How to Size a VFD Braking Resistor for an Overhauling Load
When a load drives the motor instead of the motor driving it, a descending hoist, a decelerating flywheel, or a load pulling the shaft, the motor becomes a generator and pushes energy back into the drive. A braking resistor gives that energy somewhere to go so the DC bus does not trip on overvoltage. This page is for the engineer who needs to understand how such a resistor is sized. It explains the two quantities that matter, the peak power and the average duty, and how they translate into the resistance and wattage a resistor must have.
VFD Braking Resistor Sizing in one line: To size a VFD braking resistor, work from two quantities: the peak braking power, which the load's inertia and how fast you decelerate it set, and the average power dissipated over time, which the braking duty cycle sets. The peak power and the drive's braking voltage threshold set the resistance value, low enough to absorb the peak but not so low it overloads the drive's brake chopper. The average power sets the resistor's continuous wattage rating so it does not overheat across repeated braking. Both come from the specific load and duty, so confirm against the drive and resistor manufacturer's guidance.
Understand Where the Braking Energy Comes From
A braking resistor exists because a decelerating or overhauling load turns the motor into a generator. When the motor is forced to slow a high-inertia load, or a load actively drives the shaft, the kinetic or potential energy flows back through the drive and raises the DC bus voltage. Left unmanaged that rising bus trips the drive on overvoltage, which is the failure the guide on setting VFD accel and decel times addresses by slowing the deceleration. When the process needs fast braking, the energy must instead be dissipated, and the resistor is where it goes.
The resistor works with the drive's brake chopper, which switches the resistor across the DC bus when the bus voltage climbs. The chopper and resistor together form the dynamic-braking scheme described in the page on the VFD braking resistor and brake chopper. The chopper turns the resistor on when the bus needs relief and off when it recovers, so the resistor burns the regenerated energy as heat and the bus stays below the overvoltage trip.
Distinguish this from a regenerative drive, which returns the energy instead of burning it. A resistor is the simpler and cheaper answer for intermittent braking, while an active front end that feeds energy back to the supply, described in the page on the active front end regenerative VFD, suits loads that brake often or continuously. Choosing a resistor means accepting that the braking energy leaves as heat, which is exactly why its wattage rating matters as much as its resistance.
The Two Quantities That Set the Resistor
Peak braking power sets the resistance value. The peak is how much power flows back at the hardest moment of braking, driven by the load's inertia and how quickly you decelerate it, or by how hard an overhauling load pulls. The resistance must be low enough to dissipate that peak while the bus is held near the chopper's braking threshold, but not so low that the resulting current exceeds what the drive's brake chopper can switch. The drive documentation gives a minimum resistance for its chopper, and going below it risks the chopper, so the resistance sits between that minimum and the value that handles the peak.
Average power sets the wattage rating. A resistor might survive the peak for an instant but cook if it dissipates too much energy too often, so the continuous wattage rating comes from the average power over the braking duty cycle, how much energy each braking event dumps and how frequently they happen. A hoist that lowers loads all shift long has a high duty and needs a resistor rated to dissipate that average continuously; an occasional emergency stop has a low duty and needs far less continuous rating even for the same peak.
Both quantities come from the specific load, so there is no generic answer. The inertia, the deceleration rate, the overhauling force, and the duty cycle are all site-specific, and the resistance and wattage follow from them together with the drive's braking threshold and chopper limits. Size a resistor by working these numbers for the actual machine and checking them against the drive and resistor manufacturer's guidance, rather than reaching for a value that worked on a different application with a different load and duty.
Confirm the Resistor and Protect It
Check the resistor against both failure modes before installing it. Confirm its resistance is at or above the drive's stated minimum for the brake chopper so the chopper is not overloaded, and confirm its continuous wattage covers the average braking power for the real duty so it does not overheat in service. A resistor that satisfies the peak but not the average survives testing and fails weeks later; one that satisfies the average but is below the chopper minimum stresses the drive. Both checks must pass.
Protect the resistor and the area around it, because it is designed to get hot. A braking resistor dissipates real power as heat and can reach temperatures that ignite nearby material or injure someone who touches it, so it needs to be mounted where its heat is safe, with the clearances and any thermal protection the manufacturer specifies. Many installations add a thermal cutout that trips the drive if the resistor overheats, which is a legitimate protection against a braking duty that turns out heavier than planned.
Verify the braking works under the real worst case and watch it over time. Test the hardest, most frequent braking the process will actually do and confirm the bus stays below the overvoltage trip while the resistor temperature stays within its rating. Because the DC bus voltage and any resistor thermal trips are values a monitoring system can trend, a braking scheme that starts to see the bus climb higher, or the resistor run hotter as duty creeps up, shows in the data, giving warning that the load or duty has outgrown the resistor before it fails or the drive starts tripping on overvoltage.
Frequently Asked Questions
What sets the resistance value of a braking resistor?
The peak braking power and the drive's braking voltage threshold set the resistance. It must be low enough to dissipate the peak power the load pushes back at the hardest moment of braking while the bus is held near the chopper's threshold, but not lower than the minimum resistance the drive's brake chopper can switch. The drive documentation states that minimum, and going below it risks the chopper, so the resistance sits between the chopper minimum and the value that handles the peak.
Why does the braking resistor's wattage rating matter separately?
Because a resistor can survive a brief peak yet overheat if it dissipates too much energy too often. The continuous wattage rating is set by the average braking power over the duty cycle, how much energy each braking event dumps and how frequently they occur. A high-duty application like a hoist lowering loads all shift needs a resistor rated to dissipate that average continuously, while an occasional emergency stop needs far less continuous rating even at the same peak power.
When should I use a regenerative drive instead of a braking resistor?
Use a regenerative drive, or active front end, when the load brakes often or continuously, because it returns the braking energy to the supply instead of burning it as heat, which saves energy and avoids the heat a resistor produces. A braking resistor is the simpler, cheaper choice for intermittent braking where the occasional heat is acceptable. The decision comes down to how much and how often the load regenerates, weighed against the higher cost of the regenerative front end.
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