Auto transformers - Electrician Talk - Professional Electrical Contractors Forum

chop347 · 05-25-2007, 10:04 PM

One of my guys asked why one of his smaller wall mount A/C,s he was testing. Using a Auto transformer to get the right voltage has such troubles starting up. I said it would becauce of the inrush current.
The tx has a KVA rating. With his current going up and since the KVA wont change the volts would drop off for a moment and that is causing the problem.
Am I wrong on this.

Hope for the best. Expect the worst.

brian john · 05-26-2007, 08:02 AM

This may be the reason. Standard transformers are sized with motor starting loads limitations. I am not sure if auto amfrs have this sizing option included. but it should be a consideration when sizing the auto xmfr. What is the FLA, what is the auto xmfr KVA and voltage change.

http://www.sola-hevi-duty.com/support/faq.html#Q32

chop347 · 05-26-2007, 10:41 AM

He was taking 600 down to 220. The FLA would be around 25-30 amps. And the KVA was 15. So that is what got him. The Tx should be good enough. As he was just doing a temp hook up to test a A/C he fixed.

frank · 05-28-2007, 03:10 PM

Autotransformers have a peculiar habit of becoming 'chokes' and produce back EMF forces. Usefull in some circumstances but not in the situation you have. Some manufactures install capacitors to help aleviate this problem. You would find the capacitors have a secondary use correcting the 'power factor angle' also. As a general rule though it would not be good practice to use auto transformers where high inrush currents are applied such as the use of motors etc.

Frank