Plugging, also known as reverse plugging or dynamic braking, is a method used to quickly stop a poly-phase induction motor by momentarily reversing the phase sequence of its stator windings. This process causes the motor to decelerate rapidly and come to a stop.
Here's how plugging works:
Normal Operation: During normal operation, a poly-phase induction motor is connected to a power supply, and the stator windings are energized in the correct sequence to create a rotating magnetic field. This rotating magnetic field interacts with the rotor, causing the motor to turn and produce mechanical output.
Initiating Plugging: To initiate plugging, the phase sequence of the stator windings is quickly reversed. For example, if the original sequence was A-B-C, it would be changed to C-B-A during plugging.
Reverse Rotation: The reversed rotating magnetic field now tries to turn the rotor in the opposite direction of its original rotation. This causes a braking effect, rapidly decelerating the motor's speed.
Deceleration and Stop: As the motor rapidly slows down, it eventually comes to a complete stop. The kinetic energy of the rotating mass is converted into heat energy within the motor windings.
Plugging is a relatively simple and effective method for quickly stopping an induction motor. It is often used in applications where precise and rapid stopping is required, such as certain types of industrial machinery or cranes. However, plugging can result in high mechanical and thermal stresses on the motor and the connected equipment. Excessive use of plugging can lead to mechanical wear and may also cause overheating, so it's important to use it judiciously.
It's worth noting that modern motor control methods and technology have introduced more sophisticated and controlled ways of achieving rapid deceleration and stopping, such as using variable frequency drives (VFDs) or regenerative braking systems. These methods provide better control over the deceleration process and reduce the mechanical and thermal stresses on the motor and associated equipment.