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DRIVES 601
their voltage to vary enormously. Frequency regulation is a matter of dollars and cents to a power company. All their generators, line transformers and other apparatus are designed for maximum efficiency at rated frequency, and permitting the frequency to vary means unnecessary losses. Frequency control of power lines has been so improved during the last few years that a frequencycontrolled projection motor can be relied upon, in most American theatres, to maintain constant speed at all hours of the day or night.
(6) Synchronous motors are often used to drive the non-synchronous turntables in the projection room. There may seem on the surface to be some confusion at the idea of a synchronous motor driving a non-synchronous turntable. The facts are that the motor is synchronous because it operates in svnchronism or resonance with the line frequency, while the turntable is non-synchronous because the sound taken from it is not synchronized with a moving picture.
(7) Synchronous motors are not used where only line d. c. is available. In theatres not equipped with alternating current a converter driven by d. c. can be used to generate alternating current for the operation of an a. c. amplifier. But the frequency of the alternator's output will not be absolutely constant unless its speed of operation is held absolutely constant. Why trouble to regulate the speed of a d. c. motor driving an alternator? It is as easy to regulate the speed of a d. c. motor driving a projector, and a smaller and less expensive alternator can be used.
Synchronous motors are also omitted from the equipment of theatres that show silent pictures occasionally and may wish to vary their projector speed. The speed of a truly synchronous motor cannot be varied.
(8) The use of the synchronous induction motor is only one method of securing proper projector speed. Three other methods of obtaining the same result are in common use. There are "centrifugal" speed governors, used at present only with d. c. motors. There are motor control circuits involving vacuum tubes, used with both