Diagnostic Guide for Overheating of Electric Motors: Structured Problem Analysis and Solutions

1. Problem description & Scope

Overheating of an electric motor is a critical fault that, if ignored, can lead to permanent damage to the motor, unplanned downtime and significant repair costs. This diagnostic guide has been prepared to support maintenance technicians, reliability engineers and production managers at UNITEC-D GmbH and our Benelux customers in systematically identifying the root cause of overheating in various types of electric motors, including asynchronous AC motors, DC motors and servo motors, in accordance with the relevant NEN-EN 60034-1 standards.

Symptoms of overheating include:

• Abnormally high surface temperatures (measured with thermal camera or contact thermometer).
• Smell of burnt insulation.
• Smoke development.
• Frequent switching off of the motor due to thermal protection.
• Increased power consumption without corresponding load increase.
• Increased vibrations or abnormal sounds.

Severity Rating:

• Critical: Smoke development, flames, motor stops immediately, or temperature exceeds the maximum allowable insulation class (e.g. >155°C for class F, >180°C for class H). Requires immediate stoppage and inspection. Risk of fire and serious consequences for safety and production.
• Major: Repeated shutdown due to thermal protection, significant temperature increase (+15°C above normal operation), reduced performance. Requires planned downtime and corrective action to prevent life reduction and failure.
• Minor: Occasional temperature increases (+5-10°C above normal operation), slight odor, increased power consumption. Requires monitoring and analysis to prevent escalation.

2. Safety measures

WARNING: Work on electrical machines and installations involves significant risks. ALWAYS follow applicable safety procedures and standards, such as NEN 3140 in the Netherlands and EN 50110-1 internationally. Ignoring these procedures could result in serious injury or death.

• Lockout/Tagout - LOTO: Ensure that the electrical supply to the motor is completely disconnected, de-energized, locked and marked before carrying out any inspection or maintenance work. Check the absence of voltage with a suitable voltage detector.
• Personal Protective Equipment (PPE): Always wear suitable PPE, including insulating gloves (when working under voltage or when there is a risk of arc flash), safety glasses, safety shoes and, if necessary, arc-resistant clothing (according to EN 61482).
• Stored Energy: Be aware of stored energy. Capacitors in frequency converters can maintain lethal voltages even after the power supply has been turned off. Wait for the prescribed discharge time. Rotating masses can move unexpectedly. Secure this if necessary.
• Hot Surfaces: Motors that overheat have very hot surfaces. Use heat-resistant gloves for handling and be careful when working in close proximity.
• Hazardous Substances: Old insulation materials may contain asbestos or other hazardous substances. Consult safety data sheets and wear suitable respiratory protection and gloves if this poses a risk.

3. Required Diagnostic Tools

An effective diagnosis of engine overheating requires specific measuring instruments. The following table provides an overview of recommended tools:

4. Initial Assessment Checklist

Before beginning specific diagnostic tests, a thorough initial assessment is essential. Record the points below accurately.

5. Systematic Diagnosis Flowchart

Follow this structured approach to isolate the root cause of the overheating:

