Feeding A Single-Phase Transformer from two different phases 2

[Imumahdi]

Hello, We have a 220V/2V single phase transformer with windings rated for 500V.
Is it possible to feed it from two phases (R&Y instead of R & Neutral) and what are the associated risks of doing so?

 

[waross]

Is it possible to feed it from two phases (R&Y instead of R & Neutral) and what are the associated risks of doing so?

That would depend on the supply voltage.
If we make the unpardonable sin of assumption and assume that this is a 120/208 Volt supply, then using two phases is closer to the rated voltage.
Rather than 2 Volts you will get 2 x 200/208 or 1.9 Volts.
HOWEVER if you are on a 230/400 Volt supply, the 400 Volts will saturate the transformer and if the protection does not operate quickly the transformer will self destruct in less than a minute.

windings rated for 500V.

500 Volts is probably an insulation rating for use in auto-transformer and Buck/boost applications.
That is the windings may be isolated from ground and operated at up to 500 Volts to ground.

 

[Imumahdi]

That would depend on the supply voltage.
If we make the unpardonable sin of assumption and assume that this is a 120/208 Volt supply, then using two phases is closer to the rated voltage.
Rather than 2 Volts you will get 2 x 200/208 or 1.9 Volts.
HOWEVER if you are on a 230/400 Volt supply, the 400 Volts will saturate the transformer and if the protection does not operate quickly the transformer will self destruct in less than a minute.

500 Volts is probably an insulation rating for use in auto-transformer and Buck/boost applications.
That is the windings may be isolated from ground and operated at up to 500 Volts to ground.

Thanks for pointing the saturation issue. However, the transformer was originally designed for 400V and was wounded for 230V last moment so the core and everything was meant for 400V (Single Phase) Operation. As for our supply, it's 400V 3PH / 230V 1PH. The transformer is a big sized and is rated for 5000A current.

 

[waross]

As last resort, I would try it.
I would use a very fast acting fuse with enough capacity to energize the transformer.
When a transformer saturates you have I³T on your side.
Energise the transformer with 400 Volts, protected by a fuse that will act in time to protect from saturation overcurrent.
I am not current on fuses.
Perhaps others may give more specific advice on selecting a suitable fuse.
Residual magnetism and wrong point on wave switching may pop a fuse several times before a successful energization.
If the fuse pops on the first try, don't give up, consider that this may be energization inrush.

 

[Imumahdi]

As last resort, I would try it.
I would use a very fast acting fuse with enough capacity to energize the transformer.
When a transformer saturates you have I³T on your side.
Energise the transformer with 400 Volts, protected by a fuse that will act in time to protect from saturation overcurrent.
I am not current on fuses.
Perhaps others may give more specific advice on selecting a suitable fuse.
Residual magnetism and wrong point on wave switching may pop a fuse several times before a successful energization.
If the fuse pops on the first try, don't give up, consider that this may be energization inrush.

Thank you dear for the valuable information dear. I'm willing to magnetize it using a Variac, I have a 9kVA 3 Phase 0-430V Variac and I'm thinking of using two phases from this Variac to connect to this transformer and energize it using less voltage if that helps

 

[waross]

I'm willing to magnetize it using a Variac,

Great idea. Hook in an ammeter and watch for a sudden, non-linear rise of current as you increase the voltage.
That will be the saturation knee.