JPH0866090A - Power converter device - Google Patents

Abstract

PURPOSE: To obtain a power converter device by which the operation of an AC motor is continued even when the voltage of an AC power supply is dropped so as to exceed a guarantee range. CONSTITUTION: A power conversion apparatus is provided with a converter 2 which outputs a DC voltage according to a voltage control signal from an AC power supply, with an inverter 5 which converts the DC voltage into an AC voltage at a frequency according to a frequency criterion and which drives an AC motor and with a voltage control part 8 in which a voltage criterion and the frequency criterion are generated in a decided relationship according to a speed criterion, which compares the voltage criterion with the DC voltage and which outputs the voltage control signal so as to reduce an error. Then, the power conversion apparatus is provided with a speed correction means 10 which, when the voltage control signal exceeds a limit voltage which is set to be lower than an upper-limit value, reduces the speed criterion in such a way that the voltage control signal becomes the limit voltage or lower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter for driving an AC electric motor, and more particularly to a power converter capable of continuing operation even when the voltage of an AC power supply drops.

[0002]

2. Description of the Related Art As a power converter for driving an AC motor, a DC voltage is once obtained from an AC power supply having a commercial frequency,
Those that convert to alternating current of variable frequency are widely used. FIG. 3 shows the main configuration of a conventional power conversion device of this type.

In this device, an AC voltage of an AC power supply 1 is converted into a DC voltage by a converter 2 composed of a thyristor, and the DC voltage smoothed by a smoothing circuit composed of a reactor 3 and a capacitor 4 is converted into an inverter. In step 5, the AC voltage is converted into an AC having a desired frequency, and the variable frequency variable voltage power is supplied to the induction motor 6.

On the other hand, the speed reference SP ^* set by the speed setter 9 is given to the inverter 5 as the frequency reference f ^* , and the voltage reference V ^* is supplied as a function of the frequency reference f ^* via the function generator 7. Is generated.

Voltage reference V ^* and voltage field from DC voltage
The voltage controller 8 so that the deviation of the feedback Vfbk becomes zero.
Generates a voltage control signal Vcnt and controls the converter 2 to match the DC voltage with the voltage reference. The voltage reference V ^* is generally given by a proportional function of the frequency reference f ^{* to} keep V / f constant, and the exciting current of the induction motor is generally controlled to be substantially constant.

[0006]

In the above-mentioned conventional device,
When the voltage of the AC power supply 1 decreases, the voltage controller 8 increases the voltage control signal Vcnt so as to keep the deviation between the voltage reference V ^* and the voltage feedback Vfbk at zero, but the voltage control signal Vcnt reaches the upper limit value. After that, the output of the converter 2 does not increase and the voltage reference V ^* cannot be reached. In this case, the torque generated by the induction motor may be smaller than the load torque, and the motor may stall. Further, an excessive current may flow through the power conversion device or the electric motor, which may damage the power conversion device or burn the electric motor.

Although not shown in FIG. 3, the DC voltage is stabilized by adding the current flowing in the capacitor to the output of the voltage controller 10 in order to suppress resonance caused by the DC reactor and the capacitor. However, when the voltage control signal Vcnt reaches the upper limit value, the control margin may be lost and the voltage control may become unstable.

The present invention has been made to solve the above problems, and an object thereof is to stably operate an AC electric motor without stalling even if the voltage of an AC power supply falls below a guaranteed range. An object of the present invention is to provide a power conversion device capable of continuing.

[0009]

In order to achieve the above object, the present invention provides a converter for outputting a DC voltage from an AC power source according to a voltage control signal, and a DC voltage of a frequency corresponding to a frequency reference. An inverter that converts an AC voltage to drive an AC motor and a voltage reference and the frequency reference are generated in a relationship that is determined according to a speed reference, and the error is reduced by comparing the voltage reference and the DC voltage. A voltage control unit that outputs the voltage control signal so that when the voltage control signal exceeds a limit voltage set lower than an upper limit value, the speed reference is set so that the voltage control signal becomes equal to or lower than the limit voltage. And a speed correction means for reducing

Further, the converter includes a smoothing circuit on the output side, which is composed of a reactor and a capacitor, and smoothes a DC voltage. The voltage controller corrects and controls the voltage control signal by a current flowing through the capacitor. A circuit for stabilizing the DC voltage is provided.

