In an efficient electric motor, the most insidious losses are those invisible to the eye but betrayed as heat. An abnormal temperature rise is often the first sign of a loss of efficiency or an approaching fault: a loose terminal connection, a worn bearing, an unbalanced load or clogged cooling fins reveal themselves as a hot spot. Thermography (temperature imaging with a thermal camera) is a powerful predictive-maintenance method that lets you detect these hot spots without stopping, disassembling or even touching the motor. Thermal images taken from the motor frame, terminal box, bearing housings and cable connections make hidden losses and developing faults visible at a very early stage. In this article we cover how thermography is applied to a motor, what each hot spot indicates, and the place of periodic thermal scanning in predictive maintenance. (This article is conceptual; it makes no fixed-price or numeric promise.)

Hot-spot and loss detection with thermography in an efficient electric motor

What Is Thermography and Why Use It on a Motor?

Thermography is the technique of converting the temperature distribution into a colored image by measuring the infrared radiation emitted from a body's surface. The thermal camera shows the surface temperature of the motor and its connections contactlessly, while the motor is running. This lets you distinguish at a glance the motor's normal operating temperature from abnormal hot spots that deviate from it. In a motor, an abnormal temperature rise is almost always a sign of a loss: part of the electrical energy is turning into heat rather than work, and that heat accumulates where the problem is.

Efficient motors are already designed to reduce losses; so a hot spot appearing on them usually stems not from the motor itself but from an external cause such as a poor connection, mechanical wear or load imbalance. To see conceptually where efficiency losses occur, our efficiency losses in IE4 motors: iron, copper and friction; for verifying field efficiency, our field efficiency verification with a power analyzer in efficient motors is a complement.

Difference Between Thermography and a Power Analyzer

While a power analyzer measures the motor's electrical efficiency numerically, thermography shows the physical location of the problem. The two complement each other: the analyzer says there is an efficiency drop, thermography points to where that drop concentrates (terminal, bearing or winding). We examined the difference between label efficiency and field efficiency in nameplate vs field efficiency: real savings. Our impact of maintenance on motor efficiency: bearing, lube, alignment addresses the effect of maintenance on motor efficiency.

What Does Each Hot Spot Indicate?

In a thermal image, the location of the hot spot reveals the type of problem. The interpretation of abnormal heating in different regions of the motor can be summarized as follows.

Terminal Box and Cable Connections

A local hot spot in the terminal box or at a cable connection usually indicates a loose or oxidized connection. A loose connection raises contact resistance, generates heat at that point and can progress over time to arcing, insulation damage and even fire. Therefore tightening the terminal bolts to the correct torque is critical; we addressed this in cable connection and cable lug selection in electric motors and in terminal box and cable entry orientation in IE4 motors. Thermography catches such connection-related heating before visible damage forms.

Bearing Housings

Abnormal heating at the motor's front and rear bearing housings can be a sign of insufficient or excessive lubrication, wear, misalignment or bearing current. A rise in bearing temperature is one of the earliest signs of a mechanical fault. For bearing maintenance and greasing, our bearing greasing in electric motors: grease type and NLGI; for continuous winding-temperature monitoring, our motor winding temperature monitoring: PT100 and PTC thermistor is a complement.

Early diagnosis of bearing and terminal heating with periodic thermography scanning

Frame and Cooling Surfaces

A higher-than-expected temperature across the motor frame may indicate overload, insufficient cooling or clogged cooling fins. Dust and dirt accumulating on the cooling fins block heat dissipation and heat the motor. Asymmetric temperature concentrated in a single region may be a sign of load imbalance or a phase problem. You can find the effects of overload and imbalance in electric motor failures: symptoms and causes, and vibration-related issues in vibration and balance in electric motors (ISO 10816).

Periodic Thermography and Predictive Maintenance

The real power of thermography emerges when it is done periodically rather than once. When thermal images of the same motor taken at intervals under the same load are compared, a slowly heating connection or bearing is noticed before it turns into a fault. This is the essence of predictive maintenance: instead of waiting for the failure, catching the sign early and intervening in a planned way. It reduces unplanned downtime and prevents unexpected breakdowns.

Thermography should be set up as part of a holistic maintenance program. For the maintenance schedule, our electric motor maintenance and periodic check schedule; to see early failure causes, our electric motor lifespan and early failure causes; for energy-efficiency audit and motor inventory, our energy efficiency audit and motor inventory is a resource.

A Holistic View in Efficiency and Loss Management

Thermography is only one tool of efficiency management. The real gain comes from addressing measurement, maintenance and correct motor selection together. To identify savings opportunities by measuring the load profile, our motor load-profile logging and energy-savings identification; for harmonic-related extra heating and losses, our harmonics and power quality in high-efficiency motors is important. You can review our product range under our efficient electric motors and IE4 electric motor categories, and reach more content from our home page.