1. Symptom: Motor overheated (thermal camera > 10-15°C above normal).
   1. Visual inspection and initial checks (see section 4):
      1. Is the fan intact and turning in the correct direction?
      2. Are the cooling fins and vents free of dirt, dust or obstructions?
      3. Check the ambient temperature. Is this within specifications?
      4. Is there an unusual smell or smoke?
      5. Listen for abnormal sounds (bearings, vibrations).
   2. IF cooling or ambient temperature is problematic THEN:
      1. Probable Cause: Insufficient Cooling.
      2. Go to step 6b (Insufficient Cooling).
   3. ELSE IF no obvious cooling problems THEN:
      1. Go to electrical measurements.
2. Electrical Measurements (with current clamp and multimeter, during operation):
   1. Measure the voltage (V) between the phases (L1-L2, L2-L3, L3-L1) and between each phase and earth.
   2. Measure the current (A) in each phase (L1, L2, L3).
   3. Calculate the phase imbalance for voltage and current.
   4. Measure the resistance of the motor windings (after LOTO and cooling).
3. IF current in one or more phases significantly higher than nominal (>10% for a longer period of time) THEN:
   1. Probable Cause: Overload (Mechanical or Electrical).
      1. Check the load of the driven machine. Does it run heavy?
      2. Check the frequency converter (if present): parameters, harmonic pollution.
      3. Measure the input voltage and current to the frequency converter and the output voltage and current to the motor.
      4. Go to step 6a (Overload).
4. ELSE IF current unbalance >10% or voltage unbalance >2% THEN:
   1. Probable Cause: Electrical Fault (Phase Imbalance, Over/Undervoltage).
      1. Check the power supply (transformer, mains).
      2. Check wiring and connectors for loose contacts or corrosion.
      3. Go to step 6c (Electrical Faults).
5. ELSE IF no obvious electrical abnormalities during operation and cooling is good THEN:
   1. Probable Cause: Internal Motor Fault (Insulation, Bearings, Rotor).
      1. Perform LOTO and cool motor.
      2. Measure insulation resistance (megohmmeter) between windings and earth, and between windings themselves.
      3. Take vibration measurements (if possible) on bearings.
      4. Inspect the bearings for wear, corrosion or dry running.
      5. Check the rotor for damage (broken bars, loose rings – visual inspection, possibly current analysis of start-up characteristic).
      6. Go to step 6d (Bearing failures) or 6e (Insulation degradation).

6. Error Cause Matrix

The following table ranks likely causes and diagnostic tests for common overheating symptoms.

7. Root Cause Analysis for Each Error

7a. Overload

Why it happens: Overload occurs when the motor has to produce more mechanical work than it is designed for. This can be due to too heavy a process load (e.g. a clogged pump, a V-belt that is too tight, a product mix that is too heavy), mechanical blockages in the driven machine, or incorrect settings of a frequency converter (VFD) causing the motor to operate outside its nominal operating point. A motor then draws more current than its nominal current (In), which results in higher copper losses (I²R losses) in the windings, and therefore in excessive heat development.

How to confirm: Measure the current per phase with a current clamp. If the measured current is consistently more than 5-10% above the rated current (In) on the nameplate, this indicates an overload. Also check the actual load of the driven machine. A thermal camera will show a uniform temperature increase across the entire motor housing in proportion to the current increase.

Damage if left unresolved: Long-term overload causes accelerated aging and degradation of the winding insulation. Every 10°C temperature increase above the maximum insulation class can halve the lifespan of the insulation (Arrhenius law). This ultimately leads to insulation defects, phase-to-earth or phase-to-phase short circuits and motor failure.

7b. Insufficient Cooling

Why it happens: Electric motors generate heat internally and are designed to effectively dissipate this heat to the environment. Inadequate cooling occurs when this heat dissipation mechanism is hampered. Common causes include blockages of the fan housing, cooling fins or air ducts with dust, dirt, fibers or corrosion. A defective or incorrectly mounted fan, too high an ambient temperature, insufficient free space around the motor for air circulation, or incorrect rotation of the fan can also contribute to insufficient cooling.

How to fix: Visually inspect for blockages. Check the fan for breakage or wear. Use a thermal camera to visualize the temperature distribution over the cooling fins; areas with poor cooling will be warmer. Check the airflow from the fan with an anemometer or hand feeling. Measure the ambient temperature. Compare the measured airflow with the engine specifications.

Damage if unresolved: As with overloading, insufficient cooling leads to an overall temperature increase of the motor, resulting in accelerated insulation degradation and ultimately electrical failure. Bearing grease can also oxidize and degrade more quickly due to the increased temperature.

7c. Electrical Disturbances (Voltage or Current Imbalance, Under/Over Voltage)

Why it happens: Electrical problems with the power supply can lead to overheating. A phase unbalance (difference in voltage or current between phases) causes a negative-sequence component in the rotor current, resulting in increased eddy currents and additional heat in the rotor, even at rated load. A small voltage imbalance can lead to a much larger current imbalance. Undervoltage forces the motor to draw more current to deliver the same power, leading to higher I²R losses. Overvoltage can lead to saturation of the magnetic core and increased iron losses. Harmonic pollution, especially the 5th and 7th harmonics, can also cause additional losses and heat.

