Drives:Repair or replace?
July 1st 2011

There are various cost-saving maintenance measures that can be carried out on modern DC,AC and servo drives, as Vic Harris of ERIKS explains

Most modern DC, AC and servo drives have a life of around 40,000h, or 5 years of continuous operation. Some don't last that long; for example, environmental factors such as excessive heat due to poor ventilation can reduce life expectancy by up to half. The conventional solution is to replace the drive unit. Given the relatively high cost of these devices and the potential issues of interfacing new technology into an older control system, it can make sense to consider repair rather than replacement.

Heat, dust and age The operating environment for a servo drive can make a big difference to the life expectancy of components such as fans and circuit boards. For example, high levels of localised dust and other airborne pollutants drawn through the cooling vents of the drive casing can cause dirt to build-up around the fan unit and fan motor shaft leading to malfunction or early failure. If the fan stops, other drive components such as electrolytic capacitors will overheat and eventually fail.

Inadequate cooling provision within cabinets can also drastically reduce drive life.

Common problems are caused by cabinets or enclosures that have been incorrectly specified with insufficient ventilation. This can lead to an increase in internal temperatures, which can often remain high for extended periods, putting components under duress. The graphs (below) represent the temperature changes inside two drive cabinets, as measured by ERIKS to help prevent future failure: Even in correctly designed systems, with optimum air flow characteristics, drive components will have a finite life expectancy; for example, moving parts will eventually wear and electronic devices such as capacitors will slowly degrade, making it necessary to replace these items over the course of time.

Replace or repair? A failing drive doesn't always have to be replaced. In fact, through good maintenance practice and appropriate repair the life expectancy of drives can be doubled; indeed, in many instances a simple and quick repair can put things right, often at a fraction of the cost of a new unit; however, it is important to know when it is and is not economical to repair rather than replace.

Our experience with drive technology in all kinds of applications shows that the break point for an economical repair of a standard AC drive is normally between 4.0 and 5.5kW.

Below this size, the commodity price, wide choice of interchangeable brands and ready availability can make it more economical simply to replace drive units. Conversely, large system drives, typically over 75kW, tend to be heavily customised, supported by third party maintenance contracts or based on a modular construction, which allows component parts easily to be swapped.

Between these two extremes, however, lays a large number of industrial and process drive applications, where there is considerable opportunity to reduce operating costs and extend system life through effective maintenance and, where needed, repair. It's worth noting that ERIKS' records show that the typical cost of repair is around half that of the purchase cost of a new unit, with professionally overhauled drives having a life expectancy of up to 40,000h. This is especially true for VSDs, where the capital costs tend to be higher than standard units.

One further point to bear in mind is that, unlike replacement, an effective repair should identify the root cause of drive failure, which in the case of multiple failures in a particular application can be extremely useful tool in unearthing problems that may exist elsewhere in a production system.

Maintenance matters Clichés regarding drives as fit-and-forget items, or that they last forever, can be costly misconceptions. Drives can – and do – fail.

So simple, regular, planned inspections to check for dust build-up on components such as circuit boards, or to remedy basic ventilation faults to improve airflow and lower ambient temperatures will help to prolong operating life.

To reduce downtime, inspection work can be scheduled to coincide with a planned shutdown for routine maintenance work, with pre-shutdown planning including key activities such as backing-up software for drive controls. Basic drive servicing involves cleaning the cards and fan units, replacing capacitors and checking PCB integrity.

Ideally, maintenance schedules will be determined by a programme of condition monitoring. This can provide detailed trend analyses based on readings of temperature, vibration, ultrasonic and electrical supply, enabling the deterioration in performance of a drive to be determined over time and thus a probable failure point to be predicted. For example, it has been proven that the unexpected failure rates for servo drives built into robotic systems and machine tools on automotive production lines, and monitored as part of predictive maintenance programmes, can be reduced almost to zero.

An effective predictive maintenance programme for AC, DC and servo drives means significant reductions in downtime and delays on the production line can be avoided, while increasing productivity by up to 15%. Repairing drives rather than replacing can be cost-effective and can often get to the root cause of failure, saving time and cost in the long run.

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