MRO Magazine

Tips For Troubleshooting Motors

Topping the list of the many challenges facing maintenance managers is keeping equipment and labour costs under control. Testing and troubleshooting motors and drives, which can be particularly expens...


February 1, 2009
By Colin Plastow

Topping the list of the many challenges facing maintenance managers is keeping equipment and labour costs under control. Testing and troubleshooting motors and drives, which can be particularly expensive to replace, can help maximize their operational lifetime and lead to significant cost savings.

There are three tests that technicians can use to detect problems with motors, drives and associated electrical panels and determine their cause: electrical, insulation resistance and thermal measurement.

Testing for abnormal thermal patterns

Handheld thermal imagers can collect heat signatures from any size motor, be it fractional horsepower or 1,000+ hp.

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A thermal imager is a powerful tool for long-term trending of operating conditions, yet it is also good for spot checks, to see if motors and associated panels and controls are operating too hot, and for troubleshooting, to track down the specific failed component at fault. It can also check for phase imbalance, bad connections, and abnormal heating on the electrical supply.

Using a thermal imager to test a motor’s heat signature will tell you a lot about its quality and condition. For example, if a motor is overheating, the windings will rapidly deteriorate. Every increase of 10C above its rated maximum operating temperature cuts the life of the winding insulation by 50%, even if the overheating is temporary.

If a temperature reading in the middle of a motor housing is abnormally high, a thermal image of the motor will determine cause, i. e. windings, bearings or coupling. A coupling that is running warm could be an indicator of misalignment.

There are three primary causes of abnormal thermal patterns:

1. High-resistance contact surface: Most abnormal thermal patterns are caused by a high-resistance contact surface, either a connection or a switch contact. These will usually appear warmest at the spot of high resistance, cooling off further away from the spot.

2. Load imbalances: Whether normal or out of specification, load imbalances will appear warmer throughout the phase or part of the circuit that is carrying higher current. If the entire conductor is warm, it could be undersized or overloaded. Check the rating and the actual load to determine the cause.

3. Failed component: Failed components typically look cooler than normally functioning ones. The most common example is a blown fuse. In a motor circuit this can result in a single-phase condition and possibly result in costly damage to the motor.

Insulation resistance testing

Insulation problems on motors and drives are usually caused by improper installation, environmental contamination, mechanical stress or age. Insulation resistance testing can easily be combined with regular motor maintenance to identify degradation before failure. It can also be used during installation procedures to verify system safety and performance.

Insulation testers apply a DC voltage across the insulation system and measure the resulting current. This allows the resistance of the insulation to be calculated. Typically, the test verifies high insulation resistance between a conductor and ground or high insulation resistance between adjacent conductors. Two common examples include testing motor windings for insulation from the motor frame and checking phase conductors for resistance from bonded conduit and enclosures.

Insulation multimeters combine insulation resistance functions with the other tests needed to investigate motor, drive and electrical trouble — from basic supply measurements to contact temperature. One key difference is that insulation resistance tests are performed on de-energized systems, while electrical tests (and thermal too) are almost always performed on live, operating systems.

An insulation multimeter can perform most of the tests you need to troubleshoot and maintain motors. When a motor is having problems, check the supply voltage and then use insulation testing to check the starter and control contacts, measure the insulation resistance of the line and load circuits to ground, and the winding resistance phase-to-phase and phase-to-ground.

Insulation multimeters and thermal imagers are an integral part of a technician’s arsenal in ensuring that equipment is functioning at optimal capacity. The regular testing and troubleshooting of motors and drives can maximize their operational lifetime and lead to significant cost savings.

Colin Plastow is the industrial product manager for Fluke Electronics Canada. He can be contacted atcolin.plastow@fluke.com.

Reader Service Card No. 401


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