Transformer sizing

[dklarz]

I'm a mechanical contractor engineer working with a company that builds motor driven products but we no longer have an electrical engineer. I have a fairly simple question - I have a 5Hp 230v 3Ph motor driving an oil pump. The pump oil is being cooled through a finned cooler with a fan driven by a 115v single phase motor. So I need a transformer. If the fan motor calls for 5 peak amps, does that mean I need a .5KVa transformer (115*5=575)?

Thanks - Dave

Contractin' Dave
Mechanical Design Engineer
25 years in the contracting world

 

[ScottyUK]

Dave,

Can you post a photo of the motor nameplate? In principle you're about right but there are usually other complications like overcurrent / overload protection, earthing of the secondary side of the transformer (or not) and so on which all needs to be accounted for. There are often national regulations and codes which need to be considered too - I'm guessing North America somewhere from the 'HP' rather than 'kW' and the 115V supply...?

 

[jghrist]

You may have 115 volts available from the transformer supplying the 230 volt motor. There is often a center tap on the secondary. In any case, you probably have 120/240 volts available somewhere else for feeding lighting loads.

 

[waross]

You need a 120 Volt transformer.
In North America:
Standard motor voltages are multiples of 115 Volts.
Standard system voltages are multiples of 120 Volts.
It is standard practice to use 115 Volt motors on 120 Volt supplies. That allows about 4% for line loss.
Given a choice between 4% over voltage and 4% under voltage, induction motors are happier with 4% over voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter