Infrared Inspection of Motors

This article is provided courtesy of Snell Infrared, a thermographic training company.

Most thermographers know the value of inspecting motors with infrared, but not all are cross trained to understand what to look for.

Of course you can perform a simple check of the bearings, which typically should not run warmer than the motor casing. Pillow block bearings are simple to compare with each other. Vibration analysis should be used to confirm the thermal findings of a suspect bearing.

You can also inspect the coupling. An out of alignment condition will display a unique thermal pattern as the associated mechanical forces are converted to heat. Again, correlate the thermal data with vibration.

Abnormal friction on belts and gears will also often display clear thermal patterns. Belts that are both too tight and too loose can be seen thermally. But what else should thermographers be looking at on the motor itself?

Jack Nicholas, a well known expert on motor current analysis (MCrA), presented a paper at Thermal Solutions'97 the conference sponsored by the American Society for Nondestructive Testing (ASNT), which clarifies a number of issues regarding motor inspections. Nicholas, president of Maintenance Quality Systems (410-729-1285), clearly outlines a dozen thermal signatures and discusses their correlation with MCrA and vibration analysis.

He explains that motors are rated by their maximum allowable operating temperature which is determined by the type of electrical insulating materials used in the motor. Insulation degrades rapidly when it exceeds its thermal rating. For every 10°C (18°F) rise over their rating, anticipated motor life is reduced by 50%!

Ratings for motors are based by the hottest spot allowed in the insulating system, which is inside the motor, when the motor is operating in a 40°C (104°F) ambient environment. Temperatures thermographers see on the outside of the motor are usually 20°C (36°F) less than those on the inside. Thermographers should be familiar with this information when checking motors:

Maximum Temperatures**

Class Internal External A 105°C/221°F 85°C/185°F B 130°C/266°F 110°C/230°F F 155°C/311°F 135°C/275°F H 180°C/356°F 160°C/320°F

**Maximum temperatures are based on 40°C (104°F) ambient operating temperature.

Nicholas goes on to discuss the thermal patterns associated with various motor faults:

in AC motors a hot spot or uneven heating can indicate a resistive imbalance, with the high resistance phase being cooler. in DC motors excessive resistance, which can cause problems with torque and speed controls, may be indicated by a hot spot. an overall temperature increase or uneven heating in an AC motor may result from an inductive imbalance in the stator due to a shorted coil. shorted coils in an AC synchronous will have cooler poles on the rotor while hotter poles are associated with damaged pole laminations. uneven heating between the speed control and the slip rings on an AC induction motor may be caused by shorts in the rotor coil. hot motor leads may result from shorts in coils or loss of inductance in a DC field or armature winding. an overall temperature increase or hot connective leads may result from cracked rotor bars in a squirrel cage inductive motor. shorted bars in a DC armature will appear warmer while open bars will appear cooler. dirt buildup inside a motor can cause an overall increase in temperature.

Of course Jack recommends using other technologies, especially motor current analysis and vibration analysis, to verify findings.

John Snell & Associates is working with Jack to collect images representative of all these motor problems. If you have an image that fits the bill, please forward them a copy and tell about your findings.

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