Control transformer size

[Pitt123]

I need to supply a small control transformer 480-120V to supply power to a contactor.

If contactor has a rated seal in of 140VA but has an inrus of let's say 300VA then do I need to size transformer per seal in rating with time delay fuses to account for inrush or must I size transformer per the inrush rating of coil?

 

[gar]

100812-0648 EST

Pitt123:

Get a 150 VA transformer. Run an experiment. Adjust the transformer input voltage, starting at a voltage below contactor pull-in, and determine the input voltage where the contactor just pulls in.

Is this voltage adequately below your minimum expected supply voltage? If so, then this size transformer should be adequate.

Unless you were rapidly cycling the contactor there is no thermal problem in the transformer.

Note: when running a test such as this you do not want to apply under pull-in voltage very long because you may overheat the contactor coil. A suitable technique would be to set the input voltage to the transformer, then on the secondary side have a switch to momentarily apply voltage to the contactor coil. Change voltage and do again, etc.

.

 

[jim dungar]

A true 'control power' transformer sizing, requires you to compare both the inrush and sealed VA requirements. Manufacturers provide charts that will show the output voltage regulation of the transformer (usually at 85%, 90% and 95% voltages) versus the inrush of the load.

However, I don't think I have ever seen a standard control transformer that could not simply be sized based on the sealed VA only, when dealing with a single motor starter coil.