VFD Braking Resistor Calculator

Calculate the minimum braking resistor resistance and peak power dissipation for dynamic braking on a variable-frequency drive.

Motor & drive parameters

kW (shaft power at rated load)
% — check nameplate or IEC 60034-30 table
Seconds from rated speed to 0
% — braking time ÷ total cycle time × 100
Multiply calculated values; 1.5 is typical
How dynamic braking works

When a VFD decelerates a motor faster than the mechanical load can dissipate, the motor acts as a generator and feeds energy back into the drive's DC bus. The DC bus voltage rises. If the bus voltage exceeds the drive's overvoltage trip threshold (typically around 1.35 × VAC × √2), the drive trips on an overvoltage fault.

Dynamic braking prevents this by connecting a braking resistor across the DC bus via a braking chopper (IGBT transistor) when the bus voltage rises above the braking threshold (typically around 1.15 × Vnominal DC). The resistor dissipates the regenerated energy as heat, allowing the drive to complete the deceleration without tripping.

The minimum resistor value is set by the peak current the braking chopper IGBT can handle. The power rating is set by the average energy dissipated over the duty cycle. Using a resistor with too low an ohm value may damage the chopper; using one with too low a power rating will cause thermal runaway.

Disclaimer: This calculator uses simplified industry-standard formulas and is provided for preliminary estimation only. Actual resistor selection must be verified against the VFD manufacturer's braking resistor sizing guide, the resistor manufacturer's derating curves, and applicable standards. Braking resistors operate at high temperatures and present fire and burn hazards if improperly specified or installed. Always consult a qualified engineer. See Terms of Use.