http://cr4.globalspec.com/thread/39567/engr-khan

In the first place, why do you need a transformer for a 30HP induction motor. Just connect the motor wire to the starter that get the 3 phase supply from the mains and you can run your motor without any problem.

Any particular reason for the transformer.?

there are so many others custmers using the available transformer so due high load on the transformer the motor needs its own transformer

By installing a separate transformer just for your 30HP motor will not solve the high load power problem of the existing transformer. I presume that your incoming voltage to the existing transformer is 11KV and step down to 415Vac or 380Vac and your 30Hp motor is rated at 415ac or 380Vac. Even if you install an isolation /separate transformer with the Primary input voltage of 415Vac and the Output voltage of the secondary is 415Vac, you still face the same problem, since your main distribution transformer is on high load.

What is more important is to solve the main distribution transformer problem. It is not feasible just to install another transformer for your 30HP motor. It will cost you a lot of money.

Do you have other power supply that you can take from? May be, by diverting your 30HP motor to other power source will able to solve your problem. With DOL starter, your 30HP motor will start with 275 Amps just for a few second and down to the running current of 36.6 amps.

All you need is to have a MCCB of 250 AMP from the new source.

do you mean to start motor using ATS ?

This is not a simple answer.

30HP is about 22kW @ 380-415 volts (3 phase - about 40 amps). If your existing supply can sustain a momentary current of about 300 amps (motor start current) without greatly affecting the supply voltage, go ahead & connect it. If not, you need to know;

1] the inrush current of the motor (Locked Rotor Current) or you can safely assume 8 times the FLA.

2] the maximum allowable supply voltage drop to the motor (NEMA rating or other. In any case, it's usually 10% or less for DOL starting).

After you have this information, you can calculate the transformer size, taking into account transformer impedance. The calculation will give you the correct size transformer without exceeding the rated minimum voltage of the motor.

If you have no idea what I'm talking about, consult an engineer.

Caviat: This information relates to DOL motors only.

thanks all of you reply to my question :supply transformer.but there is still ambieguity

by supply transformer, i mean that there is an isolated/seperate transformer is required for 30 hp motor and what should be the exact rating of this transformer when

1)using DOL and

2)using soft starter expain pls.

See my earlier post.

If you can't give exact details of the transformer & the motor, get a consultant.