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.