How to confirm: Measure the voltage and current in each phase with a multimeter and current clamp. Calculate the voltage and current unbalance. A voltage imbalance of more than 2% or a current imbalance of more than 10% is of concern. Check the power quality with a power analyzer to detect harmonic pollution.

Damage if left unresolved: Imbalance and abnormal voltages lead to hot spots in the windings and accelerated degradation of the insulation. This significantly reduces efficiency and lifespan. Overvoltage can also affect the dielectric strength of the insulation.

7d. Bearing errors

Why it happens: Bearings support the rotor and ensure smooth rotation. Errors such as insufficient or incorrect lubrication, excessive load, incorrect installation, contamination (dirt, moisture) or normal wear and tear cause increasing friction. This friction generates heat that can build up locally in the bearing housing and spread to the motor shaft and windings.

How to fix: Use a thermal camera to check the temperature of the bearing housings; >80°C or >20°C above the motor housing is critical. Listen for abnormal sounds (rattling, grinding) with a stethoscope or screwdriver. Perform vibration measurements in accordance with ISO 10816-3; values ​​> 4.5 mm/s RMS (at 10-1000 Hz) indicate an alarm status. Disassemble after LOTO and inspect the bearings for discoloration, pitting, roughness or lubrication problems.

Damage if left unresolved: Bearing failure can lead to overheating, excessive vibration, shaft damage, misalignment, and ultimately rotor-stator contact (rubbing), resulting in catastrophic engine failure and significant collateral damage.

7th. Insulation Degradation (Internal Motor Fault)

Why it happens: The insulation of the motor windings separates the conductors from each other and from ground. This insulation degrades due to aging, overheating, moisture, chemical attack, mechanical stress or partial discharges. When the insulation breaks down, the leakage current increases, generating additional heat. Ultimately, this can lead to a complete short circuit between phases or between a phase and the motor housing.

How to confirm: After LOTO and cooling of the motor, perform an insulation resistance measurement with a megohmmeter (see section 3). An insulation resistance of less than 1 MΩ per kV rated voltage (with a minimum of 1 MΩ) is an indication of poor insulation. Low values ​​that decrease further at higher test voltages indicate severe degradation. Partial discharge measurements can also detect incipient insulation defects.

Damage if left unresolved: Progressive insulation degradation inevitably leads to an electrical short circuit. This causes a very high current that can damage the motor and possibly the power supply, resulting in a fire hazard and total motor failure.

8. Step-by-Step Troubleshooting Procedures

The following procedures describe corrective actions for the identified root causes. Always perform LOTO before starting work.

8a. Solution for Overload

1. Reduce the load: Evaluate the driven machine. Is there a possibility to reduce the process load? Adjust process parameters, optimize material supply or remove blockages.
2. Check mechanical transmission: Inspect clutch, sprockets, belts, or chains. Ensure proper alignment (max. 0.05 mm parallel and 0.05 mm angular error for flexible couplings), tension (according to manufacturer) and lubrication.
3. Evaluate variable speed drive (VFD) settings:
   • Check the motor parameters (rated speed, current, voltage, power) in the VFD. Make sure these match the engine.
   • Check the V/Hz ratio.
   • Inspect for harmonic contamination; install harmonic filters if necessary.
4. Recheck: After adjustments, monitor motor current and temperature for a full operating cycle. The temperature must return to normal operating values ​​(e.g. ≤ 90°C surface temperature for class F motor).

8b. Solution for Insufficient Cooling

1. Cleaning: Turn off the engine (LOTO!) and thoroughly clean the fan housing, cooling fins and all ventilation openings. Use compressed air (max. 2 bar) or an industrial vacuum cleaner.
2. Fan inspection: Check the fan for damage, balance or improper mounting. Replace if necessary. Make sure the fan is oriented correctly to maximize airflow.
3. Space and surroundings: Ensure there is sufficient free space (at least 10-20 cm around) for air circulation. Consider ventilation improvement in the engine environment if the ambient temperature is consistently too high.
4. Cooling system (if equipped): Check liquid cooling systems for pressure, flow and coolant quality.
5. Recheck: Monitor engine temperature after cleaning and adjustments. The temperature must be within specifications.

