474 



BELL SYSTEM TECHNICAL JOURNAL 



The operation of the circuit is as follows: In starting switch ^i is 

 closed which applies direct current to the shunt field and armature 

 circuits of the motor. The motor accelerates as an ordinary compound 

 wound motor. Switch S2 is then closed applying three phase power 

 from the slip rings of the motor to the transformer. As the speed of 

 the motor approaches the operating point, the beat frequency between 

 the pilot generator and the oscillator will cause beats in the current 

 through the regulating field which are visible on the meter Mi. Let us 

 assume that the field rheostat has been previously adjusted so that with 



SCANNING DISK-22"DIA. 



BELLOWS 



m 



ss 



ss 



Fig. 4— Hydraulically dampedcoupling'to prevent hunting of motor. 



the shunt field alone the motor will tend to run slightly over the desired 

 operating speed. When the exact operating speed is obtained, the 

 beat frequency in the regulating field will be zero and as the motor 

 tends to accelerate, the phase relationship between the pilot generator 

 and the oscillator will reach a point tending to give maximum strength 

 to the regulating field. When this point is reached, the acceleration of 

 the motor will be checked by the increased field and the speed will tend 

 to fall until the phase of the pilot generator with respect to the 

 oscillator has shifted sufficiently so that the regulating field current is 



