SOLUTION

• The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.
• Armature current is I_a = (V – E_b)/R_a
• When the motor is running on no-load, small torque is required to overcome the friction and windage losses.
• As Eb is zero the armature current will increase to a very high value, therefore a large amount of heat will be produced which may burn the armature winding.