Transformer Rating for Motor

my question is very simple that is

Unfortunately the answer is not.

http://cr4.globalspec.com/thread/39570/Supply-Transformer-Ratings

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.

As others have said, you should probably talk with either a consultant or electrician as the transformer size depends on a number of electrical and motor variables you have not mentioned, and as such there can be no exact or guaranteed right answer with the limited information given.

The closest figure I can give you is a transformer of at least 25kW when using a soft starter and say 100kW if using a DOL starter. Note that this doesn't guarantee your motor will work if your primary supply to the new transformer is too "soft" and sags excessively due to the additional load.

If what I am saying is still confusing, refer back to my original comment above and get a professional to look at your system.

detail of my question:

induction motor specifications:

rated voltage=400

rated power =7.5KVA

rated current=14amp

transmission line=11kva

distribution line =220v single phase

now how to calculate the KVA of the required transformer for the motor :

1)drive by soft starter

2) direct on line

explain pls

rated voltage=400

rated current=14amp

distribution line =220v single phase

Sounds more like a 3hp motor, not 30hp.

Whats the overload capability of the transformer?

Why is it being fed from its own transformer?

Are you talking about the transformer rating or relay settings?

30hp = 22.4 kW

Add in some heat losses, etc.

Would a 30kVA transformer suffice? I don't know, it just seems rational, so I'm throwing it out there.

Considering :

1] You have no base load

2] Power factor of the motor : 0.8o lag

3]Type of starting : Star & delta

so 30 hp , works out to 22.38 kw

with 0.8 pf ---28 kva ratingkeeping 50 % factor of safety----42 Kva rating transformer

In supporting elkangorito's calculation, there is a very good information can be found in this link

http://www.powerqualityanddrives.com/short_circuit_transformer

- MS

This site is cool, I used it before for transformer sizing !!

Thanks MS .

This method is old but proven. All it takes is a little "expansionary knowledge" to apply the concept to reality.

i want to offer thanks to the this answer writer as you have tried best and helped me in this regard , budle of thanks

reply to ans: 9

this is a very useful discussion due towhich i reached at the design stage. there is rasie tow question which i would like to be cleared.

1) how you calculated the 4% impeadance

2) as you described

The transformer supply voltage at motor start will therefore be;

380 x .76 = 288.8 volts

in this eq. what is the fig . 0.76

discuss pls.

Let me answer it:

1) 4% is the transformer percentage impedance (or voltage impedance). It can be found in transformer name plate.

2) 1.00 - 0.24 = 0.74.

0.24 is from 24%, the percentage voltage drop at motor start.

- MS