VFD Braking Resistor Calculator

VFD Braking Resistor Calculator supports engineering calculations with transparent assumptions, practical result interpretation, and links to next-step technical resources.

Braking Energy Visualization

Core Formula

Eregen = 1/2 × J × (ω1² − ω2²)

Pdecel(avg) = Eregen / tdecel

Rmin = Vclamp / Ichopper,max

Rmax(power) = Vclamp² / Pdecel(avg)

A feasible resistor must satisfy both current limit and decel power absorption constraints.

Brake Chopper Circuit

Power Profile Over Braking Cycle

Enter required inputs to render braking duty profile and resistor window.

Inputs & Outputs

DC Bus Clamp Voltage

Chopper Max Current

Total Reflected Inertia

kg·m²

Start Speed

rpm

End Speed

rpm

Deceleration Time

Cycle Time (optional)

Candidate Braking Resistor (optional)

Regen Energy per Stop—

Avg Braking Power (decel)—

Continuous Eq. Power—

Duty Ratio—

Resistor Window

Minimum (current-limited)—

Maximum (power-limited)—

Recommended Center ValueNo feasible overlap

Window Status—

Candidate Check

Enter candidate resistor value to check chopper current and decel power adequacy.

Braking Resistor Fundamentals

During deceleration, motor kinetic energy returns to the DC bus. The brake chopper diverts this energy into the resistor to keep bus voltage below trip threshold.

Electrical Constraint

Resistor must not force chopper current above hardware current limit.

This sets a minimum allowable resistance value.

Energy Constraint

Resistor and chopper path must absorb decel power quickly enough.

This sets a maximum practical resistance value for the stop profile.

Equation Reference

Use these equations to understand how inertia and braking profile map into resistor sizing boundaries.

Topic	Equation	Interpretation
Regenerative energy	E = 1/2 × J × (ω1² − ω2²)	Mechanical kinetic energy converted into electrical energy during deceleration.
Decel average braking power	Pdecel = E / tdecel	Average resistor power demand during active braking interval.
Current-limited minimum resistor	Rmin = Vclamp / Ichopper,max	Prevents braking transistor current from exceeding hardware limit.
Power-limited maximum resistor	Rmax = Vclamp² / Pdecel	Upper resistor bound to absorb required decel power at clamp voltage.

Sizing Decision Matrix

Combine electrical limits with thermal duty behavior for robust, repeatable braking operation.

Scenario	Objective	Recommendation	Critical Checks
Frequent indexing motion	Handle repetitive stop energy without resistor overheating	Use cycle-time input to size continuous thermal rating, not only single-stop energy.	Duty ratio, enclosure ventilation, thermal derating curves
Fast emergency stop profile	Meet short deceleration time without DC-bus overvoltage trip	Verify feasible resistor window (Rmin to Rmax) and confirm pulse capability margin.	Chopper current limit, pulse overload duration, trip threshold
Heavy inertia axis	Absorb large kinetic energy at high speed transitions	Re-evaluate reflected inertia and mechanical coupling losses before final resistor value selection.	J estimation accuracy, gearbox ratio mapping, safety stop profile