Transformer Sizing

[Guardiano]

Hi,

I need to size a transformer to power a variable speed drive which itself will drive a 1200 kW electric motor. Because the displacement power factor is not a really an issue, the transformer could be sized for a 1200 k VA load. How far will distorted power factor affect the transformer sizing ? The transformer ratio is 400/690 V.
Thanks for any input.

Guardiano

 

[iop95]

If 1200kW is nominal output power and considering 98% motor efficiency, lead to 1263kW input active power. But, IM is inductive load, so have to get PF from motor datasheet; maybe at nominal power PF is about 0.9, that mean about 1400kVA input power; depending of application, if overload may occur, take care about how much / how often / duration, thermal and cooling TR specs / requirments, legal standards. I think a 1500kVA TR is much better.

 

[burnt2x]

Just consider the efficiencies of the supply equipment and the power factor of the load. We had a VFD fed 670 kW motor supplied by a 1000kVA trafo. The calculations included the motor PF and the trafo + motor + VFD efficiencies. A 2000 kVA will be the most pessimistic size I recommend.

 

[ScottyUK]

iop95,

The motor PF won't be seen from the drive input, only the drive PF. Allowing for the harmonic loading on the transformer 1.5MVA likely isn't a bad estimate for size.

Why can't the drive vendor provide assistance with sizing?


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

 

[iop95]

Yes, PF motor is won't be seen from drive input; I forgot VFD in may estimation but this PF is "transfered" to TR by VFD sizing. We discuss on nominal values only, but for a practical and economical estimation have to begin with load (load maxim value, service type) and thermal specifications of all devices, environment and legal conditions. 1200kW is nominal power of motor but this value tell us that load have to be below this value; depending of real load value and conditions sizing may conduct to very different values.

 

[waross]

1200 kW mechanical load will require more than 1200 kW electrical.

Bill
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"Why not the best?"
Jimmy Carter