# Help with motor-transformer calculation.

• posted

I have had this question for a while. Can anyone answer it?

If I wanted to run a 600V 50HP motor with a line current of approx 50A on a 480V line. I would calculate that there is a 120V difference so the autotransformer would be 120V X 1.73 X 50A or just over 10 KVA.

This is a small transformer, with small windings and small connection lugs. It just does not seem right to be connect and run a motor of this size. Something tells me that it is inadequate in size.

Is this a correct calculation? Remove dashes "-----------" to e-mail.

snipped-for-privacy@shaw.ca

• posted

It isn't right. It also wasn't right last time you posted:

bud--

• posted

Thus spake Frank White:

I don't know.

But here is a web site that is full of documents titled "Selection Steps" and such to help you decide.

And there's a tech support phone number somewhere on that site that will connect you to people who can give you a definitive answer to your question. They make these transformers.

Good luck,

• posted

I don't know which country you are in, but in the USA according to the NEC, the full load current for 575 volt 50 hp 3-phase motor is 52 amperes. This would require a minimum transformer of 50 KVA. An old engineering rule of thumb is 1kva per horsepower when sizing tranformers for motor loads. There is no listing for a 120 volt 50 hp single phase motor. I don't know where you are coming up with a "120 volt difference" or the use of an autotransformer.

• posted
• posted

So, if the start load is small, start at 480V, switch to 600 for running. The transformer is then not affected by the start inrush and is capable of handling the running conditions. Go up a bit in size 480/600 Auto (3 phase) at,say, 15KVA. Essentially the opposite of the usual autotransformer reduced voltage scheme for motor starting. It is feasible. Start current seen by the supply will be about 64% of normal (motor 80%) -none of which is through the transformer, and start torque will be about 64% of normal.

• posted

In the transformer industry we use 1.1 kVA per HP as a rule of thumb. That is why motor drive isolation transformers are rated at odd KVAs such as 220, 275, 330, 440, 550, etc. These correspond respectively to motor HP ratings of 200, 250, 300, 400, 500, etc.

Steve Lockridge, Manager Alfa Transformer

---------------------------------------------------------- Drive Isolation Transformers In Stock Thru 770 KVA! Pad Mounted Transformers In Stock Thru 3750 KVA!

----------------------------------------------------------

• posted

Dang!

• posted

But the proposed transformer is an autotransformer- that makes a difference. In this case transformer action accounts for 1/4 of the transferred power. In this case using the 1.1 ratio the result is under 14KVA -say 15 as suggested.

• posted

Here is anothe example of this on a 600V system

3 PH 40HP 460V motor FLA = 52A wire size is #6 CU

600 to 480V autotransformer would be 13.5 KVA (600V - 480V) x 1.73 x 52A = approx 11KVA

Use three 600V-120V 5 kva transformer in a buck configuration for a 15KVA bank, you will be wiring a motor that requires a #6 copper wire to feed it, to a transformer that has # 12 GA wires coming out of it. It doesnt seem right.

I bought a 30 KVA autotransformer that has #12 wires for connecton. The #6 wires that feed the motor get a bit warm during normal use, I can imagine how warm the #12 GA wiring will be.

Remove dashes "-----------" to e-mail.

snipped-for-privacy@shaw.ca

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.