Why do motors stop working?

[8rad]

I put a reedy DH in my Topcat last night and no joy. Swapped in another one I knew was good and it worked fine. I opened up the can and everything looks ok. I was just wondering why a motor would not turn. Seems to me that as long as there is power to the comm the only thing that would prevent the motor from not turning is if the windings were open at some point.

[Charlie don't surf]

If the windings were broken you would usually find smoke, or burnt stacks upon inspection. insufficent brush material, spring tension or a hung brush are causes as well. Also little debris like e clips too

[8rad]

That is why I find it puzzling. Brushes are new, spring tension is good. I tried turning the end bell a quarter turn at a time so the brushes are not hanging up and there is no smoke or funny smell. There is nothing going on. No hum, the motor does not jerk or twist when I throttle, nothing! Just dead! I can spin the shaft by hand as well. If I measure the resistance of the motor I should get what? Less than 1 ohm?

[jwscab]

probably have a dud winding, or possibly the comm is so glazed it's not making contact.

if it's a modified motor, just take it apart to inspect it.

you *can measure the ohms, and yes it will be very low, but it's hard to measure just the brushes because they will always short out one pair of comm faces. if you can pull the brushes, and measure from each pair of the comm faces you can see if one pole is shot. so you end up taking three measurements.

Oh yeah, i just read your thread a little more closely. You need to make sure the endbell is on correctly. it needs to be timed so that the brush hoods are roughly in the middle of the magnets. 90 degrees from that, the motor shouldn't actually run.

[8rad]

Mystery solved! I measured the resistance and all three stacks had ultra high resistance or open so I figured I had an open winding. Upon closer inspection I found one of the windings not going to where they clamp up top. It is too short to re-clamp so this motor is FUBAR!

What probably happened was the winding came loose, was thrown out of the armature by centrifugal force and was chopped off by the magnet. Game over!

Thanks for the help.

[jwscab]

I've never had that happen to me when winding my own.

[8rad]

When you wind your own do you have to balance it too?

[jwscab]

i never did, but i tried to wind them as identically as possible. I didn't have a good way to secure the wires, just silver solder and the old crimp, so they never lasted long anyway once the solder melted and the connections opened up. was fun right up to that point though

[Charlie don't surf]

Melted solder? What were you winding? 1X5's

[jwscab]

no....poorly geared 10-13 singles. they flew, but had no run time

[Mad Racer]

A shorted out winding will cause it to stop or smoke as said.

To check if a winding is shorted take the brushes and springs of the com. Spin the arm by hand and it should clunk over a few times.

If it's shorted it won't clunk over easy. Check it with a good motor and you will see what i mean.

[dldiaz]

I believe a shorted armature can also be revealed with an ohm-meter:
Put one lead on the commutator and the other lead on the armature shaft itself...