On a constant 750 VDC bus, a 3.5 ohm brake resistor draws about 214 A and dissipates roughly 750 x 750 / 3.5 = 160.7 kW at the moment of switching, while a 5 ohm unit on the same bus draws 150 A and dissipates 112.5 kW, per the standard sizing relation R = V² / P [S2].
The peak power gap is set by V²/R, so a 30% drop in resistance (5.0 to 3.5) buys roughly a 43% jump in instantaneous heat dissipation and braking torque, but the same 30% drop also raises peak current by 43% through the brake chopper transistor [S3].
Why Ohm Value Sets Peak Current, Not Long-Term Heat
Brake resistor selection splits into two independent ratings: minimum resistance and continuous power dissipation, per the KEB America selection guide [S3]. Minimum resistance is governed by Ohm's law applied to the drive's DC bus over-voltage threshold, which KEB lists at 840 VDC on its braking input; the formula R_min = V / I_max of the brake transistor is what protects the chopper from inrush [S3]. On a 750 VDC bus, a 3.5 ohm resistor pulls 214 A peak while a 5 ohm pulls 150 A peak, a 64 A difference that must stay inside the chopper's continuous and peak ratings.
Wattage rating, by contrast, is a thermal capacity number for the resistor element itself, not for the chopper [S3]. A 3000 W resistor can dump 3000 W average without the element failing, but the average is defined by the duty cycle: a heavy flywheel on a 10 hp saw spinning a 24 inch pulley needs only a few seconds of peak power per stop, while a high-cycle elevator regenerative load can demand continuous dissipation [S2][S4].
Energy Per Stop: The Joules Side of the Question
Ohms control how fast the energy leaves the system; joules per stop control how much energy must be removed. The kinetic energy of a 50 mph car stop is roughly 250 kJ, while a 65 mph HGV lorry is closer to 15 MJ, per Cressall's worked examples [S2]. Stopping a 600 mm x 300 mm steel flywheel at 1500 rpm releases about 375 kJ, an order of magnitude above what the DC link capacitance can absorb (3-5% of regen power, roughly 3-5 kJ on a 100 kW drive) [S2].
For a VFD on a 100 kW motor, the difference between a 3.5 ohm and a 5 ohm brake resistor at 750 VDC is the difference between shedding 160.7 kW peak and 112.5 kW peak, a 48 kW delta that translates to about 30% faster stop time on a fixed-inertia load, assuming the chopper is rated for the higher current [S2][S3].
Heat Dissipation Rate: Instantaneous vs Continuous
With a constant voltage across the resistor, lower resistance always means more instantaneous heat output: P = V²/R goes up as R goes down [S5]. The reverse observation from that thread (a 0.1 ohm resistor "throwing off less heat" than a 3.3 ohm unit) was a non-constant-voltage drive behavior, where most of the power was being radiated from the switching transistor, not the resistor [S5].
In a correctly wired VFD brake circuit, the chopper holds the bus near the trigger voltage while current is shunted through the resistor, so the 3.5 ohm element does in fact run hotter per unit time than the 5 ohm element at the same duty cycle. Thermal mass, resistor housing (wirewound, ceramic, or grid), and forced-air cooling then determine whether that heat can actually be rejected to ambient before the next stop, which is why OEM datasheets list a "100% duty" rating separately from peak power [S3][S6].
When 3.5 Ohm Is the Right Pick, and When 5 Ohm Wins
Choose 3.5 ohm when the drive's published R_min is at or below 3.5 ohm, the load has high inertia or requires aggressive decel ramps, and the duty cycle is intermittent (a few stops per minute with cool-down gaps). The Danfoss VLT MCE 101 design guide, for example, lists a minimum resistance tied to each drive's brake transistor rating; going below that floor can destroy the chopper [S6].
Choose 5 ohm (or higher) when the drive's R_min is closer to 5 ohm, the application runs many cycles per minute, or the resistor is mounted in a sealed enclosure with limited airflow. The practical-machinist thread on a 10 hp, 575 V saw illustrates the boundary case: the user found a 35 ohm and a 63 ohm candidate, both 3000 W, where Magnetek's drive floor was 29 ohm. The 35 ohm unit gave "maximum possible braking effect" while staying safely above the chopper limit [S4].
Comparison Matrix: 3.5 Ohm vs 5 Ohm at 750 VDC
At 750 VDC: peak current is 214 A on 3.5 ohm vs 150 A on 5 ohm; peak power is 160.7 kW vs 112.5 kW; stopping time on a fixed inertia is roughly 30% faster on 3.5 ohm; chopper thermal stress is ~43% higher; resistor element steady-state wattage must still meet the duty cycle average, which is independent of ohm value. Both must exceed the drive's minimum resistance floor, and the brake transistor's continuous and peak current ratings set the real ceiling, not the resistor itself [S2][S3][S4].
What the Standards and Datasheets Actually Require
VFD manufacturers publish two numbers per frame: a minimum allowed resistance and a recommended continuous wattage for a stated duty cycle [S3][S6]. The minimum resistance protects the brake transistor from over-current, while the wattage protects the resistor element from thermal overload; neither replaces the other. KEB specifies "100% duty" rating on its brake transistors, meaning the chopper can stay on continuously at its rated current without tripping [S3].
For thermal design, the resistor housing and cooling method (free air, forced air, or liquid) set the long-term heat rejection; a 3000 W resistor in a 25 °C ambient with 2 m/s airflow can sustain its rating, but the same resistor in a sealed cabinet derates to roughly 50-60% of nameplate. The Danfoss MCE 101 guide pairs each drive frame with a specific resistor ohm and wattage to keep both numbers inside the chopper's safe operating area [S6].
Limits and Failure Modes
Three failure modes dominate: chopper transistor burnout from sub-minimum resistance, resistor element open-circuit from sustained over-wattage, and connection or terminal failure from repeated thermal cycling. Going from 5 ohm to 3.5 ohm on a drive whose R_min is 4 ohm, for example, risks immediate chopper failure on the first hard stop [S3][S4].
For applications with continuous regeneration, such as a downhill conveyor or a high-cycle punch press, neither 3.5 nor 5 ohm at fixed resistance is the right answer; a regen line-side converter recovers the energy instead of burning it, and the brake resistor becomes a back-up for fault conditions only [S3]. For background on how dynamic braking fits with other motor control hardware, see the clutch-brake and electromagnetic-brake reference pages, which cover the mechanical-side equivalents. Thermal instrumentation for resistor bank monitoring falls under the heat-detector and heat-treatment-furnace categories, while bench-level validation uses the instruments grouped on the measurement-test-3 page. Related hazardous-area cabling choices for the resistor cabinet, when the drive sits in a zoned area, are mapped in the ATEX cable gland selection matrix.
Before swapping a 5 ohm for a 3.5 ohm brake resistor, confirm three numbers from the drive's datasheet: the minimum allowed resistance, the brake transistor's peak and continuous current, and the recommended continuous wattage for the application's duty cycle. If any of the three is violated, the lower-ohm swap will trade a slower stop for a blown chopper or a charred resistor housing.