Sizing and mounting a braking resistor in a VFD enclosure

Short answer: select the resistor from the exact drive model and its braking instructions, then add its average heat to the enclosure budget. Motor kW and voltage class alone cannot set the ohms, watts or mounting clearance. The hypothetical example below uses 100 ohm at 360 V for 1.5% conducting duty: about 20 W average heat. Adding an assumed 66 W drive loss gives 86 W inside the box. At 35 C ambient and a 40 C internal design target, the airflow calculation gives about 53 m³/h [31 CFM], before checking the installed fan curve.

What you need before you order

Collect the exact drive and resistor documents, your measured machine cycle and the enclosure dimensions before selecting parts.

• The exact drive model and braking section. Record its approved resistance range, brake-chopper limits, permitted duty, overload protection, cable limits and installation clearances. Use the model and manual revision on your machine.
• The resistor datasheet. Record resistance tolerance, pulse energy, average-power derating, mounting requirements, surface-temperature limits and terminal torque.
• Your machine cycle. Record energy returned per stop and stops per minute, including the worst repeated stopping or lowering cycle. Chopper conducting time can differ from the mechanical deceleration time.
• The box. Record material, exposed wall area and ambient temperature. The table below gives planning heat-transfer coefficients used on this site; mounting, airflow and surface finish change the actual heat transfer.

Stay within the resistance range permitted by the drive and the required braking power. Too little resistance overloads the chopper; too much can leave too little braking power and cause an overvoltage trip. At a fixed bus voltage, instantaneous resistor power is V²/R. That relationship helps check a selection, but the drive manual and load cycle set the usable range.

The sizing and mounting steps

1. Set the ohms from the exact manual. Select within the permitted range, allowing for resistor tolerance. Measure the isolated resistor before wiring it; measured ohms and wattage for resistor loads explains why the label alone is insufficient.

2. Check watts and pulse energy together. Use the resistor maker’s curve for pulse duration, energy per stop, repetition rate and ambient temperature. A 200 W continuous rating does not establish its short-pulse capacity. Fit the thermal protection and fault response specified for the drive and resistor.

3. Compute average braking power. Divide the energy dissipated per cycle by the cycle time. For a simplified constant-voltage pulse, power while conducting is V²/R and average power is that value times conducting duty. A real chopper modulates, so use the drive’s sizing method or measured energy for the actual cycle.

4. Put the resistor in the heat budget. Add its average dissipation to the drive’s published losses at your load and the losses from supplies and other equipment. The example assumes 66 W drive loss; it is not a value inferred for every 2.2 kW drive. Keep the drive inlet below its actual temperature limit. The enclosure sizing guide covers the mechanical space as well.

5. Choose the cooling from the sum. A box sheds P = k × A × ΔT through its walls passively. When your heat sum passes that, a fan sized Q [m³/h] = 3.1 × P ÷ ΔT does the rest, with ΔT the inside-outside rise you allow. The enclosure fan airflow calculation walks the full method, filters included.

6. Mount to the resistor instructions. Use the specified orientation, clearances and mounting surface. Route heated air away from the drive inlet and keep wiring clear of the hot body. A 2 in [50 mm] gap is only a layout allowance to check against those instructions, not a universal minimum. Use the drive’s braking terminals, cable insulation, lead-length limit and protection scheme. Fan placement around hot components covers the airflow layout.

Isolate and lock out all energy sources before work. Follow the drive’s stated DC-bus discharge time and voltage-verification procedure before touching terminals; elapsed time alone does not prove the bus is discharged.

The heat sum for a 20 x 16 x 8 in box at 35 C ambient

These are hypothetical worked-example assumptions, not specifications copied from an identified drive. Replace them with the exact drive manual, resistor datasheet and measured duty before ordering or commissioning:

• Drive: a hypothetical 2.2 kW installation; assumed 66 W of drive loss for the heat calculation.
• Resistor candidate: assumed 100 ohm and 200 W continuous rating, subject to the exact drive’s resistance limits and the resistor’s pulse energy and derating curves.
• Chopper voltage while conducting: assumed constant 360 V for the simplified calculation.
• Cycle: assumed 0.45 s of chopper conduction every 30 s, so conducting duty = 0.45 ÷ 30 = 1.5%.
• Other equipment losses: excluded from this simplified example; add them for the installed assembly.
• Box: 20 x 16 x 8 in stainless, wall-mounted, k = 3.7 W/m²K.
• Ambient: assumed 35 C [95 F]; internal-air design target: assumed 40 C [104 F], to be replaced by the installed drive’s limit.

