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I have a mitre saw that started shorting. I dismantled it to try to diagnose the problem. On inspection of the rotor, I found that the brush contacts on the rotor all have continuity to one another. Is this normal, or does it indicate a failure in the rotor windings?

This is from a 110v 13a AC Roybi mitre saw.

photo of rotor

rotor with brush contacts highlighted

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The two tests you do on that type of motor are "180 degrees" and "bar to bar". You have to measure the two pads that are 180 degrees apart. Measure each pair of pads around the motor and see if any of the measurements are a lot different than the others. It doesn't matter what the measurement is, it should just be consistent.

The bar-to-bar test is the same. You measure two adjacent bars all the way around the motor to see if any of the measurements are a lot different than the others.

All of the bars having continuity with each other is not an issue, but a third test you can do is to make sure that none of the pads/bars has continuity with the outside of the armature (the larger diameter portion in the middle of your picture).

  • I will try that. A related question, there are four wires coming from the motor. One hot, one neutral, and two others. Neutral is directly connected to wall neutral. When the switch is depressed to run the saw, current is connected to the hot wire and continuity is broken between the remaining two wires. Do you know what these two wires do? – T.D. Smith Oct 22 at 21:49
  • @T.D.Smith The other two wires are for stopping the motor quickly. All motors can act as generators, and this effectively shorts out that generator causing it to slow down. – Brad Gilbert Oct 22 at 22:43
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This looks like a universal motor (able to run on DC).

The armature wiring looks like a daisy if you spread it all out. The wire goes

  • from commutator bar 1, a couple of coils around the armature to commutator bar 2, then
  • from bar 2 a couple of coils around the armature ( turned that many degrees, so wound into different armature slots) to bar 3, then
  • from bar 2 a couple of coils around the next armature slot over to bar 4,
  • ...
  • from bar n a couple of coils around the next armature bar to bar 1.

A series wound motor has 4 leads. Armature 1, armature 2, field 1 and field 2.

  • connecting hot to A1, A2 to F1, F2 to neutral makes it run forward.
  • connecting hot to A1 ,A2 to F2, F1 to neutral, makes it run backward.
  • connecting hot to A2, A1 to F2, F1 to neutral, also makes it run forward.
  • connecting hot to A2, A1 to F1, F2 to neutral, also makes it run backward.

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