Counter Current Braking
In an induction motor, by changing the phase sequence of the input supply, the direction of the stator field can be reversed. This is also called plugging.
In practice, interchanging the supply to any two terminals of the motors, as shown in Figure 4.21, does this.
When the phase sequence of the input supply is changed, this reverses the direction of the revolving flux, which produces a torque in the reverse direction, thus applying a brake on the motor. During this braking action, the motor absorbs kinetic energy from the revolving load, causing speed reduction and bringing the motor to a rest. The motor must be switched off, as it approaches zero speed.
In high-capacity motors, if sudden torque is applied in a reverse direction (plugging), without slowing down the motor, then it can result in a mechanical damage. To avoid this, anti-plugging protection is used, which does not allow a reverse torque to be applied, unless the speed of the motor reduces below the acceptable value.
The speed–torque characteristics of an induction motor can be modified by varying the rotor resistance.
The maximum torque point can be achieved in the range of slips1–2,where the torque developed tends to brake the rotor. The torque developed can also be used to arrest the tendency of the rotor to accelerate. A high resistance is introduced in the rotor, so that the operating point shifts to the fourth quadrant.
The braking torque developed prevents any acceleration of the rotor which now works at a uniform speed, as shown in Figure 4.22.
The total braking torque (TP) may be controlled with a variable rotor resistance, which limits the braking current.