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Thread: Motor Starter Voltage Drop

  1. #1
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    Motor Starter Voltage Drop

    I believe that I am having a voltage drop issuue in a control circuit used to pull in a motor contactor. I have a control circuit that goes out a pretty long distance on a conveyor belt and then returns to an MCC to pull in the coil on a starter. After trying to run the motor I have found that the motor would run for a little bit, and the the coil on the starter would burn up/blow. When measuring the voltage across the coil (120V coil) during starting I am seeing about 110V which indicates that there is a slight voltage drop. Would this voltage drop be enough to damage the coil? I was told that with this voltage drop we would be pulling more current through the coil and that is what is causing it to blow. Is this correct?

    I could understand why the voltage drop would cause more currnet to flow, but I cannot see it mathmatically. Rearranging ohm's law V=IR you would be able to calculate the current by I=V/R. But mathmatically if you decrease the voltage in the numerator of this equation with the "R" held constant it seems like your current value would be less. Is this an accurate representation of this scenario?

    Thanks

  2. #2
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    Quote Originally Posted by mull982
    ...When measuring the voltage across the coil (120V coil) during starting I am seeing about 110V which indicates that there is a slight voltage drop. Would this voltage drop be enough to damage the coil? ...
    I've never seen or heard of undervoltage burning up motor starter coils.

    Quote Originally Posted by mull982
    ...I was told that with this voltage drop we would be pulling more current through the coil and that is what is causing it to blow. ...
    No, It won't pull more current. The coil circuit looks like an inductor with a series resistance. Lower applied voltage = lower current.

    Limiting the discussion to standard motor starter and coils, the only things I know of that will burn up the coil is the wrong voltage:
    120V on 24V coils
    DC to an AC coil

    I'd start with some troubleshooting:
    Has it ever worked as per design?
    If it did, when did it change?
    Are you really sure you are measuring the voltage at the coil?
    Are you reallly sure the coil is rated for the applied voltage?

    The only reason I mentioned the last two, is cause I been there at least once.:rolleyes:

    carl

    edited in an attempt to fix my poor wording and fat fingered typing
    Last edited by coulter; 08-08-07 at 04:20 PM.
    Using the code for a design guide is a sign of incompetance

  3. #3
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    If you do not have enough voltage (about 85% of nominal) to create a strong enough magnetic field to "pull in" the starter you will continue to draw very high currents.

    The coil of the starter acts like the stator of a motor, lower voltage = more current drawn.
    Just because you can, doesn't mean you should.

  4. #4
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    So is it safe to say that lower voltage results in more of a current draw? I dont see that happening with the simple V=IR equation. Do I have to add an inductance factor in that equation or is the circduit strictly resistance?

    Thanks

  5. #5
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    Jim -
    Quote Originally Posted by jim dunger
    ...The coil of the starter acts like the stator of a motor, lower voltage = more current drawn....
    Isn't that true only if the contactor does not pull in?

    Quote Originally Posted by mull982
    ...I have found that the motor would run for a little bit, and the the coil on the starter would burn up/blow. ...
    My understanding is the contactor in question pulled in.

    So, in the range where the contactor stayed pulled in, how would you characterize the V/I relationship?

    carl

    (Carl's bad pun of the day: "Well carl, V/I = Z":rolleyes
    Using the code for a design guide is a sign of incompetance

  6. #6
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    Motor Starter Voltage Drop

    Check voltage at control transformer without conveyor running then check the voltage at the coil with starter pulled in, this will give you an idea of how bad of a drop there is. You can also check accross the wires going out to the stop Push button which should be causing the most drop since the holding contact shunts the start PB and pretty much eliminates any drop there. Make sure your contacts on the Stop PB are good. If, all else fails the voltage drop increases with load, so if this is a large size starter you could have the start pb bring in a small pilot relay mounted in the can which can then bring in the starter.

  7. #7
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    Quote Originally Posted by mull982
    So is it safe to say that lower voltage results in more of a current draw? ...
    Not in this context. Once the contactor pulls-in, the voltage can drop pretty far before it drops out.

    Jim - If I am not understanding this correctly, I would welcome some science and references.

    Quote Originally Posted by mull982
    ... Do I have to add an inductance factor in that equation or is the circduit strictly resistance?...
    My understanding, for an AC coil, the current is mostly limited by the inductance. That is why, as Jim said, the current drops when the coil pulls in. The inductance goes up when the armature hits the coil pole piece.

    That's why DC will burn up an AC rated coil. Inductance won't limit the current.

    carl
    Using the code for a design guide is a sign of incompetance

  8. #8
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    Low voltage will definitely increase the current in an AC coil, but I would not characterize 110V on a 120V coil as "low" with any significance. Any AC coil will have at least a +-10% tolerance and NEMA specs call for -85% to +110%. So a 120V rated coil on a NEMA design contactor should be good to 102V. A lot of IEC contactors meet that spec as well.

    If the coil is burning up, are you sure there is nothing that is interfering with the pole faces of the armature coming cleanly together? Because any obstruction will interfere with the shading coil being able to function, so the armature will vibrate at twice the line frequency and the coil current will remain high. At 120Hz, sometimes you can't hear the contactor buzz. Or the shading coil itself is damaged; same difference.

    Another remote possibility is that you have the control circuit run in the same conduit as the power leads (separate but adjacent in plastic conduit will do it too). With enough distance, the control wires my be picking up an induced voltage which is raising the coil terminal voltage beyond it's limit. When you tested the control voltage, was the motor running, or were you doing it with the disconnect open? It could also be next to the power leads of a different motor, in which case you wouldn't have noticed the extra voltage if that other motor was not running.

  9. #9
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    Quote Originally Posted by Jraef
    Low voltage will definitely increase the current in an AC coil, ....
    Let me make sure I understand what you are saying. for this case and this limited case only:

    120VAC contactor coil.
    The contactor is pulled in.
    Measure the voltage across the coil and it is 10% low, 108V.

    You are saying the coil will draw more current than if the voltage is at 120V?

    carl
    Using the code for a design guide is a sign of incompetance

  10. #10
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    What is the typical current rating on a size 5 coil?

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