Problems of an Aging Motor
W
HEN AN AGING SINGLE-PHASEmotor seems to be starting slow-
ly, there are two common points of failure to examine. One is
the starting switch and the other is the starting capacitor.
The starting switch is usually a centrifugal device built to snap
open and switch off the starting circuit when the motor reaches
approximately three-quarters of its full speed. This switch usually has
weights and springs, like a fly-ball governor seen on old steam engines
in museums. Frequently, in woodworking shops, the moving compo-
nents of the switch get fouled with sawdust or pitch and become
stuck or operate sluggishly. If the switch sticks in the open position,
the motor will hum but not start; if it gets stuck in the closed posi-
tion, the motor will be quite noisy and if left on, it will get very hot
and possibly fail. Starting-switch problems can be prevented by keep-
ing motors clean and blowing away accumulated sawdust before it gets
into the motor. In a very dusty shop, it might be a good idea to switch
to a TEFC (totally enclosed, fan-cooled) motor.
The second failure point on single-phase motors is the starting
capacitor—this is a black plastic cylinder usually mounted externally
(in some cases, it’s mounted inside the motor). Occasionally, the capac-
itor will weaken or short out. When the capacitor weakens, the motor
becomes slow starting. If the capacitor shorts, the motor will be very
noisy, may start or run slowly, and exhibit the same symptoms as if the
starting switch is stuck in the closed position. When it fails completely,
the motor hums but doesn’t start just as in the case of a starting switch
that’s stuck open.
CHAPTER ONE