When an induction motor's rotor is turning slower than the synchronous speed set by the drive's output power, the motor is transforming electrical energy obtained from the drive into mechanical energy available at the drive shaft of the motor. This process is referred to as motoring. When the rotor is turning faster than the synchronous speed set by the drive's output power, the motor is transforming mechanical energy available at the drive shaft of the motor into electrical energy that can be transferred back to the drive. This process is referred to as regeneration. Therefore, if the motor is regenerating, the DC bus voltage will increase and cause an over voltage fault at the drive.
A solution can be provided by allowing the drive to feed the regenerated electrical power to a resistor which transforms it into thermal energy. This process is referred to as dynamic braking.

Brake Unit (TDBU-xxxx series)
Braking Resistor (TDBR-xxxxxxx)