1. Torque equation of DC shunt motor: ï For DC shunt motor Ifield = V/ Rsh = constant ï Hence the flux Ã¸ is constant. â´ T â Ia â¦â¦..(2) ï Hence in dc shunt motor, torque is proportional to only to the armature current. 2. Torque equation DC series motor: ï For DC series motor, the field current is equal to the armature current i.e. Ifield = Ia. ï Hence T â Ia Ia â´ T â Ia2 â¦â¦â¦(3) ï Hence in dc series motor, torque is proportional to the square of armature current.