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1132 HANDBOOK OF PROJECTION FOR
quency of the current so produced will of course always be exactly proportional to the speed of the motor at the time.
When the motor is running at the standard speed of 1200 R. P. M., which drives the projector at the rate of ninety feet of film per minute, the small generator is producing A. C. at a frequency of 720 cycles per second. Of course at a less speed than 1200 the frequency would be less than 720 and above 1200 it would be greater than 720.
The critical frequency of the tuned circuit of the motor control box is 720 cycles. By this it is meant that it will pass A. C. of that frequency, but will not pass it at any other frequency, which is the key to the action of this particular apparatus.
When you first start the motor the generator frequency will of course be below 720, hence no current can pass through the tuned circuit and the motor will pick up speed in the usual manner. However, when a speed of 1200 R. P. M. is reached, then current passes through the tuned circuit, and this so affects the control circuit of the motor that there will be no further increase in speed.
And now to take up the action in greater detail. The motor used on D. C. supply is a compound wound motor of the regular type, except that in addition to the usual shunt and series windings it has a special speed regulating winding, the current for which is supplied from the motor control box. The speed of a D. C. motor is increased when the field current is weakened and decreased when it is strengthened, which fact is taken advantage of as the means for regulating the speed of the motor. The field winding for the pilot generator is supplied with current from the mains. In the D. C.