320 ALTERNATING CURRENTS 



Again, if the field poles for some reason swing ahead of their 

 normal position, the dampers cut the rotating field in the oppo- 

 site direction or the slip becomes negative, temporarily. Induc- 

 tion generator action follows, putting a load on the rotor and 

 tending to slow it down. Therefore, these dampers always tend 

 to pull the motor back into synchronism and thus prevent hunt- 

 ing. Such windings are often used on alternators, particularly 

 of the engine-driven type, to prevent hunting. 



130. Starting the Synchronous Motor. As has been pointed 

 out, the synchronous motor is not self-starting. It must first 

 be brought nearly or actually to synchronous speed before it 

 can operate. There are several methods of accomplishing this. 



The direct-current exciter for the motor is frequently con- 

 nected directly to the motor shaft. If a direct-current source of 

 power is available, the exciter may be operated as a motor and 

 thus bring the synchronous motor up to speed. The field of the 

 synchronous motor is then excited and the motor synchronized, 

 just as with an alternator. 



If an exciter or sufficient direct-current power is not available, 

 a small induction motor, geared or direct-connected to the syn- 

 chronous motor shaft, may be used for bringing it up to speed. 

 If the induction motor is direct-connected, its synchronous speed 

 must have a higher value than that of the synchronous motor, in 

 order to compensate for the slip of the starting motor. Such 

 starting motors are often disconnected mechanically after the 

 synchronous motor has been connected to the line. The dis- 

 advantage of using an induction motor is the additional motor, 

 the gears where used, etc. This method of starting is practi- 

 cally not used at the present time. 



The synchronous motor is often used to drive a direct-current 

 generator. If sufficient direct-current power is available, the 

 generator may be used as a motor to bring the synchronous motor 

 up to speed. After the motor is synchronized, the field of the 

 direct-current machine is strengthened and it then acts as a gen- 

 erator, taking mechanical power from the synchronous motor. 



The synchronous motor may start as an induction motor. 

 First, the field circuit is opened. A polyphase alternating voltage 

 is then impressed on its stator and a rotating field is therefore set 

 up about the rotor. As a rule, it is desirable to use a compensator 



