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Ripple Current in Aged Brushed DC Motors

  1. Jan 2, 2013 #1
    I work as an engineer and I've been looking for an explanation for something that I have been puzzled by.

    Physics background of problem: In brushed DC motors, electrical coils (poles) rotate as the motor moves. A change in inductance as they move makes small variations in motor current arise.

    It would be intuitive for these ripples to decrease in magnitude over the lifetime of a motor but this doesn't seem to be the case based on data from a motor manufacturer and ripple images found in "Brush wear detection by continuous wavelet transform." (The paper however does not focus on magnitude so it can't be conclusive evidence for this) I would expect the magnets in the motor to lose their magnetism as the motor heats and cools during its life cycle. This change should be small with modern magnets but it should still decrease the back emf constant of the motor and decrease the ripple magnitude.

    How could motor ripple actually increase over the lifetime of a motor?
     
  2. jcsd
  3. Jan 3, 2013 #2

    jim hardy

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    That paper describes ripple arising from brush to commutator contact which is not variations in inductance. The author was working on a technique to infer brush condition.
    Brushes are usually some graphite bearing substance and they wear away. If the dust is allowed to accumulate it can jam the brush in its holder and it loses contact with commutator. Maintenance on large DC machines includes verifying the brushes are free in their holder so the spring keeps them snug against commutator, and replacing brushes as they wear away.

    Look in the back of your electric drill or circular saw and switch it on, observe the sparks. They will be small so long as the motor is healthy. A dirty commutator will spark more as the brushes make and break contact.
     
  4. Jan 3, 2013 #3

    berkeman

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    Staff: Mentor

    Dang, I learn something useful every single day here at the PF!

    Thanks Jim! :smile:
     
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