What’s Three Minutes of Motor Testing Worth?

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Personnel at the motor-distributor’s service center used an AT5 to perform a de-energized, non-destructive motor-circuit-analysis test on the hospital’s failed motor. This type of analysis evaluates the condition of electric-motor connections, stator, and rotor.

A non-destructive technique offered payback by determining the root cause of a critical, repeatedly failing hospital motor.

A motor distributor in The Netherlands provided a 17-kW, 400-V motor to a local hospital in 2015. The hospital rented a portable crane to install the motor on the roof, where it was used to operate a fan. In the spring of 2016, the motor suddenly failed.

Challenge

When the motor first stopped running, the hospital’s maintenance technician had reset the unit’s variable-frequency drive (VFD). Although the motor restarted, the VFD shut it down once again.

The technician then performed an insulation-to-ground test and determined the motor winding wasn’t shorted to ground. Using a digital multimeter, he measured phase resistance and learned the phases weren’t open. Since the motor testing tools indicated a “good” motor, a decision was made to replace the VFD.

After the new VFD was installed, the motor started, but, much to the technician’s chagrin, didn’t continue running. It was at this point that the hospital reached out to the motor distributor, which promptly dispatched a technician from its service center to test the motor.

The service-center technician used a meg-ohm meter and DMM to determine that the motor wasn’t grounded and the phases weren’t open—the same as the previous findings by the hospital’s maintenance technician. Given these results, the decision was made to replace the motor. The new unit started and operated normally, confirming the new VFD was working as intended. The “suspect” motor was sent to the distributor’s service center for a more thorough inspection.

Fig. 1. This figure shows the stator and rotor signatures from the motor-circuit analysis report, with phases listed at the top of the image.

Solution

At the service center, personnel used an All-Test Pro 5 (AT5) to perform a de-energized, non-destructive motor-circuit analysis (MCA) test on the failed unit. This type of test evaluates the condition of motor connections, stator, and rotor.

Using the AT5, connections were made to the three phases of the motor and a static test was performed. Next, the motor shaft was manually moved during the dynamic portion of the three-phase test. At the end of the test the instrument indicated the results shown in Fig. 1. This testing made it clear phase 2-1 (shown as “21” in the figure) had the problem.

Lessons learned

Owners/operators can reduce maintenance costs. A 17-kW, 400-V motor is not expensive, but when it is mounted on a roof and the owner has to rent a crane to lift that motor for installation and removal, the owner’s maintenance cost can become quite high. Had the hospital’s maintenance team used an effective motor-circuit-analysis device, they would have discovered that the motor was the bad actor, not the VFD. Many hours and dollars were wasted by ordering and installing a new VFD that had not been the true cause of the problem.

Distributors and suppliers can improve quality assurance. Motor distributors and suppliers should implement an additional quality-control measure prior to delivering new or off-the-shelf motors to their customers. Spending a few minutes to check the condition of motors will help distributors and suppliers avoid warranty issues and increase customer satisfaction. MT

To learn more about motor-testing tools and techniques from All-Test Pro (Old Saybrook, CT), visit alltestpro.com.