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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


Chapter Seven Pages
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©1930 RCA Photophone, Inc
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