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Universal / Commutator motor running slowly- puzzled!

Discussion in 'Electronic Repair' started by [email protected], Jul 21, 2006.

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  1. Guest

    I've got a 250W electric strimmer with a commutator type motor. It is
    running much more slowly than it should (I would guess a few hundred
    instead of few thousand rpm). Both brushes spark as it runs.

    I have checked rotor windings with a multimeter across the brushes,
    rotating the rotor readings hover around 20-40 ohms.

    There are no obvious high or low points on the commutator, and I have
    polished the contacts and cleaned them and the gaps between with
    methylated spirits.

    The brushes are about 15mm long, and are held snug against the
    commutator by springs

    The field (stator) windings are also about 30 ohm each.

    The rotor spins fairly freely, there is one point where it rubs
    slightly, but only very slightly.

    Can anyone suggest what might be wrong?


  2. Fred

    Fred Guest

    A single shorted turn won't show up on such a resistance test.

    It might be worth measuring each of the rotor segments but even then a
    shorted turn won't be obvious.

  3. Either the brushes are shagged, or the motor has developed an internal

    If its a few years old, suspect the former, if its newer, its probably
    been abused. Or was simply crap to begin with.
  4. Pete Cross

    Pete Cross Guest

    Have a look at the stringhead end, you could have a load of compacted grass
    trapped between the head and the strimmer shaft acting as a brake. Does the
    motor feel hard to turn whilst attached to the strimmer ?

  5. WTF is a 'strimmer' ??
    Regardless there are shorts in the armature. Go down to your nearest repair
    shop and ask them what is a "Growler".

    Cheers ......... Rheilly P

    Where theres a will, I want to be in it.
  6. Ron(UK)

    Ron(UK) Guest

    Leftpondians call them weedwhackers.

  7. Guest

    This may be a clue to worn bushings. If it rubs with no power applied
    imagine how much closer it will be with a strong magnetic field
    applied. Check for slop in the bushings.


  8. Which pond? There are millions of the damn things around, you know.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  9. James Sweet

    James Sweet Guest

    Are you sure it's a universal motor? The string trimmers I've seen had
    permanent magnet DC motors with a diode in series. Check and see if the
    diode is shorter or leaky, you could also run the motor on 12VDC, it
    should run slowly but smoothly.
  10. Ron(UK)

    Ron(UK) Guest

    The big pond.. the one that separates the UK from the USA.


  11. In which direction? ;-)

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  12. Dave D

    Dave D Guest

    Mine is definitely a universal motor.
    I doubt it will manage to turn at all, James- not at 5% of the line voltage.
    (Reading through the thread I'm assuming he's UK/Europe based.)

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