46 n SERVICE AND REPAIRS September 2024 www.drivesncontrols.com Failed motor bearing cuts plant output by 25% A minerals processing company based in the US relies on many large motors to power key items of equipment, such as water pumps, pulverisers and vacuum pumps. To reduce downtime and to minimise its inventory of spares, it uses similar motors wherever possible. For example, seven key items of equipment are powered by almost identical 700hp (522kW) medium-voltage motors that are now around 20 years old. In addition to holding essential spare parts, the company also keeps a complete motor that it uses to replace assets when they are removed to be overhauled every three or four years. However, some of its maintenance routines are sporadic. The servicing specialist Sulzer carries out planned maintenance and overhauls of the company’s rotating equipment, including the interchangeable 700hp motors. When one of these motors failed unexpectedly, it was sent to Sulzer’s service centre in Colton, California, for inspection and repair. Once the motor had been disassembled, an examination revealed that the drive-end bearing had overheated and failed, allowing the rotor to contact the stator, thus damaging the windings in both the stator and the rotor, as well as the bearing cap. The motor had been overhauled three years before, so Sulzer had records of its technical details. This made ordering new windings much quicker, speeding up the repair process. The rotor and stator were stripped out before checking the laminations for any defects. Some minor machining enabled hot spots to be removed, and a core loss test produced a satisfactory result. At the same time, a new bearing cap was manufactured in-house, ready for reassembly. “Under normal circumstances, the minerals company would have installed the spare motor and production would have continued unaffected,” explains Gary Patton, who manages the service centre. “However, in this case, the spare had recently been shipped to us for repairs following a premature bearing failure, caused by a problem in the application. “We found that the vacuum pumps powered by the motor were located below another process, which meant that the motor’s cooling ducts could become blocked with debris,” Patton continues. “Unless this was cleared regularly, the motor and bearings could start to overheat, as happened in this case. A simple maintenance process could have prevented repeated failures of these motors and saved time and money for the business.” Unfortunately, the lamination stacks of the motor’s rotor and the stator could not be saved, so additional parts were needed, extending the repair time. The dual outage had a significant effect on the volume of minerals that could be processed at the site, resulting in a 25% drop in output. It was essential that the failed motor was repaired as soon as possible. Once the new coils had been installed, along with the new bearings and seals, the motor was reassembled and run on a testbed before being returned to the user. The project had been expected to take five weeks, but by speeding up production of the coils and working in two shifts, Sulzer was able to cut the repair time by a week. The repairer has suggested to the minerals company that it provides a shelter for its motors to reduce the build-up of debris, allowing them to operate reliably between planned overhauls. It has also recommended adding bearing temperature sensors which could alert maintenance staff to any issues before the bearing fails again. n Carrying out routine maintenance can be a chore, but failing to perform regular checks on important pieces of machinery can result in unplanned downtime and lost productivity. This was the case for an American minerals processor where a large motor failed unexpectedly, as a result of a faulty bearing. Some simple steps could have prevented the failure.
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