JPS62254671A - Control circuit of inverter for driving motor - Google Patents

Abstract

PURPOSE:To prevent a voltage of an induction motor from abruptly varying at starting time by altering the proportional term of a voltage regulator at a predetermined change rate when starting the motor. CONSTITUTION:A deviation between a voltage command value input to voltage regulating means and actual voltage value is input to a multiplier 21. A proportional constant is fed to a soft starting integrator 23, and the output of the integrator is gradually varied at predetermined time change rate only at starting time. The result of the multiplication of a deviation between the voltage command value and the actual voltage value and a varying proportional constant by a multiplier 21 is input to an integrator 22 for a regulator. The output of the multiplier 21 is added to the output of the integrator 22, the voltage command value is further added thereto as the output signal of the voltage regulator. Thus, it can prevent a rush current from generating upon abrupt change of the voltage when a motor is started.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical field to which the invention pertains) The present invention relates to a control circuit for starting an induction motor with an inverter that outputs alternating current of variable voltage and variable frequency to drive the motor.

(Prior art and its problems) Figure 3 is a circuit diagram that outputs alternating current with variable voltage and variable frequency. In FIG. 3, AC power from an AC power supply 2 is converted to DC by a rectifier 3, and a smoothing capacitor 4
After removing the ripple contained in this DC, it is converted back into AC by the transistor inverter 5. By controlling the transistor inverter 5, the voltage and frequency of the AC output by the inverter 5 can be changed. Therefore, the induction motor 6 can be operated at a desired speed.

The speed setter 11 is for commanding the speed of the induction motor 6, and the voltage/frequency converter 12 converts a frequency command signal corresponding to this speed command signal into a pulse width modulation circuit 15.
Output to. Further, the function generator 13 sends a voltage command signal having a predetermined correlation with this frequency command signal to the voltage regulator 1.
Output to 4.

The voltage regulator 14, which is composed of a proportional-integral calculator, inputs the voltage command signal mentioned above and the voltage actual value signal fed back from the output side of the transistor inverter 5, and calculates the partial voltage of these two signals. ! 2 "Soot 1 Ko 1 Sweet 1
Output L- to Kuzukai Lupulse reconnaissance tone H missing word 1. The pulse width modulation circuit 15 generates and outputs a pulse train signal modulated to a desired pulse width from the voltage/frequency converter 12 and the voltage regulator 14 (1), so the base drive circuit 16 By amplifying the signal and supplying it to the base of each transistor constituting the transistor inverter 5, these transistors are sequentially turned on and off to control the output AC voltage and frequency to desired values.

FIG. 4 is a graph showing the characteristics of the function generator in the conventional circuit shown in FIG. 3, with the horizontal axis representing frequency and the vertical axis representing voltage. transistor inverter 5
The correlation between the AC voltage and frequency output by is this 41st
As shown in FIG. 1, when the induction motor 6 is operating at high speed (that is, when the frequency is high), the applied voltage is also high, but as the speed decreases, the voltage also decreases. However, in a very low frequency range, it is customary to apply a constant voltage to avoid the influence of excitation.

By the way, since the constant of the voltage regulator 14 is constant, when starting the induction motor 6, voltage is suddenly applied for the reason mentioned above, and an inrush current flows accordingly, causing the transistor inverter 5 to lose its rated value. This has the disadvantage that the overcurrent will activate the protection device and make it impossible to start.

(Object of the Invention) This invention provides an induction motor that is driven by an inverter that outputs variable voltage and variable frequency, and can be started smoothly by preventing sudden changes in the applied voltage. The purpose of the present invention is to provide a control circuit for a drive inverter.

(Summary of the Invention) When starting an induction motor, the present invention changes the voltage applied to the induction motor by changing the proportional term of a voltage regulator composed of a proportional-integral calculator at a predetermined rate of change. soft start. In this way, a sudden change in voltage at the time of starting is prevented, a large inrush current is prevented from occurring, and smooth starting is attempted.

(Embodiment of the Invention) FIG. 1 is a circuit diagram of a voltage regulating means showing a practical example of the present invention. As shown in FIG. 1, the deviation between the voltage command value input to the voltage adjustment means and the actual voltage value is input to the power nM21.

- The force proportionality constant passes through the soft start integrator 23, so that the output gradually changes at a predetermined time rate of change only at the time of starting, and finally reaches a constant value.

The deviation value between the voltage command value and the actual voltage value described above is multiplied by this changing proportionality constant in the multiplier 21, and the result is input to the regulator Wr1 integrator 22, and the output of this regulator integrator 22 is By adding the output of the multiplier 21 to , and further adding the voltage command value, the output signal of the voltage-to-voltage tfI device is obtained.

Fig. 21 is an operation chart showing changes in the proportionality constant when the voltage regulating means shown in Fig. 1 is used, and Fig. 2 (a) shows changes in the proportionality constant of the output from the transistor inverter. are doing.

As is clear from FIG. 2, when the inverter frequency is output to start the induction motor 6, the proportionality constant increases to a specified value according to a predetermined rate of change over time, and thereafter changes to maintain a constant value. Therefore, the voltage applied to the induction motor does not change suddenly, and therefore there is no risk of inrush current occurring.

(Effects of the Invention) According to the present invention, an inverter operates an induction motor at a desired speed by outputting alternating current power of variable voltage and variable frequency using a voltage regulating means constituted by a proportional-integral calculator. When starting the induction motor,
Since the proportional constant term of the voltage regulator mentioned above is soft-started so that it increases at a predetermined rate of change over time, there is no rush current caused by sudden changes in voltage when starting the motor, and therefore overcurrent trips are avoided. The induction motor can be started smoothly without any problems.

[Brief explanation of drawings]

2 is a circuit diagram, and FIG. 2 is an operation chart showing changes in the proportionality constant when the voltage regulating means shown in FIG. 1 is used. Figure 3 is a circuit diagram showing a conventional example of driving an induction motor with an inverter that outputs variable voltage/variable frequency alternating current;
This figure is a graph showing the characteristics of the function generator in the conventional circuit shown in FIG. 2: AC power supply, 3: Rectifier, 4: Smoothing capacitor, 5:
Transistor inverter, 6:9 conductive motor, 11: Speed setting device, 12: Voltage/frequency converter, 13: Function generator, 14: Voltage regulator, 15: Pulse width modulation circuit, 16:
Base drive circuit, 21 = multiplier, 22: regulator integrator, 23: software Figure 1 Figure 2

Claims (1)

[Claims] 1) The inverter is configured to apply a voltage having a predetermined correlation with a frequency corresponding to the speed of an induction motor driven by an inverter that outputs alternating current of variable voltage and variable frequency to the inverter via a voltage adjustment means. A control circuit for an inverter for driving an electric motor, characterized in that the inverter for driving an electric motor is provided with means for changing a proportional constant term of the voltage regulating means at a predetermined rate of change when starting the induction motor. .