AC Gear Motor Overheating: 6 Causes and How to Fix Them

A hot gear motor is a symptom, not a diagnosis. The same 70 °C housing can mean a correctly loaded motor in a warm room, a motor being asked to deliver 40% more torque than it was rated for, or a fan cowl packed solid with cardboard dust. Replacing the motor before you know which one it is means you buy the same failure twice.

This guide covers compact AC gear reducer motors from 15 W to 300 W: the six causes that account for almost every overheating complaint we see, the temperature numbers that actually define “too hot”, a ten-minute diagnostic sequence you can run without instruments beyond a clamp meter, and the fixes in order of cost.

First, separate winding heat from gearbox heat

This single step eliminates half the guesswork. An AC gearmotor has two heat sources that fail for completely different reasons, and they are separated by a few centimetres of casting.

If the gearbox is hot and the motor is not, the problem is downstream of the motor. Everything in the electrical section below is irrelevant to that fault, and this is the most common wasted afternoon in gear motor troubleshooting.

The temperature numbers that define “too hot”

“Too hot to touch” is not a specification. These are the numbers that are:

Read those together. A motor at 40 °C ambient with an 80 K rise reaches a 120 °C winding at rated load, which leaves 10 K of margin to the class B limit. That is the entire margin. Now put that motor in a sealed cabinet at 55 °C ambient, and the same load puts the winding past 130 °C. Nothing about the motor changed. The installation did.

There is no reliable conversion from “how hot the case feels” to winding temperature, because the thermal path varies with frame, mounting and airflow. What you can say is this: if the housing is uncomfortable to hold for more than a second or two — roughly above 60 °C — the winding is close enough to its limit that the current and duty should be checked rather than assumed. The customer support size chart gives the frame-by-frame ratings you need to compare against.

Cause 1: the 30-minute reversible rating used as if it were continuous

This is the most common cause we see, and it is almost always a specification misunderstanding rather than a fault.

Reversible (RK) motors in this class carry a 30-minute rated duty. That rating is not a suggestion about how you should use the motor; it is a thermal limit. The motor can deliver its rated torque for 30 minutes, after which the winding reaches its permissible temperature and the motor must cool. The 30-minute figure exists because these motors are designed for applications that reverse frequently — doors, gates, dispensers, indexing mechanisms — where the actual running time is short.

Put that motor on a conveyor that runs eight hours without stopping and you have a motor designed for 30 minutes of thermal load, asked for 480 minutes of it. It will reach its temperature limit in the first half hour and keep climbing. Nothing is broken. The rating was simply not read.

How to tell: the motor runs hot within 30–40 minutes of a cold start and then holds at an uncomfortably high temperature, current is at or below nameplate, ambient is normal, the fan is clean, and the load feels normal. That combination is a duty problem, not an overload.

Fix: for continuous duty, specify the continuous-rated version of the same frame. The reversible vs continuous duty guide explains exactly what the 30-minute rating limits physically, including why the case temperature rule exists and why reversing needs a dwell time.

Cause 2: actual load above rated torque — including the ratio mistake that hides it

Genuine overload looks different from a duty problem: current is above nameplate, the motor heats progressively, and output speed sags under load.

The trap is that overload is often introduced by a well-intentioned change in ratio. Someone increases the reduction to slow the machine down, expecting lower speed at higher torque, and the machine now stalls or trips under the same product load. Two things combine:

• Lower output speed at the same motor power means higher output torque — which is correct — but the allowable load table may already be at its ceiling, so the gearbox is now beyond its rating.
• Higher reduction means more gear stages, which means lower efficiency and more heat inside the gearbox.

The allowable load figure is a ceiling, and on most of these frames it plateaus. On the 25 W 80 mm frame it reaches 7.84 N·m (80 kg·cm) at high ratios and stays there. On the 90 mm frame it plateaus at 19.6 N·m (200 kg·cm). Adding ratio past the plateau gives you a slower shaft and a hotter gearbox, not more usable torque.

