Motor Brake Resistor Calculator
Enter DC bus voltage, peak regenerative power, deceleration time and duty to size the braking resistor.
Step-by-step
- Enter valid values to begin.
R = VDC² / Ppeak is the resistance that dumps the peak power at the chopper threshold. Continuous rating uses Ppeak × duty (conservative); if the ramp is linear the real average is about half. Brake threshold is typically about 380 V for 230 V class, 750 V for 460 V class and 1,000–1,100 V for 575 V class drives — check the drive manual. Also check the resistor’s short-time pulse energy rating.
Brake Resistor Sizing: Where the Regenerative Energy Goes
Core Engineering Principles
When a drive decelerates a high-inertia load faster than the load slows down by friction, the motor stops motoring and starts generating. That energy has nowhere to go but the DC bus, because the input rectifier is just diodes and can’t push power back to the supply. The bus capacitors charge, the voltage climbs, and when it hits the overvoltage limit the drive trips on a DC bus overvoltage fault. A brake chopper is the fix: a transistor that switches a resistor across the DC bus when the voltage reaches the brake threshold, turning the regenerated energy into heat.
The resistance is set by the power you need to dump at the threshold voltage: R = VDC² / Ppeak. A lower ohmic value means more braking power, but it can’t go below the drive manufacturer’s minimum or the chopper transistor will be overloaded. The wattage you buy is a different question. It depends on how often you brake, because a resistor sized for 30 kW peak that only brakes 10% of the time needs far fewer continuous watts.
Energy per stop ≈ ½ × Ppeak × tdecel (linear ramp)
NEC & Standard References
IEC 61800-5-1 covers drive safety including thermal limits and the touch-hot hazard of braking resistors, and IEC 61800-2 the general rating requirements. NEC Article 430 applies to the drive and motor circuit. A resistor enclosure usually needs its own thermal switch, wired into the drive’s trip chain. IEEE 519 concerns input harmonics, not braking, but remember that regenerative front-end drives are the alternative where braking is frequent. Read the drive’s minimum resistance and chopper current in the manual.1. R = 750² / 30,000 = 18.75 Ω, above the drive minimum, so acceptable. Use the standard 18 Ω value, which is at or just below the calculated figure.
2. Pcont = 30 kW × 0.10 = 3.0 kW. With a 25% margin, choose a 4 kW resistor.
3. Energy per stop = 0.5 × 30 × 8 = 120 kJ, so the resistor must also withstand that pulse.
4. Peak brake current = 750 / 18.75 = 40 A.
- Resistors get genuinely hot. Surface temperatures of several hundred degrees Celsius are normal, so mount away from cables and combustible material, with airflow.
- Always wire a thermal trip. A stuck chopper transistor puts continuous voltage across the resistor, which can burn it out or start a fire.
- Don’t go below the drive’s minimum ohms. The chopper IGBT can fail on the first hard stop.
- Fast decel on big inertia trips the bus. If there’s no resistor, lengthen the ramp or the drive will fault on DC bus overvoltage.