TIIK SYXdllfONOUS MOTOR 309 



the armature current is given by dividing the armature imped- 

 ance Z into the vector difference between the terminal voltage 

 V and the omf. E 1 '. 



Thai ia / = ^-^ = |? (71) 



Where E is the vector difference of V and E'. 



Therefore E = IZ. 



The above equation for .the armature current in the synchro- 

 motor is similar to the equation for the armature current 

 in the direct-current motor (see Vol. I, page 317, eq. 109). 



As a rule the reactance of the armature of a synchronous ma- 

 chine is high as compared with its resistance, and the current / 

 be voltage E which produces it by nearly 90. This brings 

 the current / very nearly in phase with E' and nearly 180 from 

 the back emf. E'. Therefore, I is largely energy current with 

 respect to E', which means that it supplies considerable internal 

 power to the motor. 



The rotor, by shifting its phase backward when load is applied, 

 causes the motor to take an energy current from the line which sup- 

 plies the power demanded by the increased load. 



The total power supplied to the motor per phase is 



p = vi cos e 



The total mechanical power developed is 

 P' = E'l cos (6 + a) 



The net power at the pulley is less than P' by the amount of the 

 frietional losses and the rotational core losses. 



The difference between /' ; m<l /'' is the armature copper loss. 

 It should be remembered that the mvrm/r motor speed remains 

 int. The rotor merely takes an angular position slightly 

 Of its no load position, without altering its average -peed. 

 Tin.- angular displacement of the rotor may be observed by 

 mean- of a -t rol '~ t)- 



126. Effect of Increasing the Field Excitation. When the 

 field of a direct-current shunt motor : ihened. there 18 a 



temporary increase in the armature induced emf. This decreases 

 imature current and the torque is lowered, since the change 

 in armature fa mud r than the corresponding 



change in the field. As a result, the motor slows down and its 



