Question about a 3 phase magnetic starter

[747ken]

First of all, this question is not about hvac. But I hope you can help me in any case.

I work on a 208vac, 3 phase air compressor. It uses a Siemens magnetic starter with a solid state overload relay.

The contactor has 4 terminals: V for the plus side of the coil, W for the neg side of the coil, and 2 terminals marked M. It also has an normally closed aux contact that Siemens calls a late break contact. When the contactor closes, this aux contact opens.

One of the M terminals is jumpered to the V terminal. The other M terminal is wired to the aux contact. The other side of the aux contact is wired back to the W terminal.

My question is: What is the purpose of the M terminals and the aux contact? I've looked thru a couple of my electrical text books, but can't find the answer. Thanks in advance.

[madhat]

Are there two compressors? Sounds like you have an alternating circuit. The normally closed contacts could be for an unloader solenoid to relieve the pressure off the discharge. Got a wiring diagram you can post?

[glennwith2ns]

http://www.sea.siemens.com/step/templates/lesson.mason?components:3:2:8


try this site .

It will explain things pretty good.

[NormChris]

There are dozens of ways the aux contacts could be used depending upon the application of the starter and what it is controlling.

You need to trace out the circuit diagram for the particular piece of equipment it is installed on.

Norm

[747ken]

Originally posted by glennwith2ns
http://www.sea.siemens.com/step/templates/lesson.mason?components:3:2:8


try this site .

It will explain things pretty good.


Thanks, that's an excellent site. I'll have to study it tomorrow when I have more time.

The terminals V,W,M are all low voltage (38vdc). Best I can tell, when the contactor closes, the aux contact opens and removes the ground path from the M terminal.

If I could determine what the M terminal is for, I could figure it out. From the diagram, V, W, M are all mounted on a block that I belive represents the contactor's coil.

I'll try to scan & post the diagram later.
Thanks to all replies.

[747ken]

Well I figured it out. The coil on this contactor is actually two coils in one (that's why 2 terminals are marked M). One is a 5 ohm coil, the other is 270 ohms. Both coils energize to close the contactor. When the contactor closes, the aux contact opens, causing one coil to drop out. Two coils to pull-in, one coil for holding. Pretty simple, really.