VFD braking resistors are used in order to quickly stop or decrease the speed of AC electric motors controlled by drives. Due to kinetic energy, an electric motor continues to rotate for a short time even after it has been switched off. 

As the rotor keeps rotating, the motor starts acting as a generator and feeding the drive. 

This may not only increase the stopping time of the motor, but also cause a voltage increase on DC drive’s terminal leading to an overvoltage fault and, consequently, a shut down.

In order to avoid such problems, a braking resistor of a proper resistance value and rated power should be connected to the DC drive’s terminal.

Attached is a braking resistor calculation guide for UMI products.

Attached sizing table as well.