8. A 230 V dc series motor used in lifts has a resistance of 0.2 ohm. At a speed of 1800 rpm it takes 50A. Determine the resistance to be added in series with the motor (a) to limit the speed to 3600 rpm when the current is 12.5 A, assuming that the magnetization curve is a straight line between zero and 50 A; (b) to make the speed 900 rpm when the current is 70 A, taking the flux per pole at 70 A is 20 per cent greater than that at 50 A. At what speed will the motor run, when directly connected to the 230 V mains and drawing a current of 70 A?
10. Two identical 5 kW, 220 V, 24 A dc shunt machines are to be used as the generator and motor in a Ward-Leonard system. The armature resistance of each machine is 0.5 ohm and the magnetization curve for each machine at 1500 rpm is given below:
Field current, A 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Induced emf, V 80 160 210 228 237 243 247
The generator is driven at a constant speed of 1500 rpm. (a) Determine the maximum and minimum values of the generator field current required to give the motor a speed range of 100 rpm to 1500 rpm at full load armature current of 24 A, with the motor field current held constant at 0.6 A. (b) Determine the maximum motor speed obtainable at full load armature current if the motor field current is reduced to 0.10 A and generator field current is not allowed to exceed 1.2 A.
11. A 250 V, 10 kW, 1200 rpm dc shunt motor has a full load efficiency of 80 per cent. Its field and armature resistances are 110 ohms and 0.25 ohm respectively. Calculate the value of the resistance to be inserted in series with the armature and the power lost in the armature circuit to reduce the speed to 80 per cent when (a) the load torque is constant regardless of speed, (b) the load torque is directly proportional to the speed, (c) the load torque varies as the square of the speed.
12. A 250 V dc series motor having armature and field resistance 0.25 ohm and 0.15 ohm respectively runs, at 800 rpm, while taking 50 A from the supply. Assuming the magnetic circuit of the motor to be unsaturated, determine its speed, when a divertor of 0.3 ohm resistance is connected in parallel with the field winding of the motor, which is subjected to a load torque whose torque (a) remains constant regardless of speed, (b) varies in direct proportion to the speed, (c) varies as the square of the speed.