Sizing Transformer

[Boiler79]

I need to size a transformer to step down 3 phase 460 to three phase 230. Iam running several motors using VFD's. My question is should I use the FLA of the motors or the VFD's input ratings? Total motor FLA is 150, the Drive input rating is 260.

 

[mike_kilroy]

my customer engineering departments use the motor max continuous ratings to pick the xfmr, then size CB or fusing to protect it. I have heard others suggest this may not be best code decision and try to go with the huge vfd input ratings instead. I am not a code person. I assume your big discrepancy is due to oversized drives rather than drive efficiencies (which should be in the 95+% range)

 

[jim dungar]

The NEC provides no guidlines for acutally sizing transformers, it does however address drives.
Your feeder to the drives must be sized based on the VFD input rating.

Based on the lack of information you provided, and the silght difference between using a 75kVA or a 112.5kVA transformer, at this point I would lean towards the larger transformer.

 

[petersonra]

I need to size a transformer to step down 3 phase 460 to three phase 230. Iam running several motors using VFD's. My question is should I use the FLA of the motors or the VFD's input ratings? Total motor FLA is 150, the Drive input rating is 260.

I wonder why someone would have 460 V available and buy 230 V drives.

 

[mike_kilroy]

I wonder why someone would have 460 V available and buy 230 V drives.

maybe existing 230v only old non vfd insulation motors?