Thyristor Firing

[Prospero]

We have a requirement to soft start a 800kw pump, the ramp time is expected to be around 30secs.

Our consultant tells me the soft starts will be 'block fired'.

I (partially) remember we had some furnaces which were thyristor controlled and there were issues with harmonics and high earth and neutral currents, we changed the firing from 'phase angle' to 'burst firing' (or was it the other way round!?) and the problems went away.

What do all these firing modes mean and how does one choose between them?

 

[Skogsgurra]

I think that he meant that the firing pulses are solid blocks of gate current and not of the chopped version.

The latter is more common, needs smaller firing transformers. Block firing may be desirable in some cases. But I cannot sy why, really.

The period packet switching you probably mean by "block firing" can not be used with any motor control. I do not think that your consultant meant that. At least not if he is any good.

Gunnar Englund

http://www.gke.org

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

 

[LionelHutz]

The motor has to be phase angle controlled. You will have the associated harmonics during the start. But, they really aren't so bad and it's unlikely you'll have any issues for the 30 seconds.

The block firing being referred to is a continuous block of gate current to trigger the thyristor when it should be on. The other way to trigger a thyristor is to use a series of small gate pulses. FYI, the block firing really is the best way, especially with devices over 4kV.

Your furnaces went from phase angle to pulse width modulated (so many cycles fully on then so many cycles fully off).