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600 RICHARDSON'S BLUEBOOK OF PROJECTION
more slowly, creating a continuous, sighing change of pitch, the condition is called "wows."
To repeat, the four requirements of a good drive motor are:
(a) Sufficient power.
(b) Proper starting torque.
(c) Correct speed.
(d) Absolutely steady, uniform motion.
The special methods that secure these results from electric motors used for sound projection are described in detail in the paragraphs following, but the principles of motor action as such, apart from special means used to secure special results, cannot be discussed here. There are so many different types of electric motor that a book larger than this would be needed to do justice to that subject. Such books are available in any public library. There would be little value in taking a small amount of space from matters pertaining specifically to projection in order to devote it to a sketchy and inadequate account of motors. The fundamentals of motors have already been discussed elsewhere. Here we are concerned only with methods, peculiar to the projection room, that are used to secure exceptionally accurate and steady control of motor speed.
(4) The synchronous motor is the type most widely used for projector drive. The speed of this motor depends entirely upon the frequency of the line that supplies it. Increasing the load to be moved cannot slow down the motor, although it may stop it altogether. The current through its windings has no effect upon its speed, hence its action is independent of line voltage fluctuation. If the line voltage should become too high the motor might conceivably burn out, or if the line voltage should fall too low the motor might stop, but its speed can never vary unless the line frequency changes. The very simple reason for these facts is explained fully in any good book on motors.
(5) Synchronous motors for projector drive have proven successful because power companies regulate their frequency very carefully, even when they allow