Elimination of overheating of the electric motor: thermal imaging diagnostics, current analysis, ventilation check and insulation degradation diagnostics

1. Description of the problem and scope of application

Motor overheating is a critical condition that significantly shortens equipment life, leads to unplanned production shutdowns, and potentially catastrophic failures. This manual is intended for the systematic diagnosis and elimination of malfunctions associated with an excessive increase in the operating temperature of AC and DC electric motors in industrial conditions. It is used for engines used in pumps, fans, conveyors, compressors and other rotating mechanisms.

Severity classification:

• Critical: The winding temperature exceeds the maximum permissible (usually +10°C above the nominal value indicated on the motor nameplate) or the thermal protection is activated. Requires immediate stoppage and diagnosis.
• Significant: The temperature exceeds the nominal one by +5...+10°C. Impairs efficiency and accelerates insulation degradation. Requires planned elimination.
• Minor: The temperature exceeds the nominal one by +2...+5°C. Indicates the initial stages of problems or changes in operating conditions. Requires monitoring and possible correction.

2. Safety measures

⚠ SAFETY WARNING ⚠

Before starting any diagnostic or repair work on the electric motor, be sure to do the following:

1. LOCKING / HANGING TAG (LOTO): Isolate all power sources supplying the motor and related equipment (electrical, hydraulic, pneumatic). Застосуйте процедури LOTO відповідно до ДСТУ EN 1032 та внутрішніх стандартів підприємства. Check the absence of voltage using the appropriate indicator.
2. STORED ENERGY: Make sure all energy stores (eg capacitors, springs, lifted loads, pressure in hydraulic/pneumatic systems) are discharged or locked.
3. PERSONAL PROTECTIVE EQUIPMENT (PPE): Always use appropriate PPE: safety glasses, heat-resistant gloves, dielectric shoes, protective clothing. Wear arc-resistant clothing when working with exposed electrical components.
4. HOT SURFACES: Electric motors can be hot. Allow the engine to cool before touching or use heat resistant gloves.
5. ROTATING PARTS: Make sure all moving parts (fans, shafts, couplings) are stopped and protected from accidental starting.

Failure to observe these precautions could result in serious injury or death.

3. Necessary diagnostic tools

Tool	Specification/Model	Measurement range	Purpose
Thermal imaging camera	FLIR T-series or equivalent with 320x240+ resolution	-20°C to +650°C, accuracy ±2°C	Detection of hot spots on the engine housing, bearings, terminal connections, measurement of surface temperature in dynamics. Settings: emissivity for painted surfaces 0.95.
Measuring clamps with True RMS function	Fluke 376 FC or equivalent	Струм AC/DC до 1000 А, напруга AC/DC до 1000 В	Measurement of phase currents, detection of current imbalance, voltage measurement.
Digital multimeter	Fluke 87V or equivalent, Class CAT IV 600V	Resistance up to 50 MΩ, voltage up to 1000 V	Measuring the resistance of the windings (in the off state), checking the integrity of the circuits.
Megohmmeter (insulation resistance meter)	Megger MIT420/2 or equivalent	Voltage: 500V, 1000V; Resistance: up to 100 Ω	Assessment of the condition of the insulation of the windings and its resistance relative to the case.
Digital tachometer (contact/non-contact)	Extech RPM10 or equivalent	0.5 to 99,999 rpm	Measurement of actual motor/shaft rotation speed.
Vibration analyzer	Vibrometer 2000 or equivalent, with accelerometer	Frequency range 10 Hz - 10 kHz, measurement of vibration speed (mm/s)	Detection of imbalance, inconsistency, bearing defects, loosening of fasteners. Settings: motor rotation speed.
Anemometer	Testo 425 or equivalent	0.4 to 30 m/s	Measurement of air flow rate for engine cooling.

4. Initial evaluation checklist

Before starting a detailed diagnosis, conduct a visual inspection and collect basic information.

5. Systematic diagram of diagnostics

Follow this sequence of actions to identify the root cause of overheating.

1. Overheating confirmation:
   • Action: Measure the surface temperature of the engine using a thermal imaging camera or a contact thermometer.
   • Threshold: The surface temperature exceeds the nominal by 10°C or more, or the thermal protection is activated.
   • IF → Result: Overheating confirmed. Go to point 2.
   • IF → Result: The temperature is normal. The problem is not engine overheating.
2. Analysis of electrical parameters (motor in operation):
   • Action: Measure the phase currents and voltages at the motor terminals with the help of measuring clamps.
   • Threshold:
     • Current: > Rated current on motor nameplate.
     • Current imbalance: > 5% (calculated as (I_max - I_min) / I_avg * 100%).
     • Voltage imbalance: > 2% (calculated as (V_max - V_min) / V_avg * 100%).
   • IF → Result: Current exceeds nominal (load or internal fault). Go to point 3.
   • IF → Result: Significant current imbalance (> 5%). Go to point 4.
   • IF → Result: Significant voltage imbalance (> 2%). Go to point 5.
   • IF → Result: Currents and voltages are normal, imbalance is minimal. Go to point 6.
3. Estimation of mechanical load and cooling conditions (engine in operation):

