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Invertek chooses bespoke over COTS for vital dc link choke

05 July 2015

It was during the development of its Optidrive E2 variable speed drive that Invertek turned to power specialist, REO for help concerning the development of a suitable dc link choke, choosing a custom build rather than a ‘commercial-off-the-shelf’ (COTS) approach.

Chokes assembled in a production jig housing

The brief was that the choke be both thermally and electrically efficient and ingress protected. Invertek technical director, Glyn Jones, admits this was a tall order, but one that REO embraced with enthusiasm.

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. The first prototype developed by REO consisted of an aluminium wound choke. Although it's not yet a material that's widely used in power drive systems, aluminium offers significant material cost savings over copper. While the prototype worked electrically, sustained thermal performance needed further improvement as the Optidrive E2 was destined to operate 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," recalls REO UK managing director, Steve Hughes. "Because our dc link choke is the last component to be fitted into the drive there is 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."

For the second prototype, copper was used as the winding and terminal material, its main benefits including lower losses and cooler running. REO subsequently optimised the installation of the choke, using its production tooling to customise production-jigs and thus secure the choke correctly in relation to the drive busbar. Glyn Jones emphasises the importance of achieving the best possible solution in terms of dc link choke design:

“If the choke used in our drive failed, the drive would also fail. 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 would also alarm the customer if smoke started coming out of their 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 valuable to us and a drive failure rate, due to the failure of the choke, of even one in one thousand 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.”

Further challenges
Fulfilling the next part of the brief, REO was charged with the problem of securing adequate ingress protection for the choke and its suitability for worldwide use. Some 90 percent of Invertek’s products are exported, and the challenges this poses include operation in humid ambient conditions, mains supply issues and lack of engineering support, all of which can quickly push components to their limits.

REO chose to pot the dc link choke and was able to make the unit waterproof to an ingress protection rating of IP55. The entire drive can be mounted on the wall of a control room - an improvement on the typical arrangement, which might involve installation within a cabinet located on the production floor, where there is a risk of overheating, moisture ingress and terminal corrosion.

A process of laboratory testing involving various voltage, temperature and installation options, allowed REO to achieve the Invertek specification requiring terminals on the dc link choke to be rated between 690 and 1,000V. Had a typical off-the-shelf choke been used in this installation, the inadequate standard terminal rating of 500V would almost certainly have caused a failure of the terminals by short-circuiting, resulting in possible fire and subsequent injury.

Both companies continue to work together towards further product development, as Steve Hughes explains: “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 are also looking at the possibility of providing these chokes in plastic housings. We have had good results in other applications and it can provide weight and cost benefits,” he adds. “Moreover, we can easily make prototypes using our 3D printing capability to demonstrate the principle.”

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