Thermal imaging diagnostics of electrical equipment

Thermal imaging diagnostics of electric motorsand distribution equipment is a non-contact control method in which an infrared camera records the temperature distribution over the surface and identifies areas of local overheating. The method is included in the mandatory PPR program at all large enterprises in Tashkent: it is recommended by PUE, PTEEP and the GOST R 8.619 standard. According to our statistics, up to 30% of failures of electric motors with a power of over 30 kW could be prevented if thermal imaging inspections were carried out quarterly.

The principle of thermal imaging

Any body with a temperature above absolute zero emits in the infrared range 0.9–14 microns. A thermal imager is a camera with a microbolometer array that converts infrared radiation into an electrical signal and builds a color image of the surface with a resolution of up to 0.05 °C. Three parameters are important for correct interpretation of a thermogram:

• Emissivity—for a painted motor housing ε ≈ 0.93, for uncoated aluminum ε ≈ 0.3, for copper busbars ε ≈ 0.05.
• The temperature of the reflected background is especially important when shooting shiny surfaces in the workshop.
• Distance and atmospheric transmission - when shooting from a long distance, signal attenuation is taken into account.

Modern models Fluke Ti401, Testo 875, FLIR E96, Sonel KT-560 have a matrix resolution of 320x240 or 640x480 pixels and automatically make corrections. For diagnostics of electric motors in the conditions of Tashkent, a camera with a measurement range of –20...+650 °C and thermal sensitivity NETD ≤ 0.05 °C is suitable.

Typical motor overheating points

Over the years, Elektromotors engineers have compiled a map of control points for a squirrel-cage induction motor. A standard round takes an experienced diagnostician 3–5 minutes.

1. Terminal box - loose contacts, oxidation, overheating of jumpers.
2. The frontal parts of the winding (through the inspection window or ventilation holes) - interturn short circuit, deterioration of insulation.
3. Stator housing in the area of the package - general overheating due to overload or asymmetry.
4. Bearing shields - wear of bearings, lack or excess of lubrication.
5. Coupling or pulley - misalignment, belt slippage.
6. Fan casing - cooling fins are clogged with dust and cotton (relevant for textile production).

Additionally remove the power cable, circuit breaker, contactor and frequency converter. Overheating of one phase of the cable by 5–10 °C relative to its neighbors is a clear sign of loosening of the clamp or the beginning of the core burning out.

Insulation classes and temperature limits

The permissible heating of the electric motor is determined by the heat resistance class of the insulation according to GOST 8865. This is a basic table that every OGE engineer should know.

In practice, if the frame of a class F engine is operating in a workshop with an ambient temperature of +35 °C (typical summer in Tashkent), the measured frame temperature of 100–110 °C is normal. Exceeding 130 °C at the housing means that the inside of the winding is already about 155 °C, and the insulation life is halved for every 10 °C of overheating according to Montsinger's rule. In this case, urgent electric motor diagnostics is required.

When to choose a thermal imager and when to choose vibration analysis

The two methods do not compete, but complement each other. The thermal imager is faster, cheaper and does not require shutdown. Vibration analysis is deeper, but requires contact between the sensor and a trained operator. The choice depends on the task.

For enterprises in Tashkent, where the fleet of electrical equipment is large, a two-level scheme is optimal: monthly thermal imaging inspection of the entire fleet + quarterly vibration monitoring of critical machines.

Diagnostic algorithm based on thermogram

A simple block diagram that our engineers use on site:

1. We measured the T of the body. Excess over ambient < permissible for insulation class? If yes, the engine is normal, re-check after 30 days.
2. If there is an excess, check the uniformity of heating of the three phases of the terminal box. A difference > 5 °C indicates network unbalance or turn-to-turn short circuit. Remove the currents, measure the resistance of the windings.
3. If the heating is uniform, but exceeds the norm, measure the operating current. Current > nominal by 10% means overload; mode correction or replacement of the motor with a more powerful one is required.
4. If the current is normal, but only the bearing shield is heating up, it’s time to order replacement bearings.
5. If a section of the housing heats up locally, remove the cover, perform diagnostics and, if necessary, rewind the stator.

This algorithm covers 90% of typical situations at industrial facilities. For special applications - high voltage machines, variable frequency drives, synchronous motors - additional steps are added.

Practical recommendations

• Take a thermogram at a steady-state thermal regime - no earlier than 30–60 minutes after start-up.
• Keep a safe distance and use PPE - especially when working with switchgear above 1 kV.
• Document each measurement: date, load, ambient T, ε. This is the basis for trend analysis.
• Hold a clamp meter in parallel with the thermal imager - half of the diagnoses are made using a combination of T and I.
• For asynchronous electric motors with an inverter, keep in mind that PWM interference can cause additional heating of the winding by 5–15%.

If your equipment in Tashkent operates at the thermal limit, Elektromotors engineers go to the site with a Testo thermal imager and a power quality analyzer, conduct a full thermal control, issue a protocol with thermograms and recommendations. Contact us through the contacts section - we will arrange departure within one business day.