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200v Three phase motor 4 poles

Discussion in 'High Voltage' started by Dmed, Apr 8, 2011.

  1. Dmed

    Dmed New Member

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    I'm getting 0 ohm from the windings no matter how i measure.

    PE with U or V or W terminals
    or
    between the terminals U, V and W

    Is this the right way to check? If it is, will this mean motor is gone?

    thanks in advance
     
  2. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    How many HP motor? What type ohmmeter are you measuring the windings with? There is no zero ohms.

    Ron
     
  3. Dmed

    Dmed New Member

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    cont/30 min 3.7/5.5KW ~5/7 HP
    and a regular digital multimeter
     
  4. dave

    Dave New Member

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  5. Dmed

    Dmed New Member

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    just re-measured again between PE with U or V or W terminals = open cct

    between the terminals U, V and W = 0
     
  6. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    OK, a 5 to 7 HP three phase AC motor is a large motor. The larger the motor (HP rating) the lower the coil resistance will be. While the number of poles will contribute to the resistance the higher the HP the lower the resistance still holds true. The higher the HP the larger the wire. The largest AC three phase motor I have ever worked with was 500 HP which is not quite gigantic but not small either. I work with 200 HP pump motors every now and then. Motors in the 5 to 7 HP range can typically have coil resistance sub one ohm. That is why I asked what you were using for an ohmmeter. When I measure the phase to phase resistance and resistance to case on larger motors I like to use a DLRO (Digital Low Resistance Ohmmeter). Meters like this typically deliver 1 amp (or more) through the coils and accurately measure the voltage drop. Regular DMMs typically have an upper range (low range) of 2 Ohms or greater and deliver mA to measure resistance. Not really a good choice.

    There is no zero ohms as I stated. Even a short has some resistance. The trick becomes knowing what you should be seeing. I have seen motors with shorted windings and shorted windings to case but I have never seen a motor thst was totally shorted? Not to say it can't happen but I never saw it.

    What exactly is happening? Has the load on this motor been removed as in the shaft decoupled? With the shaft decoupled have you tried to manually rotate the shaft? What exactly happens when you try to run the motor? Do breakers trip? Thermal overloads trip?

    Ron
     
  7. Dmed

    Dmed New Member

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    the problem right now is that the motor is not moving smoothly. kind of jerking move(move, stop, move, stop). that's why I'm guessing one of the winding got short.
    Motor is controlled by a adjustable speed control. Forward and reverse works, but speed does not change.
     
  8. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Just a rough guess but based on symptoms I would be looking at the VS drive for the motor. For an AC motor forward and reverse is just a matter of shifting the phase rotation. Jerking motion would point to a problem in the VS drive with one of three phases. This is where a good scope with HV isolated differential input probes is a nice to have. Personally if I were you I would not be too quick to suspect the motor based on what you have posted.

    Just My Take
    Ron
     
  9. Dmed

    Dmed New Member

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    Thanks for the suggestion.

    Motor: Fanuc model 3 AC spindle motor 3 phases 4 poles
    Controller: Leeson SM2 AC speed controller

    problem with VS drive with one of the phases, you mean at the connection point? settings on the drive? or wire the motor wrong?
    if it is the last one, how to check which one is U or V or W terminal? (the cables are labeled, but got a suspicion they might be mislabeled)
    unfortunately, the best test equipment I got is a digital multimeter and a low voltage analog scope.
     
    Last edited: Apr 11, 2011
  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Spindle motor as in a spindle motor used in a CNC (or similar) machine? Does the motor have or do you use any type of feedback from the motor to controller? I originally envisioned a simple large 3 phase motor with a simple speed controller. I missed that call! :)

    If the controller is using any feedback, check those connections thoroughly. The U, V, and W are your 3 phase inputs. These determine the motor direction of rotation. The controller can internally switch those. For example if we label the phases U, V, and W we have 3 phases spaced 120 degrees apart in time. Let's say the motor runs CW facing the shaft. If that is changed to U, W, and V internally by the controller the motor will run CCW. It is very important that the 3 phases are equal in amplitude. If for example U and V are 200 volts but W is only 160 volts the result would be jerkey motion (rotation) of the motor. That is a single guess, jerking can have other causes, especially if speed feedback is used.

    If feedback is used depending on the type, you can monitor it likely with a scope and look for irregularities. You can also look at the speed command in signal of the VS drive for glitches or irregularities.

    You CANNOT use a basic scope with a common input ground to look at the actual levels and frequency of U, V, and W signals. That requires a special isolated differential channel inputs.

    I looked but could not find a good PDF data sheet for the motor. :(

    Ron
     
  11. Zerotolerance

    Zerotolerance Member

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    Hard wire the motor direct to the line coming into the unit. In other words bypass everything and run three phase directly to the motor (use the breaker as your on/off switch), then switch two of the phases and see if it operates correctly in the other direction as well. If the motor works fine then your problem probably lies in the controller. If one phase wasnt working your motor would likely just HUMMM and not turn at all. I think your motor is ok and your problem lies within the controller.

    It doesn't matter how you wire a three phase motor just as long as all three phases are getting an = amount of voltage. If not your phase timing will be off, so make sure that all three legs are getting the same amount of voltage. The only issue you will have if you switch two of the phases, is the motor will run in reverse. If that happens you just switch two legs and you will be running forward.
     
    Last edited: Sep 27, 2011
  12. strantor

    strantor Active Member

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    Usually when a 3 phase motor that's controlled by a VFD has only single phase it will humm and turn real slow, with no torque, or not turn at all and humm. I have seen 3 phase motors connected up to line voltage do the jerk-stop-jerk-stop before when one of the phases was not connected up. That happened for about the first 7 or 8 seconds before smoke started billowing out of it. That's probably because a VFD puts out a lower voltage for a lower speed command, plus current limiting, therefore not burning up the motor (as fast). If you do decide to test it by connecting it to line voltage, keep your hand on the disconnect and have a fire extinguisher handy.

    did you disconnect the motor and check the output of the drive? because if the motor is good and the drive is bad, and you are testing voltage on the drive terminals, you will read voltage on all 3 phases, even if all 3 phases are not working, because you get feedback through the motor.
     

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