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VFD Braking Resistor Calculator

Calculate ohmic value and peak power rating for dynamic braking resistors.

Application Data

Resistor Specification

0.00 Ω (Max)
Peak Power 0.00 kW
Cont. Power 0.00 kW
* The resistance shown is the maximum value to achieve the desired braking torque. Using a slightly lower resistance is common, provided it does not exceed the VFD's minimum resistance rating (chopper limit).
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Disclaimer of Liability: This calculator is provided for educational and estimation purposes only. By using this tool, you agree to discharge AutomationView of any liability for direct, indirect, or consequential damages resulting from its use. It is your strict responsibility to independently verify all calculations, validate the results against official manufacturer documentation, and ensure compliance with all applicable safety and engineering standards before implementing any parameters in a production environment.

Sizing Dynamic Braking Resistors (DBR) for VFDs

To properly size a dynamic braking resistor for a Variable Frequency Drive (VFD) in industrial automation, engineers must determine two critical values: Resistance (Ohms) and Power Rating (Watts). Properly specifying these ensures the VFD’s braking chopper can safely dissipate regenerative energy without overloading.

1. Determining Resistance (Ω)

The resistance value is paramount for protecting the drive’s internal circuitry. Most VFD manufacturers specify a minimum allowable resistance in their manuals. Never select a resistor below this minimum, as it could damage the braking transistor.

If a specific value isn’t provided, it can be estimated using the DC bus voltage (V_dc) and the required peak braking power (P_peak):

R = (V_dc)² / P_peak

2. Determining Power Rating (Watts)

The wattage rating depends on the generated energy during deceleration and the duty cycle. The duty cycle is how often and for how long the braking occurs.

  • Peak Power (P_peak): The maximum power generated during deceleration. A common rough estimation is 1.5 times the motor power rating.
  • Average Power (P_avg): Since braking is typically intermittent, the resistor doesn’t need a continuous rating for peak power. Calculate average power using: P_avg = P_peak × Duty Cycle.

Best Practices and Manufacturer Tools

While manual calculations provide a good baseline, the most reliable approach is to use a VFD Braking Resistor Calculator provided by the drive manufacturer (such as Rockwell, Siemens, or SEW). Additionally, it is highly recommended to install a thermal overload relay to protect the resistor from overheating during unintended continuous braking events.