THE SYNCHRONOCX MnTOR 321 



so that reduced voltage is applied to the stator windings. The 

 rotating field sets up currents in the pole faces of the rotor and 

 in the amort isseur winding as well, if such exists. This is obvi- 

 ously induction motor action. As the paths of the pole-face 

 currents and of the currents in the dampers have considerable 

 inductance (see Par. 103, page .237), only a comparatively weak 

 starting torquecanbe obtained. On starting, the rotor currents 

 may be lame, and the rotor frequency is that of the stator. The 

 rotor reactance, which is proportional to the rotor inductance 

 and to the frequency, is large. This causes the rotor currents to 

 lag the induced emfs. by a considerable angle and hence, the 

 rotor currents make considerable space-angle with the flux (see 

 I JL r . 228, page 240). Therefore the motor develops little torque, 

 even with considerable line current. The motor under these 

 conditions is very similar to the squirrel-cage induction motor, 

 which has a very small starting torque. 



However, the starting torque, though small, is usually suf- 

 ficient to start the machine, which then accelerates until it is at or 

 near synchronism. Before the compensator is thrown into the 

 running position, the field switch is usually closed, so as to mini- 

 mize dist urbances to the system. If the rotor is slipping slightly, 

 it will usually pull into synchronism when the field switch is 

 closed, the field poles locking in with the poles produced by the 

 armature mmf.. Fig. 'J'.U, page 314. 



The motor may pull into synchronism before the field circuit 

 is dosed. The flux (see Fig. 291) sweeping by the salient poles 

 shows a less and less tendency to leave them as the rotor ap- 

 proaches synchronism, owing to hy- That is, the flux 

 tend- to persist in the poles after the magnetizing force is de- 

 .sed (see Vol. I, page 181. 1 IL l"7 . This action may be 

 ng enough to pull the rotor into synchronism before the field 

 circuit i> dosed. 



When the field circuit is closed, it may excite the motor poles 

 so that their polarity is opposite- to that produced by the revolv- 

 ing field. /.<.. by the armature reaction Fig, 201 , The rotor is 

 then thrown hack one pole, or in other words, it slips a pole. 



Thi< may oauae considerable disturbance to the system and for 



this rea.-on the field is usually closed when the compensator U 

 in the starting position. This difficulty may be avoided by ap- 

 II 



