79. Non-Synchronous Turntable Drive Motors. - The motors used to drive the non-synchronous
turntables are similar, except in size, to the motors used in the induction type watt-hour meters.
The action of the motor is similar to that described for the starting of the split-phase start
induction motor used for projector drive (See Section 32), but the mechanical construction is
entirely different. Instead of a squirrel-cage rotor a flat disc is used which rotates between two
sets of poles. The pole pieces are so wound that when a single-phase alternating voltage is
applied to the winding a moving magnetic field is created, which acts with the eddy currents set
up in the disc by the transformer action to produce rotation. The speed of the motor is
controlled by means of a brake, operated by a fly-weight governor. This device keeps the
speed at a consredt value. The speed can be adjusted by means of speed control knobs located
at the side of each turntable. This speed should be adjusted to 78 r.p.m.
Normal operation of the motor will produce more heat than can be comfortably tolerated while
touching any part of the coil units. This is mentioned in order that the operator will not
misconstrue this heating effect to indicate a defect.
The turntable motors should be oiled about once a month with light machine oil. (Do not use
Three-in-One oil.) The points to be oiled are the upper turntable bearing, the lower turntable
bearing and both bearings of the governor. The worm and gear should be lubricated with a light
grease. In addition to the regular lubrication, all bright metallic parts except the motor disc
should be covered with a light film of oil to prevent rusting.
80. Speed Regulation of the Non-Synchronous Turntables. - The governor will maintain a
consredt speed of the motor within a range of sudden voltage changes of 20 volts, providing all
the parts are correctly adjusted. Any adjustment made on the motor (including lubrication), will
have a certain effect on the regulation of speed and the speed adjustment should be checked
before the unit is again placed in service.
The speed may be checked as follows:
(a) Place a record on the turntable and insert a small piece of paper
under the edge of the record to serve as an indicator.
(b) Play the record in the normal manner and count the number of revolutions made by the turntable for one minute. The speed should be 78 revolutions per minute.
(c) Turning the speed regulating knob clockwise allows the motor to run faster and turning the knob counterclockwise causes the motor to slow down. Adjust by trial
until the speed is 78 revolutions per minute as determined from a full minute's count.
NOTE: The speed of the machine should be checked at least four or five times a year.
Improper speed will cause distortion.
Copyright 1930 RCA Photophone, Inc