Universal / Commutator motor running slowly- puzzled!

[[email protected]]

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

 

[Fred]

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.

 

[The Natural Philosopher]

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.

 

[Pete Cross]

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

 

[Rheilly Phoull]

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

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.

 

[Ron(UK)]

WTF is a 'strimmer' ??

Leftpondians call them weedwhackers.

Ron(UK)

 

[[email protected]]

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

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

 

[Michael A. Terrell]

Leftpondians call them weedwhackers.

Ron(UK)

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

 

[James Sweet]

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

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.

 

[Ron(UK)]

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

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

Ron(UK)

 

[Michael A. Terrell]

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

Ron(UK)

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

 

[Dave D]

[email protected] wrote:

Are you sure it's a universal motor? The string trimmers I've seen had
permanent magnet DC motors with a diode in series.

Mine is definitely a universal motor.

Check and see if the diode is shorter or leaky, you could also run the
motor on 12VDC, it should run slowly but smoothly.

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