186 



BELL SYSTEM TECHNICAL JOURNAL 



An examination of the standard commercial types of speed control 

 indicated that there was nothing exactly suitable. The nearest 

 approach to a suitable governor is the standard type of phonograph 

 governor but this friction brake type of governor has serious objections 

 if applied to a motor of considerable power output. In order, there- 

 fore, to have a control system which would be free from maintenance, 

 it was necessary to develop a special form of control system for the 

 purpose. Fig. 1 shows a photograph of the A.C. motor and its control 

 cabinet. 



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Fig. 1 — A. C. Motor with control cabinet. 



A.C. Motor Control Circuit 



Fig. 2 shows the A.C. motor circuit which consists of a repulsion 

 type of motor coupled to a small auxiliary alternator providing a 

 frequency of 720 cycles which through a control circuit is made to 

 operate a variable reactor across the armature terminals of the motor. 

 If the speed of the motor is too high, the control circuit produces a 

 maximum impedance in the reactor Li thereby reducing the armature 

 current of the motor and causing it to slow down. While if the 

 speed is too low the control circuit causes the reactor Li to have a 

 minimum of impedance increasing the armature current and causing 

 the motor to speed up. 



This reactor Li is of the D.C. saturating type having two outer 

 legs with A.C. windings and the middle leg with a D.C. winding. 

 The A.C. flux circulates around the two outer legs. The D.C. flux 

 flows from the middle leg and returns through the outer legs in 

 parallel. When D.C. flux is sent through the middle leg it saturates 



