Re: [CAD_CAM_EDM_DRO] Sizing Rectifiers

jmw@... wrote:

> Sorry, this is a little off-topic--but the group has so much
> horsepower.
>
> Suppose one is making a power supply to deliver 250 amps at 30 volts
> DC 60% duty cycle from three phase 230 using 3 single phase matched
> transformers and a full wave bridge consisting of 6 diodes. The
> ultimate load is resistive (arc welding). What amperage rating should
> the diodes have? Even at full output each diode doesn't feel the
> entire current all the time--right?

Arc welding is a pretty savage environment for semiconductors.
Even if the load is resistive, the transformer windings and welding
cables can cause substantial voltage spikes due to their inductance.
Is this SMAW (stick welding)? You need an inductor to stabilize the
arc, and you need special transformers with high leakage inductance
to limit current. With standard power transformers, the current will
not be limited to any reasonable value when the electrode touches
the workpiece.

If this is GTAW (TIG) or GMAW (MIG) then other kinds of regulation
are needed. If you have HF, you need to keep the high HF voltages
out of the rectifier, or it will pop the diodes.

You might be able to use 250 A diodes because at 3 phase, the
individual diodes only conduct 1/3 of the time. but, you will need
300 to 600 V diodes due to inductive effects.

I'm not sure what sort of bridge you are implementing with 6 diodes.
If you have 3 diodes connected from one winding end of each transformer
to the + terminal, and 3 diodes connected similarly to the - terminal,
then the trasformer will have pure DC flowing through it, which will
saturate the iron in the transformer. A full-wave rectifier with 6
diodes requires center taps on the transformer windings, and
all 6 diodes go to + (or -). All center taps are connected together.
This is not called a bridge rectifier. I think it is sometimes called
a 6-diode star.

There is a 12-diode bridge rectifier, if your transformers have no center
tap.

Jon