So I have made a single phase semi-controlled thyristor drive, which is fired by a microcontroller (diagram attached). Upon lot of struggles, I finally succeeded in getting the gate pulse the right way using microcontroller (I have not designed a feedback yet, and it works open loop). And the gate drive circuit is using a NPN switching transistor and pulse amplifier. I have made sure the right pulse go to the right SCR, say the pulses for positive half cycle goes to SCR1 and for negative half cycles goes to SCR2.
The gate pulse which I got was similar to Reliance Electric Drive, with peak gate pulse voltage 14V. And everything was fine, I tested first using a small DC motor (12V), input voltage was 20V, stepped down using voltage transformer, it worked. But then I tried with a Baldor 1 H.P permanent magnet D.C motor, with input voltage 230V, but it did not work. The motor did not start. sometimes, 1 out of 10 times, when I switch on, the motor suddenly starts and stops in maybe 500ms (like a single pulse of current applied). I have attached the ZCD I have used and also the gate pulses for various POT tunings.
SCR used is S6020L and diode D6020L.
Any help to make this circuit work will be much appreciated.
Instead of single pulse suggest pulse train for single phase DC drive due to its BACK EMF.
Thanks Dr. Gode for you reply... I think, Its difficult to control pulse train using microcontroller, as the programming using timers and counters will be very confusing.. but nevertheless, I 'll try that method.... Cause, If the POT setting is such that speed has to be very low... Then the pulses should be near the end of the half cycle and as it is a semi-controlled converter, so there is a chance the train of pulses may overlap to the next half cycle...
Today, strangely, the drive set-up started to work... the speed control is smooth... the strange thing is, it only works if I interchange L1 and L2... I have no idea why this happens...
And I also do not know, how they implement starting control over speed in D.C Drives. That is, if the speed setting is maximum, and when you start the motor, it smoothly starts and the speed slowly increases till it reaches the set speed... Can anyone help me regarding that...
Perhaps it has something to do with ZCD. means synchronization.
Its the problem with the thyristor module, I changed that and its working now...
Can anybody help me with this.. How do they isolate DC armature voltage. I took the armature voltage, fed it through bridge rectifier, then low pass filter and then through voltage divider, so the final output is 5V, which varies linearly as armature voltage changes from 0 to 200V. But I did not isolate it. What's the best method/component to use for isolation.
One my use Hall Current/Voltage sensor for Armature voltage isolation.
https://en.wikipedia.org/wiki/Hall_effect_sensor
You can use a op amp differential amplifier to "isolate" the armature voltage from the control circuit, but note that this is not galvanic isolation. The input resistances of the amplifier are usually in the Mohm range and gain is less than 1. Thus the floating dc voltage is scaled down to be compatible with the control voltage. The op amp inputs must be protected using usual diode networks against voltage spikes.
I advise you to read about the modes of operation of the dc motor driven by single-phase dc converters, and how the back emf prevents the thyristor firing unless the instantaneous ac voltage reaches the back emf for the case of discontinuous armature current operation. High armature inductance widen the current conduction period and eliminates this problem. I have a paper in this area. It is a very old one, but I think it is useful although it deals with DC motor fed from HW thristorized rectifier:
M. A. Abdel-halim, et. al., “Armature controlled d.c motor fed from h.w rectifier circuits,” Scientific Engineering Bulletin, Faculty of Engineering, Cairo University, Vol. 1979/4, 1979, pp. 65-83.
I advise you to try first pure resistive load. Then try R-L load with train of pulses. The last trial should be a load which reflects back EMF such as a battery or a dc motor.
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