322 ALTERNATING CURRENTS 



plying a weak direct-current field to the motor as it approaches 

 synchronism. This causes the armature reaction to act in con- 

 junction with the direct-current field windings, and the poles 

 then come into synchronism with the same polarity as will be 

 produced by the direct-current excitation. After the motor has 

 pulled into synchronism, it is necessary merely to strengthen 

 the direct-current field to the desired value. The starting com- 

 pensator may then be thrown quickly into the running position. 



When voltage is first applied to the synchronous motor, there 

 may be a very high voltage induced in the field winding. The 

 stator acts as the primary of a transformer, the primary having 

 a comparatively few turns. The flux produced by the stator or 

 primary cuts the field winding at synchronous speed, and as the 

 field has a very large number of turns, a very high electromotive 

 force is induced in the field. This electromotive force may be 

 sufficiently high to puncture the field winding. Therefore, the 

 field winding should be insulated for voltages considerably in 

 excess of that which normal operation requires. The field is 

 sometimes short-circuited, or is shunted by a resistance when 

 starting, in order to decrease this high voltage. The induced 

 emf . in the field decreases as the rotor comes up to speed, until 

 at synchronism it becomes zero. 



131. The Synchronous Condenser as a Corrector of Power - 

 factor. The fact that the power-factor of the synchronous motor 

 may be varied at will makes it useful in many installations, 

 particularly in those which operate at low power-factor. It will 

 be recalled that a low power-factor means larger generators, 

 more transmission copper, poorer regulation, and reduced effi- 

 ciency. Factories and mills using induction-motor drive often 

 have an over-all power-factor as low as 0.5, which is very undesir- 

 able. If it is possible to use a synchronous-motor drive in any 

 part of the installation, the motor may be operated over-excited 

 and therefore will take a leading current. This leading current 

 neutralizes some of the lagging current of the system and so 

 improves the system power-factor. 



This is illustrated in Fig. 296 for single phase or for one of 

 the phases of a polyphase system. Let V be the voltage of the 

 system and let the total current be /, lagging the voltage V by 

 an angle 0i. It is desired to obtain the size of synchronous 



