Transformer Use... in Reverse?

Hello Guest,

A few more details, i am assuming by 440 volts you mean 3 phase...L-L is 440 volts,

1) What is the capacity of transformer in Kva?

2) How do you propose to start the motor? is it by a soft starter or Y/D?

3) What is the capacity of your 3 phase? single phase generator in Kva?

The transformer is a 50KVA 3 phase transformer, the AC alternator is capable of 100KVA. By 440V, I do mean line-to-line. I plan to trim the AC alternator voltage output to the transformer via field control, I can get 0-260V output from it via this. Then the voltage to the motor will vary between 0 and 450V.

Unless you use a VFD to start the motor, the transformer and Generator both will stretched.

You did not mention the mode of starting the motor...

Good luck.

The transformer may be srretched for a very short amount of time though (in terms of minutes), it is only to be run at near rated load for a short load test and then switched off again.

The transformer is currently wired in star-delta (220-380), thus if i had the possibility to rewire delta to a star then i could apply 147V to the 220V side and get the required 440V out on the secondary side for my motor. Do you agree?

I think that you are asking about changing your generator from delta to Wye to match the transformer? (Most transformers I've worked with do not allow such an easy modification ) If so, then yes, I would agree. Although it is not an ideal situation, it is workable. It allows the connection of a neutral to the transformer from the generator. The generator would then supply 147/254 to the transformer, which in turn would supply the 440V to your motor.

Just take into account that your weak point is the transformer. To not exceed the 100% rating of the transformer, limit the current (steady state) to 113A on the low side and 66A on the high side for the voltages you want. Obviously, it is not desirable to run anything at 100% of it's rated capacity for extended periods, and 80% continuous load is usually the rule of thumb (NEC). If this cannot be reasonably controlled, remember to take the ambient temp into consideration as well. Power transformers are fairly hardy, and if you can keep it cool or allow it to cool between starts, it should serve you well.

I know this is not the absolute best solution. Given the current criteria, it is a workable one. Money, time, or both often forces one to choose such a solution. If you can, I would still recommend that you consult with your transformer manufacturer.

If the transformer is rated 50 KVA at 380 volt, that's about 76 amp per phase. So even if everything else is OK - working "wrong" way round, higher voltage than nominal, question of starting etc, it will be overloaded at 93 amp. You could ask the supplier what he thinks but I'd be amazed if he gives his blessing to higher voltage or current than nominal. He might OK the reverse working, I suppose.

Cheers........Codey

If the primary windings are rated for 380V, then driving them at 440V will produce greater saturation effects than connecting them to a 440V supply. (Because of the greater losses at this voltage, the effective step up ration will be reduced). This is likely to saturate the core or at least significantly increase the iron losses. Unfortunately your load is synchronous so you can't raise the frequency by 15% to compensate.

I've done this with smaller transformers and attained bad results. Llike everyone else has indicated you have to verify you don't exceed your kva limit or rated voltage or current.

If those factors aren't considered you have a good chance to burn the wiring inside the xfmr.

The high voltage side will have a lower current rating than the low voltage side so be sure the motor won't pull more current than the high voltage side is rated. Good luck!! I also want to do this using a pole xfmr for a tesla coil.

HI,

Unless the transformer is rated for 50/60hz and you are using it at 60hz the increased voltage will cause damage to the transformer. As others have stated you cannot go above the kva rating of the transformer. Based on your specs I would recommend the transformer be at least 112kva 50/60hz rated and only used at 60hz.

The statement above applies to Sola/Hevi-Duty transformer and I would call the tech servcie dept of the company that built the transformer just to make sure.