Motor Testing Saves Steel Mill Time, Money

Initial analysis during a quarterly inspection seemed to suggest motor degradation, but was it?

Re-testing Gerdau’s trolley motor with the ALL-TEST PRO 5 (AT5) de-energized testing instrument connected directly to the unit found that the problem was in the festoon cabling from the motor-control junction cabinet.

With an annual manufacturing capacity of approximately 10M metric tons of steel products, Gerdau (gerdau.com) is one of the largest producers and recyclers of steel in North America. Its Midlothian, TX, site maintains an inventory of about 70 cranes in a range of sizes for various uses throughout the mill and materials-recycling operations.

The in-house predictive-maintenance (PdM) team inspects the site’s overhead cranes quarterly. Alerted to a potential problem by the ALL-TEST PRO 5 de-energized motor-testing instrument from All-Test Pro (alltestpro.com, Old Saybrook, CT), technicians determined the root cause of the issue.

BACKGROUND

The materials-recycling operation’s #4 scrap-crane trolley motor was scheduled for inspection during the first week of March, 2017. An AT5 de-energized motor-testing instrument was used to perform a motor-circuit analysis (MCA), which enables a technician to determine the condition of an electric motor’s windings. The hand-held AT5 had been purchased in 2016 because it was more portable than the steel mill’s 60-lb. testing instrument that required a dedicated 110-V power source.

The AT5 was connected to the motor-control panel, and a motor-circuit analysis (MCA) was performed on the 20-hp trolley motor. Within only a few minutes, the test results showed a disconcerting insulation-to-ground readout of 10.1 megohm. This “practically new” motor had been installed onto the crane and tested in Oct. 2016. That first test had shown a reading of greater than 999 megohm. This marked difference in test results for a motor that had only been in operation six months raised red flags for the PdM team. The motor should not have deteriorated so quickly.

THE REAL PROBLEM

The PdM team scheduled a retest of the trolley motor the following week. At that time, however, they planned to connect the AT5 directly to the motor. Within minutes of performing the second test, the AT5 reported an insulation readout of greater than 999 megohm.

At this point, the team realized the issue was not with the motor. It lay in the festoon cabling from the motor-control cabinet up to the trolley motor. The cables from the drive to the festoon-trolley junction box were checked for damage and then promptly replaced.

Significant changes in test results between the time this ‘practically new’ trolley motor was first put into service and after six months of operation raised red flags with Gerdau’s PdM team in Midlothian, TX.

WHAT THEY LEARNED

Gerdau’s PdM team did everything right:

They tested the new motor at installation. This is a standard approach at the site. It’s important to test new motors at the point of installation. If a “new” unit is being installed after having been stored for a significant period of time, testing can confirm that it will operate as promised. Testing at the point of installation might help to uncover problems that can be resolved within the warranty period. From a long-term standpoint, motor testing will also establish baseline data that may come in handy for future maintenance review and decision making.

They continued the testing instead of immediately replacing the motor. They didn’t waste time trying to replace a motor that was in perfectly good condition. They knew how much time and how many resources would be required to replace the motor, including an overhead crane, multiple technicians, and additional man-hours. By getting to the root of the problem, they saved time and money.

They used a reliable testing instrument. The AT5 motor tester has proven to be an invaluable tool for motor diagnostics. Lightweight and portable, it provides fast, accurate results. These features and capabilities allowed Gerdau’s PdM team to find and resolve the real problem quickly, and then move on to other projects. EP

What is Motor Circuit Analysis?

A low voltage, non-destructive AC signal is sent through the motor windings to measure the response to these signals. The phase angle (Fi), current/frequency response (I/F), impedance (Z), resistance (R), and inductance (L) of each winding are measured and analyzed. Winding faults are indicated by variances in the response to the applied signal through the windings. These variances cause imbalance in the measured response to the applied signal. Therefore, when testing three-phase equipment such as motors, generators, or transformers, the response of each phase is compared with the other two. Phase resistance is evaluated to determine connection issues: inductance is evaluated for possible rotor problems; impedance and inductance matching are used to detect winding contamination or over-heating; insulation-to-ground tests detect conductor-to-ground issues.

For more information, visit alltestpro.com.