Keep your motors running
1 Jan 2011
Amid all the advances in condition monitoring and related technologies, motor breakdowns seem as prevalent as ever. Patrick Raleigh examines why
UK industry is largely mismanaging its motor inventory, according to industry experts, citing the high level of low-voltage (LV) motors still showing up in repair company workshops.
For some, this trend is reflected in the robustness of the market for motor repairs, ABB and Brammer, for instance, highlighting industry estimates that the rewinds business is today worth at least twice that for new industrial LV motors.
“Why is the UK rewind market so big?,” asks Steve Ruddell, ABB’s division manager, discrete automation and motion. “Today, with advances in design and materials technologies, motors should have a lifespan of 20 years, but we’re still seeing motors in repair shops [that are] less than five years old.”
One viewpoint is that - with the exception of the larger players in sectors such as oil and gas, chemicals and, to a lesser though increasing extent, the utilities - most companies still fail to understand the need to adopt motor management strategies.
“For the majority of process companies, the mentality remains ’production, production’ with the maintenance engineer still seen as something of a necessary evil,” according to Mike Brook of industrial services specialist ERIKS.
But that’s not the full story, Brook pointing out that, even in the bigger companies, many engineers have been made redundant in recent times and, as a result, their skills eliminated from the process.
To compensate for this shortage of in-house engineers, ERIKS is being called on to do a lot more on-site motor maintenance work, including preventative maintenance and condition monitoring, as well as mechanical and electronic repairs.
Meanwhile, many companies are buying motors, sticking them in and forgetting about them, in part because there is, apparently, little to do to maintain them. This, warns Brook, ignores the fact that you still have to keep your eye on motors, and make sure they are not overheating or being overloaded.
Damage control
“Most motors nowadays fail through mechanical problems [rather than electrical],” said the ERIKS expert. “Mechanically, though, you get overloads, things jamming, or too much throughput. However, the motor - because it is under a machine or hidden somewhere - gets forgotten about, gets dirt all round it, so it is not getting sufficient cooling or being greased.”
Depending on the type of motor, you generally need to look at it once a month, continued Brook. “Half the time,” he said, “it’s a squirt of grease and just putting your hand on it to make sure it is not running hot, and to listen to it and see if it is overloaded or has bearing problems.”
The load side, noted Brook, is also often an issue, as a motor can be damaged by what it is driving: “The motor always gets blamed, but it is usually a mechanical fault that has put extra load on that motor and taken it out,” he said.
“People haven’t got time. In the good old days, most places had what they called a ’grease monkey’ who went around squirting the grease in and topping the oil up.
“That’s the level of guy, the sort of semi-skilled engineer, being got rid of. He was like the eyes and ears of the chief engineer and who would flag up a problem with the motor. He might not know what it was, but he would say that there was something wrong with it.”
Nowadays, concluded Brook, these staff are being replaced, where possible, with condition monitoring and hard wiring. However, he said, that tends to go only on critical plant, or equipment that’s seriously going to cause loss of production.
Rewind in decline
For his part, Mike Herring, sales director, Whitelegg Machines, does not see that the number of repairs are growing. In general, he says, motor rewinds have been reducing for many years, primarily due to the low cost of new motors.
“Many companies will simply replace failed motors with new ones, even up to as large as 20-30kW in size,” said Herring. “Some will replace motors to obtain better efficiency and hence energy savings, but others will replace simply because it is cheaper than rewinding.”
Brook at ERIKS concurs, noting that repair work on electric motors has fallen to around 48% of his company’s turnover from about 78% over the past 10 years. For the majority of standard, flange-mounted, squirrel-cage motors up to 25-35kW, it is, today, more cost effective to replace them, he said.
Motors should last 20-30 years when operated in perfect conditions, so if a motor is failing every five years there is likely to be a reason for this, according to Herring.
“Investigating the failures and ensuring everything is as it should be is important, as to simply replace a motor time and time again will not solve the problem,” he said.
The most common failure of motors is due to problems with the bearings, which are generally estimated to represent around 40% of breakdowns. This is why vibration analysis is the most common form of condition monitoring, said Herring. Other common faults, he added, include stator failures and problems with the rotor.
A key recommendation for helping reduce motor failure is condition monitoring. This includes vibration analysis, electrical testing - both offline and online - thermal imaging, oil analysis and so on, said the Whitelegg director. It is also very important, he added, to trend the data over time, so that changes in operation and/or conditions can be seen.
