A 220 V, 20 A, 1000 rpm, separately excited d.c. motor has armature resistance of 1 ohm. The motor is supplied from 250 V dc supply via a step down chopper, for operation of motor at 500 rpm at the rated torque, the duty ratio of the chopper should be
1
0.40
2
0.44
3
0.58
4
0.50
5
0.48