Problems

Author: E.E. Kimberly

	Problem	Answer
1	A certain 230-volt shunt motor with an armature current of 80 amp produces a full-load running torque of 100 Ib-ft. The armature-circuit resistance is 0,1 ohm. If field flux is decreased 2 per cent and the speed is restrained from rising by external means such as a large constant-speed motor, what will be the new torque?
2	A 230-volt, 25-hp, 1200-rpm shunt motor has a full-load armature-circuit resistance of 0.12 ohm and a full-load efficiency of 88%. Calculate the counter emf at full load.
3	Why are interpoles desirable even when compensating windings are used?
4	A conductor carrying 1200 amp is placed in and perpendicular to a field whose intensity is 50,000 lines per sq in. Calculate the force on the conductor, in pounds per foot of length.
5	A 20-hp, 230-volt, 1200-rpm motor has a full-load current of 74 amp. Its armature resistance is 0.15 ohm. How many ohms must be provided in its starting rheostat to limit the starting current to 150 per cent of the rated full-load current? Under these conditions, how many pound-feet of starting, torque would be produced?	1.92 ohms; 131 Ib-ft
6	A 50-hp, compound-wound, 230-volt motor takes an armature current of 180 amp at full load. Its armature-circuit resistance with 180 amp flowing is 0.06 ohm. Its no-load speed with minimum shunt-field-circuit resistance is 820 rpm, and its full-load speed is 780 rpm, approximately. How much resistance must be placed in series with the armature to produce full-load torque at 500 rpm?	0.43 ohm
7	If a motor has a torque of 20 Ib-ft when the armature current is 15 amp, what will be the torque when the armature current is reduced to 12 amp and the field flux is reduced 5 per cent?
8	A shunt motor is operated with its field excited from a 230-volt supply. Variable voltage is available for the armature. The armature resistance is 0.102 ohm, exclusive of brush contact. With 230 volts across the armature, the full-load speed is 600 rpm and the armature current is 90 amp. Calculate: (a) the speed when the armature applied voltage is 100 volts and the current is 80 amp; (6) the horsepower output in (a) if 5 per cent of the armature input power is spent in windage, friction, and iron loss.