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Thread: help with spindle motor winding resistance 10hp

  1. #13
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    Default Re: help with spindle motor winding resistance 10hp

    Quote Originally Posted by Jim Dawson View Post
    Very true, I think it would get close, but that is not really the best way to test a motor. I have a LCR meter but I'm not sure the results would be meaningful. I'm running a job in the lathe so tearing into it to get some readings is not going to happen. If I had the motor on the bench, then I would spend the time to set up a proper test.

    I would take the motor to a motor shop and have it checked out. That motor is a standard 3 phase induction motor, not a servo. Running it on a 3 phase line and getting amp readings would be the best test.
    unfortunately 8milliohms sounds like a direct short across more than just one coil of wire, more like a coil group or between several coils. you'll know really quickly if you have a shorted turn. maybe with a thermal camera you could fix it before a complete rewind is required.



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    Default Re: help with spindle motor winding resistance 10hp

    Hi,
    I have a milliohm meter, not an expensive one , but still about $800USD. Does a good job down to 1mOhm at about 1%
    but will read down to 10uOhm at about 10%. Has proven to be very useful in detecting faults in transformers and high current
    paths in welders.

    At 8 mOhm is pretty low, what confidence do you have in the accuracy and repeatability of that measurement?

    I think you would be better off trying to measure the inductance of the coils, Any shorted turns would show up very dramatically
    in an inductance measurement.

    Craig



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    Default Re: help with spindle motor winding resistance 10hp

    Hi, inductance is 0.9mH, I´have used a WAYNE KERR 6425 RLC METER




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    Default Re: help with spindle motor winding resistance 10hp

    Hi,
    what about the other two phases? Inductance imbalance is quite pronounced in faulty three phase machines.

    Craig



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    Default Re: help with spindle motor winding resistance 10hp

    I have to check it, all that I have is the report from a repair center. When I test the motor with my oscilloscope and a current probe, motor draws 180 amp on start up. Repair center saids that in star configuration motor has 0,23 ohms, and in delta 0,003 ohms!!!!



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    Default Re: help with spindle motor winding resistance 10hp

    some thing does not add up

    if the 0.23 ohms is the resistance of 2 coils in series

    the resistance across 2 wires from the Delta connected motor
    will be one coil in parallel with the other 2 coils connected in in series

    the resistance will be
    the product divided by sum of the parallel resistances

    0.115 x 0.23 divided by 0.115 +0.23

    = 0.02645/0.345
    = 0.0766 ohms which is a lot more than the Repair center said


    0,003 ohms looks more like a short circuit

    John



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    Default Re: help with spindle motor winding resistance 10hp

    Thanks John-100, is what I´ve seen.
    Now I only need to measure a good motor to compare, or a FANUC motor specs to claim at repair center.
    I´m wating for FANUC EU answer






    Quote Originally Posted by john-100 View Post
    some thing does not add up

    if the 0.23 ohms is the resistance of 2 coils in series

    the resistance across 2 wires from the Delta connected motor
    will be one coil in parallel with the other 2 coils connected in in series

    the resistance will be
    the product divided by sum of the parallel resistances

    0.115 x 0.23 divided by 0.115 +0.23

    = 0.02645/0.345
    = 0.0766 ohms which is a lot more than the Repair center said


    0,003 ohms looks more like a short circuit

    John




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    Default Re: help with spindle motor winding resistance 10hp

    Anyone else has a FANUC spindle motor and a low resistance meter?



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    Default Re: help with spindle motor winding resistance 10hp

    unless you are working as a maintenance engineer and can justify buying a milliohm meter

    one option could be to connect a current limited supply and digital voltmeter to the motor connections

    if the motor winding has a resistance of 0.1 ohm or less this could give you an inexpensive way to measure the resistance

    help with spindle motor winding resistance 10hp-basic-low-ohm-test-jpg


    with a 12 ohm 20W resistor and 12v supply connected to the motor terminals with the voltmeter

    you can consider the current through the motor winding of 0.1 ohm (or less) as being a 1A constant current
    so a voltmeter will see 0.1V across a 0.1 ohm resistor or 0.01V across a 0.01 ohm resistor

    while its not as acurate as a £300 meter with 4 wire test leads with Kelvin clips

    it will be close enough to check the 3 windings are the same

    if the motor resistance is nearer to 0.5 ohm (or more) you will need to measure the current to calculate the resistance or increase the supply voltage to set the current to 1A

    John



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    Default Re: help with spindle motor winding resistance 10hp

    you can make a microohm meter with a 5$ digital scale, kelvin clips and two resistors and an adjustable resistor.

    its pretty easy: the digital scale takes the one part per thousand change in resistance of the strain gauge at full scale (which works out to about 2.4 millivolts full scale (when run from two nicad batteries)) and amplifies it about 500? times and sends it into the adc


    basically cut the wires going to the strain gauge and run all 4 wires to the kelvin clips, red to one clip black to the other, but you add two resistors (68 ohms is a good start) each in series with the red and blackwires, add the variable resistor in for callibration.

    the polarity of the white and green wires doesn't matter, run them to the kelvin clips.

    how it works is pretty simple: when you clip the kelvin clips on something, tare the meter. the two 68 ohm resistors limit the current flowing through the clips to about 17 milliamps. when you move the clips to the resistance you are intending to measure, the white and green wires now have say, 17 microvolts across the unknown resistor if you measured a .001ohm resistor. 17 microvolts will correspond to about 10 grams on a 1KG scale with two 68 ohm resistors iirc.

    because the battery is supplying the voltage, pushing current through the resistors and the kelvin clips.. and because the battery is also the reference for the ADC, battery voltage does not matter. the digital scale is basically making a resistance comparison between the resistance in series with the test resistance. as long as you have low temperature coefficient resistors, the accuracy of this measurement is pretty good, about accurate to 1 part in 1000.


    anyhow i need to get off my ass and make some of these meters, let me know if anyone wants one.



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    Default Re: help with spindle motor winding resistance 10hp

    Thanks Robowi, I can measure inductance too, but I need a good motor to compare, now my motor has 0.9mH and 0.003 ohms phase to phase.



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    Default Re: help with spindle motor winding resistance 10hp

    Make motor off line and put +05 -05 v in phase to phase and see the voltage dropping phase to phase and compare to earth



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help with spindle motor winding resistance 10hp

help with spindle motor winding resistance 10hp