Most VFD failures I get called about in the field aren’t caused by a bad drive. They’re caused by heat that had nowhere to go. The drive derates, throws an OH (overheat) or OL fault, or the fan bearing dies two years early — and the panel builder swears the enclosure was “well ventilated.” It usually was ventilated. It just wasn’t ventilated enough for the actual heat load, because nobody calculated the heat load.

Here’s how to actually get this right, instead of guessing.

Where the heat comes from

A VFD isn’t a passive box — it converts AC to DC and back to variable-frequency AC through IGBTs, and that switching isn’t 100% efficient. Depending on the drive and switching frequency, expect roughly 2–5% of the motor’s rated power dissipated as heat inside the drive itself. For a 15 kW drive running near full load, that’s 300–750 W you need to move out of the enclosure — every hour, continuously.

This is the number people skip. They size the enclosure for the drive’s physical footprint and call it done.

Ambient temperature derating is not a rounding error

Every VFD has a rated ambient temperature, usually 40°C, above which the drive can no longer deliver its full rated output without overheating. Above that point, manufacturers derate — a common figure is about 2–3% output current reduction per °C above 40°C, up to a 50°C limit on many industrial drives (check the specific datasheet — this varies by brand and frame size). Run a drive rated for 40°C in a 45°C cabinet without accounting for derating, and you’re not “pushing it a little” — you may be 10–15% short of the current the motor actually needs at full load, which shows up as nuisance trips or an undersized-looking motor.

Altitude matters too. Above roughly 1000 m, most drives derate further — commonly around 1% per 100 m — because thinner air cools less effectively. If you’re specifying drives for an alpine pumping station or a mountain-side facility, this isn’t optional reading.

Clearance requirements are minimums, not suggestions

Manufacturer datasheets specify minimum clearance around the drive — often something like 100 mm above and below, and 10 mm side-to-side for smaller frame sizes, with larger requirements for higher-power units. I still see drives mounted side-by-side with zero gap because a panel had limited DIN rail space. The drive on the downstream side of airflow is drawing in air that’s already been heated by the drive next to it — you end up cooling nothing.

Practical rule from installs I’ve done: if you’re tight on panel space, go up in enclosure size before you go tight on spacing. A bigger enclosure is a one-time cost. A drive that derates or fails early is a recurring one.

Sizing enclosure cooling: a real method, not a guess

Add up the heat load of everything inside the enclosure — drive(s), contactors, transformers, resistors. Use the manufacturer’s stated power loss figures (in W), not the drive’s kW rating.
Subtract what the enclosure itself dissipates through its steel surface via natural convection — there are standard tables (IEC 60890) relating enclosure surface area and ΔT to natural dissipation in watts.
Whatever heat is left needs forced removal — either a filtered fan (cheapest, but lets in dust and humidity), a cabinet air conditioner (for dusty, humid, or high-ambient environments), or an air-to-air / air-to-water heat exchanger (for sealed enclosures in harsh environments where you can’t let outside air in at all).

If you skip step 1 and just buy “a fan that seemed about right,” you’re guessing at a number that has a correct answer.

The failure patterns I actually see on service calls

Dust-clogged fan filters. This is the single most common cause of “the drive started tripping OH after two years of working fine.” Filters need scheduled cleaning, not reactive cleaning.
VFDs mounted above or beside a source of heat — a transformer, a compressor, direct sun on an outdoor enclosure — with the drive’s own rated ambient never adjusted for that added load.
Fan bearings past their service life. Cooling fans are consumable parts, typically rated for a few years of continuous duty, not the lifetime of the drive. If nobody’s tracking fan hours, nobody knows it’s overdue.
Cable ties and wire bundles routed directly across the heatsink fins or fan intake, quietly cutting airflow by a third.

Heat dissipation in a VFD installation isn’t really about “good airflow” as a vague goal — it’s arithmetic: heat generated, minus what the enclosure sheds naturally, equals what your cooling method has to remove, adjusted for your actual ambient temperature and altitude. Get that number right at the design stage and most of the “mystery” VFD failures never happen.

If you’re specifying a drive for a specific ambient, altitude, or enclosure and want a second set of eyes on the derating math, that’s exactly the kind of question worth asking before the panel is built rather than after it trips for the third time.

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