SYNCHRONOUS MACHINERY 



273 



OEi = E! = ET! + EI" = Iz = e.m.f. consumed by the syn- 

 chronous impedance of the armature. 



OE Q ' = EQ = e.m.f. generated in the armature by cutting the 

 flux produced by the field m.m.f. 



OE Q EQ~ component of impressed e.m.f. required to over- 

 come the generated e.m.f. EQ. 



OM f = M f = field m.m.f., 90 degrees ahead of EJ. 



M f is the value of field m.m.f. required to make the power factor 

 of the motor unity. 



Fig. 251 is the vector diagram for the motor when the current 

 has the same value as before but lags behind the impressed e.m.f. 

 by angle <j> = 60. 



Fig. 252 is the vector diagram when the current leads by angle 



= 60. 



FIG. 250. 



FIG. 251. 



Referring to these diagrams it is seen that the field excitation 

 required in a synchronous motor to produce a leading power factor 

 or to cause the current to lead the impressed e.m.f. is greater than 

 that required to produce a lagging power factor or to cause the 

 current to lag behind the impressed e.m.f. 



If, therefore, the field current of a synchronous motor is varied, 

 there is no change in speed as in the direct-current motor, but the 

 generated e.m.f. Ed changes both its value and its phase relation 

 with the impressed e.m.f. E and allows leading or lagging currents 

 to flow to make up for the change in excitation; when the field 

 current is decreased a component of current 90 degrees behind the 

 impressed e.m.f. flows in the armature and magnetizes the field 



