Induction Motor Braking-Plugging, dynamic, regenerative

Induction motor is mostly used motor in the industry as well as in domestic application because it's a simple design, reliable operation, rugged construction, and simple control gear for starting, speed control, and induction motor braking methods such as plugging electrical braking, dynamic braking, and regenerative braking.
 
The most simple method to stop the motor is to disconnect the motor terminal from supply and applied braking to the motor.
 
When rapid and more positive stopping action required mechanical or electrical braking may be employed. induction motor electrical braking method like plugging( counter-current braking), dynamic braking or rheostatic braking and regenerative braking.

Electrical Braking of Induction motor

Following are the three main types of electrical braking of induction motor
 
1) Plugging/ Reverse voltage braking 
2) Dynamic braking 
3) Regenerative braking
 

1)Plugging Braking:

Plugging-Diagram-Electrical-Braking-Induction-motor

Plugging induction motor braking is carried out by changing the phase sequence of the induction motor. this method also called as reverse voltage braking. Plugging braking is done by interchanging connections of any two leads of the motor with respect to supply terminals.

Three-phase induction motor develops torque in the same direction as the rotating magnetic field (RMF) generated by the stator winding of induction motor also in any two stator winding leads are reversed the rotating magnetic field also get reversed. hence if stator leads reverse by any means while motor rotating, torque is suddenly produced opposite to the original direction of rotation.

These reverse torque cause rotation in the opposite direction as soon as the motor stops, therefore, the arrangement must be made to disconnect stator completely from supply lines when the motor stops. A plugging controller must be used along with the switch. 
 
The plugging switch is connected to the motor and open when the motor stops. It is connected in the control circuit so that it allows operation of controllers reversing current when the motor rotates in a given direction only when the motor stops, reversing circuit get open by plugging switch and the motor gets completely disconnected from the supply. 
 
When the stator field reverses the slip suddenly increases from a small percentage to 200 %. Since the rotor and stator field are now rotating in opposite directions. The rotor induced voltage also increase by a large amount. Hence there is a large inrush of stator current. The high slip causes high rotor reactance and hence a very low power factor in the stator of induction motor. 
 
After plugging braking of motor has started when the motor stops slip is unity and the current and power factor are the same as when the motor started with full supply voltage. The above condition occurs during a very large period of time they may cause disturbances in supply. 

Regenerative Braking

Regenerative-braking-torque-speed
Regenerative braking

Regenerating braking takes place when the motor runs above the synchronous speed, when the rotor speed, becomes more than rotating field speed, the slip becomes negative. The negative slip means that the induction motor now operates as an induction generator in the second quadrant and fed back power to the supply line. The power fed to supply is given by the product of regenerative braking torque and the corresponding rotor speed. Fig shows that the amount of power return to the supply line depends upon how much rotor speed is above synchronous speed. When rotor speed falls to synchronous speed, the regenerating braking of an induction motor comes to an end. 

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