Short answer: from 3 A of set current upward, bolt every drive base to a 1/8 in [3 mm] aluminum plate through a paper-thin film of thermal paste, treat the plate as the heat sink, and size the airflow from the waste heat. The worked example runs three drives set at 3.5 A and assumed to waste 6 W each. Replace that 6 W with the loss your drive's datasheet states at your setting, because every number below moves with it. In a 30 C [86 F] shop, that load holds the air at the drives near 36 C in a sealed 20 x 16 x 8 in stainless box, or near 35 C with about 13 CFM of filtered fan air and a plate running 4 C above it. Paste by itself never finishes the job: it fills microscopic gaps, it does not spread heat across a panel.
What you need
- A 1/8 in [3 mm] 6061 aluminum plate, 2 in wider and longer than the row of drive bases. 6061 conducts about 170 W/mK.
- Four to six 1/2 in [13 mm] standoffs, so air can wash the back of the plate.
- CPU-grade thermal paste; datasheets run from 1 to 4 W/mK.
- A 3 in [80 mm] brushless fan with a washable filter, for shops whose air is clean enough to vent.
- A Type K thermocouple or an IR thermometer.
- Three datasheet numbers for your drive: the current that triggers heatsinking, the maximum case temperature, and the watts lost at your setting.
Fit the drives to the plate
- Read three numbers before you cut metal. On the example drive the datasheet calls for heatsinking above 3 A. Treat 3 A as the line where you plan a plate, and let your own sheet move it: take its threshold, its maximum case temperature (70 C is assumed below until you read yours), and its loss in watts at your current.
- Set current to the winding, then plan to measure hot. Match the setting to the motor's winding rating for the connection you wired. The example drive idles near 70% of setpoint at standstill, so full heat shows while the axes are moving at step rate. That is the condition for every measurement later.
- Size the plate from the heat. Air moving 1 to 2 m/s strips about 30 W/m2K from an aluminum face. For 18 W and a 4 C target rise, conductance must reach 18/4 = 4.5 W/K. With both faces working, area = 4.5/(2 x 30) = 0.075 m2, so a 12 x 10 in plate (120 in2, 0.077 m2) fits with a whisker to spare: it runs 18/(30 x 0.077 x 2) = 3.9 C above box air. Spreading matters too. L = sqrt(k x t / (2h)) = sqrt(170 x 0.003 / (2 x 30)) = 92 mm, so one drive's heat flattens over a circle about 7 in across. Keep every base roughly 4 in clear of the plate edges.
- Clamp the paste thin and use every hole. Clamped to 0.1 mm under a 2 in2 base, paste at 1 W/mK adds 0.08 C/W, half a degree at 6 W. The same joint left at 1 mm measures 0.78 C/W and adds nearly 5 C. Bolt through all mounting holes so the base sits flat, and keep the film thin enough to wipe away. An isolated base stays isolated: paste is not a ground path, as enclosure grounding and bonding explains. If you are still choosing the bond, the page on thermal paste versus adhesive versus tape for heat sink mounting covers the trade-offs.
Move the heat out of the box
- Size one filtered fan from the heat load. Air near room conditions carries 1206 J of heat per m3 per C (density 1.2 kg/m3, specific heat 1005 J/kg-K), which folds into CFM = 1.76 x Q/delta-T; the enclosure fan airflow calculation walks through the arithmetic. Pick a 5 C air rise because 30 C shop air plus 5 C stays under the 40 C [104 F] ambient rating most drive and electronics datasheets build on. For 18 W: 1.76 x 18/5 = 6.3 CFM. Fit at least double, about 13 CFM, because grilles and filters swallow a large slice of the free-air rating; a 3 in fan rated 20 to 40 CFM free air covers it. Blow in through the filter and cut exhaust vents with free area at least equal to the fan's open area. The box stays pressurized, so dust has one way in: through the media.
- Or stay sealed and check the rise first. Wall conductance is k x A. A 20 x 16 x 8 in box exposes 1216 in2 (0.784 m2) of stainless, worth 3.7 x 0.784 = 2.9 W/K, so 18 W lifts the inside air 18/2.9 = 6.2 C. A 30 C shop lands at 36 C, inside the limit. A 40 C shop lands at 46 C and needs the fan or a bigger box. Estimating enclosure thermal resistance from a heat load works the same method in general, and the backplate and DIN rail layout guide shows where a drive plate sits among terminals and rail.
- Order the plate drilled if you would rather skip the drill press. The 3D enclosure builder takes plate, standoffs, glands and cutouts from your layout, following the same dimensions the cutouts and glands guide sets out, and quotes from the drawing you approve. Custom enclosures are delivered in 10 business days from that approval.
