Transformer Sizing for Motor Loads

If your writ runs in the plant, then you can bypass what you would normally consider, and you can go abnormal.

Your idea is sound, but I think you should confer with the transformer manufacturer about momentary overloads, recovery time, etc.. Also consider short-term voltage drop on other loads.

Considering how fast an electrical phenomenon takes place, the starting of a large electrical motor can be called anything but momentarily.

Now to say something about the transformer sizing:

- consider what Tornado said about the voltage drop for other consumers powered with the same transformer;

- when calculating the starting current consider this case: Your transformer is out of juice for a few seconds, time in which most motors will stop. When the transformer is energized again some of the motors will start. Consider them for choosing the starting kVA if it is the most restrictive condition.

As other replies have mentioned voltdrop is important in the motor starting problem. Size the transformer to ensure you a) have adequate voltage at the motor terminals under starting conditions, and b) that at the point of common coupling of the other loads you have sufficient voltage to avoid flicker problems (this depends on the frequency of starting), c) the transformer size is adequate for the static loads (assuming the motor starting is infrequent).

The starting current of the motor is flowing only for seconds and this would not cause rating problems for a quality transformer. I am aware of a 2.8MW motor starting from a 6.3MVA transformer. The starting current is 1200A at 6.6kV, MVA is 5.8 time FL approx 13.7MVA. However it is started only infrequently, and runs continuously for long periods.

Does it depend upon the source capacity(MVA)?. If source(fault level) is weak,there will be severe voltage drop. For DOL starting what is the % voltage drop allowed and for how long?. During DOL starting if starting current is 6 to 8 time FLC,considering voltage drop during starting the starting MVA should be less than 6 to 8 times rated MVA of the motor.

Why have DOL starting?

"eugene344" has mentioned starting current is 6 to 8 times FLC

Use a soft start motor controller

As already was detailed, the main concern is the voltage drop. The transformer supports-for short time- 50% to 100% overload.

As a rule of a thumb: the transformer rated power has to be about 3 times motor power [for D.O.L. start].

I think the voltage drop for a successful DOL starting depends on motor rated power -more for small motor less for big one[30-10%].

The voltage drop at the all other motor already in operation has to be less than 10%.

If the motor will start many times per 24hr the transformer will be warmer and the its life could be shorted. So the load has to be

limited as per standard recommendation for cyclic loading [only 30-80%]. See:

IEC 6095 Loading Guide for Dry Type Power Transformers

IEC 60354-12 Loading Guide for oil immersed Power Transformers

Corrections:

1) The phrase: "If the motor will start many times per 24hr the transformer will be warmer and the its life could be shorted."

has to be:

"If the motor will start many times per 24hr the transformer will be warmer and then its life could be shorted."

2) The phrase: "So the load has to be limited as per standard recommendation for cyclic loading [only 30-80%]".

has to be: "So the overload has to be limited as per standard recommendation for cyclic loading [only 30-80%]".