How can I prove whether the motor has burnt out on my electric circular saw?
I know how to use a multimeter and so I`ve checked the continuity of the L & N
from the mains plug to each brush tip ( on the commutator) ( via the On/Off
switch) and all is OK.
There is low resistance / short circuit across the brushes looking into the
motor windings .
Does that mean the motor has burnt out as I thought that there would be an
expected low resistance across the comm anyway.
How can I prove it?
Could be shorted turns. The only way to check an armature is with AC, and
measure the impedance of each set of windings since it's unlikely all have
shorted, or by knowing what they should be.
If the machine is ok otherwise, you can usually by a new armature and
that's the most likely cause.
*Never underestimate the power of stupid people in large groups.
Dave Plowman email@example.com London SW 12
Simplest way to find out is first check the insulations all OK, then
plug it in in series with a bar fire or a big bulb. Does it run ok or
For a proper safety evaluation more than that would be required: I
have no idea sitting here if it needs such or not.
PS If not sufficiently qualified dont do it.
The resistance across the brushes, when in contact with the commutor of
the armature, will be extremely low anyway. There is just a few inches
of enamelled copper to privide any resistance in the motor, this would
be less than one ohm. The actual resistance of which could only be
measured with a milliohmeter.
A specialised piece of test gear is needed to adequately check out the
If you are getting mains voltage through to each of the brushes, then
the only possibility left will be either a faulty armature or faulty
commutator. In either case the only sensible repair for a faulty
armature would be to replace the entire motor. The cost of the
replacement motor might well be more than the replacement value of the
Embarrassingly , I`ve discovered that the fault was simply due to to a
sticking/ faulty brush!
When I tested it using a meter , all the prodding and poking must have given me
the reading OK, but it must have been an intermittent/poor contact with the
commutator as it wouldn`t work when switched on from the mains.
What puzzled me originally was I hadn`t smelt any burning etc.( and so had my
doubts about burning out the motor) and everytime I tested various parts , I
obtained OK readings .This sort of defied normal logic. This, then made me
think that perhaps there was an intermittent connection so I dismantled the
brushes and one of them was fairly stiff in it`s holder.
I cleaned them both up( and the comm ) and refitted them and now all is OK.
I knew that using a digital meter could lead you to the wrong conclusion
sometimes so I was surprised that I was led astray using my old analogue
Thanks for all the advice
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