Testing a 3 phase transformer

[innovate.invent]

Hi all.
So I have a 3 phase transformer in a wye wye configuration with the primary and secondary neutrals tied together. It was bought surplus for an arc welder.

I was hoping to build the rest of the PWM control circuitry and all that jazz to get a functioning arc welder out of it.

My problem is I tried to power up one of the primary coils on the transformer with 120v just to see if it was ok ( I didn't want to attach it straight to my rotary phase changer before testing it a bit) and it blew the breaker instantly. It actually caused the crimp on my overhead service from the street to explode. So before I try that again can anyone tell me what went wrong?
I get about an ohm across the primary coil but I am not sure what range it should actually be in.
I isolated the primary and secondary coils and there is no continuity between them.

Perhaps someone could suggest the next step I should take? I know there are tons of possible reasons for an apparent short.
Is it possible that only powering a single phase on the transformer and not all three could cause some kind of crazy inductance surge that I am not aware of?
I have yet to grasp the finer points of induction.
What kind of resistance should I expect from the primary coil? I would have thought something in the kilo-ohms.
The only things I can think to do would be to put a light bulb in series with the transformer and see what happens, and if that doesn't reveal anything then I am gonna have to take it apart and rewind it. :( Which is really not cool, so if there are any better ideas out there please share them.

Thanks

[awright]

You don't say anytiing about the size or ratings of the transformer but if you are hoping to use it as part of a welder it must be fairly large.  That implies a very low DC resistance because the DC resistance is simply that of the (heavy) wire in the winding.  In the absence of very sophisticated and precise low resistance measuring equipment (not a DVM) you won't learn much from a DC resistance measurement except for continuity or open circuit.    For a transformer of a few KVA rating I would expect a primary resistance in the neighborhood of an ohm.  Remember that if the winding had a significant resistance the primary current of several tens of ohms would dissipate a lot of power uselessly inside the windings.

Are you sure you have identified the windings properly (should be fairly easy)?

I'm not familiar with 3-phase equipment but I would think that a single primary of the unloaded 3-phase transformer could be connected across the line without problems.  Shouldn't matter whether or not the neutrals are connected.

Your description of the incoming line hardware exploding is alarming because the main breaker and/or the branch circuit breaker or fuses should have opened long before the overhead line could be damaged even with a direct short across the branch circuit.  That's what they are there for.  Better check out your main panel.

Why are you planning on using a 3-phase transformer if you actually only have single-phase power available?  I am aware of the advantages of 3-phase power in industrial settings (and, in fact, in my garage shop that has several 3-phase machines) but if you must create it with a rotary converter it would seem that you are unnecessarily complicating your life.  A welder of any significant power rating is a pretty heavy load on a phase converter that was designed to run home-type machine tools - but I am making assumptions here. 

High current draw in an unloaded transformer would seem to indicate that one of the windings is shorted.  There may have been a good reason the transformer was on surplus.

Good luck.

awright

[Hero999]

It does seem weird how the neutrals are connected together; was it easy to disconnect them?

I would have guessed it was an autotransformer by the fact that the primary and secondary neutral are the same.

I agree that the primary resistance will be, below 1Ω, if it's a large transformer. If you have a miliohm meter then you could measure the DC resistance of each primary phase, they should be the same is true for the secondary phases.

I would also be concerned that the transformer doesn't have good enough primary-secondary insulation to safely isolate the secondary from the mains so I'd connect the 0V side of the secondary DC bus to earth/ground so if the primary-secondary insulation does break down you won't be electrocuted.

Three phase is a very good idea for a DC power supply because the filter capacitors on the DC side can be much smaller or non-existent if phase shifting transformers are used to provide more phases reducing the ripple further.