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DRIVES 555
Any elementary book on motors will explain that when field current is increased a shunt-wound motor slows down, and vice versa. The reason, very briefly, is that a motor always acts as a generator, creating a countervoltage in its own armature. A stronger field current increases this counter-voltage, reducing the armature current so greatly that the motor runs slower.
(10) It is high speed in the motor of Figure 210 that forces the flyweights outward and closes the centrifugal contact, thereby short-circuiting the field resistance and increasing the field current. Thus, excessive motor speed compels the motor to slow down. Conversely, when the motor speed declines, the centrifugal contact opens, cutting in the field resistance, decreasing the field current and causing the motor to speed up. The action of this contact is a continuous, chattering make-and-break. The rotating parts of the motor are too heavy to permit the motor speed to fluctuate as rapidly as the centrifugal contact opens and closes. In consequence, the motor maintains a steady, average speed, at the rate required by the setting of the adjustable contact. The radio-type dial by which this contact is adjusted is prominent in the foreground of Figure 209.
An additional three-position switch, not shown in these drawings, provides further variation of the motor field current, and therefore greater variation in speed than is attainable by the adjustable contact.
Figures 209, 210 and 211 are so plain that further discussion of this simple method of speed control seems unnecessary.
Motor Control Cabinets
(11) Five types of speed control cabinets, all produced by one manufacturer, are used to regulate projector speed. In all systems they have been superseded by the simple synchronous motor. Many, however, are still in use. The tubeless type is diagrammed in Figure 212.
(12) The dot-dash line of Figure 212 separates the motor from the control box wiringr. In this arrangement