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
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.
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?
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.
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.
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.
I tried and indeed the sensitivity is inadequate. But for the record it gave very low voltages of 30-50V.
Mmmm, I'll try that.
Seems rock solid.
Thanks!
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.
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