Transformer Theory

[alc]

Would like to get some instruction on wiring a 240/110 transformer. Our shop is constantly discussing the proper grounding method for the transformer secondary. E.G.: We have 240VAC, 3PH, 3-wire coming into a breaker panel. One of the breakers feeds a 1KV 240/110 transformer. Since there are only three wires coming into the breaker panel, how is a neutral obtained on the secondary side of the transformer? And what about grounding? I know this is basic stuff, but this application exists on a trailer and it doesn't look right to me. The 240 panel has no ground bus bar. It also has a neutral bus bar but nothing is connected to it? Some transformer theory would be appreciated.

alc

 

[230842]

I´ll try to answer your question: The most common and simple way to obtain a neutral wire on a transformer secondary is to perform a Y connection instead of a delta. The common point of a Y is a natural neutral. That neutral can be wired as well, and in most cases, when possible, it is advisable to connect it to ground.
Neutrals on both sides of a transformer are electrically independent although they are generally interconnected. You can get only a neutral on a Y connected secondary even the primary is delta connected.
There is another way to obtain a neutral when Y connection is not possible or when mains is three wires. In such a circunstances you must use an special three phase coil with zig-zag connection.
For more details let me know.

Julian

 

[busbar]

See

http://home.att.net/~benmiller/elecsys.htm

"...typical transformer configurations that are used to supply 60 Hz power at 600 volts or less in the United States."

"UNGROUNDED DELTA" and "TWO-WIRE" describe the transformer configuraions you describe.