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Universal / Commutator motor running slowly- puzzled!
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Pete Cross
electronics forum beginner


Joined: 21 Jul 2006
Posts: 1

PostPosted: Fri Jul 21, 2006 1:46 pm    Post subject: Re: Universal / Commutator motor running slowly- puzzled! Reply with quote

Quote:
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

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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The Natural Philosopher
electronics forum beginner


Joined: 28 Apr 2005
Posts: 15

PostPosted: Fri Jul 21, 2006 12:42 pm    Post subject: Re: Universal / Commutator motor running slowly- puzzled! Reply with quote

google@river-view.freeserve.co.uk wrote:
Quote:
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



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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Fred
electronics forum beginner


Joined: 14 Oct 2005
Posts: 34

PostPosted: Fri Jul 21, 2006 12:11 pm    Post subject: Re: Universal / Commutator motor running slowly- puzzled! Reply with quote

<google@river-view.freeserve.co.uk> wrote in message
news:1153482305.621339.288610@i3g2000cwc.googlegroups.com...
Quote:
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


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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google@river-view.freeser
electronics forum beginner


Joined: 21 Jul 2006
Posts: 1

PostPosted: Fri Jul 21, 2006 11:45 am    Post subject: Universal / Commutator motor running slowly- puzzled! Reply with quote

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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Google

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