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Why Is My Motor Overheating? 8 Causes & Fixes

Release Date ：2026-10-15

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An electric motor that runs too hot is one of the most common - and most misread - warning signs in a plant. In most cases overheating comes down to three areas: electrical, load and cooling. This guide lists the eight most frequent causes with a fault table and a temperature-check method, so you know what to inspect first.

Infrared thermometer checking electric motor surface temperature

1. How Hot Is "Too Hot"? Use the Insulation Class, Not Your Hand

Permissible temperature rise depends on the insulation class: 80 K for Class B windings, 105 K for Class F and 125 K for Class H (at a 40 C ambient reference). Never judge overheating by "it feels hot" - measure the temperature rise against the insulation class. As a rule of thumb, a frame temperature consistently above 90 C deserves investigation. See our guide to motor insulation classes B/F/H for details.

2. Eight Common Overheating Causes (Roughly by Frequency)

Cause	How to identify	Fix
Overload (undersized motor / heavier load)	Running current above nameplate, sustained	Measure load factor; fit a larger motor if needed
Low voltage or voltage unbalance	Unbalance above 2%; one phase current higher	Check supply and cable drop; balance the phases
Poor ventilation	Blocked cowl, failed fan, dust build-up, high ambient	Clean air paths, replace fan, improve location airflow
Bearing trouble	Bearing temperature rises first, with noise or vibration	Check lubrication, replace bearing; see our bearing grease selection guide
Frequent starting / reversing	High start current, repeated starts	Limit starts per hour; use soft starter or VFD
Moisture / low insulation resistance	Insulation resistance below ~1 MOhm	Dry the winding; always check IR after long idle
Single-phasing	Abnormal heat, hum, uneven current	Check fuses / contactor / wiring; stop promptly
Harmonics / wrong VFD settings	High harmonic content, poor carrier settings	Add output reactor; set parameters per VFD manual

3. A Three-Step Temperature Check

Step 1: Measure frame temperature with an infrared thermometer - focus on the cooling fins, bearing housings and terminal box;
Step 2: Clamp-meter the three-phase currents against the nameplate; more than 10% above rating signals overload;
Step 3: With the motor stopped, measure winding insulation resistance; below the acceptable level means drying is needed.

4. Two Causes People Often Overlook

Voltage unbalance: at 3% unbalance, motor losses can more than double. Check the supply before blaming the motor;
Installation environment: a self-ventilated motor mounted against a wall, partially boxed in, or used above 1,000 m without derating will overheat even under normal load.

5. When Should You Replace the Motor?

Occasional overload can be fixed with better cooling or load adjustment. But if prolonged overload has discoloured the winding or dropped the insulation resistance, rewind or replacement is the safer option. When re-selecting, give the manufacturer the real load, starting frequency and ambient temperature. At SHANGDIAN (Shanghai Electric Motor Factory) we supply low-voltage three-phase induction motors and energy-saving retrofit motors sized to the actual duty, so the "bigger is safer" guesswork is removed.

Send us your current, voltage, temperature and insulation readings and our engineers will help you find the root cause of the overheating.