Sizing a Control Transformer for Various Loads

What voltage?

120 VAC

8.1 amps x voltage = VA or watts. Use this figure to size the transformer.

Yes it's more then enough

1 What are the voltages on the primary and secondary side?

2 Will there be any other loads on the transformer at a later date?

3 Will the breaker be operated occassionally or rather frequently?

4 Any other relevant details?

SAMUEL CHELLIAH

1. 110kV/33kV

2. No

3. It's regular operating breaker

I have enclosed the clculations. Transformers have to be designed for the starting currents. If you need any further clarifications please let me know

SAMUEL CHELLIAH

Sorry!!

Mr. Samuel , I have given voltage parameters for sensitive earth fault calculations not for Transformer sizing

Pls. ignore this

Mr Krishna,

Can you give me the primary and secondary voltages of the transformer? this is important to fix the rating of the transformer

The primary is 480V and secondary is 120V

Secondary loads:

Total sustained load = 8.1 A

Total inrush load = 41 A

Control Transformer size ?

I presume the currents you have indicated are the secondary side currents

SAMUEL CHELLIAH

dEAR,

Ratio starting Amp / Normal Amp = 41/8.1 = 5

As any transformer can take up the starting current 6 - 8 times , thermal effect is neglected.

You require transformer to suit 8.1 * 120V = 972 VA.

Add 15% safety for ambient variations, I suggest to go for 1.2 KVA rated transformer.

I was dealing with lift irrigation schemes where the motors are DOL (direct on line) starting. The manufacturers recommended the transformers should be rated for the starting current and not the rated current. In my experience I have seen the transformer voltage dropping drasticallyduring starting if not designed for the starting current. Hence the transformer should be designed for the starting current

SAMUEL CHELLIAH