Transformers supplied from thyristor stacks
Transformers supplied from thyristor stacks
(OP)
Why do they burst fire into transformers which have resitance heating loads from thyristor controllers
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Transformers supplied from thyristor stacks
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RE: Transformers supplied from thyristor stacks
Transformer fed from, or feeding, thyristors?
Is it the power transformer we are talking about? Or firing (gate) transformers?
Gunnar Englund
www.gke.org
RE: Transformers supplied from thyristor stacks
RE: Transformers supplied from thyristor stacks
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
www.gke.org
RE: Transformers supplied from thyristor stacks
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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Member, P3
electricuwe (Electrical)
I'd like to add a third kind of control to the two mentioned by jraef:
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RE: Transformers supplied from thyristor stacks
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.