PROBLEMS. 401 



(c) The transformer is used at rated frequency but with primary 

 electromotive force of 1,500 volts. Find P h and P e each in 

 terms of normal. 



(</) The transformer is used on a primary electromotive force 

 of 1,500 volts. Find / for which P h is normal. Show that 

 P 9 is independent of frequency with given E' . 



(e) With primary electromotive force of 1,650 volts what load 

 would give normal P e ? 



Ans. (a) 1.44 x normal P e \ (ft) 0.87 x normal P h , no change 

 in P t ; (c) 1.64 x normal/^, 1.86 x normal/^; (d) 137.2 cycles 

 per second; (e) 7.5 kilowatts. 



111. Following are the data for a shell type transformer (see 

 Figs. 207 and 208). 



A fine wire coil consists of 560 turns of No. 17 B. & S. copper 

 wire. Mean length of turn 29 J inches. 



The coarse wire coil consists of 28 turns of two No. 7 wires 

 in parallel. Mean length of turn 29! inches. 



Section of magnetic circuit is loj" x lyf", and ^ of this 

 section is iron. Volume of iron 24 J sq. in. x ioj" x -f$. 

 Thickness of lamination 14 mils. 



(a) Find the electromotive force, current, and power ratings 

 of this transformer at a frequency of 60 cycles per second, allow- 

 ing 500 circular mils per ampere, and allowing a maximum flux 

 density of 5,000 lines per square centimeter. 



(b) Find the copper loss and iron loss at full load. 



(c) Find the efficiency of the transformer at full load. 



(d) Find the all-day efficiency of the transformer when it is 

 operated at full load for three hours each day, and at zero load 

 for the remainder of the time. 



112. The losses in a direct-current shunt motor when it is 

 running at zero load are equal to 125 watts and the losses in the 

 motor when it is running at full load equal to 175 watts. The 

 full-load output of the motor is one horse-power. Find the all- 

 day efficiency of the motor when it is operated at full load for 



one minute of time every 10 minutes during the day. 



27 



