Electric motor overheating: causes and what to do

Overheating of the electric motor is the most common cause of winding failure: an increase in temperature for every 10 °C above normal reduces the insulation life by half. In this article, we will look at the six main causes of overheating, temperature standards by insulation class, how to make a preliminary diagnosis yourself, and what to do right now if the engine housing is so hot that you can’t hold your hand on it. The material is based on the practice of the Elektromotors service center in Tashkent - in our hot climate, overheating is especially common.

Normal motor temperature by insulation class

Before considering an engine overheated, it is important to understand what temperature is permissible for it. The norm depends on the heat resistance class of the insulation indicated on the nameplate.

Most modern industrial motors are manufactured with class F insulation. This means that a housing temperature of 90–100 °C is the operating mode and is not a reason to sound the alarm. But if the body heats up above 110 °C or you cannot hold your hand for more than 1-2 seconds, the engine is overloaded or has an internal malfunction.

6 main reasons for electric motor overheating

In 90% of cases, the cause of overheating falls into one of six typical scenarios. Let's look at each one.

1. Torque overload

The most common reason. The engine operates with a mechanism whose resistance moment exceeds the nominal one: the gearbox is clogged, the hopper is full, the pump bearing is jammed. The current exceeds the nominal value by 20–40%, the winding heats up according to the I²R law. Tested with current clamps on each phase under load.

2. Supply voltage unbalance and phase loss

If one of the phases is missing (wire break, starter contact burnt out), the motor continues to operate in single-phase mode, but consumes current 1.7–2 times the rated value. After 2–10 minutes the winding burns out. Voltage asymmetry of more than 5% also causes local overheating of one phase - a common problem in industrial networks in Tashkent, especially in summer at peak load.

3. Cooling system clogged

Self-cooled motors (IC411) are cooled by an impeller on the shaft through the ribs of the housing. If the fins are clogged with dust, cotton wool or cement suspension, heat transfer drops by 30–50%. This is a common cause of overheating in foundries and textile shops. The solution is to clean the housing with compressed air every 1–3 months.

4. High ambient temperature

Standard motors are designed for operation at +40 °C. In Tashkent in summer, in workshops without ventilation, the air temperature reaches +45…+50 °C. For every 10 °C increase in ambient temperature above 40 °C, the permissible engine power is reduced by 10–15%. If this factor is not taken into account when selecting, the machine will work at its limit.

5. Frequent starts and reverses

The starting current is 5–7 times higher than the rated current. An engine designed for 6–10 starts per hour, when operating in S4 mode (intermittent with starts), can make 60–100 starts and simply does not have time to cool down. Crane and elevator engines are especially affected - separate maintenance mode is provided for them.

6. Internal faults - interturn and bearings

An interturn short circuit creates a short-circuited circuit in which a current circulates tens of times higher than the working one. The engine heats up unevenly: one area of ​​the housing is much hotter than the rest. A damaged bearing has a similar effect - friction heats the bearing shield to 120–150 °C, and the heat goes to the winding. These faults are diagnosed at the service - full diagnostics and often replacement of bearings are needed.

How to make a preliminary diagnosis yourself

1. Measure the housing temperature with a pyrometer at three points: above the stator winding, on the front and rear bearing shield. Write down the values.
2. Measure the current in all three phaseswith a current clamp under operating load. Compare with the rating on the nameplate and between phases.
3. Check the voltageat the motor terminals - there should be three equal phases 380 V ±5%.
4. Listen: hum, grinding, tapping are signs of bearings; “Buzzing” at a low frequency is a sign of asymmetry.
5. Inspect the vent shroudfor dirt and make sure the impeller rotates.

Based on these five points, in 80% of cases you can understand which of the six reasons is to blame, and make a decision - fix it on the spot or take the engine for service.

Urgent action in case of overheating

If you notice that the engine is heating up above normal right now, follow these steps:

• Do not turn off the engine abruptly if it is overheated. After stopping, the blowing stops, and the winding, by inertia, heats up by another 10–20 °C - thermal shock of the insulation is possible. If possible, allow 2-3 minutes of idling to cool down.
• Reduce the load - reduce the supply of material, stop the conveyor, unload the mechanism.
• Check the current with clamps and compare by phase. Asymmetry more than 10% - stop, look for a break in the power supply.
• Do not spray with water - this is not a fire, but a thermal regime. Sudden cooling will cause cracks in the winding enamel.
• Do not start until the cause is determinedif the engine has already shut down due to thermal protection. Restarting a burnt-out winding turns a minor repair into a complete rewind.

When the engine is already damaged

Signs that overheating has already led to irreversible changes:

• persistent smell of burnt varnish from the terminal box;
• blackening or swelling of the varnish on the frontal parts of the winding;
• insulation resistance has dropped below 0.5 MOhm;
• when starting, an automatic device or RCD is triggered;
• the engine hums and does not accelerate.

In this case, you need rewinding the stator or complete rewinding the winding. The cost of the work depends on the power and insulation class - the original class F cannot be changed to A in order to save money, the service life will drop threefold.

Prevention - how to avoid overheating

1. Once a month, clean the cooling fins with compressed air, especially in dusty workshops.
2. Once a quarter, check the current in three phases and compare it with the nominal value.
3. Once a year, measure the insulation resistance with a megohmmeter and monitor the dynamics.
4. In workshops with temperatures above +40 °C, provide a power reserve of 15–20% or install motors with class H insulation.
5. On critical drives, install thermal protection in the winding (PTC thermistors or PT100), and not just a thermal relay in the cabinet.
6. Once every 2 years, perform balancing the rotor and checking the bearings - vibration indirectly causes overheating due to uneven clearance.

Elektromotors engineer advice: install simple thermal indicator stickers (thermal stickers for 80, 100, 120 °C) on key engines. This is a cheap measure, but it allows the operator to immediately see the onset of overheating without instruments.

Total

Overheating is not an independent disease, but a symptom. There is always a specific reason behind it: overload, asymmetry, dirt, climate, frequent starts or internal malfunction. You can make a diagnosis in 15 minutes with a pyrometer and current clamps. But repairing a burnt winding will cost tens of times more than timely diagnostics.

Is the engine overheating above normal or has it already burned out? Elektromotors engineers in Tashkent will conduct on-site diagnostics, find the cause of overheating and, if necessary, carry out repairs - from replacing bearings to complete rewinding. Contact us or order urgent repair. A full range of services for electrical equipment maintenance is also available.