Infrared Thermography Finds Overheating Motors [top]

By Forrest Wilcox
Level I Thermographer
J.R. Simplot Co.

Are you replacing electric motors after one to five years that should last ten years? In the past this was not unusual in the plant where I work. Furthermore, how much power is being wasted while these motors are running at less than nameplate efficiency?

The J.R. Simplot Co. plant at Pocatello, Idaho produces sulfuric and phosphoric acid and various grades of liquid and granular fertilizer. With the intense competition in this industry, we must do everything possible to insure that we produce our products at the lowest possible cost. Establishing an infrared inspection program on the motors has been a key factor in achieving our goals in this area. Most large chemical plants have hundreds of electric motors and maintaining proper motor temperature is very important if motor maintenance costs are to be kept to a minimum.

Generally speaking, each 10 C degree rise above the rated temperature cuts motor life by half. An increase of 20 C degrees above rated temperature would reduce motor life to about one-fourth of normal. Regularly scheduled infrared inspections of electric motors identify motors, which are starting to overheat. Additional inspections can identify small problems before they become major problems.

A common problem we find is restricted cooling airflow caused by build up of process residue on the fan end of the motor. Some of our big process mixer motors are mounted high above floor level where the fan end is not visible. A quick infrared inspection from floor level allows inspection without climbing up to inspect the air intake grill. If a brief shutdown is possible without affecting the plant process, the motor is shut off long enough to perform minor cleaning on the air intake grills, and a thorough motor cleaning is scheduled for the next plant down day.

Since starting our regular infrared inspections on the motors, we have had no motor failures due to lack of cooling airflow.

Figure 1 on the left was taken November 14 shows the motor running at 244.9 degrees F. Figure 2 taken the next day after minor cleaning was performed on the air intake grill shows the temperature has came down 53.8 degrees F. Considering there was a 6.1 degree F difference in ambient temperature there was a net improvement of 47.7 degrees. It was still operating over recommended temperature, but we were able to continue running it until a regularly scheduled down day when a thorough cleaning could be performed.

Figure 3 shows the dirt buildup on the cooling air grill, partially restricting the cooling air to the motor. Before we started our infrared inspections it was not unusual to find intake ports completely plugged with this material.

Very Small Resistive Imbalances Can Cause Very Significant Increases In Motor Temperature

From time to time we find motors that are overheated, are not dirty, have good cooling airflow, and are not overloaded. It takes a little more detective work, but often the problem can be identified with an infrared inspection.

Increased resistance to the flow of electricity causes an increase in temperature. An increase in resistance on one of the feeders to a three phase motor causes a resistive imbalance. This not only causes a temperature increase at the point of high resistance, it also causes voltage and current problems that result in increased motor temperature. A high resistance connection in the switchgear, disconnect, or motor connection box can usually be identified with an infrared inspection. If you have a resistive imbalance and don’t find a problem in these areas, the problem may be in the stator windings or internal motor connections, but more often than not we find the problem is in the power circuit. Often there is a problem in the motor connection box with loose connections or improperly installed, mismatched, or corroded components. A few strands of damaged wire on a motor lead can cause a significant resistive imbalance.

After we find a motor is hot, and we suspect a high resistance connection in the power circuit, we begin our inspection at the motor control center. We report any temperature differences between phases of ten F degrees or greater. Depending on the importance of the motor to our operation we may report differences of five F degrees . If this leads us to the motor connection box we perform an offline motor test if possible. On most of our critical motors however, a half hour shut down to perform tests is not an option and an infrared inspection at the motor connection box can often identify the problem.

You may not have the equipment to perform the motor tests, and an infrared inspection may be your next best option anyway. I used this 450 horsepower blower motor for this article because it so clearly demonstrates the effects of a resistive imbalance. Obviously you won’t find very many motors with this much imbalance that are still operating and most motors won’t have two separate conduits feeding them. The box cover will usually need to be removed.

Follow all safety precautions when working around moving machinery and hazardous voltage. Damage inside motor connection boxes can expose potentially hazardous live electrical surfaces. The motor should be shut down long enough to remove the cover if possible.

The motor in the following infrared image (Figure 4) had been running somewhat hot for over a week, but was not overloaded. We had sent an infrared inspection report to the manufacturer with images taken from the other side of the motor. The motor representative said that the motor was ok to continue running, and the elevated temperature was not a major problem. The motor connection box and conduit were not visible in the inspection, which was taken from the side opposite the connection box. We went back the next day and took some infrared images from the connection box side, and learned a good lesson. Always look at the box, and open it if you must. After adjusting the span on the camera the source of the heating became very evident. A loose connection was causing heating on one set of parallel feeders going to the motor and causing a severe resistive imbalance. The temperature inside the box was so hot that it was visible with the indirect image taken on the conduit and box cover. Notice the 24.8 F difference between the top and bottom conduits.

After an orderly shut down of the plant, the motor connection box cover was removed. One of the connections had a loose bolt that was causing the imbalance. The motor leads had been so hot that there was residue from soot and carbon on the interior of the box.

The motor was sent out for repair and all that was required was a cleanup and new motor leads. The price of the repair including new bearings was $2,414.00. If we had continued running the motor with the loose connection very much longer we would have had to do a complete rewind, which would have cost about $10,000.00. Replacement price of the motor would have been $19,296.00. With plant downtime costing hundreds or thousands of dollars per hour, the price of the motor is not the major expense when some of our motors fail.

The manufacturer of our motor tester recommends that resistive imbalances over one percent between phases need to be corrected. You may not be able to see a temperature rise on the motor leads until the imbalance is somewhat higher than this depending on how the motor leads are insulated.

Offline motor tests performed on the motor from the motor control center before it was disconnected and sent out for repair verified that we had very high imbalances in the power circuit. We were surprised that the motor was still able to run, and it probably would have failed in a short period.

Notice in the motor test report, how much power would be wasted if the motor could run for a year with an imbalance this high: $47,074.00

The first test was performed when the motor was taken out of service. The second test was performed after clean up and replacement of the motor leads. No rewind was necessary.

Summary

Motor cleanliness and power circuit problems are only two of the many reasons that motors overheat. Infrared inspections have helped us find these problems, and together with motor testing, vibration analysis, and high lubrication standards we are significantly reducing our motor maintenance costs.

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