310 ALTERNATING CURRENT* 



back electromotive force accordingly decreases. The armature 

 current then increases until it is again of sufficient magnitude to 

 enable the motor to carry the load. 



When the field of a synchronous motor is increased, the motor 

 cannot slow down, except momentarily, for it must run at con- 

 stant average speed. Since its speed is constant, its back emf. 

 must increase when the field is strengthened. It might seem 

 then that the motor would stop, for its induced emf. must 

 apparently become greater than its terminal voltage. In the 

 direct-current motor, an induced electromotive force exceeding 

 the terminal voltage would mean generator action with the re- 

 sult that the machine would cease to operate as a motor. 



/ ( Lags induced emf. and leads 

 motor terminal voltage ) 



E (Induced emf.) 



V m (Motor terminal voltage) 



V a ( Generator terminal voltage) E 



FIG. 286. Relation of current to voltage in motor and in generator. 



The synchronous motor, however, may operate as a motor and 

 at the same time its back emf. may exceed its terminal voltage 

 in magnitude. Under these conditions, the motor is said to be 

 over-excited. Two reactions occur which enable the motor to 

 operate with an over-excited field. First, the motor takes a 

 leading current. A leading current in a motor corresponds to a 

 lagging current in a generator. This is illustrated by Fig. 286. 

 A current / is shown lagging the induced emf. E by 90. This 

 current I is lagging with respect to both the induced emf. and 

 the generator terminal voltage and is therefore a lagging current 

 if the machine is considered as a generator. Such a current 

 weakens the field through the effect of armature reaction (see 

 Par. 63, page 134). 



When the machine is considered as a motor, the emf. E, 

 which is the component of the terminal voltage that balances 

 E, is opposite and equal to the induced emf., E. The termi- 

 nal voltage V m differs from E only by the armature impedance 



