Improved Trouble Shooting
Thermography definitely should be one of the tools that are selected for motor and rotating equipment inspection. Thermographic examination can help technicians use the other tools, such as vibration analysis, more effectively. If a thermal anomaly is found, then the other tools can be employed to help isolate the cause of the problem.
Motors and Generators
When considering electric motors and generators, operating temperatures and thermal patterns can be a valuable key in a predictive maintenance program. All motors have a normal thermal pattern as well as given maximum operating temperature. This temperature is usually stated on the nameplate of the motor and is normally given as a rise in degrees C above the ambient air temperature. Most motors are designed to operate in ambient temperatures that do not exceed 40 C. Conditions such as inadequate air flow, partial discharge, unbalanced voltage, bearing failure, insulation failure and degradation in the rotor or stator can be identified with an infrared monitoring program. Abnormal thermal patterns can also identify misalignment in couplings when these devices are used in conjunction with motors.
Belts and Pulleys
Belts and pulleys are good candidates for thermo graphic inspection. The interaction of the pulley wheel and the belt generates friction as the belt contacts and then leaves the pulley surface. Additionally, the continuous tension and compression of the belt causes internal friction. Both of these processes result in heat being generated, which can be seen with the infrared camera. Comparing the thermal patterns of several pulley belt systems can provide clues to improper operation.
The temperature distribution across a pulley sheave should be uniform if everything is working as intended. In one documented case of a pulley arrangement with several belts, several belts were found to be running hotter than others.
The belts were replaced with no improvement in the thermal distribution. Something was obviously wrong. The pulley did not appear to be worn; yet it was replaced to see if the uneven heating would disappear. To everyone's surprise the uneven thermal distribution remained. At this point some people were questioning the thermography data!
Vibration testing confirmed there was something amiss. Test data showed that the fan speed was decreasing in relation to the motor speed over a period of time. This means that the belts were slipping. At this point, all the materials that had been removed were carefully inspected. It was then hypothesized that the belts may not have come from a matched set. After checking with the stockroom clerk, it was determined that these belts had not been stored and retained as a matched set. A matched set of belts was procured and fitted to the pulley system, and the thermo gram showed a nice even thermal distribution. This is an example where thermography used with other instrumentation and the perseverance of the investigators drove the inspection process to a correct and satisfactory resolution of the problem.
Evidence of Improper Installations
Bearing problems are generally found by a comparison of surface temperatures; comparing one bearing to another working under similar conditions. Overheating conditions are documented as hot spots within the infrared camera and are usually found in comparing equipment to equipment, end bell to end bell (for the same type of bearings) and stator to end bell temperatures (determined by motor design and configuration).
An automotive facility was having continual problems with some of their motor and pump combinations. The bearings of the pumps repeatedly failed. An infrared inspection confirmed that the lower thrust bearing was indeed warmer than the other bearings in the pump. Further investigation revealed that this motor - pump combination was designed to operate in the horizontal position. In order to save floor space, the pump was mounted vertically below the motor. As a result, the lower thrust bearing was overloaded leading to premature failure.
A failure like this costs over R 105 000 to repair, not counting for lost production time (at this plant an opportunity cost of R 210 000 per minute and labor costs over R 4200 per minute.
© Thermography Consulting 2008