The Subtleties of Interpreting a Thermal Image Correctly

The most common mistake in thermography is fixating on the absolute temperature value. Yet a motor's frame temperature can normally be high depending on ambient temperature, load level and insulation class; for example, an F-class insulated motor runs perfectly normally up to a certain temperature at rated load. So what matters is not the number in a single image but three kinds of comparison: with past images of the same motor (trend analysis), with similar motors running at the same time (peer comparison), and with the motor's normal expected temperature. A point being markedly hotter than the others, or heating over time, is more meaningful than the absolute value.

The concepts of temperature-rise class and temperature rise are the basis of this interpretation. To understand up to what temperature a motor runs normally, our temperature-rise class and temperature rise in induction motors (Delta T 80K); how continuous monitoring and thermography complement each other is explained in our motor winding temperature monitoring: PT100 and PTC thermistor. While thermography gives a snapshot, PT100/PTC provides continuous protection; ideally the two are used together.

The Thermal Signature of Load Imbalance and Phase Problems

In a thermal image, one side of the motor being markedly hotter than the other usually indicates a load or supply imbalance. In a three-phase motor, a weak phase (voltage imbalance) or single-phasing causes asymmetric heating in the winding and becomes visible early through thermography. We addressed the effect of voltage imbalance on a motor in derating under voltage imbalance in induction motors, and the burnout risk of single-phasing in single-phasing (phase loss) in induction motors. When such issues are evaluated together with terminal heating, the picture becomes clear.

Fitting Thermography Into a Predictive-Maintenance Program

The value of thermography emerges when it is part of a holistic predictive-maintenance program rather than standing alone. When vibration measurement, temperature monitoring, grease analysis and thermography are used together, the different signs of a fault are cross-verified. For example, if a bearing shows both a rise in vibration and heating in the thermal image, the fault is highly likely confirmed and it is time for a planned intervention. For vibration and balance acceptance values, our vibration and balance in electric motors (ISO 10816); for the effect of maintenance on efficiency, our impact of maintenance on motor efficiency: bearing, lube, alignment is a guide.

The biggest contribution of predictive maintenance to operations is reducing unplanned downtime. When a motor stops unexpectedly there is not only repair cost but also production loss; this loss is large especially in continuously running plants. When a connection or bearing problem is caught with thermography before it turns into a fault, the intervention is done during a planned stop. To see the motor's lifespan and early failure causes holistically, see our electric motor lifespan and early failure causes; to see the place of harmonic-related extra heating in predictive maintenance, see our VFD and harmonic-related extra heating in induction motors.

Frequently Asked Questions

Does thermography measure a motor's efficiency directly?

No. Thermography does not give a numeric efficiency value; it shows where abnormal heating concentrates. A power analyzer is used for the numeric measurement of efficiency. When the two are used together, both an efficiency drop is detected and the physical source of that drop is found.

Does seeing a hot spot in a thermal image always mean a fault?

Not always. What matters is how far the hot spot deviates from the motor's normal operating temperature and how it compares with past images of the same region. A temperature above expectation or rising over time can be an early sign of a developing problem (loose connection, bearing wear, load imbalance).

How often should thermography be done?

The frequency depends on the motor's criticality and operating conditions. The general approach is to scan the motor at regular intervals tied to the maintenance schedule, ideally always under similar load. This makes the images comparable and catches slowly developing heating early.

The Correct Method in a Thermography Scan

To get reliable results from thermography, the scan must be done under the right conditions. The motor should be running under a meaningful load that allows interpretation; the thermal image of a motor running idle does not show problems under load. During measurement, the camera's angle to the surface, the distance and the surface's emissivity affect the result; shiny metal surfaces can be misleading, so measurement from matte, standard surfaces is preferred. If images of the same motor over time are to be compared, taking the measurements under similar load and ambient conditions makes the comparison meaningful.

Thermography is a method in which the motor should be evaluated not alone but together with the machine it drives. Heating seen at a coupling or bearing may not originate from the motor; a shaft misalignment or a problem on the machine side can also generate heat. Therefore thermal findings should be interpreted together with vibration and mechanical checks. For coupling- and alignment-related issues, see our flexible or rigid coupling? shaft alignment; for evaluation together with noise and vibration, see our noise and vibration in electric motors. When all these methods come together, a far more reliable picture of the motor's health is obtained.

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Thermography Checklist

  • Is the scan done while the motor runs under normal load?
  • Were the terminal box and cable connections scanned for hot spots?
  • Were the front and rear bearing housings checked for temperature?
  • Were the frame and cooling fins inspected for dirt/blockage?
  • Was asymmetric (single-region) heating assessed for load imbalance?
  • Were the images compared with past scans of the same motor?
  • Has thermography been entered into the maintenance schedule periodically?