Diagnostic Guide: Overheating in Industrial Electric Motors

1. Problem Description and Scope

This guide addresses the diagnosis and resolution of overheating problems in three-phase and single-phase alternating current (AC) electric motors, widely used in the Brazilian manufacturing industry. Motor overheating can lead to premature winding insulation failure, bearing degradation and, in severe cases, catastrophic equipment failure and production interruption. The severity classification is as follows:

• Critical: Frame or winding temperature exceeds the maximum thermal limit of the insulation class (e.g. Class F > 155°C), imminent risk of motor burnout or unexpected stoppage.
• Major: High temperature (above normal operation, but below the critical limit) with signs of incipient degradation (e.g.: burning smell, abnormal noise). May cause failure in the short term if not corrected.
• Minor: Temperature slightly above normal operating temperature, but within the safety limits of the insulation class. Requires monitoring and investigation to prevent escalation.

2. Safety Precautions

Critical Attention: Working with energized or rotating electric motors presents serious risks of electric shock, burns, crushing and other injuries. ALWAYS follow the NR-10 and NR-12 safety standards during any intervention. Ensure the engine is completely de-energized and isolated before beginning any inspection or repair.

• Lockout and Tagout (LOTO - Lockout/Tagout): Perform the LOTO procedure on all engine energy sources (electrical, mechanical, hydraulic, pneumatic) before any intervention. Check the absence of voltage with a calibrated multimeter.
• Personal Protective Equipment (PPE): Use insulating gloves, safety glasses, helmet, hearing protection, safety shoes and appropriate flame-retardant (FR) clothing, in accordance with ABNT NBR ISO 11612.
• Stored Energy: High inertia motors can continue to rotate after de-energization. Direct current (DC) circuits or capacitors can retain a dangerous charge. Wait the necessary time for discharge and energy dissipation.
• Hot Surfaces: Overheated engines can have high surface temperatures. Use thermal protective gloves.
• Confined Space: In some environments, the engine may be located in a confined space. Follow safety procedures for confined space entry.

3. Required Diagnostic Tools

Diagnostic accuracy depends on the use of appropriate and calibrated tools. Below is a table with the essential instruments:

4. Initial Assessment Checklist

Before beginning any in-depth diagnosis, collect as much information as possible about the history and current operating conditions of the engine. This step is essential to direct the diagnosis and avoid rework.

5. Systematic Diagnosis Flowchart

This flowchart guides the technician through a logical sequence of tests to identify the root cause of overheating.

1. Symptom: Motor showing overheating (checked by contact thermometer or thermal imager).
   1. Check Motor Load:
      • Measure line current (AC) with clamp meters on the three phases.
      • Compare with rated board current and service factor (FS).
      • IF Current > Nominal Current x FS: Probable Mechanical or Electrical Overload. Go to 5.1.1.
      • IF Current ≤ Nominal Current x FS: Normal load or underload. Go to 5.2.
   2. 5.1.1. Overload Diagnosis:
      • Check Mechanical Overload:
        • IF motor drives pump, fan, conveyor: check flow obstruction (dirty filters, partially closed valves), misalignment, excessive friction in the driven equipment.
        • Test Uncoupled Motor: If possible, uncouple the motor and operate at no load. Measure current and temperature.
        • IF motor operates at no load without overheating and normal current: Probable Cause: Mechanical Overload. Go to 7.1 and 8.1.
      • Check Electrical Overload (Overvoltage/Undervoltage/Voltage Unbalance):
        • Measure line voltage in the three phases with a multimeter.
        • IF Voltage > 10% of Nominal OR Voltage < 5% of Nominal: Probable Cause: Variation in Mains Voltage. Go to 7.2 and 8.2.
        • Calculate Voltage Unbalance: Unbalance = (Max. Average Voltage Deviation / Average Voltage) * 100.
        • IF Unbalance > 1%: Probable Cause: Voltage Unbalance. Go to 7.3 and 8.3.
   3. 5.2. Ventilation and Cooling Diagnosis:
      • Visual Inspection: Check dirty/obstructed fins, damaged/missing fan, incorrect direction of rotation.
      • Measure Air Flow: If possible, with an anemometer, or by sensing the air flow.
      • Check Ambient Temperature: Compare with the engine specification (generally 40°C).
      • IF Dirty/obstructed fins OR Damaged/missing fan OR Ambient Temperature > 40°C OR Poor airflow: Probable Cause: Ventilation/Environmental Problems. Go to 7.4 and 8.4.
   4. 5.3. Current Unbalance Diagnosis (Even with Normal Load):
      • Measure line currents in the three phases.
      • Calculate Current Imbalance: Imbalance = (Max. Average Current Deviation / Average Current) * 100.
      • IF Unbalance > 5%: Probable Cause: Current Unbalance (Internal or Network Electrical Problem). Go to 7.5 and 8.5.
   5. 5.4. Diagnosis of Insulation Fault or Internal Short Circuit:
      • Attention: DE-ENERGIZE AND BLOCK THE MOTOR.
      • Measure Insulation Resistance (Megohmmeter):
        • Between each phase and earth (housing).
        • Between phases (phase-phase).
        • Test voltage: 500V DC for motors up to 1000V.
        • Limits (NBR 5383-1): The insulation resistance (Ri) must be ≥ 1 MΩ (for motors in use). Values ​​below indicate degradation.
        • IF Ri < 1 MΩ: Probable Cause: Degradation of Insulation to Ground. Go to 7.6 and 8.6.
      • Measure Winding Ohmic Resistance (Multimeter):
        • Between terminals of each phase (U1-U2, V1-V2, W1-W2 for typical delta/star connections).
        • The three measurements must be very close (deviation < 2-5%).
        • IF Large difference in resistance between phases OR Very low resistance (< 0.1 Ω): Probable Cause: Internal Short circuit (between turns or phases) or Open Coil. Go to 7.6 and 8.6.
   6. 5.5. Diagnosis of Bearing Problems:
      • With the engine turned off and cold, turn the shaft manually. Feeling rough, excessive play, noise.
      • Perform vibration analysis (if available).
      • Vibration Limits (ABNT NBR ISO 10816):
        • Small motors (< 15 kW): Good (< 1.8 mm/s), Acceptable (1.8-2.8 mm/s), Inadequate (2.8-4.5 mm/s), Dangerous (> 4.5 mm/s).
        • Medium engines (15-75 kW): Good (< 2.8 mm/s), Acceptable (2.8-4.5 mm/s), Unsuitable (4.5-7.1 mm/s), Dangerous (> 7.1 mm/s).
      • IF Abnormal noise, excessive vibration, hot bearings (checked by contact thermometer): Probable Cause: Bearing Failure. Go to 7.7 and 8.7.

