Double pulsing thyristors -why?
Double pulsing thyristors -why?
(OP)
Hello all.
What is the purpose of double pulsing a thyristor, as is sometimes seen in a fully controlled three phase converter? The thyristor is already conducting so why fire it again?
Anybody know?
What is the purpose of double pulsing a thyristor, as is sometimes seen in a fully controlled three phase converter? The thyristor is already conducting so why fire it again?
Anybody know?






RE: Double pulsing thyristors -why?
Years ago, at a UPS manufacturer I used to work at, we used to hit them with a square wave that used a 1 kHz signal within the wave. If the thyristor failed to turn on with the first strike it would with the second or third...atc, etc.
RE: Double pulsing thyristors -why?
The chopped gating pulses that you (ometimes) see is because the thyristors need a certain width of the gating pulse so the thyristors turn on reliably when the load is inductive. A wide pulse has a rather high DC component and that would saturate the gating transformers. Chopping the pulse makes the use of smaller transformers possible.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Double pulsing thyristors -why?
Iv'e seen, as you say, that the second pulse is at the same time as the pulse to the negative thyristor.
But in the case of, say a converter as part of a drive, why would the current be discontinous? Wouldn't the thyristor pass current until it reaches zero degrees?
RE: Double pulsing thyristors -why?
When there is a DC motor connected and running, the motor generates a counter-EMF that stops current from flowing at any angle. Earlier the faster the motor runs and the higher the counter-EMF is. There are also other circumstances like actual current and inductivity in the circuit. But the simple counter-EMF model explains most of what happens.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Double pulsing thyristors -why?
This is difficult to achieve with small transformers, and so the second best method is to use picket fence firing where the long pulse is broken up into a series of pulses, commonly with a 5:1 mark space ratio.
Single pulse triggering where there is an extended period of time where there is no gate trigger signal can result in premature SCR commutation due to voltage disturbances on the supply etc. If single pulse firing is used, the pulse must be long enough to ensure that the load current rises well above the latching current of the SCR, and there msut be a trigger pulse at each time where another SCR is triggered while the SCR is expected to remain ON.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Double pulsing thyristors -why?
RE: Double pulsing thyristors -why?
It became apparent after a year ot two that this wasn't a great idea. The failure rate of the thyristors was extremely high. The gate resistance would go high and eventually the thyristor would fail to turn on.
The unit was quickly re-designed so that the thyristor was gated for the duration of the on period.