Drive craft

Most modern electronic drives have a finite life of around 40 000 hours or five years of continuous operation. After this period performance begins to degrade and they become likely to fail. Environmental factors such as excess heat due to poor ventilation can cut that life expectancy by half. The result is that many drives fitted during or before 2000 therefore have a high potential for failure.

But what can you expect from different types of drives? How do you make the right decision about whether to repair or replace them? And can their life expectancy can be doubled with good maintenance?

Victor Harris is electronics development director at ECA firm WYKO Industrial Services, a specialist in the repair and replacement of electronic drives. “Electronic drives have become the workhorses of industry over the last ten years: most machines and motors are driven in some shape or form by ac or dc drives, soft starters or servo amplifiers; many of which have been fitted as energy saving devices,” Harris explains.

“A large number of units that were fitted over five years ago now have a clear potential to stop working and cause disruption. The general move to save costs and maximise efficiency is also being negated in many instances by poorly maintained drives that are not reaching their expected lifespan,” Harris expands.

What’s the problem?

There are two common misconceptions about drives.

The first is that they are fit-and-forget items – they are not. The operating environment can make a huge difference to fans, boards and components. If the health of a drive is not checked it can swiftly lead to premature failure, causing serious unplanned downtime and loss of production.

Wyko also reports that it is encountering increasing numbers of units that have failed and are being replaced when a simple repair could cost half as much and extend the life of the drive to the same as is expected from a new unit.

The second misunderstanding is that drives last forever. This is not true – most modern drives have an expected operating life of 40 000 hours or roughly five years in relatively constant use. There are many components within a drive, such as fans and capacitors, that will naturally degrade and will need replacing. Boards will also be likely to need inspection and cleaning after this time, otherwise a failure could be imminent. It is a sobering thought that this now applies to many drives fitted as recently as the year 2000.

When to drive repairs

The break-even point for a standard ac variable speed drive to be economical to repair is between 4 and 5·5 kW. Below this size and the price and wide availability make it more economic to replace the unit.

Above this size, however, and the cost of a repair can be as little as half that of a new unit. The situation changes when you reach high power drives above 100 kW. These are modular, so power supplies and gate drive cards can be easily replaced, plus they usually come with maintenance contracts and are expected to operate for longer.

It is important to note that once a drive has been overhauled it is literally as good as new. Hence, given suitable conditions it is expected to last up to 40 000 hours as a mean time between failure or five years in continuous operation.

Servo technology is becoming very similar to ac variable speed drives in some respects, so is comparable in terms of being economical to repair. Servos are higher value products; hence repairing small units can be cost-effective.

Many servo amplifiers are modules built into machines, such as robots on automotive production lines. Wyko has proven that scheduled maintenance on four or five-year cycles can reduce servo control failures to an actual zero point on a whole production line. The service involves cleaning the cards, replacing capacitors and checking printed circuit board (pcb) integrity. All the work is carried out during shutdown, so there is no interruption to production.

Avoiding problems

Most modern drives are well protected from load power surges, so failure in the power stage components is rare. Most have overload protection that ensures the drive simply trips out while remaining protected and is not damaged.

A drive failure will normally reveal itself when the plant is switched back on from a period of shutdown – drives will not power up again after being powered down. There is normally no indication on the display, but the main power stages look okay.

This type of failure mode is typically caused by natural ageing of electrolytic capacitors or premature ageing due to the components overheating. The main cause of overheating is fan failure; the lack of effective cooling causes poor performance and accelerates the degradation of components. Causes for drive fan failure in general are dust and other airborne pollutants such as oil mist entering the drive casing and causing bearing failure.

Problems with cooling inside the cabinet can easily halve the life of a drive. These commonly include inadequate ventilation from poorly specified cabinets; cabinet filters not being changed; or high ambient temperatures that have not been allowed for in the design and specification of the system (typically compounded on a seasonal basis by high peak summer temperatures). A modern inverter dissipates about 2% of total power in heat; older generation drives dissipate more so adequate cooling is essential.

Once the basic steps have been taken to ensure adequate cooling of a drive when it is fitted, regular checking of fans, cabinet internals, filters and external heat sinks to prevent dust build-up is also essential.

Basic faults with cabinet ventilation can be quickly remedied, while any recommended maintenance work can be carried out during a planned shutdown. Setting up a monitoring and maintenance schedule for a site following a period of survey and maintenance is also recommended.

Repaired and ready

Repaired drives carry the same guarantee as most new products (up to 12 months). A repair always requires some element of identification of the cause of failure; in this instance there is always a recommendation given on how to avoid this problem reoccuring.

The argument for conducting routine maintenance and overhaul of drives equipment is clearly demonstrated by one of the UK’s major car plants. Servo packs on the production line are removed, cleaned, components replaced and returned to service every five years. As a result, there have been zero failures to date.

For more information circle the number on the Enquiry card or see www.emconline.co.uk/

Enquiries: Wyko 200