Short answer: a braking-resistor overload fault that starts a week or two after the enclosure AC unit dies is a heat fault first. The 22-25 K rise over a 24 C room you are seeing puts the drive intake at 46-49 C, and drives are rated at 40 C ambient unless the datasheet says more — one servo-drive maker derates continuous power 4 % per 10 K above that line, so the braking model trips on decels the machine has made for years. Log the drive-intake temperature for one shift, check the stored resistor ohms and watts against the nameplate, then restore cooling with a sealed method sized at W x 3.412 = BTU/h: a ducted 3000 BTU/h portable unit carries about 880 W of panel heat.
What the fault is telling you
Your drive does not measure the resistor's temperature. It computes resistor loading from the regenerated energy of each decel against the ohms and watts stored in its parameters, folded together with its own temperature. Two routes lead to the same trip: a resistor that is genuinely overloaded, or a healthy resistor modelled in air 20 K too hot. Read the symptoms like this:
- Fault starts one to two weeks after the AC unit dies, same program, same moves: heat. The machine did not change; the air around the model did.
- The fault survived new drives, new cables and a rebuilt gearbox: check the stored resistor data on every drive that was swapped. A replacement ships with defaults, not with your resistor.
- Fault lands on one specific decel even on a cool panel: the resistor is under-sized for that move, or its element has drifted.
- Intake logger reads over 40 C at the fault timestamp: heat, and nothing else to chase.
- Cold element resistance off the nameplate: the resistor itself.
The timing you have — dead AC, then the fault, then weeks of parts — is the classic signature of the first row below. Log before you buy anything else.
The four causes, most likely first
| Rank | Cause | Why it trips | The check that separates it |
|---|---|---|---|
| 1 | Hot enclosure, dead AC | Drive and resistor shed into air 22-25 K above the room; above 40 C the derating eats the braking margin | Fault rate falls once the intake holds below 40 C |
| 2 | Resistor data reset in a swapped drive | The new drive models braking against default ohms and watts | Stored values differ from the nameplate or the old drive's backup |
| 3 | Resistor under-sized or drifted | Regen energy per decel exceeds the element's continuous or pulse rating | Cold resistance off nameplate; the same decel faults on a cool panel |
| 4 | Fans and filters | Clogged mats or a tired heatsink fan run the drive 10-20 K above the panel average | Hand-check airflow; filter mats gray and loaded |
A washdown room lifts cause 4 toward the top of your list: hosing and foaming every 3-4 days loads filter mats fast and kills enclosure AC condensers early.
Prove heat before you change parts
Put three loggers in the panel: one at the drive air intake, one beside the braking resistor, one at the top of the box. Read the drive's device-temperature diagnostic from its parameter list and log it alongside. Lay the fault timestamps from the machine log over those traces and let one full production shift run. If the intake sits at 46-49 C and the faults cluster on the hot stretches, you have your cause — at that intake the drive has already lost 2-4 % of continuous power to derating. Park the parts list until that shift of data exists; motors, cables and gearboxes have already been spent on a fault a 40 C intake explains. The same 40 C line reaches the rest of the panel too: breaker makers publish derating tables for ambients above 40 C.
Check the stored resistor data and the element
A drive swapped during troubleshooting arrives with its own parameter set, and a wrong watt figure alone can produce this fault. Find the resistor nameplate — ohms, continuous watts, pulse or energy rating — and compare each value with the braking parameters in every replacement drive, then with the original drive's backup if you kept one. Correct any mismatch, save, power-cycle, and run a full shift before you judge anything else. For the regen-energy arithmetic behind those ohms and watts, the page on VFD brake resistor selection: ohms, watts and thermal limits walks it step by step.
Then the hardware. De-energize the panel, lock it out, and give the DC bus its stated discharge time — commonly 15 minutes — before an ohmmeter touches the resistor, because it hangs on the bus. Expect the cold reading inside the nameplate tolerance, commonly ±10 % for wirewound elements. Turn every fan on the drive and the resistor by hand while the door is open, and look at the filter mats at the same time.
Sizing the backup cooler for a 630 W panel in a 24 C room
Inputs, each with its source:
- Two servo drives at 180 W losses each, from the drives' power-loss tables: 360 W.
- One 24 V supply at 45 W, from its datasheet.
- Braking resistor at 200 W cycle average, read from the drive's regenerative-power monitor during your logged shift.
- Contactors, chokes and wiring, a 25 W allowance.
- Room 24 C, existing panel 60 x 36 x 16 in stainless, wall coefficient k = 3.7 W/m²K.
The rule in words: a sealed box sheds heat through its walls at area x k x temperature rise, so with the cooler dead the whole load takes that path. A backup cooler must carry the full load plus margin, and watts become BTU/h at W x 3.412 = BTU/h.
The arithmetic:
- Load: 360 + 45 + 200 + 25 = 630 W.
- Sealed rise: a 60 x 36 x 16 in box has 4.8 m² of surface; 630 / (3.7 x 4.8) = 630 / 17.8 = 35 K. Inside: 24 + 35 = 59 C, which is 19 K past the 40 C rating point — at 4 % per 10 K, nearly 8 % of continuous power is gone before the machine moves.
