Reversing the Direction of Rotation of a DC Shunt Motor

How is a DC Shunt Motor Reversed?

To reverse the direction of rotation of a DC shunt motor, you must change the relative direction of current flow in either the armature winding or the field winding, but not both simultaneously.

A DC shunt motor's rotation direction depends on the interaction between the magnetic field created by the field winding and the magnetic field created by the current flowing through the armature winding. To reverse the motor, this interaction needs to be altered in a specific way.

According to the provided reference:
The direction of rotation of DC shunt motor can be reversed by reversing either field terminals or armature terminals but not both.

This crucial point ensures that the net torque reverses, causing the motor to spin in the opposite direction. Reversing both the armature and field terminals would result in the same direction of torque and thus rotation.

There are two primary methods to achieve this reversal:

1. Reversing the Armature Terminals:

   • This involves swapping the connections to the armature winding while keeping the field winding connections as they were.
   • The field current direction remains unchanged, maintaining the polarity of the main magnetic field.
   • Reversing the armature connections reverses the direction of current flow through the armature.
   • This reversal of armature current reverses the magnetic field produced by the armature, leading to a reversal of the motor's torque and direction of rotation.

2. Reversing the Field Terminals:

   • This involves swapping the connections to the field winding while keeping the armature winding connections as they were.
   • The armature current direction remains unchanged.
   • Reversing the field connections reverses the direction of current flow through the field winding.
   • This reversal of field current reverses the polarity of the main magnetic field.
   • With the main field reversed and the armature current direction the same, the interaction changes, leading to a reversal of the motor's torque and direction of rotation.

Why Not Reverse Both?

As highlighted in the reference, reversing both the field and armature terminals simultaneously will not reverse the motor's direction.

• If you reverse both, the direction of the main magnetic field reverses, and the direction of the armature current reverses.
• Since both contributing factors (field flux and armature current direction) are reversed, their combined effect on torque direction cancels out the reversal, and the motor continues to spin in its original direction.

Summary Table

Here's a quick look at the outcomes:

Action	Effect on Field Current	Effect on Armature Current	Resulting Direction of Rotation
Original Connections	Normal	Normal	Forward
Reverse Armature Only	Normal	Reversed	Reversed
Reverse Field Only	Reversed	Normal	Reversed
Reverse Both Armature & Field	Reversed	Reversed	Original (Forward)

By understanding these methods, you can effectively control the rotational direction of a DC shunt motor for various applications.