6. Failure and Cause Matrix

This table summarizes the symptoms, probable causes and diagnostic tests for engine overheating.

7. Root Cause Analysis for Each Failure

7.1. Mechanical Overload

Why it happens: The motor is forced to deliver more torque than its rated capacity. This can be caused by a blockage in the driven equipment (e.g. pump with a clogged impeller), increased fluid viscosity, misalignment between motor and load, excessive friction, or simply undersizing the motor for the current application. The increase in torque leads to an increase in current in the stator winding, increasing the I²R (Joule) losses and, consequently, the temperature.

How to confirm: The line current exceeds the rated current multiplied by the service factor. The engine overheats even with adequate ventilation. When decoupling the motor, the current and temperature return to normal. Inspection of the driven equipment reveals the source of the mechanical resistance.

Damage if not resolved: Continuous overload quickly degrades winding insulation, reducing motor life exponentially. It can also cause premature bearing and shaft failure due to excessive mechanical stress.

7.2. Network Voltage Variation (Under/Overvoltage)

Why it happens: An undervoltage (below 90% of the nominal) causes the motor to draw more current to maintain constant output power, increasing I²R losses. An overvoltage (above 110% of the nominal) increases the magnetization current and losses in the iron (losses due to hysteresis and eddy currents), in addition to saturating the core, leading to an increase in temperature.

How to confirm: Measure line voltages at the motor input with a digital multimeter. Values ​​outside the acceptable range (±10% of nominal voltage) confirm this cause. Observe whether the variation is punctual or persistent.

Damage if not resolved: Both undervoltage and overvoltage cause overheating of the insulation and, in the long term, its failure. Overvoltage can also cause electrical arcs at terminals and capacitor degradation (in single-phase motors).

7.3. Voltage Unbalance

Why it happens: Voltage differences between the phases of the three-phase motor. Even a small voltage imbalance (above 1%) can cause a much larger current imbalance (5 to 10 times greater), leading to significantly high I²R losses in one or more phases. This is often caused by uneven load distribution on the grid, faulty power factor correction capacitors, or utility issues.

How to confirm: Measure the line voltages on the three phases and calculate the percentage of unbalance. An imbalance > 1% is critical. The thermal imager may show one phase hotter than the others.

Damage if not resolved: Localized overheating in the winding of the most demanded phase, leading to premature insulation failure. Drastic reduction in engine efficiency and useful life. It may cause abnormal vibration and noise.

