
Problem 
Answer 
1 
A 10hp motor whose fullload efficiency is 87.5 per cent cost $160. What must be the selling price of a second motor having a fullload efficiency of 89 per cent, if fullload operation only is considered and the two motors are to be exactly comparable economically? The cost of energy is 2 cents per hphr. The time of operation is 3000 hours per year. Assume the life of either motor to be 25 years, and the interest rate to be 6 per cent. The duration of the service is 75 years. 
$308 
2 
A 150hp, 220volt motor that costs $1200 has a guaranteed efficiency of 91 per cent at full load. The cost of energy is 2 cents per hphr. The motor has a life of 25 years and is to be used 2500 hours per year. The duration of the service is ,50 years. If, instead, a 440volt motor with an efficiency of 90 per cent were bought at the same price, what would be the increase in the amount of the present worth of the service? 

3 
If in Problem 1 the hours of use per year were 500, what price must be paid for the motor of higher efficiency to cause the present worth of the service to be the same when performed by either motor? 

4 
A factory load of 500 kva of 460volt, 3phase motors is to be supplied by a threewire line from a bank of transformers 300 ft away. What size of soft copper Type W insulated conductors must be used to satisfy the code requirements for outdoor overhead distribution? (See Appendix for currentcarrying capacities of conductors.) 
630 amp, 600,000 cir mils 
5 
If the available voltage at the transformers in Problem 4 is 460 volts, linetoline, what will be the linetoline voltage available at the motors when the line current is 630 amp? 
454 volts 
6 
If the motors of Problem 5 are guaranteed to perform satisfactorily at a voltage 10 per cent less than their rated voltage and the current drawn by them when fully loaded at 414 volts is 680 amp, what would be the maximum length of line for which the 600,000 cir mil conductor would be acceptable? 
1368ft 
7 
A city has a bank of three singlephase transformers, rated 200 kva each, supplying a sewagedisposal plant. Each transformer has a core loss of 1100 watts and a fullload copper loss of 1300 watts. In accord with the trend in standardization of voltages, the power company from which energy is purchased plans to raise its distribution voltage from 2200 to 2460 volts. New transformers have a core loss of 800 watts each. What will be the yearly saving in energy cost? The cost of energy is 1 cent per kwhr. Assume as an approximation that the total iron loss is proportional to the square of the maximum flux density. 
$65.26 