What is a braking resistor in a VFD system?

A braking resistor dissipates regenerative energy as heat when a motor decelerates faster than it would coast to stop. The VFD's brake chopper switches the DC bus to the resistor when voltage exceeds a threshold (typically 115-130% of nominal DC bus voltage).

How do I calculate the braking resistor value?

R_min = V²dc / P_brake, where Vdc is the DC bus voltage (typically √2 × V_line) and P_brake is the maximum braking power. The resistor must handle peak braking power without exceeding the brake chopper's current limit.

What is the DC bus voltage for common supply voltages?

DC bus ≈ √2 × V_line. For 400V supply: Vdc ≈ 565V (brake threshold ~750V). For 480V: Vdc ≈ 680V (threshold ~800V). For 230V: Vdc ≈ 325V (threshold ~400V).

How do I determine braking power?

P_brake = J × ω² / (2 × t_decel), where J is total inertia (kg·m²), ω is angular velocity (rad/s), and t_decel is deceleration time. For quick estimation: P_brake ≈ P_motor × (rated_speed/decel_time) for full-speed stops.

What duty cycle should I use?

Duty cycle = braking_time / cycle_time × 100%. Typical: 5-10% for occasional stops, 20-40% for frequent cycling. The continuous power rating of the resistor = peak power × duty cycle. Check the VFD manual for maximum allowed duty cycle.

Free Engineering Tool

Braking Resistor Calculator

Size a dynamic braking resistor for VFD applications. R = V²_dc / P_brake. Calculate resistance, power rating, and energy per braking cycle.

VFD BrakingR = V²/PEnergy / Cycle

Results

Min Braking Resistance

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Peak Braking Power

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Energy per Cycle

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Continuous Power Rating

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DC Bus Voltage

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Braking Resistance

Kinetic Energy & Braking Power

⚠️ Always verify with VFD manufacturer datasheet. Minimum resistance must not exceed brake chopper current limit.

Braking Resistor Calculator | VFD Dynamic Braking | Free Online Tool

Published by admin on February 15, 2026 February 15, 2026

Results

Braking Resistance

Kinetic Energy & Braking Power