Dynamic braking resistors are used on AC variable frequency drives (VFDS) to dissipate energy that is produced in the motor as the drive provides braking torque to stop the motor. The resistors are either used alone for decelerating or in conjunction with compressed-air brakes for stopping.

Resistors integrated into the power unit dissipate electrical energy created by electric braking systems. The dynamic braking resistor is connected to the DC bus and will see voltages as high as 800 volts during braking conditions.

The drive manufacturer normally determines the power rating (watts) needed to prevent overheating during braking duty.

The peak braking current is determined by the specified resistance value. Each drive manufacturer specifies a resistance range with a minimum to prevent overcurrent and damage to the drive and a maximum value to give adequate lower dissipation capability.

A three-phase variable frequency drive (VFD) consists of three basic components- rectifier, DC line, and inverter-and a control system to manage these three components. The rectifier converts the three-phase 50Hz AC input to a DC signal.

Depending on the system, an inductor, a capacitor, or combination of these components smoothes the DC signal (reduces voltage ripple) in the DC link part of the VFD. The inverter circuit converts the DC signal into a variable frequency AC voltage to control the speed of the induction motor.