Is the thyristor phase-shift control waveform of a 6w single-phase capacitor AC motor normal?

tzcl

Is the thyristor phase-shift control waveform of a 6w single-phase capacitor AC motor normal? [Copy link]

Single-phase AC motor, no-load test. Use an oscilloscope to measure the voltage waveform at both ends of the motor and observe the thyristor opening status

The voltage waveform at both ends of the motor, the trigger angle of the first figure is 2.5ms , the trigger angle of the second figure is 6.5ms , and the trigger angle of the third figure is greater than 7.5ms . The fourth figure shows the zero-crossing signal and the thyristor trigger signal without a motor.

One question is: there is a big jitter in the motor waveform, is it normal?

The second problem is that after more than 7.5ms , the motor thyristor can't be cut off in the reverse direction, but is turned on in the forward direction, and the motor rotates slowly. The motor does not stop during the whole process. Is this normal?

chunyang

The waveform is normal and the phenomenon is normal. When the thyristor controls the inductive load, it is important to add an RC absorption circuit. Otherwise, when the thyristor is turned off, once the back electromotive force generated by the inductive load exceeds the voltage rise rate allowed by the thyristor, the thyristor will be triggered and unable to turn off. Also pay attention to zero-crossing detection. Both inductive and capacitive loads will cause the voltage phase and current phase to be offset, which may also cause problems with the thyristor switch.

tzcl

chunyang published on 2020-3-18 13:01 The waveform is normal and the phenomenon is normal. When the thyristor controls the inductive load, it is important to add an RC absorption circuit, otherwise the back electromotive force generated by the inductive load when the thyristor is turned off will be...

Thank you for your answer! The thyristor is connected to an RC circuit. The zero-crossing signal is perfect when the motor is not connected. After the motor is connected, it is a little deformed. I think it should be normal because of the influence of the thyristor control circuit.

The waveforms of the power supply and zero-crossing signal during the motor driving process.

When the trigger pulse is between 2.5ms and 7.5ms, the speed changes slightly from 280r/s to 250r/s. After 7.5ms, the speed drops to 25r/s immediately, without any transition speed. Directly adjusting the trigger angle cannot achieve a continuous change in speed from 280r/s to 25r/s. Is there something wrong with the zero-crossing check you mentioned? You said, "Also pay attention to zero-crossing detection. Both inductive loads and capacitive loads will cause the voltage phase and current phase to be misaligned, which may also cause problems with the thyristor switch." How to eliminate the problems caused by the voltage and current misalignment?

chunyang

tzcl posted on 2020-3-18 17:00 Thank you for your answer! The thyristor is connected to an RC circuit. The zero-crossing signal is perfect when the motor is not connected. After the motor is connected, it is a little deformed. I think...

Solution: add an RC absorption circuit with appropriate parameters, and the zero-crossing detection circuit should be reasonable. However, it is useless to say that it is correct. It is better to post your specific circuit and analyze the specific problem.

tzcl

chunyang posted on 2020-3-19 11:17 Solution: Add an RC absorption circuit with appropriate parameters and a reasonable zero-crossing detection circuit. However, it is useless to say that it is correct. It is better to post your specific circuit, ...

In theory, there is no problem with the circuit. I just can't figure out where to modify it.

nian1206

Give a thumbs up, the poster is really a good person for sharing his information so completely.