Transformers supplied from thyristor stacks 1

[ianjinks]

Why do they burst fire into transformers which have resitance heating loads from thyristor controllers

 

[Skogsgurra]

Transformer fed from, or feeding, thyristors?

Is it the power transformer we are talking about? Or firing (gate) transformers?

Gunnar Englund

http://www.gke.org

 

[ianjinks]

This is the power to a transfomer approx 100A 3phase, which heats an electric resistance furnace

 

[Skogsgurra]

Does it do it even when running at full power? Or only when running with reduced power? In the latter case, it is possible that you have a problem with thyristors conducting unsymmetrically so that you get a DC component in the primary winding.

That saturates the transformer core and makes it draw excessive primary current.

It is also possible that you need to use a "K-rated" transformer. That is a transformer with windings and core laminations specially adapted to thyristor wave-forms.

But check the DC component first. You can do it with a current clamp (DC, of course) and a DMM. An oscilloscope is even better. It will show you exactly what is going on in the primary.

Gunnar Englund

http://www.gke.org

 

[jraef]

In SCR power controllers for resistive loads such as this, there are 2 kinds of firing techniques. One is Phase Angle firing, the same as is done for motor control, which turns the SCRs on for a portion of each cycle. These types of power controllers are the ones you must use for secondary transformer applications like that, and it is done all the time in order to reduce the current handled by the controller when the power requirement is high. The drawback is that Phase Angle firing creates line noise and harmonics continuously.

Another type of SCR controller uses what is called a Zero Cross Varialbe Time Base firing method, known for short as a Zero Cross controller. This has an advantage in that the SCRs are only turned on at the zero cross point, so they do not create line noise or extra harmonics, and are frequently used in sensitive equipment such as semiconductor manufacturing equipment. The downside of this firing scheme is that it can only be used on full voltage resistors, no transformer in the circuit. By turning on at the zero cross, the SCR is always providing full conduction. So what is done to give the same effect as reducing the voltage is to reduce the number of cycles the SCR is turned on for. The SCRs are then turned on in short bursts of cycles, then left off for a short "burst", then turned on again. Firing into a purely resistive load, this has the effect of reducing the RMS voltage to the resistor. These bursts are varied in duration by cycles of On and Off states. You cannot (or at least should not) use these on the primary of a transformer because the transformer field will be colapsing and expanding at full values, so the magnetization inrush will greatly effect the current values seen, and make for erratic behavior on voltage output. If you have a Zero Cross power controller and it s feeding a transformer, someone probably misapllied it.

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[electricuwe]

I'd like to add a third kind of control to the two mentioned by jraef:

You can control the output voltage of a transformer by phase angle control on the primary winding. With only a single short pulse to trigger for each halfwave it might happen, depending on power factor of load taking magnetising current into account, that the thyristor is not able to trigger at that time it should because it's anode voltage is still negative. As a result, a DC-voltage would be applied to the primary and the transformer would saturate. To avoid this kind of fault it's stronly recommended to use long pulses (180°) or burst for such controllers.

Typical application, beside resistance heating, is high current power supplies for electroplating or high voltage power supplies.