6. (i) A 20 kW, 500 V d.c. shunt wound motor draws a current of 45 A when running at full load with a speed of 600 rev min-1. On no load, the current drawn from the supply is 5 A. If the armature resistance is 0.3 ohms and the shunt field resistance is 220 ohms, calculate the armature current, speed and efficiency of the motor when the torque falls to half of its full load value, assuming that the flux per pole does not change.
(Hint: don't forget that all armature current is supplying torque. One must decide at some points what is the armature current producing
torque and at others, to determine generator emfs, the total armature current).
(ii) State what you would expect to happen to the speed if a resistance is inserted into the field circuit.
(iii) If a resistance of 50 ??is inserted in series with the shunt field, calculate the armature current required to maintain half full-load
torque. Calculate also the speed of the motor when supplying this torque.
The expert examines shunt wound motor drawing a current of 45A running at full load. What you would expect to happen to the speed if a resistance is inserted into the field circuit is determined.