140 ELEMENTS OF ELECTRICAL ENGINEERING. 



Example j. Compound motor (slwrt-sli tint). A given com- 

 pound motor is supplied, with 50 amperes of current from 110- 

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

 motor, having given the following data : 



R s = 55 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 = 1 10 volts X 50 amperes = 5,500 watts. 



(b) Series field loss = 0.078 x (50 amp.) 2 = 195 watts. 



(c) Shunt field loss = R s x (^-~ [ -& Y = 204.7 watts. 



(d) Armature loss = R( 50 -- ~.~ c X 5 j = 208 watts. 



5,500 195 204.7 2 8 7 



(e) Efficiency = - = 0.762. 



5,500 



Example 4.. Series mo for. A given series motor takes 50 

 amperes of current with an electromotive force of no volts be- 

 tween its terminals. Under these conditions the motor has a 

 definite speed and a definite stray power loss. It is required to 

 calculate the efficiency of the motor under the given conditions, 

 having given the following data : 



R c = o. 1 2 ohm (hot), 



R a = o. I 5 ohm (hot), 



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



Solution : 



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



(b) Series field loss = R c x (5o) 2 = 300 watts. 



(c) Armature loss = R a x (5o) 2 = 375 watts. 



tJ\Tf^' 5,500300375 



(d) Efficiency = - - -- - =0.75. 



5,500 



It is important to remember that the stray power loss of a 

 series motor varies greatly with the load on the motor, or in 



