45kva transformer, 380 primary to 208 secondary 2

[desertsheik]

I have a 3 phase 45kva transformer, the primary side will be 3 phase 380 and the secondary side will be 3 phase 208.
What is the formula for figuring the amps on the primary side and the formula for the secondary side.I want to install a disconnect on the primary side. This is a 50 hz system and what problems could I expect to my 120v equipment?

 

[qassure]

Hi desertsheik!
you have not specified the load or the KVA rating of the transformer. How ever the following formulae may be used for rough estimate.
F.L Secondary current= KVA rating X 1000/Secondary Voltage
F.L Primary current=F.L.Secondary currentXTransformer ratio
Transformer ratio = Secondary Voltage/ Primary voltage.

For detailed estimate copper & iron losses of both primary & secondary should be considered.
Regarding your 120V equipment, your secondary voltage is 208V ,3 phase, where single phase voltage will be approximately 120V.hence you may connect and operate your equipment to one of phase and nutral, with out any problem.

 

[davidbeach]

Not quite.

The transformer is given as 45kVA.

Current in either winding is kVA*1000/sqrt(3)*voltage.

It is often easier to work in the form kVA/sqrt(3)*kV.

So, the primary would be 45/sqrt(3)*.38 and secondary would be 45/0.36 or 125A. Work with it long enough and you know immediately that sqrt(3)*.208 is .36 and sqrt(3)*.48 is .831.

120V equipment, with induction motors, designed for 60Hz systems, will run at 5/6 normal speed. Universal motors should be less affected by the reduced frequency. Anything that uses zero crossings for a clock function will also run slow.