Paul Knock, Global Sales Manager, SKF
Every motor that leaves the factory is in perfect shape mechanically and electrically, and that’s thanks to a combination of exacting manufacturing practices and rigorous testing. SKF’s manufacturer-focused products, such as the Baker WinAST static motor analyzer, can ensure that the windings are correctly wound and the insulation is in perfect condition.
But what happens after deployment?
Are Your Motors Keeping Dark Secrets?
A row of smoothly humming motors powering a productive facility is something that warms a plant manager’s heart. Everything’s running as designed, production is going to plan, and the threat of unscheduled downtime seems far away.
But is everything as it seems? Are the motors in peak condition? Or are there degradations and insulation failures which, while not apparent now, may one day bite you in the assets?
The truth is, all electric motors suffer degradation over time, from heat, vibration, magnetically-induced stresses, contamination and environmental factors. And while the motors may appear to be running just fine, they may already be nearing the end of their useful lives. Running motors to failure may be the simplest way to maintain a plant, but the cost of unscheduled downtime far exceeds the much wiser investment in a comprehensive predictive maintenance (PdM) program. It’s always better to find and fix latent problems before they become real showstoppers.
So can the practice of PdM be applied to electric motors? Absolutely, yes. SKF’s line of static analyzers and dynamic monitoring systems provides all the tools needed to uncover those dark secrets.
Static Analysis: Revealing Degraded Insulation and Coil Imbalances
The most common mode of motor failure is mechanical, usually when the bearings fail. However, in second place is electrical failure, which generally starts with insulation degradation in the stator windings, though other failures can occur, such as broken bars or armature problems. Turn-to-turn arcing leads rapidly to other, more serious failures as it can damage winding insulation, ground wall insulation and the copper itself. But it’s hard to determine if turn-to-turn arcing is happening, especially if it’s deep within a random-wound coil in a motor that’s in service.
This is where standards-compliant test equipment, such as the Baker DX and AWA families from SKF, comes in. Using electrical tests which are applied when the motor isn’t running (hence the term “static”), these analysers can expose a range of insulation degradations and failures, helping the maintenance manager to make a judgment call on whether the motor will continue to run for a long time, or whether plans for its servicing or replacement should be made.
Several test types are available, each one focused on certain aspects of the motor’s health. Resistance, inductance and capacitance tests can confirm whether a motor’s stator coils are well balanced. DC HiPot testing checks for ground wall insulation failures, as well as checking the insulation’s polarization index, which is an indicator of aging. Surge testing, which consists of applying very short duration, high voltage pulses to the windings, is the best, and indeed the only reliable way to expose turn-to-turn insulation weaknesses. These high voltage tests are truly predictive in finding insulation issues in their infancy, allowing weeks or months of monitoring during continued operation. In contrast, most low voltage tests find faults after the motor has already failed. This is autopsy, not predictive maintenance. Most industrial motors should undergo static testing once every six or twelve months.
Dynamic Monitoring: Healthy Power + Healthy Load = Healthy Motor
But does this mean that you only get to check on a motor’s health every six or twelve months? That’s where SKF’s dynamic monitoring products come in. Designed to measure the voltages and currents in real time on a running motor, the EXP4000 can expose a wide range of issues, such as poor power quality, start-up issues, harmonics, phase imbalances and noise from VFD drives. And by applying the mathematics of motor science, the instrument dynamically monitors the torque demanded by the load, making it possible to see loading issues in real time and allowing electricians and mechanics to determine if a problem is in the motor or with the load. There are many cases of motors inexplicably failing in certain applications, where the EXP4000 has identified the underlying causes through continuous monitoring.
While the EXP4000 is an instrument that a technician takes on a test route, the SKF NetEP On-Line Motor Analysis System is a permanently-installed, network-connected dynamic monitor that can keep an eye on up to 32 motors, 24 hours a day, seven days a week. Multiple NetEP devices can be added to monitor any number of motors. A maintenance manager in his office,or using a laptop in Starbucks, can see at a glance whether any motors are having issues, and the detailed data in NetEP’s database allows analysis of a motor’s behaviour and operating conditions over an extended period, which allows infrequent and potentially damaging power or loading conditions to be caught and understood.
Reliability Requires Measurement
While motors can be expensive, unscheduled downtime can cost far more so it behoves maintenance and reliability managers to know the health of their motors. Often, finding an insulation problem early can reduce a repair to just a rewind, whereas it could be a rewind plus restacking the motor core laminations if the insulation failure burns to ground, typically tripling the cost of the repair. With SKF’s analysers, it’s possible not only to get a picture of current motor condition, but also to understand whether future failures are on the cards. Armed with this knowledge, scheduled maintenance and replacement planning will keep the plant operating smoothly.
To learn more, attend SKF’s session on Predictive Maintenance Through Electrical Testing and Motor Diagnostics, on February 8th, at 3:15pm and see us at SKF’s booth in the exhibition hall.