Engineering question
Which calculations are useful for a VFD braking resistor, and why can motor kW alone never determine the final resistor?
A dynamic braking resistor converts regenerated DC-bus energy into heat when a driven load decelerates or overhauls. Selection has at least two distinct parts: calculate the mechanical energy and event duty, then check an electrically compatible resistor against the exact VFD or braking unit. Motor kW alone says little about reflected inertia, speed, stop time or gravitational energy.
The manufacturer's minimum allowed resistance is a protection boundary for the brake chopper. A lower resistance can demand excessive current and damage the chopper even if the resistor itself has a high power rating. Continuous watts, short-time pulse energy, resistance tolerance, enclosure temperature and ventilation are separate checks.
Calculation basis
Formulas and units
Rotational braking energy
E = ½J(ω1² − ω2²) + additional mechanical energy
J must be total inertia referred to the motor shaft; ω = 2πn/60.
Mean event power
Pevent = E / tstop
This screens the energy removal rate but does not define the resistor's pulse curve.
Candidate electrical power and current
PR ≈ Vchop²/R; IR = Vchop/R
Use the manufacturer-specified chopper threshold and permitted resistance range.
Average thermal load
Pthermal,avg = event energy × events per hour / 3600
Include all braking energy actually dissipated in the resistor and the real cycle profile.
Worked example
Apply the formula
Total referred inertia 5 kg·m² decelerates from 1500 RPM to rest in 5 s, 12 times per hour.
- 1ω = 2π × 1500/60 = 157.08 rad/s.
- 2E = 0.5 × 5 × 157.08² ≈ 61.69 kJ.
- 3Mean braking power during the stop ≈ 61.69/5 = 12.34 kW.
- 4Average mechanical energy rate at 12 events/h ≈ 0.206 kW.
Result: The resistor must tolerate the event energy and instantaneous electrical duty, while its resistance must remain at or above the exact drive's minimum. This example is not a product recommendation.
Open VFD Braking Resistor CalculatorSelection workflow
- Identify VFD model, frame, DC-bus/braking option and manufacturer Rmin.
- Calculate reflected rotational, linear and gravitational energy for the worst credible event.
- Define stop time, events per hour, emergency stops and back-to-back duty.
- Check resistance, chopper current, pulse-energy curve and continuous thermal rating.
- Apply enclosure, ambient, altitude, ventilation and resistor tolerance derating.
- Provide guarding, cable temperature rating, earthing and thermal protection.
When a resistor may be the wrong architecture
Frequent high-energy deceleration or continuous overhauling may exceed practical resistor duty. A regenerative front end, common DC bus or mechanical/process change may be more appropriate. The choice requires a drive-system and energy-flow review, not only a larger resistor.
Common mistakes
- Selecting from motor kW only
- Choosing resistance below manufacturer Rmin
- Checking average watts but not pulse energy
- Ignoring repeated or emergency stop sequences
Troubleshooting checks
- DC overvoltage trip: verify stop ramp, actual inertia, chopper enable and resistor circuit.
- Resistor overheats: verify event rate, ventilation, duty and continuous rating.
- Brake transistor fault: verify resistance, wiring and insulation.
- Stop time varies: inspect torque/current limits, DC-bus voltage and mechanical load.
Assumptions
- Entered inertia is referred to the motor shaft
- Mechanical losses and regenerated energy path are treated conservatively
- Manufacturer chopper data is available for final selection
Limitations
- Does not recommend a resistor model
- Does not predict drive DC-bus control or regenerative efficiency
- Load torque, gear efficiency, vertical potential energy and emergency duty are project-specific
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