Diane DeTar wrote in 120421 MMDigest:

> I have had the problem of intermittent motors several times.  In
> every case it was because of bad brushes.  There was usually some
> fine "grit" on one of the brushes and it would stop on that bad
> place every time.  I would hit the motor and it would start again.

It can also be due to an open-circuit winding in the armature.  The
motor will run more or less okay, but if it happens to stop with a
brush on a dead segment of the commutator, it will not start again
until moved physically.  I have seen this problem several times in
small universal motors in calculating machines.

John Wolff
Melbourne, Australia
http://www.vicnet.net.au/~wolff