Use average heat for the steady-state enclosure estimate, and pulse energy for the resistor’s short-term thermal check. The simplified wall calculation is total heat divided by k × A. A fan selection also needs the system resistance from filters, grilles and the airflow path.

The arithmetic:

• Peak chopper power: 360 V × 360 V ÷ 100 Ω = 129,600 ÷ 100 = 1296 W, about 1.3 kW.
• Average resistor heat: 1296 W × 0.015 = 19.4 W — call it 20 W, a tenth of the 200 W body.
• Total heat: 66 W + 20 W = 86 W.
• Surface: 2 × (20×16 + 20×8 + 16×8) = 1216 in² = 0.78 m²; the wall-mounted back adds little, so count 0.6 m².
• Passive capacity at 35 C: 3.7 × 0.6 × (40 − 35) = 11 W. Sealed, this box would sit at 35 + 86 ÷ 2.22 = 74 C [165 F].
• Fan: 3.1 × 86 ÷ 5 = 53 m³/h [31 CFM], allowing a 5 C inside-outside rise.

For these assumptions, use active cooling or revise the heat load and enclosure size: the estimated 11 W passive capacity is below the 86 W load. Select a fan that delivers the calculated airflow against the installed filters and grilles, with margin for fouling. A free-air rating above 53 m³/h alone is insufficient. Thermostat control for an enclosure fan covers control placement. The assumed resistor heat alone gives about 9 C rise; the drive losses dominate this example. VFD enclosure rating, size and cooling for one drive covers the enclosure decision.

When the box must stay sealed

A washdown or outdoor panel specified for Type 4X / IP66 needs cooling hardware suitable for that enclosure rating. An ordinary IP54 fan-filter does not preserve it. Our boxes are rated NEMA 4X / IP66 by the manufacturer and are not UL listed. Confirm the completed assembly after every penetration; a component label alone does not rate the assembly. Two options are:

• Move the resistor outside. Where the drive instructions permit it, use a guarded housing and lead arrangement suitable for the environment. In this example it removes about 20 W average from the main box. Select conductors for voltage, temperature, protection and installation conditions, not only the example’s 3.6 A conducting current.
• Cool without open ventilation. Select a suitably rated heat exchanger or sealed cooling unit for the actual thermal load and ambient conditions; outdoor drive enclosure cooling for high-power equipment explains that choice.

If you are specifying the box now, custom enclosures let you set the material, the three dimensions and every cutout, including the resistor's clearance in the layout — delivered in 10 business days, counted from your approval of the drawing.

Checks when the job is done

• Stop time and energy. Confirm the required stop under the worst load and repeated cycle, watching drive faults and resistor temperature. A longer stop increases time duty at a fixed cycle interval; recompute energy per stop instead of assuming the heat falls.
• Air temperature. Run the worst repeated duty until temperatures stabilize at the highest intended ambient. Measure at the drive inlet and compare with its model-specific limit; 40 C is only this example’s design target.
• Resistor case. Measure with a method suitable for the surface and compare with the resistor’s operating limits and derating curve. Account for surface emissivity if using infrared measurement; a low average wattage does not guarantee a safe pulse temperature.
• Fan behavior. The fan should reach full speed when the thermostat calls, and the intake filter should still pass air; a blocked filter takes a big bite out of your 53 m³/h.

The mistakes that cost the drive or the rating

• Ohms outside the approved range. At the example’s 360 V, 80 ohm draws 4.5 A while 100 ohm draws 3.6 A. Neither calculation identifies the drive’s allowable current; obtain that limit from the exact manual.
• Sizing on the nameplate alone. Check both energy per pulse and average dissipation. The example’s 200 W label does not prove its suitability for a 1296 W pulse, and the example’s 20 W average applies only to its assumed cycle.
• The resistor in dead air. Hot exhaust can push the drive inlet above its limit and derate the resistor; braking resistor overload from a hot panel explains the shared heat load.
• Cable insulation against the body. Use the specified separation and temperature-rated wiring; the resistor surface can exceed the surrounding air temperature substantially.
• A fan-filter on a sealed box. Choose cooling hardware approved for the intended assembly rating; an IP54 filter does not preserve a Type 4X / IP66 enclosure.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions