier 8A includes the capacitor Cf
Is connected only to the resistor Rf. A capacitor Cf for stabilizing the current control amplifier in the low frequency region,
Select resistor Rf.

Here, the transfer function of the current control amplifier 8A is generally expressed by the following equation (1). G (s) = (1 + Rf · Cf · s) / (β · Ri · Cf · s) (1) s: Laplace operator β: Gain by voltage dividing feedback circuit (β = 1) Resistance of resistors R1 and R2 The value is ignored because it is sufficiently smaller than the impedance of the feedback circuit. Therefore, the transfer function of the current control amplifier 8A when the high-speed switching element SW is fully turned on is expressed by the following equation (2). G (s) = (1 + Rf · Cf · s) / (Ri · Cf · s) (2)

Next, when the primary frequency of the AC motor becomes high, the gate control circuit 20 switches the high-speed switch element SW.
And outputs a signal that turns off all the signals. Therefore, in the feedback circuit of the current control amplifier 8A, the capacitor Cf and the resistor Rf are connected in series, and the gain by the voltage dividing feedback circuits R1 and R2 is amplified. The detailed description of this voltage division feedback circuit is omitted because it is in the literature. Current control amplifier 8A in this case
When the transfer function is obtained, β = １ ／ in the equation (1), and the following equation (3) is obtained. G (s) = (1 + Rf · Cf · s) / (1/2 · Ri · Cf · s) (3) s: Laplace operator 1/2: Gain by voltage division feedback circuit (β = １ ／)

By rewriting equation (3), the following equation (4) is obtained. G (s) = 2 (1 + Rf · Cf · s) / (Ri · Cf · s) (4) Comparing the equations (2) and (4), the equation (4) shows that the gain of the current control amplifier is higher. Is doubled.

Now, as an example, the resistance of the voltage dividing feedback circuit is set to R1
= R2, it is clear from equation (1) that the gain changes by changing the resistance value. Therefore, the high-speed switching element S
It is possible to vary the gain of the current control amplifier 8A by turning W on and off. FIG. 2A shows output characteristics of the gate control circuit 20 in the case of the above control. In the figure, the high-speed switching element SW
The switch has a hysteresis width. In addition,
The capacitor C1 connected in parallel with the resistor R1 is unnecessary at the time of the above control.

Next, the present invention is applicable to a case where current control accuracy is required more than the above control or a case where a torque fluctuation of an output load of an AC motor due to a shock at a moment when the high-speed switching element SW is switched becomes a problem. Another embodiment will be described.

The configuration of the current control amplifier 8A is the same as that of FIG. The difference is that the on-duty control is performed when the output signal of the gate control circuit 20 shifts from all ON to all OFF in addition to the above-mentioned SW ON and OFF. The output characteristic of the gate control circuit 20 in this case is shown in FIG.
(B).

As an example of performing the on-duty control,
The frequency of the triangular wave as a carrier is increased to several tens of kHz, and the velocity element is compared with the triangular wave. Here, simply by connecting the high-speed switch element SW in parallel with the resistor R1, the frequency component of the triangular wave as the carrier wave appears at the output of the current control amplifier 8A. Therefore, a capacitor C1 is connected in parallel with the resistor R1 for the purpose of removing this. Since the frequency of the triangular wave as the carrier is sufficiently higher than the primary frequency and the resistance value of the voltage dividing feedback resistor circuit is low, the delay due to the capacitor C1 is not a problem for the current control, and the on-duty control is continuously controlled. it can. If the frequency of the triangular wave as the carrier is increased, the value of the capacitor C1 can be reduced.

As described above, by performing the on-duty control by the high-speed switching element SW, the resistance value of the voltage dividing feedback circuit is continuously varied, and the gain of the current control amplifier is also changed from the equation (2) to the equation (4). Instead of instantaneously changing
Equation (2) continuously changes to equation (4).

Therefore, if the gate control circuit 20 determines the primary frequency at which the high-speed switching element SW is fully turned on, on-duty controlled, and completely off, the above-mentioned problem, that is, when the current control accuracy is required or when the high-speed switching element SW is required. The torque fluctuation of the output load of the AC motor due to the shock at the moment when the SW is switched is solved.

[0023]

As described above, according to the present invention, even when the primary current command of the AC motor varies in frequency over a wide range, the primary frequency element of the AC motor is used in the voltage dividing feedback circuit for determining the gain of the proportional integration circuit. This is applied to a high-speed switching element connected in parallel with a resistor, and the high-speed switching element makes it possible to vary the voltage division ratio of the voltage-dividing feedback circuit, thereby keeping the gain of the current control amplifier substantially constant. As a result, there is an excellent effect that appropriate and stable current control can be performed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a current control amplifier according to an embodiment of the present invention.

FIG. 2 is an input / output characteristic diagram of a gate control circuit for controlling a high-speed switching element in the current control amplifier of FIG. 1; FIG. 2A is an input / output characteristic diagram when the high-speed switching element is turned on and off; FIG. 4B is an input / output characteristic diagram when the high-speed switch element is fully on, on-duty controlled, and completely off.

FIG. 3 is a circuit diagram of a conventional control device for an AC motor.

FIG. 4 is a circuit diagram of the current control amplifier of FIG. 3;

[Explanation of symbols]

1: AC power supply, 2: Rectifier circuit, 3: PWM inverter,
4: AC motor, 5: speed sensor, 6: current detector,
7: speed control unit, 8, 8A: current control amplifier, 9: triangular wave oscillator, 10: PWM circuit, 20: gate control circuit,
SW: High-speed switch element.

Claims (1)

(57) [Scope of request for utility model registration] 1. A current flowing to the AC motor by the current detector
Detecting the detection signal and the current command signal of the AC motor.
Current for calculating the AC current flowing to the AC motor from the signal
Control means, and an output signal of the current control means
The AC motor through a circuit and a PWM inverter
The control device for an AC motor to be controlled, wherein the current control
The means includes a detection signal detected by the current detector and the current.
A command signal is compared and amplified, and a comparison amplified signal is sent to the PWM circuit.
Operational amplifier for outputting a signal, and input resistance of the operational amplifier
And a capacitor and a resistor constituting a feedback circuit of the operational amplifier.
Proportional integration circuit consisting of a resistor and multiple resistors connected in series
Connected to the output side of the operational amplifier and
A voltage-dividing feedback circuit for setting the gain of the circuit;
Connected in parallel with some resistors in the path
A high-speed switching element that is turned off, and the high-speed switching element is turned on and off according to the rotation speed of the AC motor.
A control device for an AC motor, wherein the control device is controlled by a gate control circuit .