I'm trying to fix a vacuum cleaner.
When switched on the motor fires for about half second on, half second
off...
The wiring all checks out so I'm left with the motor itself. Brushes all
seem fine, plenty of life, strong springs and good contacts.
What else should I check, and how do I check it?
TIA
--
Mike W

"visionset" wrote in message
...

I'm trying to fix a vacuum cleaner.
When switched on the motor fires for about half second on, half second
off...
The wiring all checks out so I'm left with the motor itself. Brushes all
seem fine, plenty of life, strong springs and good contacts.
What else should I check, and how do I check it?

I've now had it in more bits, cleaned the commutator to v. shiny,
reassembled and no improvement.
???

--
Mike W

On 28 Feb, 17:32, "visionset" wrote:
"visionset" wrote in message
...

I'm trying to fix a vacuum cleaner.
When switched on the motor fires for about half second on, half second
off...
The wiring all checks out so I'm left with the motor itself. Brushes all
seem fine, plenty of life, strong springs and good contacts.
What else should I check, and how do I check it?

I've now had it in more bits, cleaned the commutator to v. shiny,
reassembled and no improvement.
???

hopefully a multimeter will let you know where the o/c is.


NT

Sounds like an open circuit armature winding, you need a Mulitmeter across
it and turn it slowly by hand checking there is continuity reading between
the brushes for each segment of the commutator.

Sam Farrell
"visionset" wrote in message
...
I'm trying to fix a vacuum cleaner.
When switched on the motor fires for about half second on, half second
off...
The wiring all checks out so I'm left with the motor itself. Brushes all
seem fine, plenty of life, strong springs and good contacts.
What else should I check, and how do I check it?
TIA
--
Mike W

"Sam Farrell" wrote in message
...
Sounds like an open circuit armature winding, you need a Mulitmeter
across it and turn it slowly by hand checking there is continuity reading
between the brushes for each segment of the commutator.


Thanks Sam,
I attached my mulit meter ;-) to both brush holders and turned motor
(armature) very slowly.
Readings were between 1.5 and 4.5ohms - no open circuits.

Any other clues?

--
Mike W

On Mar 1, 8:33 am, "visionset" wrote:
"Sam Farrell" wrote in message

...

Sounds like an open circuit armature winding, you need a Mulitmeter
across it and turn it slowly by hand checking there is continuity reading
between the brushes for each segment of the commutator.

Thanks Sam,
I attached my mulit meter ;-) to both brush holders and turned motor
(armature) very slowly.
Readings were between 1.5 and 4.5ohms - no open circuits.

Clip an analogue multimeter to the brush holders and measure the
voltage when it's running, taking due care to not electrocute
yourself.
I doubt that a digital meter will show a fast variation like that.
If the voltage goes up and down then the problem must be in the wiring
to the brush holders.
Check that the armature isn't sliding backwards and forwards.

"Matty F" wrote in message
oups.com...
On Mar 1, 8:33 am, "visionset" wrote:
"Sam Farrell" wrote in message

...

Sounds like an open circuit armature winding, you need a Mulitmeter
across it and turn it slowly by hand checking there is continuity
reading
between the brushes for each segment of the commutator.

Thanks Sam,
I attached my mulit meter ;-) to both brush holders and turned motor
(armature) very slowly.
Readings were between 1.5 and 4.5ohms - no open circuits.

Clip an analogue multimeter to the brush holders and measure the
voltage when it's running, taking due care to not electrocute
yourself.
I doubt that a digital meter will show a fast variation like that.

Okay I'll have to try it with my digital - it's all I have. But the stop
start problem is ~1 second cycle so should be able to observe a change in
reading.

If the voltage goes up and down then the problem must be in the wiring
to the brush holders.

Can you explain why the conclusion - my electronics is a very rusty.

--
Mike W

On Mar 1, 10:59 am, "visionset" wrote:
"Matty F" wrote in message

Clip an analogue multimeter to the brush holders and measure the
voltage when it's running, taking due care to not electrocute
yourself.
I doubt that a digital meter will show a fast variation like that.

Okay I'll have to try it with my digital - it's all I have. But the stop
start problem is ~1 second cycle so should be able to observe a change in
reading.

If the voltage goes up and down then the problem must be in the wiring
to the brush holders.

Can you explain why the conclusion - my electronics is a very rusty.