Fix: measure current at the actual production load and compare with the nameplate. If it is above the rating, the motor is undersized for the duty — go up a frame. The torque calculation guide shows the conversion from load to required output torque, including the starting factor.

Cause 3: ambient and altitude derating

Every rating on the datasheet assumes 40 °C ambient and installation below 1,000 m altitude. Both assumptions are frequently wrong in practice.

Ambient. The motor is cooled by air at ambient temperature. If the air around it is hotter than 40 °C, the motor cannot reject the same amount of heat at the same load. Three common ways this happens without anyone noticing:

• The motor is inside a sealed cabinet. The cabinet temperature — not the room temperature — is the ambient. A cabinet in direct sun in a tropical plant can run 15 °C above room temperature.
• The motor is mounted downstream of a hot machine section: above an oven, next to a heat tunnel, near an extruder barrel.
• The motor is in still air with no cross-flow. Air temperature may be fine; the local air is being reheated by the motor itself.

Above 40 °C, the continuous capability reduces. The size of the reduction is published as an ambient derating curve — ask the supplier for it rather than extrapolating from a rule of thumb, because the curve depends on the frame and the insulation class.

Altitude. Above roughly 1,000 m, lower air density reduces cooling. The effect is small at 1,200 m and significant at 3,000 m. If the machine ships to a highland site, state the altitude at enquiry stage.

Fix: measure the actual air temperature at the motor, not in the room. If it exceeds 40 °C, either reduce the load, improve the airflow, or specify a larger frame with thermal headroom.

Cause 4: the cooling path is blocked or absent

A totally enclosed fan-cooled motor relies on a shaft-mounted fan blowing air along fins under a cowl. Four things block that path, and all four are common:

1. Dust and lint packed into the fan cowl. Textile, woodworking and cardboard applications clog the cowl within months. This is the single most common field failure on textile machinery.
2. A broken or missing fan blade. Easy to miss if the cowl is in place. Spin it by hand with the power off.
3. Painting or coating over the fins. A repainted motor with the fins filled is a motor with a smaller heat sink.
4. Mounting in an enclosed space with no airflow. The motor is not blocked; it is simply sitting in its own exhaust air.

Fix: clean the cowl and fins on a schedule matched to the dust load, not to a generic interval. In a textile plant that can mean monthly. The parts identification guide shows the cowl and fan arrangement so a maintenance technician knows what they are looking at.

Cause 5: the electrical supply

Five supply conditions produce excess heat with no mechanical fault at all.

The capacitor point deserves emphasis because it is the one that gets missed. A single-phase motor’s run capacitor is matched to its winding and voltage: a 220 V unit and a 110 V unit take very different capacitance values. A wrong-value capacitor, or one that has drifted with age, causes exactly the symptoms people describe as “the motor is overheating” — hot winding, poor starting, normal mechanical load. Check it before condemning the motor.

The frequency point connects back to the sizing question. A motor specified from a 60 Hz datasheet and installed on a 50 Hz supply runs slower and cooler at the fan, while the machine still asks for the same torque. That combination is a slow thermal overload. The single-phase vs three-phase and 50/60 Hz guide covers the selection implications, and the three-phase wiring guide covers connection and protection.

Cause 6: mechanical drag

If the electrical side checks out and the motor still overheats, the load is heavier than it should be. Five sources:

• Over-tight belt or chain. Adds radial load to the output bearing and increases friction continuously. A belt should have a small amount of deflection, not be drum-tight.
• Misalignment between the output shaft and the driven shaft. Produces vibration, bearing heat, and eventually a worn coupling. A rigid coupling on a misaligned pair is a guaranteed failure.
• Bearing damage or failed lubrication. Listen for rumble or squeal, and feel for roughness with the drive disconnected.
• Gearbox lubricant wrong or overfilled. Grease-filled compact gearboxes are sealed for life; an overfilled or wrong-grade refill causes churning losses and heat.
• Brake drag, where a brake is fitted. A brake that does not fully release adds continuous load that shows up as heat and high current with nothing obviously wrong downstream.

