Plugging or Reverse Current Braking

In plugging or reverse current braking, the armature terminals or the supply polarity of a separately excited or shunt DC motor is reversed while the motor is running. As a result, during plugging, the supply voltage V and the induced armature voltage Eb (also known as back EMF) act in the same direction. This causes the effective voltage across the armature circuit to become (V + Eb), nearly twice the supply voltage. The armature current reverses, producing a high braking torque. To limit the armature current to a safe level, an external current-limiting resistor is connected in series with the armature.

The circuit diagram and characteristics of a separately excited DC motor are shown in the figure below:

Where:
V — Supply voltage
Rb — External resistance
Ia — Armature current
If — Field current

Similarly, the connection diagram and characteristics of a series motor under plugging are shown in the figure below:

For braking, either the armature terminals or the field terminals of a series motor are reversed, but both must not be reversed simultaneously; otherwise, the motor will continue normal operation.

At zero speed, the braking torque is not zero. Therefore, when the motor is used to stop a load, it must be disconnected from the power supply at or near zero speed. If the motor remains connected to the supply, it will begin to accelerate in the reverse direction. To achieve this disconnection, centrifugal switches are commonly used.

This method of braking, known as plugging or reverse current braking, is highly inefficient because, in addition to the energy returned by the load, the energy supplied by the source is also dissipated as heat in the resistors.

Applications of Plugging

Plugging is commonly used for the following purposes:

1.Elevator control

2.Rolling mills

3.Printing presses

4.Machine tools, etc.

The above describes the basic principle and characteristics of plugging or reverse current braking.