# Faulty Circular saw

• posted on December 19, 2003, 10:54 pm
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?
Any suggestions?
TIA
reteplav
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• posted on December 19, 2003, 11:08 pm
Reteplav scribbled :

I would expect a very low resistance , its when the reistance is high you have problems - worn or broken brushes or pitted com.
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Gary
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• posted on December 20, 2003, 12:27 am

The resistance would be low, yes.

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 snipped-for-privacy@argonet.co.uk London SW 12
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• posted on December 20, 2003, 3:34 pm

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 not.
For a proper safety evaluation more than that would be required: I have no idea sitting here if it needs such or not.
Regards, NT
PS If not sufficiently qualified dont do it.
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• posted on December 20, 2003, 1:06 pm
"Reteplav" wrote on 19/12/2003 :-

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 armature.
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 saw.
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Regards,
Harry (M1BYT)...
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• posted on December 20, 2003, 2:16 pm

Dunno about every maker, but it used to be easy to get just replacement armatures. For quite a bit less than an entire motor. Might make the difference to a viable repair.
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• posted on December 20, 2003, 5:13 pm
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 multimeter.
Thanks for all the advice
reteplav
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