Inrush question

[winnie]

IMHO a good reason to use thyristor soft start to limit transformer inrush is that you need to have some sort of hardware to bypass the resistor anyway; in any application where that bypass is a thyristor, then I'd consider phase controlling that thyristor and ditching the resistor.

You would be trading the design/size of the resistor for the complexity of the thyristor control. I don't know which wins.

If there is no load on the transformer, then a low wattage resistor with very sloppy resistor bypass is quite likely sufficient; the transformer itself limits any long duration heavy current flow through the resistor, and the magnetizing current will be low, so you don't need a big resistor and you don't need accurate timing. On the other hand, if you are starting a loaded transformer, then the design is likely much more critical.

At any given power level, precharge on a transformer is probably of lower total energy and shorter duration than precharge on a rectifier capacitor bank.

-Jon

[S'mise]

Originally Posted by Besoeker

Using a resistor to limit inrush current isn't like a soft start circuit.
The resistor is just two discrete stages. It's in circuit or shorted.
By contrast, the thyristor soft start provides a continuously variable voltage.
Clever and expensive? I wouldn't have thought so.
Simple light dimmers can use either triacs at the low power end or thyristors at higher powers. It's fairly well-established technology. Light dimmers are not generally very expensive.

Yes, I think of a soft start to be just that; two stages. Resistors used for an instant and then taken out of circuit. Thyristors are essentially a high current SCR. (Either on or off) I was talking about tsr switches that fire the thyristors for an instant before supplying line voltage. Variable voltage is not the objective here.

See link.

http://www.einschaltstrombegrenzer.c...wieme-2004.pdf

[Besoeker]

Originally Posted by S'mise

Yes, I think of a soft start to be just that; two stages. Resistors used for an instant and then taken out of circuit. Thyristors are essentially a high current SCR. (Either on or off) I was talking about tsr switches that fire the thyristors for an instant before supplying line voltage. Variable voltage is not the objective here.

See link.

http://www.einschaltstrombegrenzer.c...wieme-2004.pdf

Yes.......interesting link.
Bad translation but it seems to be a version of the (not so new) technique for switching at the right point in the cycle to achieve minimum inrush. For best results you'd need som idea of remnance too.

Slide 40 for three phase is exactly the configuration that would be used for a variable voltage soft starter.

Thyristors are not specifically high current SCRs.
Thyristor* is the term generally used in UK for the same type of device that is called the SCR in the US.
The SCR, being the term for silicon controlled rectifier is possibly a better description than thyristor which, I seem to recall, is derived from thyratron and transistor.
*I suppose, to be more accurate, the SCR is a subset.