268 SYNCHRONOUS MOTORS. 



assist and sometimes oppose the rotation, and the motor 

 would probably cease to rotate. 



The essential point about a synchronous motor is that it 

 can only operate when rotating synchronously, or in step 

 with the current supplied to it. 



Effect of Increase in Number of Poles. As explained above, 

 a 2-pole synchronous motor will make exactly one 

 revolution for each cycle of the alternating current supplied 

 to it. Thus a 2-pole machine supplied with current having 

 a periodicity of 25 would make 25 x 60 = 1,500 revs. per. 

 minute. 



If the motor has a greater number of poles, the speed will 

 be such that an armature conductor passes from one pole 

 to the next of the same character in the time of each period 

 of the current. Hence, if the machine has 12 pairs of poles, 

 it will take the time of 12 periods to make one revolution. 

 Thus, 



n x 60 

 Speed of motor = revs, per minute. 



When n = periodicity of current supplied. 

 p number of pairs of poles. 



Method of Starting a Synchronous Motor. As just explained, 

 the action of a- synchronous motor depends on its 

 running at the speed of synchronism, and the machine 

 will not rotate if switched on to an alternating supply while 

 stationary. It must, therefore, be started by some external 

 means from rest, and, when it has been brought up to the 

 correct speed, it may be switched on to the alternating 

 circuit, and will then continue to rotate. 



Synchronous motors are often started by means of direct- 

 current motors either intended for that special purpose, or 

 intended as direct-current generators for supplying the 

 exciting current required by the synchronous motor. The 

 use of a direct-current motor necessitates a supply of direct 

 current which is sometimes not available before the machine 

 is started, e.g. in cases where the motor drives its own exciter. 



Sometimes a synchronous motor can be started from rest 

 by the action of the eddy currents formed in the pole faces 

 of the magnets. This can only be done in the case of 2 or 

 3 phase machines, and will be referred to in discussing them. 



In cases where only a single motor is to be driven by 

 an alternator, as is frequently the case in testing, the motor 



