What is a Dynamic Braking Resistor in VFD?

Dynamic Braking Resistor

Dynamic braking resistors (DBR) are used with variable frequency drives (VFDs) to dissipate regenerated power that is produced in the motor as the drive provides braking torque to stop the motor. The dynamic braking resistor is connected with the DC bus and will keep the DC voltage from exceeding the rated limit of the drive.

A variable frequency drive has the major components as,

» Rectifier → converts the main power supply with fixed frequency into fixed DC voltage.

» DC Bus → has the combination of capacitors, inductors, etc and will smoothen the DC voltage received from the rectifier.

» Inverter → converts the smooth fixed DC voltage into AC voltage using pulse width modulation technique. This simulated AC voltage is directly fed to the electric motor.

» Control Circuit → main purpose of the control circuit is to main the AC voltage and frequency in the inverter section as well as provides the feedback of the running status as well as parameterize the variable frequency drive.

Why dynamic braking resistors are used with VFDs?

When we stop an electric motor from the high speed, due to the application such as blower, drum, etc. The deaccelerating load on a motor causes the electric motor to run faster than the speed set by the drive. As a result, an electric motor acts as a generator and transforms mechanical energy from the motor shaft to electrical energy.

The AC power generated due to this fed back into the AC drive and causes the DC bus voltage to increase. Once the DC bus voltage reaches a certain value, the drive will trip on the DC bus overvoltage to protect itself.

The dynamic braking resistor is the most cost-effective and simplest way to dissipate this energy in the form of heat. For a motor with a smaller speed, the DBR can absorb the heat themselves safely. The DBR is fitted with an external heat shield for a motor with a higher speed that can absorb a greater amount of heat.

The resistance value of the DBR and heat absorption threshold have to be accurately designed for each application. The manufacturer generally determines the power rating to prevent overheating during braking duty. We may also consider the thermal capacity of the resistor that it does not heat up during a single stop for a shorter stop time.