- Cooler: 630 x 3.412 = 2150 BTU/h. A 3000 BTU/h class unit is 3000 / 3.412 = 880 W of capacity, so it leaves 250 W of headroom, 40 % over the load.
Decision: at 630 W, a 3000 BTU/h ducted unit carries this panel with real margin. When your sum passes 880 W, that class is exhausted — step up one size or run two units, then recheck the intake against 40 C after a full shift of running.
Restore cooling without opening the panel to the room
That room is hosed and foamed, so the enclosure stays closed and every temporary method passes through sealed openings. Your box is rated NEMA 4X / IP66 by the manufacturer; not UL listed — and an assembly holds only the rating of its weakest part, so a cutout with a hose through it is an open hole until you blank it. Cut the opening to the duct collar size, hinge a blanking cover over it, and the rating returns the day the unit comes off.
| Method | How it connects | What to watch |
|---|---|---|
| Ducted portable AC, 3000 BTU/h class | Cold duct to a sealed inlet collar; exhaust fan kit on a second cutout | Cold discharge runs near saturation; sweat is the risk |
| Server-rack portable AC | Hole-saw opening with a collar; cold hose in, unit exhausts away | Same dew-point risk; add units for capacity |
| Large portable AC, 29000 BTU/h class | Cold nozzles ducted into a large panel | Needs 230 V, floor space and a drain route in a wet room |
| Vortex cooler | Small opening plus an instrument-air line | Large compressed-air draw; the cold end can fall below the room dew point |
| Air-to-water heat exchanger | Closed loop; the box stays sealed | Needs a water supply and drip-free fittings above the electronics |
Aim the cold air at the drive intake rather than the top of the box, where it stratifies. In a 24 C room at 50 % RH the dew point sits near 13 C, so run the unit a few degrees below room temperature instead of at its coldest setting, keep the duct exterior dry, and route condensate away from the door seam. Water on the heatsinks means raise the setpoint now; finding the cause of water inside a sealed enclosure takes it from there. For placing and running the temporary unit, portable air conditioner cooling for a control cabinet covers the setup. The compressed-air side of the vortex route is in vortex cooler air supply and sizing checks.
Make the fault stay away
- Put enclosure AC filter cleaning on the PM schedule monthly in a washdown area; loaded mats choke the condenser long before the compressor fails.
- Standardize panel AC units across the site to a handful of sizes, so one or two spares cover dozens of panels.
- Fit two smaller units on one large panel: when one dies you keep half the cooling, and the intake stays under 40 C while the spare arrives.
- Keep one portable unit ready with its collars and blanking covers pre-cut, so the changeover is an hour of work.
- A made-to-order stainless enclosure is delivered in 10 business days, counted from your order — from your approval of the drawing for custom work — so a panel split or relocation is a near-term fix.
When to re-specify the enclosure
With the fault gone and the intake held below 40 C, ask whether the box itself put you on this cliff. Three signs say yes:
- The intake runs near 40 C even with cooling healthy. You need surface area, and how to size an electrical enclosure works the same k = 3.7 W/m²K arithmetic. A 48 x 36 x 16 in 316L wall-mount enclosure sheds a given load at a lower rise than a shallow box.
- The braking resistor dominates the heat load. Move it into its own housing beside or above the panel, the arrangement a box on top of a machine cabinet for a brake resistor works through. An element averaging under 60 W holds a sealed 20 x 16 x 8 in 316L enclosure about 21 K over the room. Anything hotter needs a larger box, because a vent drops the assembly below the washdown rating.
- The environment: hosed and foamed every 3-4 days means 316L and sealed cooling only. Run the choice through the enclosure selector, and compare the closed-loop methods in sealed enclosure cooling options for hot ambients before the next AC unit goes in.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 316L stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How hot can the inside of the panel get before the braking-resistor fault appears?
Plan on 40 C at the drive intake as the line. Drives are rated there unless the datasheet says more, and one servo-drive maker cuts continuous power 4 % per 10 K above 40 C ambient, so the braking model loses margin before anything is overloaded. In a 24 C room, a 25 K rise puts the intake at 49 C and a hard decel can trip it. Hold the intake under 40 C and the fault stops.
Can I raise the drive's braking-resistor overload threshold until the new AC unit arrives?
Leave the threshold where it is. It stands in for the resistor's real continuous power, and raising it runs the element past its rating toward burnout, with the fault arriving mid-deceleration instead of on the display. Cool the panel instead: at 630 W of losses a 3000 BTU/h unit carries the load with 250 W of headroom.
Does the temporary duct opening cost the enclosure its rating?
While a hose or duct passes through the cutout, yes: the assembly drops to the rating of that opening. Blank it with a hinged cover or blanking plate whenever the unit is off, keep the collar sealed, and the box returns to its manufacturer NEMA 4X rating. Cut the opening to the collar size so the blank seals flat.
What if the fault still comes back with the panel cool?
Then heat was a contributor, not the whole cause. Compare the stored resistor ohms and watts against the nameplate in every swapped drive, measure the cold element inside its tolerance, commonly ±10 % for wirewound, and note whether the fault lands on one specific decel. That points to an under-sized resistor, which no amount of cooling fixes.