Fix: uncouple the motor from the driven machine and run it free. If it runs cool unloaded and heats up when coupled, the fault is downstream. That single test decides whether you are buying a motor or fixing a machine.

One more case that belongs here: a motor run from a variable frequency drive at low speed for long periods. The shaft-mounted fan moves much less air at low rpm, so cooling falls while the torque demand does not. If the application needs sustained low-speed operation at high torque, that is a cooling design problem, and a standard TEFC motor is the wrong starting point. The speed control guide covers the torque loss and thermal implications.

A ten-minute diagnostic sequence

1. Note the ambient temperature at the motor — not the room, the air at the motor. Write it down.
2. Locate the heat with your hand: motor barrel, gearbox housing, or bearing end. This decides which half of this article applies.
3. Measure current on every phase under normal production load. Compare with the nameplate.
4. Measure voltage at the motor terminals under load. Compare with the nameplate band.
5. Confirm the duty rating — is this a 30-minute reversible motor in continuous service?
6. Inspect the cooling path — cowl clear, fan intact, fins exposed, air able to escape.
7. Check the capacitor on single-phase units against the specified value.
8. Uncouple and run free. Cool when free, hot when coupled means mechanical drag.
9. Compare with a like-for-like machine. If one of five identical units runs hot, the motor is fine and the other four are the baseline.

Steps 3, 4 and 8 catch the overwhelming majority of real faults. Everything before them is preparation, and everything after is confirmation.

Fix table: symptom to action

When to replace instead of repair

Replace rather than investigate further when the winding has already discoloured or smells — insulation damage is not reversible. Replace when the bearing housing is worn or the shaft is scored, because at this frame size the repair cost approaches the replacement cost. Replace when current is correct, ambient is correct, the fan is clean and the motor still overheats: that points to an internal winding fault.

Do not bypass a thermal protector to stop nuisance tripping. Automatic-reset protectors mask the problem by cycling. A motor that trips regularly has a fault that needs finding, and the protector is reporting it correctly.

Frequently asked questions

How hot should an AC gear motor get in normal operation?

At 40 °C ambient and rated load, a typical small gearmotor with class B insulation runs a winding temperature rise of up to 80 K, which puts the winding near 120 °C. The housing will feel very hot to the touch at that point and that is normal. What is not normal is a motor that keeps getting hotter after the first hour, or one that is noticeably hotter than identical units on the same line.

Why does my gear motor overheat after 30 minutes?

Because it is almost certainly a reversible motor with a 30-minute rated duty being run continuously. The thermal limit is reached in the first half hour and then exceeded. Check the model code — an RK designation indicates the reversible 30-minute rating, while a continuous-duty version is a different specification.

Can I just run it slower to stop it overheating?

Sometimes, but not reliably. A shaft-mounted fan cools less at lower speed, so running slower reduces the cooling as well as the load. If the overheating is caused by overload, reducing speed without reducing load torque does not help. Establish the cause first.

Does a hot gearbox mean the motor is overheating?

No, and this is the most common misdiagnosis. Gearbox heat comes from gear friction, lubricant condition and bearing preload. A gearbox can be hot while the motor winding is well within its limit. Check where the heat actually is before replacing anything.

What causes a single-phase gear motor to overheat with no load problem?

The run capacitor is the first suspect. A wrong-value or degraded capacitor produces poor starting torque and elevated running current with a perfectly normal mechanical load. Compare the fitted value and voltage rating with the specification before looking further.

Will cleaning the fan cowl really fix it?

In dusty environments, frequently yes. On textile and woodworking machinery a blocked cowl is the single most common cause of overheating, and cleaning it restores the original thermal margin at no parts cost. The lesson is the maintenance interval, not the cleaning.

If you have a motor that runs hot and the electrical and cooling checks are clean, send the nameplate details, the measured current per phase, the ambient temperature and the duty cycle. We will tell you whether it is a rating problem, a sizing problem, or a machine problem — and if the honest answer is that the frame is wrong for the duty, we will say so.