Electrical braking of polyphase induction motors is a technique used to rapidly stop or decelerate the motor by converting its kinetic energy into electrical energy. This method of braking is commonly employed in industrial applications where precise control over the motor's stopping behavior is required, such as in conveyor systems, cranes, elevators, and various automation processes. Electrical braking is often preferred over mechanical braking due to its better controllability and reduced wear and tear on mechanical components.
There are a few common methods of implementing electrical braking in polyphase induction motors:
Regenerative Braking: This method involves reversing the connections to the motor, effectively turning it into a generator. The generated electrical energy is then fed back into the power supply system or dissipated as heat through resistors. Regenerative braking is especially useful in applications where the motor is decelerating while lifting a load, as the generated energy can be recycled back into the system.
Dynamic Braking: In dynamic braking, a braking resistor is connected across the motor terminals. When the motor is being decelerated, its windings are short-circuited through the resistor, causing the motor to act as a generator and dissipate energy as heat. This method is effective for rapidly stopping the motor but may be less efficient than regenerative braking in terms of energy recovery.
Plugging or Reverse Current Braking: Plugging involves abruptly reversing the motor's phase sequence, causing it to generate torque in the opposite direction. This sudden reversal of torque helps in rapidly stopping the motor. However, this method can be quite harsh on the motor and the connected load, so it's often used with caution.
VFD (Variable Frequency Drive) Braking: If the polyphase induction motor is controlled by a Variable Frequency Drive, the drive itself can be configured to provide controlled braking. The VFD can control the frequency and voltage supplied to the motor during braking, allowing for smoother deceleration and energy dissipation.
DC Injection Braking: This method involves injecting a controlled DC voltage into the stator windings of the motor, which creates a stationary magnetic field. This stationary field opposes the motor's rotation and provides braking action. DC injection braking is generally smooth and controllable.
The choice of method depends on factors like the desired braking performance, energy recovery capability, the nature of the load, and the specific requirements of the application. It's important to note that proper control and protection mechanisms should be in place to prevent overheating and damage to the motor during the braking process.