I assume that it's a 230v (thereabouts) AC vacuum cleaner, so there
should be a continuous 230v AC at the brush holders when it's running.
My cheap digital multimeter samples about once per second so that
would be no good to see the problem. Maybe you could use a 230v light
to check the voltage at the brush holders. But you need to be careful
about electric shock so don't be touching anything when the power is
on.
Another possibility is that the commutator is not perfectly round.
That would make the brushes jump up and down when the motor runs fast.
Or (but unlikely) if the whole armature is sliding backwards or
forwards in the end bearings when the motor is running.

"Matty F" wrote in message
oups.com...
On Mar 1, 10:59 am, "visionset" wrote:
"Matty F" wrote in message

Clip an analogue multimeter to the brush holders and measure the
voltage when it's running, taking due care to not electrocute
yourself.
I doubt that a digital meter will show a fast variation like that.

Okay I'll have to try it with my digital - it's all I have. But the stop
start problem is ~1 second cycle so should be able to observe a change in
reading.

If the voltage goes up and down then the problem must be in the wiring
to the brush holders.

Can you explain why the conclusion - my electronics is a very rusty.

I assume that it's a 230v (thereabouts) AC vacuum cleaner, so there
should be a continuous 230v AC at the brush holders when it's running.
My cheap digital multimeter samples about once per second so that
would be no good to see the problem.

I tried and indeed the sensitivity is inadequate. But for the record it
gave very low voltages of 30-50V.

Maybe you could use a 230v light
to check the voltage at the brush holders.

Mmmm, I'll try that.

But you need to be careful
about electric shock so don't be touching anything when the power is
on.

Sure thing.

Another possibility is that the commutator is not perfectly round.
That would make the brushes jump up and down when the motor runs fast.
Or (but unlikely) if the whole armature is sliding backwards or
forwards in the end bearings when the motor is running.

Seems rock solid.

Thanks!
--
Mike W

Matty F wrote:

I assume that it's a 230v (thereabouts) AC vacuum cleaner, so there
should be a continuous 230v AC at the brush holders when it's running.

No, no, no - it will be a series wound motor and some voltage will be
dropped across the field coils. The heavier the mechanical load on the
motor the higher this drop will be.

The test suggested is still useful though. If the voltage across the
brushes rises when the motor cuts out it points to an o/c in the
armature (v. unlikely, IMO). OTOH if the voltage falls the intermittent
o/c is elsewhere. Use a test lamp (15 W pygmy bulb) or a robust old AVO
7 or similar. If there is an intermittent o/c you might zap your lovely
DMM with the multi-kilovolt spikes that will appear across the brushes.

IME the problem is more likely to be an intermittent connection in the
internal wiring of the motor itself (including the RF suppressor chokes)
- or the appliance switch even, or the flex. Intermittent breaks in
flexes subject to a lot of rough use are quite common. Give everything
a good tug or waggle while the motor is running.

--
Andy

"Andy Wade" wrote in message
...
Matty F wrote:

I assume that it's a 230v (thereabouts) AC vacuum cleaner, so there
should be a continuous 230v AC at the brush holders when it's running.

No, no, no - it will be a series wound motor and some voltage will be
dropped across the field coils. The heavier the mechanical load on the
motor the higher this drop will be.

The test suggested is still useful though. If the voltage across the
brushes rises when the motor cuts out it points to an o/c in the armature
(v. unlikely, IMO). OTOH if the voltage falls the intermittent o/c is
elsewhere. Use a test lamp (15 W pygmy bulb) or a robust old AVO 7 or
similar. If there is an intermittent o/c you might zap your lovely DMM
with the multi-kilovolt spikes that will appear across the brushes.

IME the problem is more likely to be an intermittent connection in the
internal wiring of the motor itself (including the RF suppressor chokes) -
or the appliance switch even, or the flex. Intermittent breaks in flexes
subject to a lot of rough use are quite common. Give everything a good
tug or waggle while the motor is running.

I "didn't repair" (!!!) a vacuum once, that went on and off due to a shorted
winding and self resetting thermal fuse. Basically ran for about 10s then
off for 5s then on again. Carefully inspecting the aramture windings (and
noticing sparks from brushes) revealed "welded" armature wires. Didn't
register a short with a meter just say 2.5Ohm as opposed 4.5 Ohm, basically
on the limits of my analogue meter. Ended up in bin after finding
replacement motor was £46.