   (Relevant if the current exceeds the nominal)

   • Action:
     • Check the mechanical load on the motor.
     • Огляньте систему охолодження двигуна.
   • Threshold:
     • Load: Obvious excessive load on the drive mechanism.
     • Cooling: Contamination of ventilation holes, damage to the fan, limited air access.
   • IF → Result: Excessive mechanical load. Go to point 7.
   • IF → Result: Cooling problems. Go to point 8.
   • IF → Result: Load and cooling are normal. Go to point 9 (internal electrical fault).
4. Current imbalance (engine in operation):

   (If significant current imbalance is detected)

   • Action: Check the quality of the contacts in the motor terminal box and in the starter protection equipment.
   • Threshold: Weakened, oxidized or burnt contacts detected.
   • IF → Result: Bad contacts. Go to point 10.
   • IF → Result: Contacts are normal. Go to point 9 (internal electrical fault).
5. Voltage imbalance (engine in operation):

   (If significant voltage imbalance is detected)

   • Action: Check the power supply (transformer, distribution busbars, cables) to the motor protection equipment.
   • Threshold: A significant voltage imbalance (> 2%) or a voltage drop in one of the input phases has been detected.
   • IF → Result: Problems with the power network. Contact the energy service of the enterprise.
   • IF → Result: The power network is normal. Go to step 4 (possible problem with contacts or controls).
6. Diagnostics of mechanical malfunctions (motor is working, currents are normal, but there is overheating):
   • Action: Measure the vibration on the bearing shields of the motor using a vibration analyzer. Listen to the bearings with a stethoscope.
   • Threshold:
     • Vibration speed: > 4.5 mm/s (in accordance with ISO 10816-1 for engines with a power of 15-75 kW, base stiffness class B) — alarm.
     • Noise: characteristic grinding, hum, creaking of bearings.
   • IF → Result: Increased vibration or noise. Go to Item 11 (Bearing Problems/Mismatch).
   • IF → Result: Vibration and noise are normal. Go to step 8 (cooling recheck, internal loss).
7. Overload detection (motor off):

   (Relevant if current was higher than rated)

   • Action: Disconnect the motor from the drive mechanism. Rotate the motor shaft and gear shaft by hand.
   • Threshold: The shaft of the mechanism rotates with resistance, or a malfunction is detected in the drive mechanism (jamming, excessive friction).
   • IF → Result: Problem in the drive mechanism. Eliminate the mechanical malfunction (see item 12).
   • IF → Result: The mechanism rotates freely. Go to step 9 (internal motor electrical fault).
8. Estimating cooling efficiency (engine off):
   • Action:
     • Inspect the fan (blade integrity).
     • Перевірте вентиляційні канали та ребра охолодження на забруднення.
     • Use an anemometer to measure the speed of the air flow at the exit of the engine (if possible).
   • Threshold: Damaged fan, blocked ducts, air velocity below nominal (< 5 m/s for most engines).
   • IF → Result: Cooling problems. Go to point 13.
   • IF → Result: Cooling is normal. Go to point 9 (internal electrical fault).
9. Diagnostics of electrical isolation (engine off, LOTO):

   (Relevant if the previous stages did not reveal obvious reasons, or an internal electrical malfunction is suspected)

   • Action:
     • Measure the insulation resistance between each phase and the case, as well as between phases (if the circuit allows). Use a megohmmeter at a voltage of 500V or 1000V according to the voltage class of the motor.
     • Measure the resistance of the motor windings with a multimeter.
   • Threshold:
     • Isolation resistance: < 1 MΩ (DSTU EN 60204-1) – critical, < 5 MΩ – requires attention.
     • Winding resistance: Significant difference (> 2%) between phase resistances.
   • IF → Result: Low insulation resistance. Go to point 14.
   • IF → Result: Winding resistance imbalance. Go to point 15.
   • IF → Result: Insulation and winding resistance are normal. Consider other factors (eg harmonics, high ambient temperature, hidden faults).
10. Contact problems (engine off, LOTO):

    (If bad contacts are detected)

    • Action:
      • Visually inspect the contacts in the motor terminal box, contactors, automatic switches.
      • Measure the voltage drop across the contacts under load (if possible without risk) or the resistance of the contacts in the off state.
    • Threshold: Oxidized, burned, weakened contacts, voltage drop > 0.1 V.
    • IF → Result: Bad contacts. Go to point 16.
11. Bearing problems / misalignment (engine off, LOTO):

