POWER LOSSES IN GENERATORS AND MOTORS. 139 



Example i. Shunt motor. A given shunt motor is supplied 

 with 50 amperes of current from no-volt mains, and it is re- 

 quired to calculate the efficiency of the motor, having given the 

 following data : 



R s = 44 ohms ( hot ), 



R a = o. 14 ohm ( hot), 



Stray power loss = 700 watts at the given voltage and speed. 



Solution : 



(a) Power intake = 1 10 volts x 50 amp.= 5,500 watts. 



(E V 

 M = 275 watts. 



(E \ 2 

 50 -g J =316 watts. 



5,500 275 316 700 

 (d) Efficiency = * - = 0.765. 



Example 2. Compound motor (long -shunt). A given com- 

 pound motor is supplied with 50 amperes of current from no- 

 volt mains, and it is required to calculate the efficiency of the 

 motor, having given the following data : 



R s = 5 5 ohms (hot), 



R c = 0.078 ohm (hot), 



R a = 0.09 ohm (hot), 



Stray power loss = 700 watts at given voltage and speed. 



Solution : 



(a) Power intake = 110 volts x 50 amperes = 5,500 watts. 



/ E V 



(b) Shunt field loss = R t ( -^ \ = 220.0 watts. 



(E \ 2 

 SO ~ \ = 179.7 watts. 



(E \ 2 

 50 - J = 207.4 watts. 



(,) Efficiency = ~ 22 ~ , o . 7 6 2 . 



