VFD brake resistor calculator
Size the dynamic brake resistor for a VFD: enter motor power, reflected inertia, the speed range and the stop time, and read the peak braking power, resistor kW and ohms.
Braking energy & power
Total inertia = motor + load reflected to the motor shaft.
Braking duty
Formula: E = 0.5 × J × ω² with ω = 2π × RPM / 60, then P_avg = E / t. A linear deceleration ramp peaks at twice the average power.
Resistor rating
The resistor must absorb the peak braking power, but its continuous rating only needs to cover the duty cycle — a high-stop-count application needs a higher continuous kW.
The DC bus is roughly 1.35 × line RMS — 540 V for a 400 V supply and 310 V for 230 V. Resistor ohms: R = V_DC² / P_peak. The 1.35 factor is a conservative low-side estimate: real drives brake at a higher chopper level (typically 1.6–1.7 × line RMS), so check the drive manual's chopper voltage before finalising the resistor.
For high-stop applications check the braking time per minute against the duty you selected — the resistor continuously dissipates P_peak × duty. AO Ctrl stocks the drives, braking resistors and motors for variable-speed control:
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Frequently asked questions
How do I size a brake resistor for a VFD?
The resistor must absorb the peak braking power, computed from the rotational kinetic energy E = 0.5 × J × ω² (with ω = 2π × RPM / 60) divided by the deceleration time. A linear deceleration ramp peaks at twice that average, and the continuous kW rating is peak power × duty cycle.
How do I calculate the brake resistor ohms?
Use R = V_DC² / P_peak. The DC bus is roughly 1.35 × line RMS — 540 V for a 400 V supply and 310 V for 230 V — so a 400 V drive gets a higher ohms value than the same power on a 230 V drive.
What braking duty cycle should I select?
The calculator offers Light 5 %, Standard 10 %, Heavy 25 % and Continuous 50 %. Pick the one that matches how often the drive actually decelerates — a high-stop-count application needs a higher continuous resistor kW because the resistor dissipates P_peak × duty continuously.
AO Ctrl provides this tool for reference only.