    (If increased vibration or noise is detected)

    • Action:
      • Disconnect the engine from the drive mechanism.
      • Check the motor shaft play. Rotate the shaft by hand, evaluate the smoothness of rotation and the presence of extraneous noises.
      • Check engine and drive gear alignment.
    • Threshold:
      • Shaft backlash: noticeable radial or axial backlash.
      • Rotation: rough, with jamming, grinding.
      • Misalignment: > 0.05 mm (radial), > 0.1 mm (angular).
    • IF → Result: Bearing problems. Go to point 17.
    • IF → Result: Mismatch. Go to point 18.
12. Detection of excessive load in the mechanism (engine off):

    (If resistance is detected when scrolling the drive mechanism)

    • Action: Sequentially disconnect the elements of the drive mechanism (reducer, pump, fan) and rotate them by hand to locate the source of resistance.
    • Threshold: A source of excessive friction, jamming, or malfunction has been detected.
    • IF → Result: Localized mechanical fault. Go to point 12 (continuation of the elimination of excessive load).

6. Matrix "fault-cause"

7. Analysis of the root causes of each malfunction

7.1. Excessive mechanical load

Reason: The motor is forced to produce more torque than its rated capabilities due to increased resistance in the drive mechanism. This can be caused by jammed bearings, friction, damage to working elements (eg fan blades, pump rotor), improper adjustment or process change.

Confirmation: The actual current of the motor significantly exceeds the nominal (by 10-20% or more). After turning off the engine, the drive mechanism rotates with noticeable resistance or jams. The temperature of the engine drops sharply when working without load.

Consequences without elimination: Rapid overheating of windings, accelerated degradation of insulation, failure of bearings, activation of thermal protection, complete failure of the motor.

7.2. Insufficient cooling

Reason: Loss of efficiency of the engine cooling system. This may be due to contamination of ventilation channels and cooling fins with dust, dirt, oil; damage to the blades of the external fan; the absence of a protective fan cover; limited space around the engine, which prevents the free flow of air.

Confirmation: Visually visible contamination of the cooling fins or damage to the fan. Анемометр показує низьку швидкість повітря на виході. Engine temperature does not decrease during no-load operation or decreases slightly.

Consequences without elimination: Constant overheating of windings, accelerated degradation of insulation, reduction of engine efficiency, reduction of service life.

7.3. Imbalance of phase currents/voltages

Reason: Uneven distribution of current between phases or uneven voltage of the supply network. Current imbalance can be caused by bad contacts in the motor terminal box, contactors, circuit breakers, or an internal defect in the motor winding (for example, inter-turn short circuit). Voltage imbalance is often an external power network problem caused by uneven load distribution across phases or faults in transformers/cables.

Confirmation: Measurement of phase currents with measuring clamps shows a difference of > 5%. Measurement of phase voltages shows a difference > 2%. Local overheating in the area of ​​one connection or one phase of the winding.

Consequences without elimination: Overheating of windings, especially in the most loaded phase, additional losses in copper and steel, torque ripples, increased vibration, accelerated degradation of insulation.

7.4. Worn/damaged bearings or misalignment

Reason: Increased friction in the bearings due to wear, lack or degradation of lubricant, corrosion, or due to incorrect installation (misalignment, incorrect tension of belts/chains). Misalignment of the motor shaft and the drive mechanism causes additional radial and axial loads on the bearings.

Confirmation: Local overheating of bearing shields (detected by a thermal imager). Increased vibration (vibration rate > 4.5 mm/s). Extraneous noises (creaking, humming). After turning off the engine, the shaft rotates smoothly or with jams. Coaxial measurement shows a deviation.

Consequences without elimination: Overheating of bearings, destruction of lubricant, mechanical damage to rotor/stator windings due to shaft subsidence, complete engine jamming.

7.5. Degradation of insulation / inter-turn short circuit

Причина: Зниження електричного опору ізоляції обмоток через старіння, перегрів, вологу, агресивне середовище, механічні пошкодження. An inter-turn short-circuit is a direct short-circuit between adjacent turns of one winding, which leads to a significant increase in current in this part of the winding and its local overheating.

Confirmation: Low insulation resistance (< 1 MΩ) when measured with a megohmmeter. Winding resistance imbalance (> 2%). A local hot spot on the motor housing that corresponds to the damaged winding area. The characteristic smell of burnt insulation.

Consequences without elimination: Rapid progression of damage to complete phase-to-phase shorting or shorting to the housing, leading to complete motor failure, tripping of protection and possible network damage.

