I studies a lot of books regarding phase controlled thyristors, and how we control speed of the D.C motor. We vary the firing angle of the firing pulse given to Gate of the thyristors, say from 22 degree to 84 degree, and the output D.C voltage is controlled... But coming to industry, actually checking the gate pulse of various d.c drives, I realized something different is going on...
1. I checked Reliance Electric DC Drive, the drive was working and the motor speed changes as I vary my POT. So I checked the gate pulse on the oscilloscope and found that the gate pulse is not varying as I vary the POT. It stands stationary at around 22 degree, for the both the half cycle. I have attached the DSO output, the shape of the pulse looks good with no high freq noise, with max volt of 14V. But do anyone working with D.C Drives or working in industry has any idea how the speed of the D.C motor changes inspite of firing pulse remaining stationary in the same place
2. I checked it with other drive, Beel controls, its an old drive from 1990's, the gate was not a single pulse, but multiple pulses with lot of high freq noise - 1000HZ, and as i vary the pot the high freq noise and the height of the pulses kept on increasing.
Is the DC drive 1-phase to DC or 3-phase to DC?
also can you show some more results for various output voltages/Speed??
What you have read is what is actually practiced. There is some problem in detection. please check again more carefully.
1) The voltage waveform distortion reveals that the motor is lightly loaded, resulting in discontinuous conduction of current (dc motor current exists for only a short time and then goes to zero). Thus, the expected relationship between fining angle and average dc voltage will not hold true. Check dc current.
2) However, there must be some change in firing angle. Your oscilloscope triggering is with respect to a voltage in ch 1. What voltage is it?
3) In several schemes, in order to reduce gate power dissipation and the size of the pulse isolation transformer, a train of high frequency pulses is used instead of a single solid pulse. Still, starting position decides firing angle.
Professor, The voltage is from the transformer inside the drive, and I did not connect any load to the D.C drive when I was checking the gate pulse with scope, and of course you cannot.... If I connect the load and check the gate pulse with oscilloscope probes, It is like shorting the output of P.T... I did that once, it began to burn... fire started...
I agree with your 3rd point... I read that somewhere too...
Ronak, its a single phase Drive.... I do not have any more results, I gave the drive back...
Joseph, it is still not clear to me: you did not connect any load to the DC motor or you did not connect any load to the DC drive System ? In the second case, the system is without any current flow !
Pulses can be checked on-line safely using an isolated channel DSO. Even if you have a 2-channel non-isolated one, use a voltage transformer to check the ac voltage through isolation and monitor the gate pulses without problem. However, the earth pin of power cord must not be connected to earth but left floating. The earth pin is actually connected to the clip of every probe and is the main cause of circuit damages. Oscilloscope will be safe without earth connection provided voltages are within safe limits. Of course, the metal parts of body will be live so avoid touching them when connected to circuit.
Dear Joseph,
What Sujit sir is stating is correct....i also think some measurement error must be there...also Always when you try to debug power and control circuits you should use an isolated DSO.....
Thanks Professor Biswas and Ronal Nemade...
So I was wrong... Because of PI controller, the gate pulse did not vary as I varied the POT, as there was no load connected and the error must have been maximum.
And I do not have an isolated scope, and do not know how to measure with voltage transformer... its sounds difficult to a guy like me with no experience.
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