How to keep drives well serviced to prolong life
Variable speed drives are among the most reliable pieces of equipment. Many sit there for year after year performing their allotted function. Yet, they cannot be totally ignored if they are to continue to work reliably - good, regular servicing is a must.
Although not among the most expensive items of capital equipment, variable speed drives are not a trivial investment and getting the most out of them means ensuring they are looked after.
Regular servicing can maximise the life of a drive – conversely, poor or irregular servicing can lead to constant breakdowns and the consequent disruptions to operations and production.
A variable speed drive is essentially a combination of a small computer and a power supply, and should be provided the same care normally given to such components. nder a planned maintenance scheme, a service company should visit your plant at agreed intervals, ensuring all connections are tight, that the ventilation is working and that the drive is free from dust and moisture.
But how often should a drive be serviced? This depends to a large extent on the environment. A drive working in the dust-laden atmosphere of a quarry might need to be serviced every six months, while another, ensconced in the benign, relatively dust free surroundings of a switch room, may only need servicing at intervals of a year to 18 months.
Dust and its effect on the cooling airflow through the drive are a major factor. Variable speed drives are essentially solid state, with the cooling fan being the only major moving part. The fan must always be checked at each service but in normal use should be good for around six years of continuous use. Yet the fan cannot maintain a good airflow if the filters are blocked with dust and these should always also be checked at each service.
Keeping the drive dust free helps the cooling fan keep the electronics at the correct running temperature. Major drive manufacturers such as ABB design their drives to run at a high ambient temperature of around 40 degrees C, yet the power capacitor should ideally be kept at 25 degrees. Every seven degree increase in temperature can reduce the life of the capacitors by half.
Fans have up to a six year life before the first replacement becomes necessary, while in year nine of a drive’s life, small electronic boards may need to be changed together with some capacitors. Larger capacitor banks typically need replacing after 12 years, though at elevated ambient temperatures, this can be reduced to two years.
Although the capacitor itself should only need replacing every 12 years or so, this can be a long time in the evolution of drives. With a ten year old drive below 37 kW, it may well be cheaper to replace the whole drive with a newer more modern unit than to replace the capacitor.
The time required for each service usually depends on the size of the drive – typically an hour for the smaller dives and anything up to two days for larger drives. Mechanical and connective components also need to be checked, particularly ribbon cables and fibre optic connections.
The benefits of a proper drive inspection and maintenance system can be substantial. A manufacturer of filtration materials in northern England discovered it could save about 20 hours of production time annually through improved reliability of its drives. The company operates about 60 drives from various manufacturers. Every year, a couple of these drives used to fail, causing on average 40 hours of downtime per year.
A maintenance programme was initiated, aiming to reduce the failure rate from approximately eight percent to zero. An engineer from ABB Drives Alliance visited the site, replaced the cooling fans in all drives, stripped down and cleaned the drives internally, checked the connections, captured the parameters to external backup medium, changed the air filters, re-assembled the drives and checked the operation. Three years after the maintenance programme started, no fault has been experienced.
Who should service?
Choosing who to service the drive is another question for the user. It could be the drive’s supplier, the installer or the manufacturer. It depends largely on the age and type of the drive. For older or larger and more complex drives, the drive’s manufacturer will generally carry out servicing, as they will have the in house expertise to support them. For instance, ABB’s Drives Alliance, a network of installers and drives specialists, will generally service low voltage drives, leaving the larger medium voltage drives to ABB.
Another safeguard is security back ups of the drive parameters. A modern process plant can have hundreds of variable speed drives. A good company, when commissioning a drive, will also take a security backup of the software parameters. If and when disaster strikes, you don’t have to pour over your maintenance files to identify the parameters for the relevant drive.
The data is backed up from the last tried and tested parameter set during commissioning, which means you don’t have to run trials on your application all over again. In the case of a catastrophic failure of your drive, due for instance to mechanical damage, the manufacturer can take a new drive from its shelf, restore the existing software parameters and install it in the plant, where it will work as well as its predecessor from the moment it is switched on.
Look for nationwide service
It is also a good policy, particularly if you have several sites in different parts of the country, to choose a company that can offer servicing nationwide. Although drives users might think that opting for a big company will guarantee this, not all drives manufacturers offer this level of back up.
Another factor is the level of emergency service the user needs. A four hour response time in the event of a breakdown might be critical, particularly if the process can lose thousands of pounds in revenue for every hour the drive is out of action. With drives up to 55kW, a good servicing company will hold a full spare unit in store, with spare boards held for drives up to 500kW.
In many cases it is possible to repair a failed drive, enabling the user to get more life out of their investment. Typical repairs include replacement of fuses, input/output circuitry, capacitors or cooling fans. Particularly if the unit is large or reasonably new, this is often the most viable option. This may also be the quickest way to get a critical application back into service. Look for a company that operates a 24-hour repair service and one that can repair all makes of drives, even those that are no longer manufactured. A good manufacturer or its agents will evaluate the cost of repairs versus replacement and offer the best advice.
Spares availability is critical. The best approach is the suitcase system, where a technician will carry a suitcase of spares that can be used to repair up to 95% of common faults, allowing drives to be brought back to life with the minimum of delay and disruption.
Quick reaction critical
This ability of a drive vendor or installer to respond quickly is particularly vital for companies in the food and beverage industries. A drive may sit working perfectly happily in, for instance, a dairy for many years, but if it goes down, the whole dairy can be brought to a halt. With little or no storage time, milk can spoil if it is not processed into finished product and despatched quickly.
But quick response is also useful in many other industries. Chipboard manufacturer Sonae Industria in Merseyside saved tens of thousands of pounds worth of lost production through swift action when its largest drive, used for the main dryer in the wood preparation area, went out of action due to a short circuit. The local ABB Drives Alliance partner quickly traced the fault and replacement parts were found, on a Saturday, in Finland, flown in and met by engineers to fit the parts on arrival.
"We are a 24/365 operation and downtime costs us £10,000 per hour; the 48-hour period that the drive was out of action cost us £480,000. The swift action saved us at least a day, avoiding the loss of a further £240,000 ” says Electrical Manager John McNerlin.
However the best way is obviously to prevent emergencies from happening in the first place. A good vendor or its agents will offer a life cycle audit that will allow the user to plan for the maintenance intervals and predict the drive’s obsolescence, providing for a smooth transition to replacement technology and ensuring that its operations continue uninterrupted as variable speed drives evolve.