DOL starting motor, 2 dif sized txf's

[Latango]

Quick question:

If you use a generator (say 750kVA) as a power supply, stepping up through a 500kVA transformer with around 5% impedance, then try to DOL start a 180kW SC motor hooked up to a fan. Will the generator see a significantly lower startup current than if a 1500kVA 5% impedance transformer is used?

 

[ScottyUK]

Assuming both are 5% impedance the ohmic value of the 500kVA transformer impedance will be 3x that of the 1500kVA transformer. The added impedance will reduce the starting current, but it's impossible to say by how much without a lot of other information.


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If we learn from our mistakes I'm getting a great education!

 

[Latango]

Yeah to be honest I don't HAVE any more information, only that the generator was tripping its circuit breaker on o/c while DOL'ing the fan while hooked up to a 1500kVA transformer, but not when hooked up to a 500kVA transformer. There is no fault on the 1500kVA, no fault gases in the oil. The transformer was powered up first, then the fan powered up. I can't find out what curve the OCR on the generator is set up with, csa of the cables, length of the cables, startup load of the fan, the type of fan or anything like it. It's even over the other side of the country so I can't go have a look.

 

[jraef]

Starting energy will be the same, but if there is significant voltage drop caused by the smaller transformer saturating, the current may end up lower on the transformer secondary so the length of time that the current remains high might be significantly different.

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[waross]

Normally either transformer would start the motor. The lower impedance transformer (1500KVA) will allow a little more current to be drawn from the generator. You are falling into a small window where the small difference makes a difference.
I don't see many sophisticated breakers on a generator of that size. The breaker is probably a thermal magnetic which is tripping on instantaneous current.
The instantaneous (magnetic) trips may be set one notch higher.

Bill
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Jimmy Carter

 

[7anoter4]

I agree with waross.The difference is only 5-6% and in both cases
the generator has to trip as the overload will be more than 80%[minimum,depends on voltage drop].