Troubleshooting guide

Why Is an Electric Motor Overheating?

The common causes of motor overheating (overload, voltage problems, poor cooling, duty, drives and bearings), what to check for each, and a worked loss calculation.

Electrical & Power · Last reviewed October 15, 2026

Motor heat comes from the motor's own losses. When the heat made is more than the heat the motor can shed, the temperature climbs. So the question is always one of two things: is the motor making more heat than it should, or is it losing less heat than it should?

Touch is not a measurement. A motor that is too hot to hold can still be inside its rating, and one that feels warm can be over it. Compare measured temperature with the nameplate insulation class and ambient rating. Measurements on energized equipment belong with qualified personnel following lockout/tagout and site procedures.

Causes that make more heat

Cause	What points to it	What to check
Overload	Current above nameplate full-load amps; driven equipment loaded harder than before	Measured current against nameplate; process or load changes; jammed or dragging equipment
Low or high voltage	Voltage outside about ±10% of nameplate; current rising as voltage falls	Voltage at the motor terminals under load, not at the panel
Voltage unbalance	Unequal phase currents, often much larger than the voltage unbalance causes them to be	Phase-to-phase voltages and the three line currents. Motors are normally derated once unbalance exceeds about 1%
Single-phasing	One phase lost: a blown fuse or open contact. Hum, high current on the two remaining phases	Fuses, contactors, connections. The motor may keep running if lightly loaded
Too many starts	Overheating soon after repeated starts, with normal running current	Starts per hour against the manufacturer limit. Each start draws several times full-load current
Bearing or alignment problem	Heat concentrated at a bearing end, noise, vibration	Lubrication, alignment, belt tension, vibration readings

Causes that remove less heat

Cause	What points to it	What to check
Blocked cooling	Dust, lint or debris on the frame, fins or fan guard; a broken fan	Clean surfaces and an unobstructed fan intake
High ambient or altitude	Hot room, enclosed space, or a site above the rated altitude	Ambient temperature against the nameplate (often 40 °C) and altitude (often up to 1,000 m)
Slow speed on a VFD	Self-cooled motor running slowly for long periods; its fan moves less air	Speed range and load. A constant-torque load at low speed may need an inverter-duty motor or a separate cooling fan
Drive effects	Motor on a VFD runs hotter than on the line	Carrier frequency, cable length, motor insulation rating, output filters

Why it matters

Insulation life falls quickly with temperature. A common rule of thumb is that each 10 °C above the insulation's temperature rating roughly halves its life. Treat it as an illustration, not a prediction for a specific motor.

Worked example: where does the heat come from?

A motor delivers 15 kW of mechanical output at 90% efficiency.

Input power = output ÷ efficiency = 15 ÷ 0.90 = 16.67 kW.
Losses = input − output = 16.67 − 15 = 1.67 kW.
That 1.67 kW is heat the motor must shed. If an overload takes the output to 18 kW at the same efficiency, losses rise to 2.0 kW, about 20% more heat.

This is a simplified balance. Efficiency also changes with load, so it will not predict the winding temperature or show that a motor is overheating.

Write a useful problem statement

Motor identification: nameplate hp, voltage, full-load amps, service factor, insulation class, ambient rating.
Symptom: what was observed, where, and when it started.
Measurements taken: voltage, current per phase, temperature, how and by whom.
Recent changes: load, process, drive settings, maintenance, environment.
Keep observations separate from conclusions. "Temperature rose after the process changed" is more useful than "the motor is defective".

A second example: voltage unbalance

A technician measures phase-to-phase voltages of 460 V, 455 V and 450 V at the motor terminals.

Average voltage = (460 + 455 + 450) ÷ 3 = 455 V.
Largest deviation from the average = 5 V.
Voltage unbalance = 5 ÷ 455 × 100 = 1.1%.

Motor standards (NEMA MG 1) call for derating once unbalance goes above about 1% and advise against running a motor above 5%. A small voltage unbalance causes a much larger current unbalance, often several times larger, so a hot winding on one phase can come from a voltage difference that looks minor on the meter.

Preventive checks

Keep the frame, fins and fan cover clean, and keep the air intake clear.
Trend running current, temperature and vibration so a slow change shows up before a failure.
Take a thermographic survey under load from time to time, and compare the three phases.
Follow the lubrication interval on the nameplate or in the manual.
After any drive or process change, check the settings against the motor nameplate again.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

What temperature is too hot?

It depends on the insulation class, the ambient rating and the service factor on the nameplate, and on how the temperature is measured. Use the manufacturer data, not a single number.

Can a motor overheat while drawing normal current?

Yes. Blocked cooling, high ambient, low-speed operation on a drive or too many starts can overheat a motor with normal running current.

Is a hot bearing the same as a hot winding?

No. Heat at one bearing end points to lubrication, alignment or load problems. Heat across the whole frame points to electrical or cooling causes.

How do I measure voltage unbalance?

Measure the three phase-to-phase voltages at the motor terminals with the motor running. Divide the largest deviation from the average by the average, and multiply by 100.

What are the common insulation classes?

Class B (130 °C), class F (155 °C) and class H (180 °C) are the common maximum operating temperatures for motor winding insulation. The nameplate gives the class and the allowed temperature rise.

Is the service factor a reason to run a motor overloaded?

No. The service factor is a reserve for occasional overload. Running continuously in the service-factor range shortens insulation life.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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