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Uncover Your Motor's Hot Spots

Sep 1, 2001 12:00 PM, By John Paschal, P.E., Editorial Consultant

Excessive heat — especially when it relates to motors — is a killer, but infrared thermography can identify this deadly condition.

All motors get hot when they operate, but sometimes it's difficult to know when or if temperatures are excessive. Identifying hot spots can be even more difficult if you lack the correct equipment for the job. Modern infrared inspection tools are invaluable for the prevention of overheated motors and the problems they can cause. These devices offer exceptionally accurate and revealing information for low-cost predictive maintenance.

You can use thermal overload detectors and electronic overload devices in motor controllers to mimic motor heat and to de-energize the motor before excessive heat damages it. Some large motors also contain embedded remote thermal units (RTUs) to measure local winding temperature. It can be easy, however, to develop a false sense of security and neglect the parts of the motor these devices don't measure, such as the bearings and the shaft. During a motor event, bearing lubricant can get so hot that it cokes and ceases to lubricate. The subsequent extreme heat can cause the bearings to fail. At this point, the bearing race typically becomes friction-welded to the shaft.

Several conditions can lead to destructive motor events. Shafts can be misaligned at the load coupling, causing the shaft to flex continuously during rotation. Minor cases of misalignment can cause the shaft to heat up. However, results are more severe in the case of bad misalignment, where work hardening and shaft breakage can occur. Dirt or other foreign objects can also cause bearings to fail prematurely. When a motor's air passages become clogged with debris, its windings can overheat and fail. It's no secret these situations present the potential for motor failure. The problem comes in knowing how to detect these conditions before serious damage sets in.

You can tell whether a motor — or part of a motor — is very hot by viewing it through a thermoscan camera. Most models of these cameras even provide temperature indicators to digitally note the surface temperature of an object within the viewfinder. When the readout temperature approaches the maximum operating temperature of the insulation or grease, this alerts the operator to shut down the motor for maintenance. But the most valuable information the camera can provide is an image of one part that is much hotter than its surrounding components. Remember, just because a part of the motor is hot doesn't mean it's approaching dangerous temperature levels. It's important to know what materials the parts are made of to determine safe heating levels. Keep in mind the heated internal motor parts are operating at a higher temperature than the digital temperature value indicated on the camera.

You can obtain accurate thermoscans quickly and conveniently with one of the many hand-held cameras on the market. Thermoscans have been used for many years for preventive maintenance of switchgear, but they are just as useful, if not more so, for examining motors. Just aim and shoot to instantly identify overheated bearings, misaligned motor shafts, plugged cooling air passages, or loose or corroded wire joints within the motor or motor junction box. You can use the cameras to identify improper wiring connections or splices, corona, arcing brushes, heating of brushes or slip rings and commutator, poor contacts, a shorted winding, or an overheated surge capacitor.

You can also use thermoscan testing as a nondestructive method for discovering motor overload and deep corrosion or rust in the stator housing. The camera will depict corrosion as lines of differing temperature outlining the corrosion depressions in the metal.

A thermoscan can determine the temperature rise of a motor to an accuracy of about 2°C. Let your electrical system relays continue to provide motor protection against overload, low voltage, and loss of phase, and use your thermoscan unit to complete the protection scheme for large motors.

Just after installation, allow each motor to warm up and scan it during operation to check for shaft misalignment. Afterward, you should scan them periodically. Keep a file of photos for each motor and compare them to new photos so you can check for differences in operating temperature over time. This procedure enhances the thermoscan's predictive analysis and improves your chances of saving money and reducing downtime over the long haul.

Thermoscan cameras exist for almost every need. Different cameras are available for different temperatures. One thermoscan camera operates from -20°C to +160°C, and another unit measures temperatures as high as +450°C. Simple cameras are available for taking snapshots, while more sophisticated units can also provide full, printed reports. Regardless of which thermoscan camera you select, this technology will take your motor's preventive maintenance program to a new level of ease, validity, and value.

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© 2012 Penton Business Media, Inc.

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