CHAPTER X. 

 SYNCHRONOUS MACHINES. 



EXPERIMENT IO-A. Study and Operation of a Synchronous 

 Motor. 



PART I. INTRODUCTORY. 



I. Structure and Use. A synchronous motor has the same 

 structure as a synchronous generator (Exp. 3~A). As in a gen- 

 erator, the field is separately excited by direct current ; the arma- 

 ture is supplied from the line with alternating current, either 

 single-phase or polyphase. The speed is fixed by the frequency 

 of the supply circuit, that is, the motor runs at synchronous speed 

 or not at all. In starting a synchronous motor it is necessary to 

 first bring it to speed, usually by external means,* and this limi- 

 tation has no doubt prevented its more general use. 



The fixed speed in many cases is a positive advantage, as for 

 example in the operation of frequency converters (Exp. 9~A). 



A synchronous motor has the important advantage that its 

 power factor is adjustable and it can be made to take leading or 

 lagging current by varying the field excitation ; its leading current 

 can be used to offset the lagging current taken by induction motors 

 and other apparatus/)* thus improving the power factor of the 

 system. Synchronous motors are sometimes installed solely for 

 this object. 



A synchronous converter has the same general characteristics 

 as a synchronous motor and most of the following statements will 

 apply to a converter as well as to a motor. 



*(ia). An auxiliary starting motor is often provided. The means 

 for starting may be within the motor; thus, some single-phase synchronous 

 motors are provided with an extra winding and commutator so as to start 

 as a series motor. For self-starting polyphase motors, see n. 



tFor operating an induction generator (Exp. 9-6) it is necessary to 

 have a synchronous machine connected with the system. 



304 



