The armature and field resistance of a 100 V separately excited d.c. motor are 0.4 and 100 respectively. It takes an armature current

The armature and field resistance of a 100 V separately excited d.c. motor are 0.4 Ω and 100 Ω respectively. It takes an armature current of 25 A to drive a constant torque load at 1200 r.p.m. A chopper is used to control the speed of the motor. 

 

 
Find the on period (TON) to reduce the speed to 800 r.p.m. at a chopper frequency of 500 Hz.