JPH0440953B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a speed control device for an induction motor.

[Prior art]

A conventional speed control device for this kind of induction motor includes a controller such as a thyristor interposed between the induction motor and an AC power source, and controls the firing angle of a firing signal applied to the controller. It is known that the speed of the induction motor is controlled by this.

FIG. 1 is a characteristic diagram showing the relationship between the voltage and current applied to the induction motor when a firing signal is supplied to the controller at a firing angle of 80 degrees in the speed control device for an induction motor according to the prior art. It is. In the figure, A shows the power supply voltage, B shows the ignition signal, and C and D show the voltage and current applied to the induction motor.
In the figure, when the ignition signal P is supplied at time T1, the controller is turned on and current flows through the induction motor.
Here, since the current flowing through the induction motor lags behind the applied voltage in phase, it flows until time T2, at which point the controller is turned off. Thereafter, when the next firing signal Q is applied at time T3, the controller is turned on again and current flows through the induction motor.
By repeating this process, the induction motor can be rotated at a predetermined speed.
The rotation speed is determined by the firing angle of the firing signal.

Further, FIG. 2 shows the relationship between the voltage and current applied to the induction motor when a firing signal is applied at a firing angle of 45 degrees. In the figure, A shows the power supply voltage, B shows the ignition signal, and C and D show the voltage and current applied to the induction motor. Since the current flowing through the induction motor lags behind the applied voltage in phase angle, the ignition signals P, Q, R
If is output at 45 degrees, the ignition signal Q is output before the controller turns off, so the ignition signal becomes invalid, a half-wave current flows through the induction motor, and the induction motor does not rotate. Therefore, if the firing angle is made too fast in an attempt to increase the speed of the induction motor, the induction motor will stop. Therefore, when controlling the speed of the induction motor by changing the firing angle, it is necessary to limit the firing angle range. However, in order to avoid problems caused by the firing angle being too early, the cut portion of the voltage applied to the induction motor cannot be made zero.
There is a drawback that when the voltage applied to the induction motor is cut, the starting torque rapidly weakens.

On the other hand, as is clear from the characteristic curve of slippage and torque shown in Fig. 3, the induction motor has a slippage of 1.
The induction motor has a characteristic that the starting torque is small when The characteristic is that the torque at startup suddenly weakens.

[Summary of the invention]

The present invention has been made in view of the drawbacks of the prior art, and by supplying a DC voltage as an ignition signal to the gate of the controller at startup, it is possible to obtain a large torque at startup and to reduce the rotation speed. To provide a speed control device for an induction motor that can reliably shift to firing angle control even when the speed does not reach a set level or when the speed rises quickly at startup.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 5 to 1.
This will be explained based on Figure 0. In the figure, 1 is a full-wave rectifier that performs full-wave rectification of the power supply voltage, and 2 is a zero-point detection circuit that detects the zero point of the full-wave rectified waveform output from the full-wave rectifier 1 and outputs a zero-point detection signal A. 3 detects the rotational speed of the induction motor M, and outputs a speed detection signal B.
4 is a waveform shaping circuit that shapes the waveform of the speed detection signal B output from the speed detection section 3. Further, 5 is a speed setting section that outputs a speed setting signal C so as to set the rotational speed of the induction motor M to a desired speed.

Here, the specific configuration of the speed detection section 3 will be explained in the sixth section.
As shown in the figure. In the figure, 6 is the rotation axis of the induction motor M.
A disk 7 with four notches 7A formed around it is attached to the rotating shaft 6, and an auto-interrupter consisting of a light emitting source 8 such as a light emitting diode and a light receiving part 9 such as an autotransistor is mounted across the notches 7A. By providing 10, the rotational speed of the induction motor M can be detected.

Reference numeral 11 denotes a microcomputer, and the microcomputer 11 includes an input circuit 12 that A/D converts and inputs a signal from the outside, a CPU 13, a memory 14, and a D/A converter that converts a signal from the CPU 13 and outputs it. It is composed of an output circuit 15. Here, the microcomputer 11 inputs the zero point detection signal A, speed detection signal B, and speed setting signal C, and outputs the speed detection signal B and speed setting signal C.
The ignition signal D is controlled based on the comparison. Further, 16 is a photoelectric relay to which the ignition signal D is input, and the photoelectric relay 16 has a light emitting element 17 such as a light emitting diode that emits light when the ignition signal D is input.
and a light receiving element 8 such as a phototransistor that induces a pulse voltage when the light from the light emitting element 17 is input.
8 and the pulse voltage induced by the light receiving element 18 are input as gate signals, and the AC power source 19 and the motor M
and a triax 20 as a controller that connects and disconnects the connection between the two.

