If the stator windings are connected directly to the AC lines the motor would
come up to speed too rapidly, so as to put undue strain on the equipment, and there
would be danger of stripping the gears. Therefore, resistors are added in the line
circuit to slow the starting. Either two 40 ohm resistors (one in each of two lines)
or three 20 ohm resistors (one in each line) are used.
A fan is mounted on the rotor shaft within the housing to cool the windings.
Oil the motor regularly every day. The oil cups are at each end of the motor
housing.
If the motor fails to start, look for blown fuses in the line circuit, or burned
out starting resistors. If one of the resistors is burned out, it may be shorted with
a piece of wire, but the machine should not be run in this condition any longer than
necessary because the high rate of starting would cause undue strain on the equipment.
30. Single-Phase Motors.- There are two types of single-phase motors used with
RCA Photophone equipment. They are both induction motors and the main difference between them arises from the different starting methods used. Single-phase
induction motors will not start as such, but they will continue to run once they are
started. The stators of these motors are wound with four poles as in the case of
the three-phase synchronous motors. When a single-phase alternating voltage is applied to such a winding a magnetic field is created which alternates in direction
twice each cycle, in the same manner as the alternating current which produces the
field, but this field does not rotate as in the case of a three-phase motor. A current is set up in the rotor winding which produces an equal turning force in both directions at the same time, so the motor will not start. But, after the rotor has attained a slight speed, the motor will have a turning torque in one direction because
the voltage generated by the rotation of the rotor creates another magnetic field.
This magnetic field combines with the field of the stator winding to produce a rotating field, and the motor is kept running. The speed of the motor never reaches
synchronous speed, or 1800 r.p.m. in case of a 60 cycle circuit, because a certain
amount of slipping of the rotor with respect to the rotating field is required in
order to produce a current in the rotor. This difference in speed between synchronous speed and actual speed is called the "slip" of the motor. The "slip speed"
is constant for a constant load. Therefore, the motor will run at a constant speed
of about 1725 r.p.m. when driving a projector because the load (the friction of the
projector) remains constant.
Two different methods of starting are used on the two types of single-phase
motors used for projector drive. One is called "repulsion starting" and the other
is called "split-phase starting." These methods will be described in the two succeeding sections.
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