A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

The correct answer is: D. Er (net resultant voltage) in armature is increased.

When the load is increased, the torque required to drive the load also increases. The synchronous motor draws more current to provide the additional torque. The increased current causes a greater voltage drop in the stator windings, which reduces the net resultant voltage (Er) in the armature. This reduction in Er causes the motor to slow down slightly, which increases the back emf (Eb). The increased Eb partially offsets the reduction in Er, so the net result is that the armature current increases.

Here is a more detailed explanation of each option:

• A. Speed of the motor is reduced. This is not the correct answer because the speed of a synchronous motor is constant at synchronous speed, which is determined by the frequency of the power supply and the number of poles on the stator.
• B. Power factor is decreased. This is not the correct answer because the power factor of a synchronous motor is typically very high, close to unity. The power factor is affected by the load, but the change in power factor is not significant when the load is increased.
• C. Eb (back e.m.f.) becomes less than V (applied voltage). This is not the correct answer because the back emf (Eb) of a synchronous motor is always equal to the applied voltage (V) when the motor is operating at synchronous speed.
• D. Er (net resultant voltage) in armature is increased. This is the correct answer because the net resultant voltage (Er) in the armature is equal to the applied voltage (V) minus the voltage drop in the stator windings. When the load is increased, the current increases, which causes a greater voltage drop in the stator windings. This reduction in Er causes the motor to slow down slightly, which increases the back emf (Eb). The increased Eb partially offsets the reduction in Er, so the net result is that the armature current increases.