Usually motors and converting equipment are rated also in volts and in amperes. The rating in amperes should never be exceeded, even though the machine may not be delivering its rating in horse-power or kilowatts, because the heating (which is the limiting factor) depends upon the current through the machine.

(B) PROJECTOR DRIVE MOTORS

27. DC Projector Drive Motors.- The DC projector motor is a standard shunt-wound direct current motor. Figures 15 and 16 show a picture of this motor, the motor terminal and resistor box, and the controller or drum-switch. This motor is supplied with RCA Photophone installations where direct current is the standard supply at the theatre, or in theatres where alternating current is the standard supply but variable speed motors are required. A suitable AC to DC motor generator set is used to supply the direct current for these motors when they are used in AC districts. A description of this motor-generator set is given in section 33.

As stated in section 25 the speed of a DC motor can be varied by changing the resistance of the motor field circuit. A three-position speed-control snap-switch on the motor terminal and resistor box gives a speed control by this method of three steps-"High," "Normal" and "Low." Sound film should always be run with the switch in the "Normal" position. The motor speed may be varied when desirable when using silent film by using the "High" or "Low" position. A close adjustment of the speed can be had for any position of the three-position switch by means of the dial adjustment on the motor. The adjustable dial is a part of a centrifugal speed-control switch shown in Figure 17. This switch consists of an adjustable stationary contact; a fly-weight operated moving contact; a slip ring and a brush, for making an electrical connection to the moving contact; and a circuit closing contact spring and stud, which make a connection to the fixed contact.

7