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    How to test field windings & armature windings of the DC motors ?

    How to test field windings & armature windings of the DC motors ?

    DMM shows very low resistance almost short (about 0.05 ohms) ... what to do ?


    thanks

    •   Alt31st January 2012, 17:18

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    Re: How to test field windings & armature windings of the DC motors ?

    What is declared power of that motor? What type of motor ?

    0,05R is very low resistance, high current will go.


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    •   Alt31st January 2012, 17:55

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    Re: How to test field windings & armature windings of the DC motors ?

    You could be reading the correct value depending on what sort of motor you are testing. A starter motor for a big car will take 200A at 10V while it is cranking the engine over. R = 10/200 = .05 ohm. What do you hope to find while testing it? A visual inspection will show if the commutator is burnt or the field coils have been over heated.
    Frank


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    •   Alt31st January 2012, 18:38

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    Re: How to test field windings & armature windings of the DC motors ?

    How to test field windings & armature windings of the DC motors ?

    DMM shows very low resistance almost short (about 0.05 ohms) ... what to do ?


    thanks



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    Re: How to test field windings & armature windings of the DC motors ?

    Quote Originally Posted by munzir View Post
    How to test field windings & armature windings of the DC motors ?

    DMM shows very low resistance almost short (about 0.05 ohms) ... what to do ?


    thanks

    I dont think that is smart to put that baby on some power supply Or you can try on some batteries.



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    Re: How to test field windings & armature windings of the DC motors ?

    Few tips when you check the brushed dc-motor,

    -disconnect all wires from motor

    -turn motor slowly with hand and check is the rotor turning without jamming or stucking
    ....if so >> mechanical fail ... check bearings ...is field magnet loose...other loose parts.

    Try to move the motor shaft axial and radial directions to find out is there right clearence in bearings.

    -if motor has field winding(s) , look voltage and current values for field in name plate.
    -calculate resistance and check by measuring

    Usually brushed DC-motor has two, four or six brush holders.
    -take away brushes so that only one pair is left in place in opposite sides ( difference 180 degrees )
    -connect ohm-meter to armature wires and turn slowly rotor so that you can read ohm value for
    every commutator plate pair. No higher resistance values are allowed.

    Check every brush , no arc-marks , no grooves , equal lenght , not too short , copper conductor to brush is undamaged ,
    if spring attached to brush , check that spring is undamaged and it makes enought pressure against commutator plates

    Check brush-holders , brush must move freely in holder , check that brush size is suitable with holder.

    If separate springs fo brushes are used , check pressure of the springs and test freely movement of springs

    Check that all wires and contacts are in good condition

    check the commutator plates surface condition
    -no deep grooves
    -no burned arc-marking
    >> if arc-marks clearly occurs in one or two places , there could be armature coil failure
    >> if arc-marks are random or all around , there is fail with brushes . ( low brush pressure , too short brush )

    Test with low voltage adjustable power supply ( current limited or over current protected )

    For motor with fixed magnet fied , connect supply and adjust voltage so that motor start slowly run. ( adjust current limit )
    -check that motor run smoothly
    -brake motor with hand , still should run smooth
    -stop motor with hand and slowly let turn 360 degrees , no loss of torque allowed in any angle.
    >> if point of torque-loss found , mark the position and check reason. ( fail in armature winding , commutator , brushes , brush-holders )

    If motor has field winding connect mains isolated supply and adjust current 25...50% of name plate rating.
    Do above tests

    It is a great help in troubleshooting , if you know exact how the fault appeared and how the motor behaved in failed situation.

    Good to remember, DC-motor commonly has tachometer included
    and failed tacho can cause fault conditions ,
    which could affect like fault on the motor.
    So allways check the tachometer too.

    There was some,
    friends can tell additional tips...........
    ---> Hike with wise men , then you will be wise <---


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