The embodiment of the speed control device for an induction motor according to the present invention is constructed as described above, and its operation will be explained next. First, a desired rotational speed for rotating the induction motor M is input through the speed setting section 5 (step S1), and then the induction motor M is started. At this start-up, the microcomputer 11 outputs the ignition signal D as shown in FIG.
becomes conductive, the power supply voltage is directly applied to the induction motor M as shown in FIG. 7C, and the current shown in FIG. 7D flows. For this reason, a large starting torque can be obtained. Thereafter, the microcomputer 11 determines whether the rotational speed of the induction motor M has reached a set level prestored in the memory (step S3), and when the rotational speed of the induction motor M reaches the set level, the microcomputer 11 A firing signal D having a predetermined firing angle calculated from the rotational speed of the electric motor M and the set speed set by the speed setting unit 5 is output, and thereafter the induction motor M is driven by firing angle control. The setting level for cutting off the DC voltage as the ignition signal may be stored in a memory in advance, or the speed setting signal C may be used. Furthermore, if the induction motor reaches a set level of speed at startup, it can sufficiently maintain the specified speed during subsequent driving even if the applied voltage is cut by about 10%.

In the present invention, a timer means is provided as an internal function of the microcomputer or externally attached to the microcomputer, and the timer means outputs a signal at a set time after starting the induction motor. Cut the DC voltage output as a signal, and then use the ignition signal D.
The rotational speed is controlled by controlling the firing angle. FIG. 9 shows a flowchart for such control. Step 30 is control based on the signal output from the timer means.

In the present invention, a comparison means for comparing the speed of the induction motor with a set level and a timer means for outputting a signal at a set time after starting the induction motor are provided as internal functions of the microcomputer or externally attached thereto. The content is to shift to firing angle control by cutting off the DC voltage outputted as the firing signal based on the signal outputted first from the comparison means or timer means. This improves reliability. FIG. 10 shows a flowchart of such control. In the same flowchart, even if the comparison means does not output a signal and the rotation speed does not reach the set level, when the timer means outputs a signal after the set time, the DC voltage as the ignition signal is cut off. That will happen. Furthermore, when the speed rises quickly at startup, it is possible to shift to firing angle control quickly.

〔Effect of the invention〕

As explained above, according to the present invention, since a DC voltage is applied as the ignition signal of the controller at the time of starting the induction motor, a large torque can be obtained at the time of starting. Further, since the DC voltage is cut off by the output of the speed comparison means and the timer means, the firing angle control can be reliably performed, thereby improving reliability.

[Brief explanation of the drawing]

Figures 1 and 2 relate to a speed control device for an induction motor according to the prior art, and show the power supply voltage, firing signal, and voltage applied to the induction motor when the firing angle is set to 80 degrees as shown in Figure 1. Fig. 3 is a characteristic curve diagram of slippage and torque in an induction motor; Fig. 4 is a characteristic curve diagram showing the relationship between the cut width of applied voltage and torque at startup in an induction motor; 5 to 8 show embodiments of the present invention, FIG. 5 is a block diagram of a speed control device for an induction motor, FIG. 6 shows the configuration of a speed detection section, FIG. 6A is a front view, and FIG. Figure 6B is a front view of Figure 6A, Figure 7 is a time chart showing the power supply voltage, ignition signal, voltage and current applied to the induction motor in the present invention, Figures 8, 9, and 10th
The figure is a flow diagram showing the operation. 3...Speed detection section, 5...Speed setting section, 11...
...Microcomputer, 20...Triack (controller), 19...Power source, M...Motor. In addition,
In the figures, the same reference numerals are used for the same or corresponding parts.

Claims (1)

[Claims] 1. An AC power supply voltage is supplied to the induction motor through a controller that is turned on by an ignition signal supplied to the gate and held until the current drops below a certain value, and is also supplied to the gate of the controller. In an induction motor speed control device that performs speed control by controlling the firing angle of a firing signal, when starting the induction motor, a DC voltage is supplied to the gate as a firing signal to turn on the controller. a control means for setting the induction motor to a state, and a timer means for outputting a signal after a set time has elapsed after starting the induction motor;
Comparing means for comparing the speed of the induction motor with a set level is provided, and the supply of the DC voltage is stopped based on the first output signal of the output signal of the comparing means and the output signal of the timer means. A speed control device for an induction motor, characterized in that the speed control device shifts to firing angle control in this manner.