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Thread: Air Compressor Starter/Pressure Switch

  1. #1

    Default Air Compressor Starter/Pressure Switch

    Can someone explain the correct wiring for a pressure switch
    to a magnetic contact and relay. The power is 220v three phase.

    Thanks

    Dave

  2. #2

    Default

    Yessir....

    You have the three phase conductors coming in the "top" terminals of the contactor. The 3 motor wires connect to the "bottom" terminals of the contactor.

    The coil of the contator is where the slightly complicated connections are. Normally, the coil of a 220 volt air compressor is also a 220 volt coil. Double check this (written on the coil somewhere). Run a jumper wire from of the hot wires from the top terminal of the contactor (any one) goes to ONE terminal of the magnetic coil. Run another jumper from another hot wire from the top of the contactor (not the same one you used before) to one terminal of the pressure switch. A wire from the other terminal of the pressure switch goes to the other terminal of the magnetic coil. Basically, you're using the pressure switch to "make and break" one of the wires leading to the magnetic coil.

    Best of luck... repost if you get stuck or find your magnetic coil voltage is something other than 208 or 220 or 240 (any of these will be okay). Remember, if this unit is 3 phase, you need 3 phase power to hook it up. You can't run this in the garage of your house, unless you have 3 phase at your house (not normal at all).

  3. #3

    Default

    Thanks for the reply. I understand what to do, but I'm not sure
    what terminals to use. I also have to run two wires to the
    side of the relay that is attached to the contactor?? This
    is what the terminals on the contactor are.


    A1 , A2


    3 , 1-L1 , L2 , L3


    2, T1 , T2, T3

    The coil is which terminals? Which terminals go to the side
    of the relay? Let me know if you need more info.

    Thanks

    Dave

  4. #4
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    Default

    The coil terminals are A1 & A2.

    The 3-phase supply connects to the contactor at terminals L1, L2, & L3 that should be in a row across the top.

    The connection to the motor overload relay is made at contactor terminals T1, T2, & T3 that should be in a row across the bottom.

    Just follow MD's instructions for wiring the pressure switch and verify that the contactor's coil voltage is between 208V and 240V by reading the printing on it (don't connect it if it's only 120V).

    Use any two of L1, L2, or L3 as per MD's instructions to wire the switch to the coil.

    Homer
    Last edited by Homer; April 10th, 2004 at 10:06 AM.

  5. #5
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    Default

    Looking back at your original post I will add some comments just to be complete.

    Do you have an overload relay (heater) for this motor or is the motor itself a thermally protected variety?

    Typically the overload snaps on the bottom of the contactor and attaches to the "T" terminals. The motor is then wired to the overload relay terminals.

    Also, have you sized your overcurrent device (fuse/breaker) for this motor?

    Homer
    Last edited by Homer; April 10th, 2004 at 10:05 AM.

  6. #6

    Default

    You are correct. The overload relay attaches directly to the bottom
    of the contactor.I have the motor wired from the "T" terminals on the relay. There are two terminals on the side of the relay also. Do any wires go to these?
    The coil is 220 and wired like MD described.

    Thanks

    Dave

  7. #7
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    The two terminals you see on the overload relay are an auxilliary contact. A normally closed contact (N.C.) will be provided so you can use the overload status for anything you wish (usually 120V control circuit). You don't need it for a simple application.

    Also, auxilliary contacts on the contactor (N.O. and/or N.C.) can be used in a start/stop control circuit. You don't need these either since you're using a pressure switch. The N.O. is usually used to "seal in" the start button contact until the stop button contact breaks the control circuit.

    Homer

  8. #8

    Default

    Thanks for the help.

    Dave

  9. #9

    Default

    I thought of another question. Do you always run the one
    hot wire directly to the coil? Since the pressure switch
    has two poles, can you run both of the hot wires
    through the pressure switch?

    Thanks

    Dave

  10. #10
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    Default

    You are correct. If your pressure switch has two poles, you can wire it as you suggest.

    When using a contactor for a motor you only need a single pole switch in the control circuit. This is the wiring configuration that MD gave you.

    If you were using a small 240V single phase motor without a contactor, and were using the pressure switch directly, then the two poles would be necessary.

    Homer
    Last edited by Homer; April 10th, 2004 at 01:31 PM.

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