[0011]

When the voltage of the AC power supply decreases and the DC voltage decreases, the voltage controller increases the voltage control signal so as to compensate for the decreased DC voltage. In this case, when the voltage control signal exceeds the limit voltage, a signal is output from the speed correction means, the speed reference is reduced so that the voltage control signal becomes equal to or lower than the limit voltage, and the operation of the AC motor is continued. Further, in this case, since the voltage control signal becomes equal to or lower than the limit voltage, a control margin remains, and the function of DC voltage stabilization control can be enabled, so that stable operation can be continued.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A block diagram of one embodiment of the present invention is shown in FIG. The configuration for supplying the variable frequency variable voltage power to the induction motor 6 is the same as in FIG. Figure 3 is different from the speed correction to generate a speed correction amount SPcmp so that the voltage control signal Vcnt and limits the voltage reference Vcnt ^* deviation when the voltage control signal Vcnt becomes limited voltage reference Vcnt ^* greater zero The instrument 10 is installed, and the speed correction amount SPcm is added to the speed reference SP ^*.
The point where p is added is the actual speed reference. The speed compensator 10 is configured to perform proportional-plus-integral operation only when its output has a negative value and not output a positive value.

Here, the operation when the voltage Vin of the AC power supply 1 is lowered when the voltage reference and the frequency reference are given by a proportional function will be described with reference to FIG. When the voltage Vin of the AC power supply 1 is kept within the guaranteed voltage fluctuation range, the output signal (voltage control signal) Vcnt of the voltage controller 8 becomes the value of the limit voltage reference Vcnt ^* as shown in the section 1. The operation is performed according to the set speed reference SP ^* without reaching.

When the voltage Vin of the AC power supply 1 greatly decreases beyond the guaranteed voltage fluctuation range, the output signal Vcnt of the voltage controller 8 is changed to the limit voltage reference Vcnt as shown in section 2.
It acts so as to compensate and control the decrease of the voltage Vin beyond the value of ^* . When the output signal Vcnt of the voltage regulator 8 exceeds the limit voltage reference Vcnt ^*, speed correction signal SPcmp a negative value from the speed compensator 10 is outputted, the actual speed reference as shown in section 3 S _PR ^* voltage The control signal Vcnt decreases until it stabilizes at the value of the limit voltage reference Vcnt ^* . Thus, the speed compensator 10 outputs the speed correction signal SPcmp before the voltage control signal Vcnt reaches the upper limit value, and the actual speed reference S
_{Since PR} ^* is reduced, the relationship between the output voltage of the inverter and the frequency can be maintained at the relationship set by the function unit 7, and the induction motor 6 can be continuously operated without stall.

[0015]

According to the present invention, when the voltage of the AC power supply drops, the speed reference is reduced to maintain the relationship between the frequency and the voltage given by the function generator, and Prevents excessive current from flowing to the motor,
It is possible to continue the operation of the induction motor.

When the voltage of the AC power supply drops,
By limiting the voltage control signal to below the upper limit value, a power converter that has a control margin, maintains a stabilizing control function that suppresses the oscillation of the DC voltage, and can continue stable operation of the induction motor. Can be provided.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram for explaining the operation of the present invention.

FIG. 3 is a block diagram of a conventional device

[Explanation of symbols]

 1 ... AC power supply 2 ... Converter 3 ... Reactor 4 ... Capacitor 5 ... Inverter 6 ... Induction motor 7 ... Function generator 8 ... Voltage controller 9 ... Speed corrector

Claims (2)

[Claims] 1. A converter for outputting a DC voltage according to a voltage control signal from an AC power source, and an inverter for converting the DC voltage into an AC voltage having a frequency according to a frequency reference to drive an AC motor. A voltage control unit that generates a voltage reference and the frequency reference according to a speed reference, and outputs the voltage control signal so as to reduce the error by comparing the voltage reference with the DC voltage; When the voltage control signal exceeds a limit voltage set lower than an upper limit value, a power correction means is provided for reducing the speed reference so that the voltage control signal becomes equal to or lower than the limit voltage. Converter. 2. The power converter according to claim 1, wherein the converter includes a smoothing circuit which is composed of a reactor and a capacitor on an output side and smoothes a DC voltage, and the voltage control unit flows to the capacitor. A power conversion device comprising a circuit for correcting and controlling the voltage control signal with a current to perform stabilization control of the DC voltage.