CHAPTER XI 

 COMMUTATING ALTERNATING CURRENT MOTORS 



SINGLE-PHASE SERIES MOTOR; REPULSION MOTOR) 

 COMPENSATED REPULSION MOTOR 



101. Field for the Single-phase Series Motor. As was 

 explained in Chapter IV a motor suitable for electric traction 

 must have a high starting torque, a variable speed-load 

 curve and good efficiency when operating at widely differ- 

 ing speeds. 



The induction motor runs at nearly constant speed 

 irrespective of load and, if it is attempted to regulate the 

 speed, either complex connections or decreased efficiency 

 is necessary. The cascade connection of two motors 

 and the scheme wherein the number of poles is varied both 

 give only a definite change in speed; instead of having 

 one speed of efficient operation the motor has two speeds, 

 but neither of these schemes give a variable speed motor 

 and in addition the switching and wiring become com- 

 plicated. If rotor resistance is relied upon for speed varia- 

 tion a low efficiency is obtained at the same time. 



The synchronous motor has a rather small starting torque 

 (when started as an induction motor) and operates at 

 constant speed, irrespective of load; it is evidently not 

 suited for railway purposes. 



The single-phase scries motor , however, has starting 

 and running characteristics very similar to those of the 

 c-c. series motor and is thus suited for railway work. This 

 type of a-c. motor has been developed as a railway motor 



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