Transformer sizing
Transformer sizing
(OP)
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
Thanks - Dave
Contractin' Dave
Mechanical Design Engineer
25 years in the contracting world






RE: Transformer sizing
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...?
RE: Transformer sizing
RE: Transformer sizing
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
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"Why not the best?"
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