Although there are technical differences between AC and DC motors, sufficient commonalities exist between them to indicate their need for attention by maintenance teams. “For both types of motor we generally focus on the condition of bearings and windings as key health indicators,” explains Peter Wright, ABB Motion Services quality & customer experience manager. “Because windings carry high currents and are subject to heating and cooling cycles, their insulation becomes a classic failure point for motors. For bearings, problems are usually mechanical, such as an unbalanced or misaligned drivetrain, as well as under- or over-lubrication.”
When focusing on DC motors, a good sign of correct motor operational health is brush wear and commutator patina.
“Rapidly wearing brushes can be an indication of incorrectly-sized brush gear,” says Wright. “Brushes can also be damaged by electrical overloading or under-loading, winding faults, and voltage surges.”
It is important for motor inspection activities to focus on the complete drivetrain. This strategy will help to pinpoint specific problems, such as when the motor is driving an unbalanced fan, which can cause high levels of vibration that ultimately lead to bearing failure.
“For this reason, traditional monitoring, carried out during normal operation with the motor running, relies largely on measuring vibration and temperature, although current and voltage in the cables feeding the motor may also be monitored,” says Wright. “It’s now becoming more common and cost- effective to use wireless smart sensors to gather this kind of data.
“They provide a cost-effective way of taking measurements on an hourly basis, making it possible to spot trends that might not be captured by monthly route-based monitoring using handheld equipment.”
SMART MOTORS
Unfortunately, an electric motor cannot volunteer the data that indicates its health, unlike a variable speed drive (VSD).
“That means we rely on smart sensors, which combine the capability to monitor vibration, temperature, acoustic signals and magnetic fields, converting motors into smart, wirelessly connected devices that report their own health data,” says Wright. “However, it’s not just the collection of data that’s important, it’s how we make use of it. When data is analysed and processed by algorithms, service experts can help customers make better decisions. They can then draw on the deep insights they gain into the status and condition of their assets to take timely action.”
Predictive maintenance based on condition-based monitoring allows engineers can plan and schedule any required work for times when it will cause least disruption.
“Furthermore, it can be possible to leverage knowledge about the health of a motor to adjust the way it’s used, such as by reducing its load so that it might be ‘nursed’ to the next scheduled downtime,” explains Wright, who goes on to suggest that performing a root cause analysis is always advisable in case an issue is found.
“For example, if the fault resulted from an imbalance in the fan or poor-quality bearings, it will keep recurring,” he states. “A partner with service expertise is essential to make effective use of the condition data collected, so it can advise if a repair or upgrade is appropriate, or if replacement might be the most cost-effective approach.”
Jason Tweedy, head of condition monitoring - UK at Brammer Buck & Hickman, says that the best inspections are audible and visual checks while the motor is running. He adds: “The persons best suited to conducing these inspections are the on-site operations engineers, who know their equipment better than anyone else and will therefore be aware of a slight change in noise or movement, giving them the ability to action an appropriate response,” he says.
Tweedy adds that further checks are possible when the motor is isolated, which will be specific to certain types of motor: inspecting brushes on DC motors, checking lubrication and ensuring there is no damage to couplings, for example. Also, checking for an excessively or unusually hot motor is important as overheating is one of the main reasons for motor failure.
“Ideally, routine checks should be performed daily,” he says. “Specialist checks, like greasing and brush inspection, will depend on the motor, its size, running hours and operating environment, but as an approximate guide, every three months would be suitable.”
In terms of tools, Tweedy recommends an off-the-shelf thermal camera as a good place to start. “Thermal cameras are relatively inexpensive and can provide useful information if used regularly to inspect motors,” he explains. Grease guns or specialist lubrication can help extend bearing life, he adds.
According to Brammer Buck & Hickman, most motors fail due to incorrect installation or use; for example, a misaligned motor or shock load to the transmission. “If issues such as misalignment or lack of lubrication are found, act immediately,” concludes Tweedy. “If you’re unsure what to do, contact an expert. Motors are the work horses of virtually all plants. If they stop, your business stops.”
BOX: LISTEN UP
According to John Drew, site services support manager at electromechanical services provider Rotomec, “listening for unusual noises at either end of the motor could be an indication of bearing failure. Here, the motor would require removal for inspection at the earliest possible convenience to avoid a catastrophic failure, which could potentially write it off. Furthermore, while not all motors have bearing grease points, it’s important to use the same grease throughout the life of the motor and/or bearing. Mixing grease grades or types can lead to premature bearing failure, as the grease will harden and not mix correctly in the housing.”
Drew is keen to point out that while most of these checks can take place during motor operation, it is best to do them at time when turning off the motor is possible, if required.
“Most checks can be made with a good quality insulation tester, multimeter and vibration analyser,” he says. “Common issues we see include belt-driven fans with motor bearing failure, where the likely cause is incorrect tensioning of the belts and/or misalignment of the drive pulleys. We also sometimes see new motors that have burnt out their windings almost immediately after fitting. It’s so important to check and test the control gear prior to installing a new motor.
“There are reasons behind any failure,” continues Drew. “It’s easy to address the failure but also easy to overlook the root cause. That’s why regular maintenance is essential, where using an experienced electromechanical services provider will help to keep consistency of service and offer the best protection against premature failure.”