Determination of 3-Phase Power Distribution Transformers

[superspider]

Guys,

I have question regarding determination of 3-phase power distribution transformers rating for such a load.
For example if we had a load(motor) with 290HP (217kW) with 0.88 PF .
What I did,
S=217/0.88=247 kVA
and then we find any transformer rated close to this value say like 300 kVA

Is that the way suppose to be done or NOT?


Could you please correct me if am wrong??
thanks

 

[avogel]

To check your apparent power result, you can calculate the FLA to see if it matches nameplate on the motor : I = 247000 / (V * .88 * 1.73)

Your answer should be close. But I must say there are special considerations when sizing a transformer and motor feed. I would not recommend someone not familiar with the electric code attempt to do so. Otherwise you run the risk of undersizing conductors and overcurrent protection.

 

[UtilityDave]

The calculation you suggest doesn't take into account the efficiency of the motor.

You should divide by that as well. It should be something like .85 more or less.

I agree that there are many other things to think about with a motor that size.

 

[dpc]

For sizing transformers based on connected motor horsepower, using an approximation of about 1 kVA/hp usually works out pretty well.

This factors in the efficiency and power factor of the motor. For very large medium-voltage motors, it becomes a bit overly conservative, but for low voltage motors, it generally keeps everyone happy.

However, this does not specifically address starting the motor. When the motor hp is a large fraction of the transformer rating, the motor starting method and acceptable voltage drop must also be considered.

 

[superspider]

Thanks all
I think I have to think again about that