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IndustryArena Forum > Machine Controllers Software and Solutions > Fanuc > help with spindle motor winding resistance 10hp
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  1. #21
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    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

    Attachment 412452


    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

  2. #22
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    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.

  3. #23
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    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.

  4. #24
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    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

  5. #25
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    Re: help with spindle motor winding resistance 10hp

    Thanks for all, motor was OK, report from remanufacturer had one more zero, in delta is 0,09 ohms, not 0,009 ohms. The problem was a intermitent failure in one of the current meters, isolated amplifiers in servo driver.

    So, for this kind of motor these resistance values are ok, 0,23 in star, and 0,09 in delta

  6. #26
    Community Moderator Al_The_Man's Avatar
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    Re: help with spindle motor winding resistance 10hp

    If you had 200-220v 3ph available you could have simply tried it across the line and measure the current in each phase!
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  7. #27
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    Re: help with spindle motor winding resistance 10hp

    Thanks Man, but a low inductor motor with 0.9mH will draw 700Amp reactive current at 50Hz and 200V !!!!! Is a motor designed to work only with a VFD.

  8. #28
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    Re: help with spindle motor winding resistance 10hp

    Quote Originally Posted by harrimansat View Post
    Thanks Man, but a low inductor motor with 0.9mH will draw 700Amp reactive current at 50Hz and 200V !!!!! Is a motor designed to work only with a VFD.
    And how does the shorted turn secondary (squirrel cage) figure into this reading?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  9. #29
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    Re: help with spindle motor winding resistance 10hp

    It will increase the current, in my previous calculus I have used only coil inductance.

  10. #30
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    Re: help with spindle motor winding resistance 10hp

    and it will dissipate 44.000 wats in 0.09 ohms!!!! this is the reason why motors designed for inverters can't be connected directly to mains. Try it and you will see what happens.

  11. #31
    Community Moderator Al_The_Man's Avatar
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    Re: help with spindle motor winding resistance 10hp

    Quote Originally Posted by harrimansat View Post
    and it will dissipate 44.000 wats in 0.09 ohms!!!! this is the reason why motors designed for inverters can be connected directly to mains. Try it and you will see what happens.
    You are talking stall amps!
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  12. #32
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    Re: help with spindle motor winding resistance 10hp

    its a 200 volt motor. will run fairly saturated on 240v 3 phase. no reason you couldn't just plug it in and see if it runs though.

    if there is a shorted turn in a 10hp motor you should see a lot of fireworks pretty quickly.

    its the much smaller 1hp and less that might have a shorted turn without blowing up.

  13. #33
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    Quote Originally Posted by Eldon_Joh View Post
    its a 200 volt motor. will run fairly saturated on 240v 3 phase. no reason you couldn't just plug it in and see if it runs though.

    if there is a shorted turn in a 10hp motor you should see a lot of fireworks pretty quickly.

    its the much smaller 1hp and less that might have a shorted turn without blowing up.
    Fairly? Do you had ever connected a 400V servo motor to mains?

  14. #34
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    Re: help with spindle motor winding resistance 10hp

    Quote Originally Posted by harrimansat View Post
    Fairly? Do you had ever connected a 400V servo motor to mains?
    synchronous motors such as most "sevos" are permanent magnet, of course you can't just plug them in.

    if they have a shorted turn they will cog and be difficult to rotate by hand, but still produce voltage, just less of it on the phase with the shorted turn,.

  15. #35
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    Re: help with spindle motor winding resistance 10hp

    Thanks Robowi, do you know how I can calculate the current in a motor with no load, motor has 0.9mH 0.09ohms at 200V 50 Hz 4 poles

  16. #36
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    Re: help with spindle motor winding resistance 10hp

    Hi,
    a calculation like that is not so simple, The data you have might adequately describe the electrical properties of the stator but what you don't know is
    the back EMF generated by the rotor once it starts to spin.

    If there were no rotor and therefore no back EMF then the current calculation goes something like this:

    I(amps) = Vin / ( Rstator + jx2x50xPIx Lstator )
    I =200 / (0.09 + j x 0.282)

    Thus the current is primarily inductive;
    I (approx.)=200/2.82
    I =707 A (inductive)

    Its not uncommon for a simple calculation like this to produce astonishing results. This situation would occur if you directly connected 230V AC to the motor and flipped
    the switch. It would almost certainly take out the fuses and/or circuit breakers. That would be the inrush current. Once the motor starts spinning the back EMF will
    reduce the current substantially.

    Lets do the calculation but instead of 50Hz input voltage lets assume 400Hz as would be supplied by a VFD at maximum speed.
    I= 200/(0.09 +j x 2.26)
    I=88A (inductive)

    Even at 400 Hz the inrush current is still very high, but prehaps not faulty either.

    I understand you are looking for some clear test to determine whether the motor is faulty or not. Doing a calculation of peak inrush current
    is not especially informative.

