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# Problems

Author: E.E. Kimberly

 Problem Answer 1 A 10-hp motor whose full-load efficiency is 87.5 per cent cost \$160. What must be the selling price of a second motor having a full-load efficiency of 89 per cent, if full-load operation only is considered and the two motors are to be exactly comparable economically? The cost of energy is 2 cents per hp-hr. 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 150-hp, 220-volt motor that costs \$1200 has a guaranteed efficiency of 91 per cent at full load. The cost of energy is 2 cents per hp-hr. 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 440-volt 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 kv-a of 460-volt, 3-phase motors is to be supplied by a three-wire 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 current-carrying capacities of conductors.) 630 amp, 600,000 cir mils 5 If the available voltage at the transformers in Problem 4 is 460 volts, line-to-line, what will be the line-to-line 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 single-phase transformers, rated 200 kv-a each, supplying a sewage-disposal plant. Each transformer has a core loss of 1100 watts and a full-load 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 kw-hr. Assume as an approximation that the total iron loss is proportional to the square of the maximum flux density. \$65.26

Last Update: 2011-01-18