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Industrial drives: what does the future hold?

02 November 2013

When Vacon's Stephen Takhar considers the future of industrial ac variable speed drives, he sees no radical changes on the horizon, but he does foresee numerous small but important developments.

When considering the future of ac variable speed drives (VSDs), one thing is certain: because of the large energy savings that VSDs provide in many applications, they will be adopted more and more widely. This widespread adoption is itself a driver for change because, when drives are deployed in large numbers, factors like harmonic generation become increasingly significant.

VSD manufacturers have, in fact, been addressing this issue for many years but in the past, it has often been necessary to use separate filters to achieve satisfactory harmonic performance. Today’s drives, however, offer much better harmonic control and often, particularly in the smaller sizes, feature internal filters that save space, reduce assembly and wiring time and, overall, save money.

This trend for enhanced harmonic performance, without the need for system integrators and drive users to make their own provisions for harmonic mitigation, will undoubtedly continue.

Another issue that becomes increasingly important when VSDs are used in larger numbers is heat. This is a particular concern when multiple drives are installed in a confined space, such as inside a control panel or in a small equipment room. 

Part of the solution, of course, is to increase the inherent efficiency of the VSDs, so that they generate less heat, and leading VSD manufacturers have already made great strides in this direction. In particular, the efficiency of VSDs running with light loads has improved significantly in recent years, which is important because many VSDs run lightly loaded most of the time, needing to develop full output only to deal with occasional peak requirements.

There is a limit, however, to how far intrinsic efficiency can realistically be improved, which implies that VSDs will always generate a significant amount of heat. The usual way of removing this heat is to use natural or forced ventilation.

Ventilation, however, brings its own problems. It involves the movement of air, which encourages the ingress of dust and contaminants into the equipment unless filters are used. But filters are prone to clogging, which means that unless they are regularly maintained, there is a distinct likelihood of unplanned VSD shutdowns due to overheating, or even equipment failures.

An attractive solution to these problems is offered by liquid cooled drives. These are typically more compact than their air-cooled equivalents and, when using them, it’s easy to produce systems that are completely sealed against the ingress of contaminants, allowing reliable operation to be achieved even in tough operating environments. 

Liquid-cooled VSDs are already available, but at the present time they are typically used only in high power applications of, say, 100kW or more. Factors restricting wider adoption are concerns about cost, complexity and coolant leakage. In fact, all of these issues have been addressed and, in the future, the undoubted benefits of liquid-cooled drives will lead to them being used much more frequently, even for applications requiring only modest power ratings.

We’ve already touched on VSD efficiency, and energy saving is without doubt one of today’s hottest topics. It’s not only VSD efficiency that needs to be considered, however; the efficiency of the motor itself is at least as important. This has led to increasing interest in ultra-efficient IE4 motors; indeed, it is possible that the use of such motors may one day become mandatory, at least in certain applications.

However, achieving IE4 efficiency is difficult with conventional induction motors, so motor suppliers are turning to alternative motor technologies including, in many cases, permanent magnet (PM) synchronous motors

Fortunately, these can be readily controlled by VSDs very similar to those currently in use. To achieve the best performance and control with PM motors, however, some fine-tuning of the VSD is needed. It is a virtual certainty, therefore, that future drives will offer options – probably user selectable – to optimise their operation not only with induction motors but also with PM motors.

Energy efficiency is, of course, only a part of a wider concern for protecting the environment and it would be folly to fit VSDs as energy saving devices – effective as they are in this role – without considering the overall environmental impact of the VSDs themselves. Leading manufacturers are already taking this into account, and are offering drives that have been designed with ease of recycling at the end of their lives firmly in mind. 

One of the biggest steps forward in this direction is the gradual elimination of electrolytic capacitors, which were once a key component of all VSDs. These capacitors use an electrolyte in the form of a paste. This paste is difficult to dispose of safely when the capacitor reaches the end of its life, and is potentially damaging to the environment. 

Newer VSDs are starting to use film capacitors in place of electrolytics, which not only reduces their environmental impact but also significantly improves their reliability, as electrolytics are notable prone to gradual deterioration with age and to premature failure if subjected to high temperatures. With these benefits in mind, it’s a safe prediction that electrolytics in VSDs will disappear over the next few years.

To summarise, it’s fair to say that nothing that’s been mentioned in this article could reasonably be described as a step change in VSD technology. It would, nevertheless, be unfair to dismiss the developments described as insignificant. Improved VSD efficiency, better harmonic performance, compatibility with PM motors, easier heat removal and reduced environmental impact are all benefits that will be widely welcomed and that promise make tomorrow’s VSDs even better products than their predecessors. 

As final note, it’s worth mentioning that many of the developments discussed are starting to be featured in products that are already on the market. When choosing VSDs, therefore, it’s a good idea to bear them in mind so that you can, in effect, specify tomorrow’s VSD today!

Stephen Takhar is managing director of Vacon UK

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