8c. Solution for Electrical Faults

1. Voltage and current imbalance:
   • Inspect wiring, terminals, and connectors for loose connections, corrosion, or damage. Tighten all connections with the correct torque (consult NEN 1010 and manufacturer specifications).
   • Check the power supply (distribution board, transformer) for unbalance. This could be a problem that extends beyond the engine itself.
   • Balance the load on the three-phase network as much as possible.
2. Under/overvoltage:
   • Check the voltage regulation of the power supply.
   • Consider a voltage stabilizer if the mains voltage consistently falls outside the tolerances (NEN-EN 60034-1: +/- 5%).
3. Recheck: After corrections, re-measure the voltages and currents per phase to confirm that the unbalance and voltages are within acceptable limits.

8d. Fix for Bearing Errors

1. LOTO and engine disassembly:
   • WARNING: Ensure engine is properly supported and mechanically safe before disassembly.
   • Remove the engine from the driven machine.
   • Disassemble the motor to access the bearings.
2. Bearing replacement:
   • Always replace both bearings (front and rear) as a set. Only use bearings that meet OEM specifications or better industry standards (e.g. SKF Explorer, FAG F'ag, Timken).
   • Ensure a clean working environment to prevent contamination.
   • Use the correct tools for disassembly and assembly (bearing pullers, induction heaters for assembly) to prevent damage to the shaft and bearing.
   • Check the tolerances of the shaft and bearing seats.
3. Lubrication: Lubricate the new bearings with the correct type of grease (consistency, base oil, additives) and the correct amount according to the bearing manufacturer's or motor manufacturer's specifications.
4. Post-assembly verification: After assembly, perform a test run without load and check the temperature of the bearing housings with a thermal camera and listen for abnormal noises. Take vibration measurements.

8th. Solution for Insulation Degradation

1. Dismantling LOTO and motor:
   • WARNING: Do not touch windings with bare hands; skin oil can damage insulation.
   • Remove the engine for thorough inspection.
2. Inspection and Cleaning: Visually inspect the windings for discoloration, cracking, delamination, or traces of moisture or chemicals. Carefully clean the windings with a suitable insulation cleaning agent (e.g. dielectric spray) and compressed air.
3. Drying (if moisture is the cause): If moisture is the main cause, dry the motor in a controlled manner in an oven or using electric heating (max. 105°C) until the insulation resistance is stable and acceptable (several measurements over time).
4. Rewicking (in case of serious degradation): In the case of serious or irreparable insulation degradation, rewicking is the only solution. This must be carried out by a specialized company that follows the NEN-EN 60034- standards for electrical machines.
5. Verification: After repair (drying, rewinding), perform another insulation resistance measurement to confirm that the values ​​are within acceptable limits.

9. Preventive Measures

Prevention is critical to maximize the lifespan of electric motors and minimize unplanned downtime.

10. Spare Parts & Components

Having the right spare parts available on time is essential for quick and efficient repairs. UNITEC-D GmbH offers an extensive range of components that meet the highest quality standards.

For a complete overview of available spare parts and detailed specifications, visit our UNITEC-D e-catalog: www.unitecd.com/e-catalog/

11. References

• NEN-EN 60034-1: Rotating electrical machines – Part 1: Nominal data and properties.
• NEN-EN 60034-27: Rotating electrical machines – Part 27: Partial discharge measurement on stator windings.
• NEN 3140: Operation of electrical installations - Low voltage.
• EN 50110-1: Operation of electrical installations – Part 1: General requirements.
• ISO 10816-3: Mechanical vibration – Evaluation of machine vibration by measurements on non-rotating parts – Part 3: Industrial machines with nominal power above 15 kW and nominal speeds between 120 r/min and 15 000 r/min when measured in situ.
• IEEE 43: Recommended Practice for Testing Insulation Resistance of Rotating Machinery.
• OEM Maintenance and Installation Manuals for specific motor types (e.g. Siemens, ABB, SEW Eurodrive).
• UNITEC-D Internal Knowledge Base: Related Maintenance Guides on Bearing Maintenance, Alignment and Vibration Analysis.