Invertek commissioned from REO a potted DC link choke that was electrically and thermally efficient, could diffuse heat into the air stream and provide ingress protection from humidity

The market for variable speed drive (VSD) based motor control is growing rapidly. A result of increasing global consumption, rising electricity prices and frantic infrastructure development in newly industrialised countries, the global market is expected to witness a compound annual growth rate of 6.4 per cent from 2014, reaching a total value of $22.7 billion by 2019. Underlying this trend is a concerted effort by VSD manufacturers to undergo collaborative development for further product innovation.

Today’s consumer culture is symptomatic of efforts that are ‘good enough’, ‘close enough’ and ‘it’ll do’ in attitude. However, for those few individuals and organisations that pluck up the courage to venture beyond the good and into the great, there are some serious rewards to be reaped. Would many of today’s engineering milestones have been achieved if Thomas Edison had succumbed to thousands of failed inventions?

When quizzed, Edison quipped, “I have not failed 10,000 times. I have not failed once. I have succeeded in proving that those 10,000 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.” He went on to invent the light bulb, the video camera and the record player.

The world of industrial engineering has come a long way since then. Today, most industrial sectors now have access to commercial off-the-shelf components that are designed to fulfil a purpose. Industries the world over, from food and beverage and packaging to maritime and offshore applications, make use of these components in applications from conveyors, compressors and elevators, to pumps, fans and extruders.

Invertek is one such company that has made a name for itself by going beyond the status quo. With a focus on energy saving of up to 50 per cent and reduced maintenance and downtime, the company has become one of the world’s fastest-growing VSD manufacturers, with a compound annual growth rate of 25 per cent. It specialises in the areas of pumps, building heating ventilation and air conditioning in addition to combustion, marine and elevator applications.

It was research conducted in its high-tech innovation facility that prompted Invertek to create a new product, the Optidrive E2. The findings showed that there was demand for a high-powered VSD for use in a variety of applications: from dry to humid conditions with temperatures between -20 and +50 degrees, to environments ranging from mild atmospheric salinity to aggressive ammonia exposure in chemical plants. This was certainly not something that could be built with off-the-shelf components.

It was with this in mind that Invertek turned to power quality specialist REO UK for support. REO is an original manufacturer of high-quality power equipment, including electronic controllers, components and electrical regulators. With 90 years’ experience as a global brand, the company was the obvious choice when it came to wound components and toroidal cores.

Variable speed drive – enduring design


Production jig to ensure high connection tolerance

“The brief given to REO was to create a potted DC link choke that was both electrically and thermally efficient,” explained Glyn Jones, technical director at Invertek. “As well as this, the unit was required to diffuse heat effectively into the air stream, provide ingress protection from water and humidity, be easy to install, cost effective and capable of being produced for a worldwide supply chain. It was without a doubt, a tall order for any manufacturer but one which REO embraced.”

The first prototype developed by REO consisted of an aluminium wound choke. Although it is not yet a material that is widely used in power-drive systems, aluminium offers significant material cost savings over copper as a result of the rising commodity price of copper in international markets. Although the prototype worked electrically, sustained thermal performance could be improved further, for use in continuous production environments.

“One of the challenges we faced was fitting the traditionally heavy-duty, large electrical components into a VSD designed for precision motor control,” explained Steve Hughes, managing director of REO UK. “Because our DC link choke is the last component to be fitted into the drive, there’s a very low mechanical tolerance. We had to ensure that it was easy to integrate into Invertek’s existing infrastructure, otherwise the whole drive would require redesigning.”

The solution was two-fold, for the second prototype copper was used as the winding and terminal material. Because the purpose of a DC link choke is to provide smoothing of the current waveform, reducing the DC bus voltage-ripple and minimising current harmonics, copper exhibited lower losses and ran cooler as a result. After this, REO optimised the installation of the choke, using its sophisticated production tooling to customise production-jigs and secure the choke in the right position onto the bus bar.

In the same way that a chain is only as good as its weakest link, even minor component failure can have lasting consequences for both the reputation of the drive manufacturer and OEM (original equipment manufacturer) supplier. “If the choke used in our drive failed, the drive would also fail,” continued Jones. “Because it’s a high-powered element, the choke failing can cause a lot of damage to the drive. It would almost certainly result in the circuit breaker clearing the fault, but causing alarm to the customer if smoke started coming out of the drive. Although this isn’t in itself very dangerous, it causes a lot of embarrassment for us as a drive manufacturer and we can’t accept this.

“Our reputation as a reliable drives maker is very valuable to us and a drive failure rate, due to the failure of the choke, of even one in 1000 would not be acceptable. We expect absolutely zero failures if we want to be taken seriously and I’m happy to report that, to date, no REO products have failed.”

Challenges and future innovation

variable speed drive

Foil wound bobbin

It was not all plain sailing, however. Fulfilling the next part of the brief involved ingress protection and suitability for worldwide use. Although headquartered in the UK, some 90 per cent of Invertek’s products are exported for use around the world. Part of the challenge of manufacturing a VSD for general use is overcoming the challenges posed by countries like India where high humidity levels, in addition to supply issues and an often underdeveloped engineering infrastructure, can quickly push components to their limits.

Using a potted design, REO enclosed the DC link choke and was able to make the unit waterproof to an ingress protection rating of IP55. In conjunction with the keymount holes on the VSD, the entire drive can be mounted onto the wall of a control room. This is an improvement on the typical setup, which involves installation into a drive cabinet on the production floor. This raises problems of effective thermal management, increased chance of ingress and terminal corrosion.

REO was able to pass these challenges by not only demonstrating its installation expertise but also testing capability, using its state-of-the-art test laboratory. It was here that various voltage, temperature and installation options were performance tested under rigorous conditions before being sent off for type testing and external regulatory validation.

This process allowed REO to achieve the Invertek specification requiring terminals on the DC link choke to be rated between 690-1000V. Had a typical off-the-shelf choke been used in this installation, the inadequate standard terminal rating of 500V would most certainly have caused a failure of the terminals by short-circuiting, resulting in possible fire and subsequent injury. At the very least, considerable wear and increased energy use would have significantly raised costs.

Despite the numerous challenges posed by a stringent brief, the collaborative approach between Invertek and REO produced not only a reliable product but also a sustainable partnership between the two companies. Research and development conducted in both organisations is being used to continue innovation in product development.

“We always aim to use a consultative approach,” said Hughes. “Having finished this project, we’re already working on a cold weld process that will realise material cost savings by using aluminium windings on the core and copper on the terminals. This will eliminate any contact corrosion and benefit from cheaper aluminium prices and better thermal dissipation.

“We’re also looking at the possibility of providing these chokes using plastic housings. We’ve had good results of this in other applications and it can provide weight and cost benefits. We can easily make prototypes using our 3D printing capability to demonstrate the principle.”

The effort to pursue perfection continues to show that greatness is not achieved by never failing, but rather a continuous process of improvement – even if it means another 10,000 tries. O'RiordanMechdesign,industry,manufacturing,United Kingdom
The market for variable speed drive (VSD) based motor control is growing rapidly. A result of increasing global consumption, rising electricity prices and frantic infrastructure development in newly industrialised countries, the global market is expected to witness a compound annual growth rate of 6.4 per cent from 2014, reaching...