In Plugging or Reverse Current Braking the armature terminals or the supply polarity of a separately excited or shunt motor when running are reversed. Therefore, in Plugging the supply voltage V and the induced voltage Eb which is also called back EMF will act in the same direction.
Thus, during plugging the effective voltage across the armature will be (V + Eb) which is almost twice the supply voltage. The armature current is reversed, and a high braking torque is produced. An external current limiting resistor is connected in series with the armature to limit the armature current to a safe value. The connection diagram of DC separately excited motor and its characteristics is shown in the figure below.
- V is the supply voltage
- Rb is the external resistance
- Ia is the armature current
- If is the field current.
Similarly, the connection diagram and the characteristic of the series motor in plugging mode is shown in the figure below.
For braking, a series motor either the armature terminals or field terminals are reversed. But both armature and field terminals are not reversed together. Reversing of both the terminals will give only normal working operation.
At the zero speed, the braking torque is not zero. The motor must be disconnected from the supply at or near zero speed when the motor is used for stopping a load. If the motor is not disconnected from the supply mains, the motor will speed up in the reverse direction. For disconnecting the supply centrifugal switches are used.
The method of braking, known as Plugging or Reverse Current Braking is a highly insufficient method because, in addition to the power supplied by the load, power supplied by the source is also wasted in resistance.
Applications of Plugging
The Plugging is commonly used for the following purposes listed below.
- In controlling elevators
- Rolling Mills
- Printing Presses
- Machine tools, etc.