If for any reason the centrifugal switch fails to short-circuit the commutator
when the motor is up to speed, do not let the motor run in this condition,
as the
resistor will get hot and burn out. Stop the motor at once and see what is causing
the mechanism to bind. If the show is in progress, give the motor a slight boost
with the crank.
If the motor fails to start, look for blown fuses in the supply circuits.
If the mechanism should bind so as to cause the commutator short-circuiting
disc to "stick" in the running position, the motor will hum when the switch is
turned on, but will not start. It can be started by cranking the machine, but the
mechanism should be repaired at the first opportunity. Keep the motor clean at
all times and check the condition of the brushes occasionally.
Oil cups are located at each end of the housing and should be filled daily.
32. Split-Phase Starting Single-Phase Induction Motor. -This motor is used for
the projector drive of PG-13 equipment. (See Figure 21.) It is started
by means of a split-phase winding which, in conjunction with the running winding,
produces a rotating field similar to that of the three-phase motor. The split-phase,
or starting, winding is connected in series with a resistor mounted in a box on the
end of the motor, and this series combination is connected across the power line
through a centrifugally operated switch.
On starting, the centrifugal switch is closed, and both windings function to
produce a rotating field which acts upon the squirrel-cage rotor to effect rotation. As the motor comes up to speed the fly-weights of the centrifugal device on
the motor shaft travel radially away from the shaft, and in so doing strike a metal
pin which operates the switch to disconnect the starting winding.
When the power is turned off, the fly-weights are drawn in towards the shaft by
small springs. As the weights are retracted, they strike another metal pin, which
closes the switch that connects the starting winding and resistor in parallel with
the running winding, to be ready for another start.
There are no brushes or other continually moving contacts anywhere in this
motor. The speed of the motor, while not synchronous, is constant and dependent
upon the frequency of the power supply. The speed of a four-pole synchronous
motor operated on a 60 cycle circuit is 1800 revolutions per minute, but the speed
of this induction motor is approximately 1725 revolutions per minute.
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