JPS5911273B2 - Induction motor speed control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a speed control device for an induction motor, and more particularly to a speed control device for an induction motor equipped with a primary voltage limiting circuit.

FIG. 1 is a circuit diagram showing a conventional example of this type of control device,
In the figure, 1 is an AC power supply, 2 is a control element circuit composed of a thyristor, etc., 3 is an induction motor, 4 is a finger speed generator for detecting the rotational speed of the induction motor 3, 5 is a speed setting signal, and 6 is a speed setting signal. A differential amplifier for speed control, T an ignition circuit for phase control, 8 a voltage limiting circuit with a dead zone that does not operate below a predetermined input, and 9 the output voltage of the control element circuit 2, that is, the induction motor 3 Voltage detection 1 to detect the primary voltage of
0 circuit, 10 to 12 are parts constituting the voltage detection circuit 9, 10 is a resistor for dividing the primary voltage, 11 is a variable resistor for adjusting the limit voltage, and 12 is a rectifying diode.

Here, as the element configuration of the control element circuit 2, 15 thyristors connected in antiparallel, a silicon bidirectional control element, or a mixed antiparallel connection of thyristors and diodes are used. In the conventional device configured as described above, the induction motor 3 is rotated at zero voltage from the control element circuit 2 connected to the AC power source 1, and the rotation speed of the induction motor 3 is controlled by the finger speed generator 4. The signal is detected as a speed feedback signal, which is compared with the speed setting signal 5 and amplified by the differential amplifier 6.

This compared and amplified deviation signal is applied to the ignition circuit T as a phase control command, and its output controls the induction motor 3 while controlling the phase of the control element circuit 2.
On the other hand, the primary voltage of the induction motor 3 is applied to the differential amplifier 6 through the voltage limiting circuit 8 as the voltage feedback signal 9a of the voltage detection circuit 9, and is compared with the speed setting signal 5 in the same manner as the speed feedback 30 signal described above. amplified.

Here, the output characteristics of the voltage limiting circuit 8 are as shown in FIG. 2, and the voltage limiting circuit 8 does not operate when the input, that is, the primary voltage is below Vo, but operates when the voltage exceeds Vo, and its output is Since the voltage feedback signal 9a is subtracted from the speed setting signal 5, the output voltage of the control element circuit 2 drops and the primary voltage of the induction motor 3 is limited. Since the conventional device is configured as described above, a voltage limiting circuit 8 with a dead band is required, and a comparison circuit must be configured using a differential amplifier 6 as an amplifier for speed control. This has the disadvantage that the control device becomes complicated.

The present invention was made to eliminate these drawbacks, and is composed of a resistor circuit and a transistor circuit, and aims to reduce cost by simplifying the configuration.

An embodiment of the present invention will be described below with reference to FIG.

In the figure, the same reference numerals as in FIG. 1 indicate the same or equivalent parts, and 13 is a transistor circuit that operates with the voltage feedback signal of the voltage detection circuit 9 when the primary voltage of the induction motor 3 exceeds a predetermined value; 18 is a part constituting a transistor circuit, 14, 16, 18 are resistors, 15 is a smoothing capacitor, 17 is a transistor, 19 is a resistor to which the speed setting signal 5 and the speed feedback signal are applied, and 20 is a point. This is an input resistor to the arc circuit 7. It is configured as above,
Those with the same symbols as those in FIG. However, since the voltage drop across the resistor 19 varies due to fluctuations in the rotational speed of the induction motor 3, the voltage drop across the resistor 20 due to the speed setting signal 5 is:
It changes in response to fluctuations in the rotational speed of the induction motor 3.

The voltage drop across this resistor 20 is added to the ignition circuit, and the induction motor 3 is otherwise controlled in the same manner as the conventional example shown in FIG. On the other hand, the primary voltage of the induction motor 3 is passed through the voltage detection circuit 9, smoothed by the capacitor 15, resistors 16, 18 and the transistor 1.
It is added between the base and emitter of 7.

When this voltage exceeds the base-emitter threshold voltage, the transistor 17 becomes conductive, and the collector current flows through the resistor 14, the collector-emitter of the transistor 17,
Since it flows through resistor 18 and a voltage drop occurs across resistor 14, the speed setting signal 5 across resistor 20 drops and the primary voltage of induction motor 3 is limited. Therefore, the resistance value of the variable resistor 11 determines the primary voltage value of the induction motor 3 that reaches the threshold voltage between the base and emitter of the transistor 17, so that a limit voltage value can be set. As described above, according to the present invention, the voltage drop across the resistor is used to compare the speed setting signal and the speed feedback signal, and the voltage drop across the resistor is generated in accordance with the primary voltage feedback signal of the induction motor. By lowering the speed setting signal, the primary voltage is limited, which eliminates the need for a differential amplifier, and the use of the threshold voltage of the transistor as the dead zone of the voltage feedback signal simplifies the configuration. Therefore, the device can be provided at low cost.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example, Fig. 2 is a characteristic diagram of a circuit that inputs the primary voltage of an induction motor and outputs a voltage feedback signal, and Fig. 3 is a circuit diagram showing an embodiment of the present invention. It is. In the figures, the same reference numerals indicate the same or corresponding parts, and 2
1 is a control element circuit, 3 is an induction motor, 7 is an ignition circuit, 9 is a voltage detection circuit, 13 is a transistor circuit, and 17 is a transistor.

Claims (1)

[Claims] 1. An ignition circuit 7 that operates based on a speed setting signal, a control element circuit 2 that performs phase control based on the signal of this ignition circuit 7, and a voltage that detects the output voltage of this control element circuit 2 and outputs a voltage feedback signal. In the speed control device for an induction motor, the speed control device includes a detection circuit 9 and a speed indicator for detecting the rotational speed of the induction motor 3 rotated by the output voltage of the control element circuit 2 and outputting a speed feedback signal. The signal is based on
A transistor 17 is provided which operates when a signal voltage applied between the emitters exceeds a predetermined value.
7, the speed setting signal is lowered by the voltage drop across the resistor due to the collector current, and the speed setting signal is lowered by the voltage drop across the resistor due to the speed feedback signal, thereby lowering the rotational speed. Speed control device.