Modern, computer-controlled test equipment helps to perform effective condition monitoring once the motor is in service. However, it can also be vital in ensuring that new or rewound motors are fit for their purpose.
“New motors can - and do - fail shortly after installation, meaning that more and more repair companies are now testing all motors that pass through their shops, storing the data and then issuing test reports to back up their quality assurance,” said Herring.
Motor end users, he added, are starting to demand computer-generated test reports for new and rewound motors as a matter of course.
For Jeremy Salisbury, head of marketing at MRO company Brammer, the most common causes of problems with motors include issues with power quality, which can lead to increased temperature and overheating.
Another area, said Salisbury, concerns the condition of the insulation, stator, rotor and the air gap between the rotor and stator.
Problems in these areas often result in winding failure and electrically induced vibration, which in turn leads to bearing failure, according to the Brammer manager.
Repair or replace?
Many motors are being repaired, sometimes repeatedly, simply because companies lack the willingness or capability to draw up a management plan to deal with the sometimes complex issues involved in the repair-versus-replace decision, industry experts believe.
“Clearly, if you have an old motor that has been rewound many times, perhaps with core damage, it would be best not to rewind it,” said Mike Herring of Whitelegg Machines. “Core damage causes hot spots, reduces efficiency and the additional heat can lead to premature failure of the windings.”
Making the case for better motor management planning, Steve Ruddell of ABB said many companies think it is much easier when a process motor fails to have someone collect it, rewind it and reinstall it. But, he noted, if this is a critical, continuous process application, taking it off line could cost hundreds of pounds per hour.
Moreover, he added, the cost of a rewind depends on how quickly you need it returned. If it’s quickly then it’s a case of “take me to your wallet” - typically 70% of the value of the motor, falling to 40% if you’re not in such a hurry.
By contrast, said Ruddell, you can rip out a perfectly good motor and replace with a high-efficiency model and still get a return on investment in three years on energy costs alone.
“When chatting to engineering teams about replacement policies, they tend to look glum, and don’t really have an answer,” said Ruddell. “Engineers at plants are not bad, there’s just not enough of them, as teams have been cut back. But I’m confused as to why UK industry, in trying to survive, is carrying on with this huge waste.”
Jeremy Salisbury at Brammer agrees that UK industry is rewinding too many older electric motors when they fail, rather than replacing them with new, more energy-efficient models. This can lead not only to higher than necessary energy costs, but also to unplanned downtime and increased repair costs. He attributes this, in part, to a lack of understanding of the true running costs of electric motors - and the benefits that more energy-efficient electric motors offer.
This lack of understanding often leads to motors being rewound many times during their lifetime, with their efficiency continuing to decline over time when compared with a new, energy-efficient model.
Many companies do not have a clear plan in place for what to do when a motor does fail - and, without having considered the whole life cost of the motor asset base, it can often be seen as an easier and lower-cost option to have the motor collected and rewound.
Salisbury contrasts this with the success of corrugated board manufacturer Abbey Corrugated in adopting best practice in managing hundreds of motors at its site in Blunham, Bedfordshire. The company, he said, is saving between £1,000 and £10,000 per unit over a 10-year life cycle - depending on the size of the motor - by using only energy-efficient products when replacing motors at the site.
An electric motor can be quick and cheap to repair, so having it rewound and reconditioned can be the obvious choice when it fails. However, a rewound motor will typically suffer a drop in efficiency of between 0.5% and 2%, unless the work is carried out to a high standard.
This means a rewound motor may well have an efficiency several percentage points below that of an EFF1 (IE2) motor. While the initial repair cost may be lower, the reduced efficiency will mean increased running costs that can quickly outweigh the initial saving in capital.
Industry suppliers also point out that it is important for end users to understand the total lifetime cost, combining the purchase and installation outlay together with the energy running costs over the entire motor life.
“This usually shows that specifying an energy-efficient EEF1/IE2 motor has a lower total cost of ownership than opting for the repair solution with the seemingly lower initial cost,” said Salisbury.
Last word, though, to an ERIKS spokesman, who said: “If you speak to a repairer they will suggest a repair, if you speak to a new product supplier, guess what they will say. Few offer a proper balanced view as to what is the best solution and how to plan for such an eventuality via a motor management plan.”
