306 ALTERNATING 



If, however, conductor a can in some manner be brought under 

 the next pole, which is a south pole, for the half -cycle during 

 which the current is in the reverse direction, the resulting torque 

 will still be from left to right and a tendency toward continuous 

 motion will result. Therefore, in a synchronous motor a given 

 conductor must move from one pole to the next in each half -cycle, 

 if the machine is to operate continuously. This applies to the 

 rotating-armature type of machine. If the machine is of the 

 rotating-field type, any given conductor must be passed by one 

 pole every half -cycle. In any event the synchronous motor must 

 operate at constant speed, if the frequency is constant. There 

 may be momentary fluctuations of speed, but if the average speed 

 differs by even a small amount from this constant value, the 

 average torque will ultimately become zero and the motor will 

 come to a standstill. The relation of speed, number of poles 

 and frequency is the same as for the alternator and for the rotating 



field of the induction motor. That is, the speed S = p r.p.m., 



where / is the frequency and P the number of poles (see Pars. 

 3 and 101, pages 7 and 235). 



Example. A 500-kv-a., 2,300-volt, 10-pole synchronous motor operates 

 on a 60-cycle three-phase system. What is its speed? 



120 X 60 

 S = = 720 r.p.m. Ans. 



125. Effect of Loading the Synchronous Motor. If a load 

 be applied to a direct-current shunt motor, the speed is slightly 

 decreased. This reduces the back emf., E. The line must 

 supply a voltage -\-E, equal and opposite to the back emf. E 

 and in addition, must supply the voltage necessary to overcome 

 the IR a drop in the armature. 



That is, 



V = E + IR a 



where V is the fixed terminal voltage, / the armature current 

 and R a the armature resistance. 

 The current 



/ = V + (-W = V~_A 



R a Ra 



