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DC Motor Problems

Discussion in 'Sensors and Actuators' started by wtaddington, Feb 4, 2013.

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  1. wtaddington


    Feb 4, 2013
    I have a 24V 350W DC motor to run on an electric bike. Two of the four stator magnets were broken in shipment, but I glued all the big pieces back in place. The motor spins free and will run on 12V. It does not seem to run the right speed, but I will attribute that to the decreased stator field. It will not however run on 24V. When connected to 24V it will nudge slightly and just sit there. I have removed the brush assemble and measured 12V and 24V across the brushes. When the motor is assembled and connected to the 24V source it measures no voltage. When connected to 12V everything seems fine.

    Since it will run on 12V I must assume the circuit is correctly configured. I believe that the two stator magnets(both side by side) have a weakened field, so the field created by 24V is to large to work correctly. The 12V field is not as strong so it works with the weakened stator field.

    Does anyone know if I am on track with this thinking.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I'm not sure why a motor should run on a weak field, but not on a strong one.
    Could it be a mechanical problem? Certainly the magnets, after glueing, do not fulfill the original tolerances. The gap between rotor and stator is probably very small to increase effectivity of the motor. The glued mechanic may not be as stable (with respect to position, tolerances and rigidity) as originally foreseen. Now a strong field will attract rotor and stator so much that the gap is reduced to zero and the motor stops. A weak field may not be stron enough so a small gap still remains allowing the motor to turn.

    My tip: If something is broken in shipment, return it. It will be next to impossible to get a refund or exchange once you tinkered with the item.
  3. wtaddington


    Feb 4, 2013
    I know. I bought it almost a year ago. I inspected the box and parts at delivery, but never installed it. From just a look I could not tell the problem I had. The gap closing sounds reasonable. because the field is strong on one side (no broke magnets) and weak on the other (two broke magnets.) The armature tilts to the strong side closing the gap and stopping the motor.

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