THE SYNCHRONOUS MOTOR. 157 



74. The polyphase synchronous motor. The foregoing articles 

 (and also the remainder of this chapter) refer explicitly to the 

 single-phase synchronous motor, that is to a single-phase alter- 

 nator taking power as a motor from a single-phase generator. 

 The entire discussion applies equally well, however, to the poly- 

 phase synchronous motor, that is, to a polyphase alternator tak- 

 ing power as a motor from a polyphase generator. In such a case 

 each armature winding of the motor takes current from one arma- 

 ture winding of the generator and the total power intake is nP", 

 where P" is the intake of each armature winding or phase and 

 n is the number, of phases. To apply any portion of the dis- 

 cussion given in this chapter to a polyphase synchronous motor 

 it is only necessary to remember that the quantities A, B, R, 

 X, P' , P fr y E, and / refer to one phase only of the system. 



The single-phase synchronous motor will run in either direc- 

 tion, but the polyphase synchronous motor with given connections 

 to the polyphase supply mains will run in a certain direction only. 



75. The rotary converter. The rotary converter, or, as it is 

 frequently called the synchronous converter, is described in the 

 next chapter. This machine, as ordinarily used, is at once a 

 synchronous alternating-current motor and a direct-current gen- 

 erator, it is usually polyphase, and it exhibits all of the features 

 of the synchronous motor as to stability of running, as to hunting, 

 and as to starting. The next chapter is devoted chiefly to those 

 aspects of the rotary converter in which it differs from a synchro- 

 nous motor developing mechanical power, such, for example, as 

 the relation between its voltages, its armature reaction, and its 

 armature heating. 



76. The over-excited synchronous motor. A synchronous 

 motor of which the electromotive force (effective) is greater than 

 the electromotive force of the generator or supply mains from 

 which it takes current is called an over-excited synchronous motor. 

 In all of the foregoing discussion the machine B which acts as a 

 synchronous motor is supposed to have a smaller voltage than 

 the machine A which acts as the generator. It is, evident, how- 



