Compressor Post Identification

[lcrabtree]

Can someone tell me how to identfy which post are what on a compressor?

I know there is a run, start, and overload post.  Don't know which one is which.

[johnieG]

depends on what manufacture it is...maybe post a picture? or what machine is it in?

[lcrabtree]

OK, here are a few pictures of the overall compressor and a close-up of the compressor terminals.  This is a Vendo V-110 machine with a 1/4 HP compressor.  The ID tag that was on the compressor deck is gone.

Does anyone know what the resistance across these terminals should be?

[johnieG]

That helps! here you go...

[Larry]

Referring to JohnieG's picture.


In the picture...

C - common is on top.

R - Run is on the bottom right.

S - Start is on the bottom left.

This is not always the case.  Sometimes coomon is on the top left, Start is on the top right, and Run could be on the bottom.  The way to find out which terminal is S, R, and C is to use an ohm meter. 

The value from C to R will be the least amount.

The value from C to S will be the second highest value.

The value from S to R will be the highest value.

For example (Johnies pic) if you placed the meter on C to R the value is 1 ohm.

C to S the value is 3 ohms.

S to R the value will be 4 ohms ( the total of the other two values C to R and C to S).

The way to determine the common is that once you find the two terminals with the highest value, common would be the other terminal that is left.  So if you put your meter on the bottom two terminal post and that gives you your highest value, then the other post is common.  Once you know which one is common you can check from the common to the other post to determine which post is the start and which one is the run.

Just remember the lowest ohm value is from C to R, and the ohm value in the middle is from C to S.

This only works on single phase compressors.  In three phase the ohms are the same between the post. 


If you have no ohms between the post, the compressor windings are open and the compressor is bad.

One other thing to check for is continuity to ground.  Scratch off a liitle paint on the compressor and place one end of the meter on the bare metal and then place the other end to each terminal post (S, R, and C).  You should have no continuity.  If you do, the compressor is grounded which means it's garbage.