    Until recently I worked for a company that had a big three phase Variac. Had you presented the motor to me for testing I would have followed the path
    you have taken but arrived at the conclusion that to attempt to connect direct to three phase power was going to end in disaster.
    I would however have been able to use the Variac and so present as little as 5V-10V AC three phase to it. That is I could test it in such a manner
    that even if it were faulty there would still be no sparks or smoke!

    In absence of a three phase Variac you will have to use a VFD but ensure that the V,f gradient is such that the current remains low until you can establish
    whether its good or not.

    Craig

  17. #37
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    Re: help with spindle motor winding resistance 10hp

    Thanks for your answer Craig, motor is good, the problem was in the VFD. I want to mathematical model the motor. I was wondering if I could connect directly to 220V. One member forum has done something similar:

    "I don't have a nameplate on the motor, but found a picture of one on the net for a Model 15 motor which has the same power rating.
    Volt 200 3 ph, 4 pole induction. RPM 1500 / 4500, amp min. 60/70, 15 kw /18.5 kw. As I need to figure out if I have a motor or drive problem (or both) I hooked the motor to the incoming 220 VAC 3 phase at 50 hz for 1 minutes and got about 1570 rpm and a steady U - 103, V - 105 and W - 106 amps.
    I dont know how to interpret the Fanuc nameplate though. Some say amps are during no load, others say full load. I tried to find out if JEC 37 differs from NEMA but got nowhere. Does anyone know if the current draw is abnormally high or if it is ok?
    Maybe the nameplate amps are not applicable for running at 50 hz from wall power?"

    https://www.cnczone.com/forums/fanuc...e-problem.html

    Miguel

  18. #38
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    Re: help with spindle motor winding resistance 10hp

    so usually induction motors draw about 30% nameplate full load amps at no load.

    a 15KW motor pulling 100 amps at no load, sounds like you have saturated the motor by applying too many volts per hz. but the fact that it started up means the coils are good. if they are shorted out you'd have known by now.


    anyhow, regarding modeling an induction motor, the formulas are well known, but you will need to make a number of measurements. it isn't as simple as just measuring the dc resistance of the coils and the ac inductance (well, you might get close if you measure the inductance at 50 or 60hz)
    typically induction motors draw about 5-7 times full load amps when you simply plug them in. basically the rotor bars in the stator have a frequency dependant inductance. once the motor gets up to speed, the 3hz slip permits the magnetic flux to penetrate deep into the rotor. when you first plug it in the magnetic field doesn't penetrate very far into the rotor and electrically it is a much higher resistance. different rotor bar designs generate the different torque-slip curves.

    a 200 volt motor plugged into 220vac may not be a big deal, a 200 volt motor plugged into 250vac3 phase is going to be completely saturated and draw a lot of amps.

  19. #39
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    Re: help with spindle motor winding resistance 10hp

    Hi,
    when I went to University and studied electrical machines, in New Zealand, what you refer to as 4 pole I would call 2 pole-pair-per-phase,
    sometimes called PPP in the literature.

    A one PPP motor energized by a 50Hz .three phase supply has a synchronous speed of 3000 rpm.
    A two PPP motor has a synchronous speed of 1500 rpm.

    An induction motor will always be spin at a little less than synchronous speed, say 2850 rpm for a 1 PPP motor and 1425 rpm for a 2 PPP motor.

    I am a bit suspicious that your claim of 1570 rpm when energized by 50Hz as that is above synchronous speed, impossible for both induction
    AND synchronous motors.

    You have quoted some numbers that came off what you believe to be a similar, maybe identical motor? You quoted speeds of 1500 to 4500 rpm.
    That would suggest to me that the motor is intended for inverter operation and further suggests that the design frequency range is 50 to 150Hz.
    If the 200V rated voltage is applied at 150Hz then I would expect the applied voltage at 50 Hz to be 200 x 150/50 =66.66V. If this piece of induction/guess work
    is correct then to hook up a 230V 50Hz supply would HOPLESSLY saturate the motor can you could be sure that the motor was faulty thereafter because
    YOU BLEW IT UP!

    For testing purposes you need to reduce the 230V three phase supply down to less than 50V three phase. Can you go around town begging/stealing/bororowing
    three identical single phase transformers say 230V input 24V output?. Then you could energize your motor with 24V three phase and both you and it are
    likely to survive the experience.

    Craig

  20. #40
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    Re: help with spindle motor winding resistance 10hp

    Quote Originally Posted by harrimansat View Post
    Thanks for all, motor was OK, report from remanufacturer had one more zero, in delta is 0,09 ohms, not 0,009 ohms. The problem was a intermitent failure in one of the current meters, isolated amplifiers in servo driver.

    So, for this kind of motor these resistance values are ok, 0,23 in star, and 0,09 in delta
    could you give me some more info on how you fixed the servo driver?
    I have a 6055 drive that has good wave forms on the drive board but is still giving AL-12 and the motor is ok like yours.

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