Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....


Author: E.E. Kimberly

  Problem Answer
1 A separately excited generator requires 69.7 hp to drive it at full load of 100 amp and 500 volts. Its armature resistance is 0.1 ohm. What would be the continuous rated horsepower of this generator when run as a motor on a 550-volt line?
2 Give instructions for calibrating a 230-volt d-c motor.
3 A 1-hp, 220-volt, 1800-rpm shunt motor has an armature resistance of 2.2 ohms and a field resistance of 320 ohms. When running without load, but at rated voltage, the armature current is 2.0 amp. What is the conventional efficiency when the load is such that the armature current is 6 amp.?
4 A 25-kw, 230-volt shunt generator has a full-load speed of 1200 rpm. Its friction and windage and iron loss is 700 watts. The resistance of the armature circuit at full load is 0.15 ohm. The field current is 2.5 amp. Calculate the full-load efficiency.
5 A 75-kw, 250-volt shunt generator has a windage, friction, and iron loss of 4750 watts at rated speed. The armature-circuit resistance at full load is 0.05 ohm. The field-circuit resistance is 50 ohms. Calculate the full-load efficiency.
6 A centrifugal pump with an efficiency of 59% is designed to deliver 3000 gal of water per minute to an overhead tank against an effective pressure head of 50 ft and a suction lift of 10 ft. Calculate the horsepower required of the driving motor, and select the proper standard rating from the table in the Appendix.
7 What horsepower would be required of a motor to operate a hoist that is to lift 10 tons at the rate of 50 ft per min? The efficiency of the hoist is 72%.
8 A 30-hp, 230-volt shunt motor has a full-load armature current of 110 amp. The armature-circuit resistance, not including brushes, is 0.125 ohm. The no-load armature current is 4.2 amp. Calculate the horsepower output when the armature current is 90 amp.
9 A 230-volt shunt motor, rated 50 hp, has an armature current of 180 amp at full load. When the motor is run at no load at rated speed and field current, the armature current is 8 amp and the voltage across the armature is 110 volts. The field current is 2.4 amp. The armature-circuit resistance, exclusive of brush contact, is 0.1 ohm. Calculate the efficiency at full load. 86.2%

Last Update: 2011-02-23