VFD Braking Resistor Calculator
By Santaji GadepatilinUpdated October 27, 2026
When a motor driven by a VFD slows down, it acts as a generator and pushes energy back into the drive's DC bus. A braking resistor burns off that energy as heat so the drive does not trip on over-voltage; see this guide to VFD braking resistor sizing and the common causes of VFD overload trips.
How to use the VFD braking resistor calculator
Enter the DC bus voltage at which the braking chopper switches on, the peak braking power and the duty cycle. You get the maximum resistor value, the average power rating and the peak current.
For more background, read VFD Sizing and Selection Guide.
VFD braking resistor formulas
Rmax = VDC² ÷ Pbrake · Pavg = Pbrake × duty
Example
A 400 V drive (chopper at 760 V) braking 15 kW for 10% of the time:
- Rmax = 760² ÷ 15,000 = 38.5 Ω
- Average power = 1.5 kW
Choose a resistor between the drive’s minimum resistance and 38.5 Ω, rated at least 1.5 kW continuous.
Typical chopper thresholds
| Drive supply | Chopper threshold |
|---|---|
| 230 V | 375–400 V DC |
| 400–415 V | 750–800 V DC |
| 480 V | 800–850 V DC |
| 690 V | 1,100–1,200 V DC |
Braking power from load inertia
For a flywheel or conveyor: P ≈ J × ω × Δω ÷ t (J in kg·m², ω in rad/s, t in s). Kinetic energy to dissipate is ½Jω².
Alternatives
For long or frequent braking — cranes, centrifuges, downhill conveyors — a regenerative (active front-end) drive returns energy to the supply instead of wasting it as heat.
Frequently asked questions
How do I size a braking resistor?
The resistance must be low enough to absorb the peak braking power — R ≤ V_dc² ÷ P — but not below the drive’s minimum allowed value. The power rating must cover the average braking power.
What is the braking power?
The rate at which energy flows back from the motor and load while decelerating. For a quick estimate, take the motor rating for full-torque braking, or calculate from inertia and deceleration time.
What is the duty cycle?
The fraction of time spent braking. A 10-second stop every 100 seconds is 10%. Cranes and lifts can be 20–50%.
What happens if the resistor is too small in ohms?
The chopper current exceeds its rating and the drive trips or the chopper is damaged. Always respect the drive's minimum resistance.