8. Step-by-step troubleshooting procedures

⚠ REMEMBER: Always follow LOTO procedures and use appropriate PPE! ⚠

8.1. Elimination of excessive mechanical load

1. Troubleshooting: Disconnect the motor from the drive mechanism. Manually rotate the motor shaft and mechanism shaft separately to determine where the source of resistance is.
2. Diagnostics of the mechanism: Perform diagnostics of the drive mechanism: check bearings, reducers, impellers, seals. Eliminate the cause of the binding or excessive friction.
3. Load check: Make sure the working load corresponds to the rated power of the motor. If necessary, reduce the load or consider replacing the motor with a higher power motor (after consulting with engineers).
4. Verification: Assemble the mechanism, connect the motor. Measure the phase currents when working under load. Make sure the currents are within ± 5% of the nominal values.

8.2. Recovery of cooling efficiency

1. Cleaning: Carefully clean the ventilation ducts, cooling fins and the outer surface of the engine from dust, dirt, oil and other contaminants. Use compressed air or flushing (if allowed for this type of engine).
2. Checking the fan: Inspect the fan blades for damage (cracks, chips). Replace the damaged fan. Make sure the fan shroud is installed correctly and does not restrict airflow.
3. Environmental Optimization: Provide sufficient space around the engine for free air circulation. If possible, lower the ambient temperature or install additional cooling systems.
4. Verification: After cleaning and/or replacing the fan, start the engine. Measure the temperature with a thermal imager. Compare with the original data. Make sure the temperature has dropped to acceptable values.

8.3. Elimination of current/voltage imbalance and bad contacts

1. Diagnosis of contacts: Check all electrical connections: in the motor terminal box, in contactors, automatic switches, cable joints. Inspect them for oxidation, burnt areas, weakening.
2. Restoration of contacts: Clean oxidized contacts, replace burnt ones. Tighten all bolted connections to the recommended torque. For most M8 terminals - 20-25 Nm.
3. Перевірка джерела живлення: Якщо проблема дисбалансу напруг походить від зовнішньої мережі, проінформуйте енергетичну службу. They must carry out diagnostics of the transformer, cable lines, switchboards.
4. Verification: After reestablishing contacts and/or eliminating network problems, start the engine. Measure phase currents and voltages. Make sure current unbalance < 5% and voltage unbalance < 2%. Check the temperature of the terminal connections with a thermal imager.

8.4. Replacement of bearings and alignment of shafts

1. Dismantling: Disconnect the engine from the drive mechanism. Dismantle the bearing shields.
2. Bearing replacement: Replace the bearings with new ones that meet the engine manufacturer's specifications (accuracy class, lubricant type, thermal clearance). Use special bearing removal/installation tools (eg pullers, induction heaters) to prevent damage. Make sure that the lubricant is properly filled (as a rule, fill to 1/3 of the volume of the bearing assembly).
3. Alignment (adjustment): When reassembling the engine, perform exact alignment (centering) of the engine and drive shafts. Use a laser leveling system. Tolerances: radial misalignment < 0.03 mm, angular misalignment < 0.05 mm per 100 mm of coupling diameter.
4. Verification: After replacing the bearings and alignment, start the engine. Measure the vibration. Make sure that the vibration speed is < 2.8 mm/s (operating range ISO 10816-1). Carry out thermal imaging control of the bearing shields.

8.5. Diagnosis and repair of insulation / windings

⚠ CAUTION: Repairing damaged insulation or motor windings requires specialized skills and equipment. In most cases, it is recommended to replace the engine or transfer it to a specialized repair workshop. ⚠

1. Defect confirmation: Repeat the insulation resistance and winding resistance measurements.
2. Localization: If possible, use inductive testing or a surge test to localize the inter-turn short circuit.
3. Solution:
   • Low insulation resistance without inter-turn shorting: Try to dry the motor in a drying cabinet (temperature 80-100°C) until the insulation resistance is restored > 5 MΩ.
   • Inter-turn short circuit: Motor replacement or winding windings in a specialized workshop.
4. Verification: After repair or drying, repeat all electrical measurements (insulation resistance, winding resistance, current imbalance).

9. Preventive measures

10. Spare parts and components

To order quality spare parts and components, visit the electronic catalog UNITEC-D.

11. Links

• DSTU EN 60204-1:2018 (EN 60204-1:2018, IDT) Machine safety. Electrical equipment of machines. Part 1. General requirements.
• ISO 10816-1:1995 Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts — Part 1: General guidelines.
• ISO 492:2014 Rolling bearings — Radial bearings — Dimensions, general plan.
• OEM maintenance manuals for electric motors (depending on the manufacturer).
• Internal standards of the enterprise on LOTO procedures and safety techniques.