Regenerative braking in Three phase Induction motors

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Regenerative Braking in Induction Motors

Braking of induction motors is essential for controlling speed, safety, and energy efficiency in industrial drives. Among the various braking techniques, regenerative braking is highly efficient since it converts the motor’s kinetic energy into electrical energy and feeds it back into the supply system.


Principle of Regenerative Braking

  • In a normal stator-fed induction motor, regenerative braking is possible if the number of poles of the machine can be changed during running conditions using special arrangements.

  • This technique is applicable in squirrel cage induction motors since:

    • Number of poles in the stator = Number of poles in the rotor (though not necessarily equal to the number of phases).

Method of Operation

  1. Pole Changing Method

    • When braking is required, the number of poles is increased (usually by a factor of 2).

    • The synchronous speed of the revolving magnetic field reduces to half.

    • Slip becomes negative, and the motor enters the generating mode.

    • Consequently, the motor speed reduces.

    • Once the speed approaches the new synchronous speed, the power supply is switched off.

    • Finally, mechanical braking is applied to bring the motor to a complete stop.

  2. Energy Feedback

    • During braking, the kinetic energy of the rotor is converted into electrical energy.

    • This energy is fed back to the power supply mains.

    • Hence, the method is termed regenerative braking.

Role of Variable Frequency Drives (VFDs)

With modern power electronics and VFD technology, regenerative braking has become more advanced:

  • Even without changing the number of poles, the supply frequency can be reduced.

  • The V/f ratio is maintained constant to avoid over-fluxing.

  • A reduced supply frequency lowers the synchronous speed, forcing the motor into regenerative mode.

  • This allows braking almost down to standstill conditions.

  • Applicable to both squirrel cage and slip ring induction motors.

Regenerative Braking Characteristics

  • At synchronous speed, slip = 0 → motor neither generates nor consumes active power.

  • When the rotor speed is slightly above synchronous speed → slip becomes negative, and power flows back to the supply.

  • This negative slip region corresponds to regenerative braking mode.

Advantages of Regenerative Braking

  • Energy saving since braking energy is recovered and fed to mains.

  • Smooth speed control without overheating resistors (as in dynamic braking).

  • Useful for frequent start-stop industrial drives, elevators, cranes, and traction systems.

  • Improves overall system efficiency and reduces operating cost.

✅ Summary:
Regenerative braking in induction motors is achieved by either pole-changing techniques or variable frequency supply methods. In both cases, the motor operates in generating mode when slip becomes negative, returning power to the grid. With the advent of VFDs, regenerative braking is possible even without pole changing, making it widely applicable to modern industrial drives.






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