Sealed box or filtered fan: the call
| Situation | The numbers | The call |
|---|---|---|
| Clean shop, air can vent | 18 W needs 6.3 CFM; fit 13 CFM filtered | One 3 in filtered fan blowing in, exhaust vent free area at least the fan's open area |
| Dust, swarf or washdown around | Sealed 20 x 16 x 8 in stainless rises 6.2 C at 18 W | Sealed box with the plate; air at the drives lands near 36 C in a 30 C shop |
| Sealed and the shop hits 40 C | 40 + 6.2 = 46 C at the drives | Grow the box or add the filtered fan; 46 C fails a 40 C ambient rating |
| Plate still runs hot | Double the area, halve the rise: 16 x 14 in gives 2.1 C | Grow the plate before you add fan speed |
Checks when done
- Let it reach steady state. A 20 x 16 x 8 in box in 1.5 mm stainless weighs about 9.3 kg. With 500 J/kg-K over a 2.9 W/K wall, the time constant is 9.3 x 500/2.9 = 1600 s, about 27 min. Read every 15 min and stop when two readings agree within 1 C; expect a couple of hours.
- Measure with the axes moving at full set current, because standstill idles near 70%.
- Check the plate beside a base. The fan example should read about 39 C: 35 C air plus the 3.9 C plate rise. The base adds the paste half-degree on top.
- Check the air high in the box, where hot air pools; where to measure ambient temperature inside an enclosure covers the spots that mislead. Hold it at or below 40 C.
- Bank the margin. A 39 C base under the assumed 70 C case limit leaves 31 C. At a 40 C summer shop the same box sits near 49 C and still holds 21 C. If your numbers come out thinner, cut current before you argue with the plate.
Mistakes that cost the drive or the rating
- Leaving the supply out of the load. At an assumed 85% efficiency, a supply delivering 100 W wastes 17.6 W, about as much as all three example drives. Add its datasheet figure before you size plate or fan.
- Counting push-pull fans as double flow. Fans in series move what one fan moves. Spend that money on vent area or a better filter instead.
- Keeping the Type 4X claim after adding vents. A fan-filter rated IP54 caps the whole assembly at IP54: splash-proof, not jet-proof, and the Type 4X versus IP66 comparison shows why the lowest-rated part sets the number. Our stainless boxes come rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the rating travels only as far as the lowest-rated part you bolt on.
- Mounting to paint or plastic. Metal to metal with paste between, or the base cooks inside its own footprint. When several drives crowd one plate, several heatsinks sharing one enclosure covers the airflow around them, and sizing a VFD enclosure from heat and clearance runs the same arithmetic at heavier loads.
- Believing a ten-minute reading. That 27-minute time constant means a sealed box reading 32 C at ten minutes is still 4 C short of its 36 C steady state.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a drive set below 3 A still need the aluminum plate?
On the example drive the datasheet asks for heatsinking only above 3 A, so at 2.8 A its own base on 1/2 in standoffs in moving air is enough. The sealed-box rise must stay modest: 18 W in the 20 x 16 x 8 in stainless box lifts the air 6 C, to 36 C at a 30 C shop. Read the threshold on your own datasheet before skipping the plate; the drive run closest to its limit trips first in a warm shop.
How much thermal paste goes under a drive base?
A rice-grain drop per 2 in2 of base, clamped to a film near 0.1 mm: about 0.08 C/W, half a degree at 6 W. Any CPU-grade paste from 1 to 4 W/mK works. The same joint at 1 mm thick measures 0.78 C/W and adds nearly 5 C. No paste beats the plate itself: paste conducts 1 to 4 W/mK against about 170 W/mK for the aluminum.
Can two fans, one pushing and one pulling, double the airflow?
They cannot. Fans in series move the same air one fan moves, so size one filtered fan from CFM = 1.76 x Q/delta-T: 1.76 x 18/5 = 6.3 CFM for the example's 18 W at a 5 C rise. Fit at least 13 CFM to cover filter loading. Spend the second fan's budget on exhaust vent area at least equal to the fan's open area.
What helps more when the plate runs hot: more plate or less current?
Less current wins per click: the dominant I2R-type losses fall with the square of current. Dropping the setting from 3.5 A to 2.8 A cuts them about 36%, from an assumed 6 W to under 4 W. More area still works: growing the plate from 12 x 10 in to 16 x 14 in drops its rise from 3.9 C to 2.1 C at 30 W/m2K over both faces. Do both when the margin is thin.
