# Sizing a 3 phase transformer

• posted

I have been fighting high amp problems with a compressor 10 hp motor running at 240V 3 phase. I am getting 100+Amps using my clamp-on meter during the initial start before it blows my 50 amp breaker. I am considering bumping up the voltage after my 15 hp phase converter to

480 using a transformer. As I understand it the KVA = KV * A * 1.73 for a 3 phase transformer. So a 30 KVA transformer should yield 36Amps at 480V. If I use this as the secondary I need 240 72 amp input for the primary.

I assume I would need a tranformer to handle BOTH motors even though one is before and one after it? So 25Hp = 22.4 amps? And 30KVA =

30,000/746 = 40 HP bigger than my combined motors. Am I sizing this 30 KVA transformer correctly or do I need to add in other factors and need a bigger transformer?
• posted

As I understand your question you have a 240V 3 phase motor running from a single phase 240V supply with a phase converter between them, and you want to connect a 240/480V 3 phase transformer to the output of the phase converter and connect the motor as 480V to lower the 240V single phase starting current. If correct, the transformer won't help.

The 1st paragraph only mentions a 10 HP motor; it isn't obvious what " "BOTH" motors are or how they are connected. Increase breaker size? Service size?

Bud--

• posted

I agree with the other poster. I believe your going to have to

1. get a bigger phase converter
2. change the motors to single phase
3. install a soft start, which MIGHT create more problems.

The 100 amps is about right for a the motor starting current. (6 x FLA)

• posted

The motor before will not affect the transformer rating. The motor and the applied voltage, not the HP rating, determines the starting current. If you try, as it appears that you want to, running a 240V motor at 480V, you can expect an inrush current at the motor of approximately double what you now get (along with enough toruque to do serious damage). I strongly suggest that you get someone who knows what they are doing -what it will cost you will be a lot less than the cost of replacing the motor and equipment.

• posted

According to the National Electric Code, the full load va of a three phase motor is not HP x 746. Motors are not 100 percent efficient. For example, the full load current for a 30HP 3 phase motor at 460V is 40 amps. 40 x

1.73 x 460 = 31,832 va. This is 1061 va per HP.

The motor should run as well on 240 volts as it will on 480 volts. Your problem is the inrush current. You need a star-delta starter, or a soft-start starter, or a bigger breaker and converter.

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