Explanation:

When the supply polarity to the armature terminal of a separately excited DC motor is reversed, the motor will run under plugging condition. Let's understand why this happens.

1. Separately Excited DC Motor:
A separately excited DC motor is a type of DC motor in which the field winding (excitation) is supplied from a separate DC source. The armature and field windings are connected in series or parallel. The direction of rotation of the motor depends on the polarity of the supply voltage applied to the armature.

2. Polarity Reversal:
When the supply polarity to the armature terminal is reversed, the direction of the armature current also reverses. As a result, the direction of the magnetic field produced by the armature also changes. The magnetic field produced by the field winding remains the same since it is connected to a separate DC source.

3. Plugging Condition:
In plugging condition, the armature current and the back EMF both oppose the rotation of the motor. When the polarity is reversed, the armature current tries to flow in the opposite direction, which creates a torque in the opposite direction. This torque opposes the existing rotation and tries to bring the motor to a sudden stop.

4. Effect on the Motor:
When the motor is running under normal motoring condition, the armature current flows in the same direction as the supply voltage, and the motor rotates in a specific direction. But when the supply polarity is reversed, the armature current flows in the opposite direction, and the motor experiences a sudden change in torque.

5. Additional Factors:
In addition to the reversed polarity, there are other factors that can affect the motor's behavior in this situation. These factors include the mechanical load on the motor, the inertia of the motor, and the speed at which the polarity is reversed. These factors can influence the motor's response and determine whether it will come to a sudden stop or continue running in the opposite direction.

Therefore, when the supply polarity to the armature terminal of a separately excited DC motor is reversed, the motor will run under plugging condition. It is important to note that plugging is an undesirable condition and can lead to mechanical stress on the motor and other connected equipment.