"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.