7.4. Ventilation and Environmental Problems

Why it happens: The engine dissipates heat to the environment mainly through forced convection (fan and fins). Any obstruction to this air flow (dust, dirt, debris, excessive proximity to walls or other equipment) prevents effective heat dissipation. A damaged, broken fan or fan with incorrect rotation direction (after maintenance) or an ambient temperature above the engine specification (generally 40°C) will also compromise cooling.

How to confirm: Visual inspection of the housing fins and fan impeller. Airflow test. Measurement of ambient temperature. Checking the fan rotation direction. Thermographic analysis to identify areas of heat accumulation due to ventilation failure.

Damage if not resolved: The generalized increase in motor temperature accelerates the degradation of the entire insulation system (windings, terminals) and lubrication of the bearings, significantly reducing the useful life and increasing the probability of electrical and mechanical failures.

7.5. Current Imbalance (Internal Problem or Phases)

Why it happens: Differences in currents between phases, even with balanced voltage. This can be caused by damaged windings (shorted turns), loose or corroded connections at the terminals, or defects in the power system that do not manifest themselves as voltage unbalance.

How to confirm: Measurement of line currents in the three phases with a clamp meter. Current imbalance > 5% is a strong indicator. A thermal imager can identify the terminal or region of the motor that is hottest due to high current.

Damage if not resolved: Localized overheating that quickly degrades the insulation of the phase with the highest current. Risk of short circuit failure and permanent damage to the motor.

7.6. Insulation Degradation or Internal Short Circuit

Why it happens: Motor winding insulation can degrade over time due to thermal, electrical, mechanical and environmental stress (humidity, chemical agents). This degradation creates low resistance paths, leading to short circuits between turns, between phases or between phase and ground. Short-circuit currents generate intense heat, resulting in localized overheating and, eventually, engine burnout.

How to confirm: With the motor de-energized, the megohmmeter will detect low insulation resistance (below 1 MΩ) to earth or between phases. The multimeter will reveal very low or uneven ohmic resistances between the windings, indicating shorted turns or an open winding. The burning smell is a strong indicator.

Damage if unresolved: Catastrophic motor failure, requiring rewinding or replacement. It may cause damage to other electrical components in the power circuit due to high short circuit current.

7.7. Bearing Failure

Why it happens: Worn, poorly lubricated, incorrectly installed or damaged bearings generate excessive friction. This friction manifests itself as heat (in the bearing region), noise and vibration. The heat generated by the bearings can be conducted to the engine housing, contributing to general or localized overheating.

How to confirm: Vibration analysis (peaks at characteristic bearing frequencies). Thermal imager will show hot spots on the bearings. Abnormal noises (creaking, grinding) and, with the engine off, roughness or excessive play when turning the shaft manually. Inadequate lubrication (insufficient, excessive, contaminated).

Damage if not resolved: Destruction of the bearing, which can lead to rotor locking, causing severe damage to the windings and the motor shaft. In extreme cases, it can cause a fire due to extreme heat and sparks.

8. Step-by-Step Resolution Procedures

For each root cause identified, follow specific corrective steps:

8.1. Resolution for Mechanical Overload

1. De-energize and Lockout: Perform LOTO on the engine and driven equipment.
2. Investigate Load: Examine the driven equipment (pump, fan, reducer, conveyor) to identify obstructions, wear, misalignment or excessive friction.
3. Check Alignment: Use laser aligner to ensure radial and angular misalignment are within tolerances (generally < 0.05 mm for flexible couplings).
4. Lubricate Components: Check and lubricate bearings and gearboxes of driven equipment according to the manufacturer's manual.
5. Reevaluate Sizing: If the overload is persistent and there is no mechanical failure, consider resizing the motor to a higher power, compatible with the load required by the application.
6. Verification: After correction, operate the motor at full load and monitor current and temperature using a thermal imager. The temperature must stabilize within the limits of the insulation class.

8.2. Resolution for Network Voltage Variation

1. Identify Origin: Using a multimeter, measure voltage at the input on the engine panel and at the dealership delivery point.
2. Correct on Site: If the variation is internal, check transformers, cables and connections in the plant. Retighten terminals with a torque wrench (e.g. M8 terminals: 25 Nm).
3. Voltage Regulator: Consider installing a voltage regulator or tap-changer on the supply transformer if the variation is due to the utility company.
4. Check: Monitor voltage during load peaks. The voltage must remain within the range ±5% of the motor nominal.

8.3. Resolution for Voltage Unbalance

1. De-energize and Lockout: Perform LOTO on the motor and power panel.
2. Inspect Connections: Check and retighten all terminals on the motor, terminal box, contactor, circuit breaker and distribution board. Use a torque wrench.
3. Check Single-Phase Loads: Examine the distribution of single-phase loads in the system. Redistribute the loads to balance the phases.
4. Capacitors: If there are capacitor banks for power factor correction, test them individually. Replace defective ones.
5. Check: Measure line voltages again. The imbalance must be < 1%.

