Double pulsing thyristors -why? 2

[GTstartup]

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?

 

[sibeen]

To make sure it does turn on.

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.

 

[Skogsgurra]

It is a little bit more complicated than that. If you look at the pulses, you will see that they are 60 degrees apart. The reason is that there are always two thyristors in the current path, one in the positive part of the bridge and one in the negative part. Now, if the A+ and B- thyristors were conducting and you need to gate C- on, there is a possibility that A+ has stopped conducting because of low load (discontinuous current). It is, therefore necessary to make it conduct again. That is why there are always double gating pulses in a bridge rectifier.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GTstartup]

Thanks Scogs,

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?

 

[Skogsgurra]

That is when the load is resistive. Then current stops when there's no voltage to "press" it through thyristor and load, i.e. at around zero degrees.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

The best form of gate control is to use a hard fire technique where the gat pulse is held on for the whole duration that the SCR should remain ON.
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

 

[GTstartup]

Thanks guys for an answer I couldn't get by hours of internet searching

 

[sibeen]

Expanding upon Mark's comment, The best form of gate control is to use a hard fire technique where the gat pulse is held on for the whole duration that the SCR should remain ON., in one set of UPS units that I used to work on it was decided that only a short duration pulse would be used to turn the thyristor on.

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.