Contact in Relay

[ibraheema2]

Dear,
How contact in relay work and how the schmatic wiring diagram for air comparssor with mult relay can understand. please with example.
Are you know any good book about relays, type of relay , contact , ... etc
best regards,
ibrahim

[suemarkp]

Thats a rather broad question, and I can't tell exactly what you are asking. But here's a start:

A contactor and a relay are the same thing. Usually, the heavier amperage relays are called contactors, but not always. I'll call it a relay from now on since thats shorter to type.

A relay allows a little bit of power to switch a lot of amps. A relay will have at least 5 pins -- two for "coil", one for "common", a "NC" or Normally Closed pin, and a "NO" or Normally Open pin. The Common and NC will make contact when no power is applied to the relay coil. When power is applied to the relay coil, the Common-NC connection is broken but the Common-NO connection is made. Some relays have multiple sets of contacts affected by the coil, so you may have additional Common/NC/NO pins that work together but yet are isolated from the other Common/NC/NO pins (two sets of contacts would be double pole, three sets triple pole, and so on). The coil has a voltage rating which may be different from the contact ratings (Common, NC, NO). The amount of current drawn by the coil is rather small, usually well less than 1 amp.

Some common relays are as follows:
24VAC coil with 240V/20A rated contacts. This would be used in an Air Conditioning compressor or air handler.
240VAC coil with 240V/30A rated contacts. This would be used on a large motor such as for an air compressor.
12VDC coil with 12V/30ADC rated contacts. This would be used in a car to switch on the headlights.

I don't think I can answer your specific question, but others here may be able to if you have a specific compressor (make and model number) you want to know about. But I can give this general answer. A motor is usually controlled with a relay (on-off switch to coil, motor terminals to relay contacts). There may be a heat sensor in the motor circuit that detects if the motor could be over heating. When these thermal switches close, they may remove power from the relay coil which turns off the motor. I don't know why you have multiple relays, as most motors should be able to be controlled with a double pole, triple pole, or 4 pole relay.

The schematic symbol for the coil is:
o
(
(
(
o

The schematic symbol for the contacts is:

o_____o Com
o

where the top left is NC, bottom left is NO, and top right is Common.

[Homer]

Here is an example of a 3-phase motor circuit.

There are two relays in the circuit. One is the contactor (3-pole in this case) and the other is an overload relay (a relay without a coil).

The contactor coil is energized when the start button (N.O.) is pressed. Once the contactor auxillary contact (N.O) closes, it "seals in" the circuit. It will run until the stop button(N.C.) is pressed, breaking the control circuit.

A typical relay or contactor coil only passes a tiny amount of current while a large current flows through its contacts.

The overload is a different type of relay from what you typically see.

It consists of heaters that conduct the full load current. They will trip the overload relay when they get too warm(too much current). The N.C. overload auxilliary will also break the control circuit as it will open once the current is too high.

Homer

[suemarkp]

Isn't that a 4-pole relay -- three for the main motor windings and a 4th to latch the start button? I'm assuming the vertical bars labeled "M" or "Ma" are N.O. contactor switches (poles). I would imagine that the "Ma" pole is rather light duty compared to the other three.

Is the M with a circle around it the relay coil?

[Homer]

That's a 3-pole overload relay with a N.C. auxilliary contact using trade terms. The contactor is also a 3 pole with a N.O. auxilliary contact.

They do function as you suggest. The 3 poles are rated based on the motor load and the auxilliary is only rated for a few (<10A) amps.

Some OL relays have an integrated N.C. auxilliary (like a 4th weak pole) and others need to have the auxilliary contact added to it.

Likewise, some contactors have either an integrated N.O. auxilliary or have the auxilliary contact added.

The N.O. contactor auxilliary in the drawing is labeled Ma and the contactor is labeled M (both coil and main contacts). The OL heaters and the N.C. OL relay auxilliary are labelled OL.

Homer

[Wgoodrich]

Homer is right. The holding circuit relay [aka control relay] is not counted as a pole on a contactor of this type. Only those contacts carrying the load of the equipment is counted as a pole.

Good Luck

Wg

[ibraheema2]

Dear Members,
Thanks for your replay.
But No member tell about good book about relays,
Please if you have any ideal plase tell me asap.
best regards :o

[mdshunk]

Abraham- I have quite a large electrical book library, and don't seem to have anything that specifically relates to contactors. I do have a small book with some technical specifications on contactors put out by the SEIMENS company, but it's all in German. I was leafing through some of the classroom textbook type of books, and they all have just a page or two on the matter. I do have a .PDF "E book", that was part of a US Army electrician training program that has some excellent information on relays and contactors, complete with pictures and cutaways. If you care to private message me with either your EMail address or street address, I'd be pleased to EMail you this HUGE file or burn you a copy on a CD and send it to you. If you care not to go this route, then "UGLY's Electrical References" has diagrams for most of the popular ways to wire up relays, but contains very little written text for the person trying to learn. It's more of a "cheat sheet" type of reference booklet for electricians who already know. Best I got.