SOLUTION

When the brush axis is parallel or perpendicular to the magnetic axis then no torque will be developed because the net voltage developed in the armature conductor is zero. When the brushes are at 90° electrical to the field axis or pole pieces then no torque is developed because the equal induced voltages in two halves of the armature winding will oppose each other hence there will be no induced current in the armature winding, therefore, no flux will be developed by the armature winding.