How to calculate a braking resistor — formulas and examples
Selecting a braking resistor for a variable frequency drive comes down to two parameters: resistance (Ohm) and power dissipation (Watts). Get either one wrong and the resistor burns out or the VFD cannot stop the motor. Below are the formulas with worked examples.
Step 1: determine minimum resistance
Minimum resistance is limited by the maximum current the braking transistor (built-in chopper) or external braking module can handle. Formula:
R_min = U_dc / I_brake_max
- U_dc — DC bus voltage at braking threshold. For 380V mains — typically 700-720V. For 220V — 380-400V.
- I_brake_max — maximum allowable braking current (from VFD or module datasheet).
Example: INVT GD20-022G, 22 kW, 380V. DC threshold = 715V, max braking current = 93A.
R_min = 715 / 93 = 7.7 Ohm
So the resistor must be at least 7.7 Ohm. In practice manufacturers recommend 8-10 Ohm for this power rating.
Step 2: calculate resistor power
Resistor power depends on braking energy and duty cycle (ED%). Two approaches.
Simplified formula (covers most cases)
P_resistor = P_motor x k_brake x ED%
- P_motor — motor rated power (kW)
- k_brake — braking factor (typically 1.0-1.5)
- ED% — braking duty cycle percentage
Example: 22 kW motor, conveyor, braking 5 seconds every 60 seconds.
ED% = 5/60 = 8.3%
P_resistor = 22 x 1.0 x 0.083 = 1.83 kW
Choose a 2 kW resistor or higher (with 20-30% margin — 2.5 kW).
Exact formula (kinetic energy method)
For high-inertia loads (centrifuges, flywheels):
E_brake = 0.5 x J x (omega1 squared - omega2 squared)
- J — total moment of inertia, load + rotor (kg m squared)
- omega1 — initial angular velocity (rad/s) = 2 pi x n1 / 60
- omega2 — final angular velocity (rad/s)
Example: centrifuge, J = 5 kg m squared, braking from 1500 to 0 RPM in 4 seconds, cycle 40 seconds.
omega1 = 2 pi x 1500 / 60 = 157 rad/s
E_brake = 0.5 x 5 x 157 squared = 61,622 J, roughly 61.6 kJ
P_peak = 61,622 / 4 = 15,406 W, roughly 15.4 kW
ED% = 4/40 = 10%
P_resistor_avg = 15.4 x 0.10 = 1.54 kW
The resistor must handle 1.54 kW continuous and peaks up to 15.4 kW.
Recommended resistor table
Approximate selection for three-phase 380V VFDs (ED = 10%):
| VFD power | Recommended resistance | Min. resistor power |
|---|---|---|
| 1.5 kW | 200-300 Ohm | 200 W |
| 4 kW | 75-100 Ohm | 500 W |
| 7.5 kW | 40-50 Ohm | 1 kW |
| 15 kW | 20-25 Ohm | 2 kW |
| 22 kW | 8-13 Ohm | 3 kW |
| 37 kW | 6-8 Ohm | 5 kW |
| 55 kW | 4-6 Ohm | 7 kW |
These values are for a typical 10% ED. If your braking cycle is more frequent, power increases proportionally.
Mistakes we see all the time
- Resistance too low — braking current exceeds the chopper rating. Result: burned transistor in the VFD.
- Power too low — resistor overheats and burns out.
- Copying values from another project — different cycle, different load, different ED%.
- Ignoring peaks — average power 2 kW but peak 20 kW. A 2.5 kW resistor will not survive the peak.
Frequently Asked Questions
Where do I get the moment of inertia J?
From the motor datasheet (rotor inertia) and mechanical calculation of the load. For standard mechanisms J is usually small. For flywheels and centrifuges it must be calculated separately.
What happens if the resistance is too high?
The VFD cannot brake the motor fast enough. Braking current is too small. The motor decelerates slowly or the VFD trips on OV.
Can I connect several resistors in parallel?
Yes. Two 20 Ohm resistors in parallel = 10 Ohm, and the power rating doubles. Standard practice for high power.
What temperature can a braking resistor handle?
Wire-wound types — up to 300-350 degrees C on the surface. Ceramic types — up to 400 degrees C. Ventilation is mandatory.
Where can I find the recommended resistance for my VFD?
In the VFD manual, section "Braking Resistor Selection". If you cannot find it — contact us and we will match by model.
Summary
Braking resistor calculation is straightforward: find minimum resistance from VFD docs, calculate power using the formula, add 20-30% margin.
Braking resistors of various power ratings are in our catalogue. We will help you match one to your variable frequency drive.
