Dynamic Braking Resistor

HOW DYNAMIC BRAKING RESISTORS WORK

State of the art AC Variable Frequency Drives (VFD) are commonplace today, creating the need for reliable, proven Dynamic Braking Resistors that can be delivered quickly, completely assembled, and ready for convenient installation at the job-site. Dynamic Braking Resistors are used with AC VFD's to produce a braking torque in the motor during overhauling conditions. The dynamic braking resistor is connected across the DC bus and will see voltages as high as 800 volts.

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 as illustrated. The rectifier converts the three-phase 60Hz 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.

During braking, the VFD ramps the frequency to zero. The rotational energy of the motor and load are driven back through the inverter to the DC bus.

^{More information}

^Source

To add on to what Mizuti gave you, a braking resistor is used when you need to ramp down to a stop very fast. Without a resistor, if you ramp down too fast you will fault the drive on bus over-voltage. If you set the drive decel time a bit longer it won't fault but you will definitely reduce the life of the VFD, especially little cheapies like A/B flex drives. They pop. Size the resistor correctly for your system or get help with it, otherwise it will glow brightly several times before burning in half.

If you need better, more accurate fast stopping than this then you need to be using a servo drive.

The application and wiring/parameter information is commonly included in the VFD manual or on the VFD supplier's website as an application note.

The braking resistor is not needed in most motor applications so check the manual and website first to see if you even need it.