July-August 2021

14 | Plant & Works Engineering www.pwemag.co.uk July/August 2021 Maintenance Matters Focus on: Asset Management V ariable speed drives (VSDs) are extremely reliable and can provide many years of incident-free operation, and a lifespan of several decades. However, their high reliability can sometimes create a false sense of security. If a VSD is operating in good conditions - away from dusty environments, extreme temperatures and hazardous areas – then it can have a mean time between failure (MTBF) of over half a million hours or the equivalent to 62 years. However, VSDs are not indestructible, and every unit, if left without maintenance, will eventually fail. If the VSD or its components are obsolete it may be very difficult to repair quickly, potentially leading to costly downtime. Having to replace an obsolete drive at short notice is also a headache that operators can do without, particularly if production backlogs are mounting up. While VSDs may not be the most expensive capital equipment in a facility, they have a high in-service value, and the cost of a device failure can be great in terms of lost production. Even so, it is estimated that up to 80 percent of VSDs in the UK are operated without any kind of systematic maintenance program in place. This makes it more likely that, when failures occur, they will occur unexpectedly. There are three main reasons why a VSD can typically fail: Using the wrong drive – The VSD may have been chosen to incorrect specifications, or maybe wrongly sized and are therefore having to work too hard for the given application, leading to premature wear. This can also be caused by running a drive over capacity. Incorrect installation – Poorly dimensioned cabinets can create airflow issues, while excessively high or low ambient temperatures can affect operation if not mitigated. Loose connections can also be a potential culprit in drive failures, while areas with high levels of moisture can lead to corrosion of circuit boards over time. Poor maintenance – The risk of VSD failure is increased if basic maintenance duties are not carried out, for instance replacing fans per manufacturer’s recommendation, checking connections, annual inspections and cleaning. Fans may become worn or blocked leading to overheating and failure of capacitors. For every 10 degrees rise in operating temperature, capacitor life is halved, and conversely, every 10 degrees reduction will double its lifespan. The first two causes can be largely remedied by using a trusted, approved partner to supply and install the VSD. However, providing regular maintenance is an ongoing challenge and not a one-off fix. This can make it hard to implement effectively, particularly if local maintenance resources are stretched. Basic maintenance tips Whilst VSDs are often employed to carry out a simple job (such as speed controlling a motor), they are highly complex devices. Maintenance is often best left to qualified professionals, specifically the manufacturer or an approved third party. However, there are some basic maintenance duties that can be carried out locally which can prevent more complex issues from developing. VSDs must be kept clean, as dust on hardware can contribute to poor airflow and even short circuits. This can be alleviated by periodically spraying air through the heat sink fan. However, the air must be oil-free and contain no water. VSDs must also be kept dry to prevent moisture from getting in and causing corrosion on circuit boards. Keeping connections tight is also important, as heat cycles and mechanical vibrations can loosen them over time, which can potentially cause arcing. This, in turn, leads to nuisance overvoltage faults, clearing of input fuses, or damage to protective components. Whole life approach In the event of a complete failure, if a VSD cannot be repaired economically then it may be time to replace it. However, this is not a decision that should be taken lightly, and certainly not one you would ordinarily wish to rush. Choosing the right replacement VSDs under pressure, amid mounting production backlogs, can result in flawed decision-making or incorrect installation. It is far better to have a plan in place beforehand that can be activated so that downtime can be kept to a minimum, and the right VSD is selected for the right application. This is best achieved by taking a whole life A whole life cycle approach can help reduce the risk of sudden failures and provides a smooth pathway for managing replacements and upgrades. PWE reports. How a whole life approach boosts production reliability