Updated: Mar 2, 2021
There are three main ways of immediate stopping the motor in running condition. The motor can be stopped by applying brakes. The braking torque just acts in opposite direction of the motoring torque. The method of braking are regenerative braking,dynamic braking and plugging. Among all the three methods of braking,plugging provides the highest counter torque and motor comes to stop in no time. However, the braking torque is too high and it produce stress on mechanical as well as electrical system. Therefore, the plugging is generally avoided. But, for those application where motor needs to stop fast,plugging is the best method.
The torque of the DC motor is proportional to the product of the armature current and field flux. In Plugging or Reverse Current or counter current Braking, the direction of the armature current is changed by applying the reverse polarity voltage to separately excited DC shunt motor. The braking torque can also be generated by reversing the direction of field current,however, the motor takes more time to stop because of large inductance of the field winding.
When the armature supply is reversed, the back emf induced in the armature remains unaltered as far as its polarity is concerned. In dc motor the back emf always opposes the applied armature voltage. In motoring case voltage across armature winding is (V-Eb), however at the time of plugging, voltage V and the induced emf Eb acts in the same direction, and thus the voltage across the armature winding is (V+Eb). Thus the armature winding experiences almost two times of the supply voltage.
With armature current reversal, the motor produce high braking torque. The braking torque depends on the magnitude of the armature current. However, it must be ensured that the higher armature current above its rated capacity must not be allowed to flow for longer period of time, else the winding insulation may fail. To limit the armature current to safe value a resistance in the series with armature winding is connected.
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:
In a DC series motor, either field winding or armature winding terminal are reversed. The filed and armature of the DC series motor is connected in the series and torque can be reversed by either changing the field polarity or armature polarity. If the polarity of field and armature both changed, then motor will give normal operation.
The sensing of zero speed during braking operation must be done to interrupt the supply to stop the motor because at zero speed the torque of the motor is not zero. If not done, the motor will speed up in the reverse direction.The zero speed switch or centrifugal switch is used to interrupt the supply when motor reaches at zero speed.
The efficiency of the plugging is very low because the power is wasted in the resistance.
Applications of Plugging
The Plugging is commonly used for the following applications where rapid stop of motor is desired.