8.4. Resolution for Ventilation and Environmental Problems

1. De-energize and Block: Perform LOTO.
2. Cleaning: Remove all dust, dirt and debris from the housing cooling fins, protective grille and fan impeller. Use compressed air (with appropriate PPE) or an industrial vacuum cleaner.
3. Fan: Inspect the fan for damage (broken, loose blades). Replace if necessary. Check the direction of rotation; must direct air over the fins.
4. Space: Ensure there is adequate clearance around the engine (minimum 1 meter for large engines) to allow unrestricted airflow.
5. Environment: If the ambient temperature is consistently high, consider installing auxiliary ventilation, thermal insulation, or relocating the engine to a cooler location if possible.
6. Check: Monitor the engine temperature with a thermal imager after cleaning and/or repair. The housing temperature must be uniform and within the operating range.

8.5. Resolution for Current Unbalance (Internal Problem or Phases)

1. De-energize and Block: Perform LOTO.
2. Inspect Connections: Check and retighten all terminals on the motor, terminal box, contactor, circuit breaker and distribution board. Use a torque wrench to ensure adequate electrical contact (e.g. M6 screws: 12 Nm).
3. Test Cables: Inspect power cables to identify physical damage or high resistance points. Perform continuity and resistance test.
4. Check Contacts: Inspect relay and contactor contacts for corrosion or carbonization. Clean or replace as necessary.
5. Winding Diagnosis: If the steps above do not solve the problem, and the unbalance persists, it is likely that there is a problem with the motor windings (shorted turns). Rewinding or replacing the motor is usually necessary.
6. Verification: After the corrections, measure the line currents in the three phases. The imbalance must be < 5%.

8.6. Resolution for Insulation Degradation or Internal Short Circuit

1. De-energize and Block: Perform LOTO.
2. Confirm Diagnosis: Repeat the megohmmeter and ohmic resistance tests to confirm the failure.
3. Rewinding: If insulation degradation or short circuit is confirmed, the motor must be sent to a specialized rewinding service. Make sure the service uses insulating materials of the correct class (e.g. Class F or H) and follows NBR IEC 60034-1 and NBR 5383-1.
4. Replacement: In many cases, depending on the cost of rewinding versus the cost of a new motor, replacement may be the most economical and reliable option.
5. Investigate Cause of Failure: Try to understand why the insulation failed (overload, overvoltage, humidity, vibration) to apply preventative measures to other motors.
6. Check: After rewinding or replacing, perform all electrical tests (insulation resistance, ohmic resistance, current/voltage unbalance) before energizing.

8.7. Resolution for Bearing Failure

1. De-energize and Block: Perform LOTO.
2. Bearing Replacement: Remove the bearing covers and replace the bearings. Caution: Use an appropriate bearing puller. Heat the new bearing in an inductive or oil oven (up to 120°C, never more) for expansion assembly. Never force the bearing with a hammer directly on the outer ring.
3. Lubrication: Apply the correct amount and type of lubricant (grease or oil) according to the motor and bearing manufacturer's specifications. Avoid over or under lubrication.
4. Seals: Inspect the bearing seals. Replace if damaged to avoid contamination.
5. Alignment: Reconfirm the alignment between the motor and the load after mounting the bearing.
6. Check: Operate the engine and perform vibration analysis and bearing temperature monitoring. Levels must be within normal limits.

9. Preventive Measures

Preventative and predictive maintenance is essential to prevent overheating and extend the life of electric motors.

10. Spare Parts and Components

Having the correct replacement parts available is essential for quick and efficient problem resolution. Consult the UNITEC-D e-catalog to find the suitable components for your engine.

Access our e-catalog for a complete portfolio of parts and components: www.unitecd.com/e-catalog/

11. References

• ABNT NBR 5383-1: Rotating electrical machines - Part 1: Testing of rotating electrical machines.
• ABNT NBR IEC 60034-1: Rotating electrical machines - Part 1: Nominal and operating characteristics.
• ABNT NBR ISO 10816: Assessment of machine vibration by measurements on non-rotating parts.
• ABNT NBR ISO 11612: Protective clothing against heat and flame.
• NR-10: Safety in Electrical Installations and Services.
• NR-12: Workplace Safety in Machines and Equipment.
• Manuals from Engine Manufacturers (Weg, Siemens, ABB, etc.).
• Related UNITEC-D Maintenance Guides: www.unitecd.com/maintenance-guides/