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Thread: Coil testing

  1. #1
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    Coil testing

    I have a Siemens Coil 120volt for 100amp lighting contractor. It requires momentary power to open and close contacts. Coil has two sets off poles that I assume would be moving the magnet in two different directions, but I have O resistance on one set off poles and correct resistance on the other set. Does one assume there is a short in the coil?
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  2. #2
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    If you are measuring the coil contacts directly then you apparently have one coil with an open winding, thus defective
    If you are measuring thru contacts, it may just be the normal holding contact open.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
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    Does "O resistance" in your post mean zero resistance (a short) or Overload resistance, an open circuit?

  4. #4
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    short

    Quote Originally Posted by GoldDigger View Post
    Does "O resistance" in your post mean zero resistance (a short) or Overload resistance, an open circuit?
    0 meaning short.
    Im assuming, i don't have any diagram or info for the coil, that,referring to the pic, the poles you see are two sets of coil connections within the contactor.

  5. #5
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    0 as meaning the two probes of the ohm meter are touching each other directly (shorted), or 0 as you hold the two leads into the air touching nothing (open)?
    Tom
    TBLO

  6. #6
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    A shorted coil would typically show a blown fuse or control circuit damage somewhere. An open coil would show lack of operation.
    Tom
    TBLO

  7. #7
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    Quote Originally Posted by ptonsparky View Post
    A shorted coil would typically show a blown fuse or control circuit damage somewhere. An open coil would show lack of operation.
    The control circuit is momentary contact 120volt to close and open, three position on/on/off switch. The contractor is closed and will not open when 120volts is applied. No issues with control.

  8. #8
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    Quote Originally Posted by pfactor2 View Post
    The control circuit is momentary contact 120volt to close and open, three position on/on/off switch. The contractor is closed and will not open when 120volts is applied. No issues with control.
    If you have zero resistance, I'd say the coil is shorted. It should draw high current when you try to energize it, unless it already did that at some point and took out some component in the control circuit.

  9. #9
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    Unless things have changed there are two things you might check before you install a new coil:
    1. Be sure the armature moves freely. If it does not seat correctly the coil will burn out.
    2. If its like the mechanically held contactors I've worked on in the past, the control circuit to the coil often goes thru a set of auxiliary contacts that opens and removes coil voltage once the contactor is set. make sure the contacts are working properly.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  10. #10
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    hard to tell without more information. coils are generally low resistance and can be close to zero ohms when measured with a common voltmeter.

    best bet is to test it on your bench and see if it works by applying the appropriate voltage to the coil(s).

    if it works great, if it does not work throw it away and buy a new one.

    it is just basically a giant latch relay.
    Bob

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