Universal / Commutator motor running slowly- puzzled!

Jul 21, 2006 11 Replies

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google 20 years ago

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?

Regards,

Nigel

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Fred 20 years ago

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.

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The Natural Philosopher 20 years ago

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

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

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Pete Cross 20 years ago

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 ?

Pete

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Rheilly Phoull 20 years ago

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.

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Ron(UK) 20 years ago

Leftpondians call them weedwhackers.

Ron(UK)

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vangard 20 years ago

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.

Van

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Michael A. Terrell 20 years ago

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

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James Sweet 20 years ago

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.

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Ron(UK) 20 years ago

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

Ron(UK)

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Michael A. Terrell 20 years ago

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

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Dave D 19 years ago

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.)

Dave

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