Buck Boost transformer capacity question

[W2PHL]

This seems too simple so that's why I'm asking.

I have 208v mains (split three phase). If I want 220v at 20 amps, do I just wire a 12v 20amp transformer in boost configuration or is there more to figuring the required capacity of the boost transformer?

I have this lashed up and the boost transformer gets pretty warm with no load. The transfomer is from a crappy imported PS. Could that be why it's getting warm or does the 120 degree phase angle on the mains have something to do with it?

Thanks!

Phil

[Opcom]

Does your 208V supply have a neutral and is Y configuration with 4 wires, like the colored part of the drawing, as if the colored part were the generator or mains?

Is this what you did? It will heat up the transformer and not work well.

[Opcom]

this would work better because the transformer's current is in phase with the load current..

BUT - each side of the 220V will not have the same potential to Neutral, so it can only work with a 220V load that uses no neutral, like the primary of a large plate transformer.

The "un-transformered" side of the 220 will have 120V to neutral.

This is likely messed up as you are asking for 220V, but the usual voltage associated with 208 is 120, and the usual 220-240V issue should be considered, and I am no expert at 3-phase. I hope someone will answer this and clarify any errors.

The diagram shows using a transformer that converts 208V to 12V, and this corresponds to a transformer that converts 240V to 14V. The same ratio. Either can be used. The secondary has to carry the 20 amps and be insulated for >240VAC. The primary needs its own fuse. At 20A the transformer puts out 280VA, and the primary should draw about 1.4A.

You asked for 220V, not 240V. Is this the intention? If you want 240V, add 20V to the secondary winding's voltage considering the 208V input voltage. That is, use a 208/32V transformer or a 240V/37V transformer. Secondary current is the same. 208V primary current would then be more, like 3.2A.

[W2ZE]

If its 208 4 wire wye ( Y ), to get 240v single phase, use 2 phases plus the neutral. If you are voltage specific for a piece of equipment, then use a buck xfmr at the equipment side, not the panel side.

[W2PHL]

Thanks for the help.  Your first diagram is accurate. From what I understand I have legs 1 and 2 from the three-phase supply. So each leg is 120v and between the two legs 208v because of the 120 degree phase angle.

I’m powering an AL-1500 Amp. It has taps for 220 and 240. I thought using 220 would be less stress on the boost transformer.

So when you say "it will not work well" it's because of the phase angle?

I guess I would have the same problem (heating) if I bought a 240v variac to use on my 208v service, yes?

Your second diagram is interesting. It should work since both the plate and filament transformers are 240v.

Or I could leave well enough alone. I get 3.3kv on the plate at idle(at 208v) and the spec calls for 3.6kv.

Thanks for the answers, much appreciated!

[Opcom]

If its 208 4 wire wye ( Y ), to get 240v single phase, use 2 phases plus the neutral. If you are voltage specific for a piece of equipment, then use a buck xfmr at the equipment side, not the panel side.

This would give 120V from line to neutral, and 208V from line to line, right?

[Opcom]

Thanks for the help.  Your first diagram is accurate. From what I understand I have legs 1 and 2 from the three-phase supply. So each leg is 120v and between the two legs 208v because of the 120 degree phase angle.

I’m powering an AL-1500 Amp. It has taps for 220 and 240. I thought using 220 would be less stress on the boost transformer.

So when you say "it will not work well" it's because of the phase angle?

I guess I would have the same problem (heating) if I bought a 240v variac to use on my 208v service, yes?

Your second diagram is interesting. It should work since both the plate and filament transformers are 240v.

Or I could leave well enough alone. I get 3.3kv on the plate at idle(at 208v) and the spec calls for 3.6kv.

Thanks for the answers, much appreciated!

I think the "it will not work well" is because the transformer is working inefficiently due to the phase in the secondary winding trying to be different than the phase in the primary which is usually against the transformer's better judgement.
The second diagram does not try to have different phasing of the primary and secondary transformer currents. If you are happy with the high voltage, and only have to deal with the filament, you could apply a small solution just to that. You could use a smallish 240V variac across the 208V to boost as requred, to make the filament voltage correct. And the blower motor and controls as well.

[W7TFO]

Advice 1...run your 240V stuff on 208 and consider it easier on things.

Advice 2...Use a 240V Variac across 208V set up for 140% output taps and that will make things happy as long as there is no neutral load.

Advice 3...If #1 or 2 doesn't fit into your plan, buy a big enough dry transformer at the electrical supply (Not Home Despot) to take 120V primary to 120/240V secondary.  Most of these have split primary and secondary so's you can get just what you want easily. 

Our home primary service is 277/480 3Ph, so there are a few transformers about doing what is needed.

Now I need an 0-50 RF Amps for my 75M rig...