THE SYNCHRONOUS MOTOR 



329 



It is also instructive to keep the lamp load or the induction 

 r load constant and at the same time to obtain a V-curve 

 and find the relation of V m to the synchronous motor field cur- 

 rent. The results of such a test are shown in Fig. 304. V m is 

 considerably lower than V for low values of field current, but 

 after unity jxnver-f actor is reached, V m exceeds V a . 



F-Curve , Motor Volti = 



Field Current 



Fio. 304. Effect of field current on motor 

 voltage at constant load. 



K.W.Load 



:{03. Relation of fid. I 

 ( urrt-nt to load at motor, 

 motor voltage constant. 



134. Industrial Applications of the Synchronous Motor. 



Single-phase synchronous motors are rarely used in practice. 

 Like the single-phase induction motor, the direction in which they 

 rotate is determined by the direction in which they are started. 

 Unlike the polyphase synchronous motor, they will not start 

 by induction motor action but must be brought up to speed by 

 other means. Polyphase synchronous motors are commonly 



The inherent disadvantages of the synchronous motor are that 

 it requires a direct -current supply for its excitation, its starting 



torque is very small, ami the motor is very sensitive to system 

 disturbances and may fall out of step when these occur. On the 

 other hand, the ease with which its power-factor can be con trolled 

 is a distinct advantage, often outweighing all the disadvantages. 

 The fact that its speed is constant is of little moment, since 

 induction mot > <M;dly in the larger sizes, have only 1 or 2 



-lation. 



The ^vnchronnus motor is used only in the larger sizes where 

 the COSt of attendance per kilovnll-ainpere 18 low. Moreover, it 

 should not In- ii-rd \\hrp- n Hidden applications of t he load, 



as it may drop nut Q| >tep under such condition.-. An important 

 field nf ii-.- is in connection with inntnr-L'enerator sets wh< 



