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BELL SYSTEM TECHNICAL JOURNAL 



per second per second or a little over one revolution per second per 

 second. For comparison consider a ^-h.p. unregulated shunt motor. 

 If the line voltage increases 10 per cent, it will cause an increase in 

 speed from 4 per cent to 8 per cent depending on the magnetic satura- 

 tion in its field circuit. This increase in speed will take place in a 

 half second or more depending upon the moment of inertia of the 

 load. Thus the acceleration in the case of a 1060-r.p.m. speed would 

 be much greater than one revolution per second per second. 



Fig. 1 — Assembled motor 



Since this problem of speed regulation is a separate one from that 

 of the synchronization, the description of the regulating circuit is 

 taken up later on. 



Motor Design 



In accordance with the phase displacement requirement as explained 

 previously it was decided to build the synchronous motors with 120 

 pairs of poles, thus giving a frequency of 2125 cycles at 1062.5 r.p.m. 

 which was the exact speed finally employed. For the sake of 

 mechanical simplicity these machines were made of the variable 

 reluctance type which gives one cycle per rotor tooth, thus requiring 

 120 teeth. The variable reluctance construction also simplifies the 

 coil arrangement, the machine having only eight armature coils 



