314 



ALTERNATING CURRENTS 



reference to the current in the usual manner and the IZ drop 

 obtained. When IZ is added to V, however, E, which is 

 opposite and equal to the back emf., becomes numerically much 

 less than V. That is, the phase shift of the IZ drop is in such a 

 direction that the machine runs as a motor with a very consider- 

 ably reduced back emf. 



The synchronous motor with salient poles will usually operate 

 even if the field current is reduced to zero. The alternating 

 current in the stator winding will produce a rotating field, just as 

 in the induction motor. Figure 291 shows such a rotating field 

 for a four-pole machine without a rotor. At the particular 

 instant shown there are two N-poles vertically opposite, and two 

 S-poles horizontally opposite. If a four-pole, salient-pole rotor 



FIG. 291. Interlocking action of salient poles with rotating magnetic field. 



without excitation be placed in this field, the magnetic lines from 

 the stator will attempt to make the rotor take such a position that 

 the magnetic reluctance is a minimum or the flux is a maximum. 

 In order to accomplish this result, the pole pieces of the rotor when 

 running become locked in with the poles produced by the stator 

 winding, as shown in Fig. 291 (6). These rotating stator poles 

 pull the salient poles of the rotor around with them and in this 

 manner enable the motor to carry a limited load without direct- 

 current excitation. Although the motor may carry a limited 

 load without any direct-current excitation, its power-factor 

 will be very low and the current will be lagging, which is undesir- 

 able. It is to be noted that under these conditions, in the ab- 

 sence of direct-current excitation, the motor takes its entire 

 excitation from the alternating-current lines in the same manner 



