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Drive maintenance: better to preserve than replace

29 July 2011

Take care of your drive and it will give you years of fault-free duty. So says maintenance specialist ERIKS, which can offer a more economical approach to this particular area of asset management. We follow up last month’s feature, which examined the repair or replacement options for ageing automation equipment.

Even with the rigorous demands placed on most industrial ac, dc and servo drives, you can expect them to survive for 40,000 hours, or five years of continuous operation. However, environmental factors such as poor heat dissipation can significantly reduce the operational life of these devices - and by as much as 50% in some cases. Conventional wisdom is simply to replace a drive unit when it starts to go wrong, but there is an alternative, more economical approach.

We tend to think of drives as commodities these days, but in reality they remain relatively high cost pieces of equipment. Moreover, when taking the replacement route, there are potential issues concerning the interfacing of new technology into an older control system. It really does makes sense to consider repair rather than replacement.

Difficult operating conditions
The type of operating environment can make a huge difference to the life expectancy of, say, a servo drive – particularly vulnerable component parts such as fans and circuit boards. Where dust and other airborne pollutants are present, these can enter the drive casing and accelerate fan bearing failure. This will in turn cause drive components such as electrolytic capacitors to overheat, leading to breakdown.

A drastic reduction in drive life can also be expected if there is inadequate cooling provision within enclosures. The most common problems are caused by poorly specified cabinets with limited or insufficient ventilation capacity, poorly maintained cabinet filters, or where no allowance has been made in the design for high ambient temperatures. The graphs on these pages show the temperature variations over time inside two drive cabinets, as measured by ERIKS during an assignment to prevent future failure:

A modern inverter will dissipate around 2% of total power in the form of heat, and older-generation drives produce even more. If airflow is restricted, the temperature within the enclosure will rise and compromise heat-sensitive electronic components. Notwithstanding these adverse conditions, as drive components are subject to natural degradation and a finite life expectancy, it is inevitable that these items will need to be replaced over the course of time.

Consider the options
Replacing a failing drive is not necessarily the only solution. In fact, through good maintenance practice and appropriate repairs, the life expectancy of drives can often be doubled. Even simple repairs can put things right, frequently at a fraction of the cost of a new unit. That said, it is important to know when it is, and when it is not economical to repair rather than replace an existing unit.

An analysis of ac drives used in a wide range of different applications, shows that the break point for an economical repair of a standard unit is typically between 4.0 and 5.5kW. The cost of overhauling a drive can be half the cost of purchasing a new unit, and it can be expected to last for up to 40,000 hours (mean time between failure) or five years in continuous operation. With smaller unit sizes, the commodity price, brand interchange ability and wide availability make it more economical to replace the unit, while above 75kW drives are not only modular and thus configured for easy component replacement, but are also often available with maintenance contract options.

A similar argument applies to servo drives, although as servos command a higher price, the repair of smaller units can be a cost-effective proposition.

It is important to bear in mind that, unlike the fitting of a replacement unit, a drive repair will identify the root cause of failure. If this lies outside of the drive unit, then it is possible to avoid the problem of recurring and unidentified faults, thereby saving considerable time and expense.

Improving operating conditions
Poor maintenance often arises from the misconception that drives are ‘fit-and-forget’ items, or that they last for ever. With simple, regularly planned inspections - checking for dust build-up on circuit boards, for example - it is possible to prolong the life of the device and improve machine uptime. Basic ventilation faults can also be quickly remedied to improve airflow and reduce ambient temperatures within enclosures or in the vicinity of machines.

To reduce downtime, the impact of such measures can also be minimised to coincide with a planned shutdown for other routine maintenance work. Note that if maintenance operations are to run smoothly, other elements of the service, such as backing-up software for drive controls prior to repair must also be considered.

Condition based monitoring is recommended in order to determine when an asset is likely to fail. Key areas include temperature, vibration, and electrical supply parameters. Moreover, testing drives while in operation ensures that any assessment of the condition of drives is carried out without compromising the production schedule.

This has proven to be the case for servo amplifiers built into machines such as robots and CNC machines deployed on automotive production lines. With scheduled maintenance on four- or five-year cycles, the predictive monitoring techniques has reduced servo control failures to zero on an entire production line. The servicing, which involves cleaning PCBs, replacing capacitors and checking the integrity of connectors, is carried out during convenient shutdown periods.

Where a large number of drives are being utilised, maintaining and operating these units requires a management programme if optimum performance levels are to be achieved. This involves surveying and cataloguing drive type, age and working conditions, and carrying out a thermographic survey to check for tell-tale hot connections and faulty thermal management components. Temperature data loggers are also utilised to monitor airflow temperatures into the bottom of drive heat sinks and can identify any problems with the airflow circulation.

Clearly, implementing better drives monitoring and maintenance, through regular inspections and simple improvements, along with cost effective component repair where appropriate, can pay dividends. In addition, an effective predictive maintenance programme covering modern ac, dc and servo drives can increase production line output by as much as 15%, as well as reducing unplanned reactive maintenance and downtime.

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