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Product category: Industrial Drives/Controls
News Release from: Control Techniques | Subject: Unidrive SP
Edited by the Engineeringtalk Editorial Team on 10 July 2006

Novel construction method boosts drive
density

Control Techniques set out to produce the most compact high power AC drives on the market with the design of its latest flagship AC drives.

Control Techniques set out to produce the most compact high power AC drives on the market with the design of its latest flagship AC drives This was achieved through a radical rethink of the traditional construction of large drives, the result being a more compact range with additional customer benefits, not the least being competitive pricing and the very highest level of performance

The new high power range of Unidrive SP AC drives uses a chassis that is constructed using SMC (sheet moulding compound).

This allows highly detailed and strong structures to be produced in one fast and cost-effective operation.

The power terminals are fixed within the moulding during manufacture process to form an integral part of the chassis, the strength of the material allows an entire 75kg drive to be safely lifted and positioned using a bracket fixed only to the power terminals.

The use of advanced composites for the drives' main component carrying chassis has resulted in Control Techniques being chosen as a finalist in the internationally recognised JEC competition "Innovative Use of Composite Materials", and the company showed the drives at the JEC Composites Show in March 2006.

"When we were looking at the design of the larger sizes of the Unidrive SP range, we took a completely fresh look at every aspect of drive construction", explains Control Techniques' Mechanical Engineering Design Manager, Tony Fleming.

"The concept of replacing a conventional sheet steel chassis with a 'plastic' alternative offered some worthwhile technical benefits".

"Not only was there the potential for reducing manufacturing costs, but, if the right material was chosen, we could consider reducing creepage and clearance within the product (safe distances between conducting surfaces to prevent flashover)".

"This, in turn, gave us the possibility of reducing the product footprint dramatically - realising the goal of an increased power to size ratio".

"As far as we are aware the use of SMC materials are a first for large AC variable-speed drives".

The current competition in this arena traditionally uses folded and painted sheet metal fabrications for these heavy assemblies.

These are costly to manufacture and involve many auxiliary supports for electrical insulation.

Increased creepage and clearance allowance leads also to larger product sizes.

The quest for increasing power/size ratios is never ending, in order to minimise the size of control cubicles on the factory floor.

The use of plastics in large, load-bearing applications required qualifying for suitability prior to volume manufacture.

Stress-strain behaviour, with time and impact resistance of plastics was of prime consideration.

These were all the more important for environments where shock and vibration can be experienced such as on cranes.

A host of other considerations also enter the equation.

These include a need for excellent high-voltage, electrical tracking resistance and a desire for halogen-free flame retardant polymers to satisfy up-and-coming RoHS legislation and the ability to pass the stringent UL tests.

EMC (electromagnetic compatibility) of the plastic frame might be considered, by some engineers, to be a problem.

Control Techniques' development engineers found, that careful design allowed the Unidrive SP chassis to exceed the necessary regulatory and reliability concerns.

Considering all of the above perhaps explains why metal fabrications have been around for so long.

It was decided that the best candidates for the job were gas-injection thermoplastics, injection moulded thermoset BMC (bulk moulded compounds) and thermosetting glass-reinforced, polyester sheet moulding compound (SMC).

Initial tests showed that injection moulded BMC did not satisfy the strength requirements - nonuniformity of glass length and glass distribution being the problem.

Gas-injection thermoplastics, although many structural versions are now on the market, were also discounted for all of the previous reasons.

So, the focus moved to SMC.

SMC is currently used with success in the automotive business, yet has made little impact in the complex-shape required for drives' chassis.

It is manufactured in a continuous inline process with the material being sheathed top and bottom with a plastic film.

A paste is prepared comprising resin, styrene, heat activating catalysts, inert fillers, release agents and thickeners.

This is spread onto the bottom film and chopped glass fibres are randomly distributed onto the paste.

The top film is then introduced and the sandwich is rolled into a pre-determined thickness.

The main process for moulding SMC material is compression.

The film is stripped off and the material is cut up into suitable pieces, comprising the 'charge' for the mould tool.

Heated moulds are used and compression pressure is applied.

The base resin, being a thermosetting material cures and hardens.

SMC components are hard, rigid mouldings with excellent electrical resistance properties.

The material has excellent dimensional-tolerance, virtually no shrinkage and not only meets the criteria set down by Control Techniques' design engineers, but occupies the unique position of having relative tooling simplicity using compression moulding techniques.

Dialogue with SMC manufacturers, OEMs and the European Alliance for SMC, indicated that the capability to formulate halogen-free resins would meet the Underwriters Laboratories (UL) "yellow card" recognition if Control Techniques was to proceed.

It would be misleading to define the tooling as low-cost, but it is still less than injection or transfer moulding.

One plus point with compression moulding, is that tooling modularity becomes financially feasible.

The overriding motive here for the large Unidrive SP range, was the search for a "telescopic" tooling solution in a common bolster.

A conscious decision to constrain the product width, retain constant depth and design constant-geometry chassis end-features yielded a workable tooling solution - with length variation coming only from growing the centre insert of the tool.

This approach offered the best return on investment.

When opportunities like this are tied into a major development programme, like Unidrive SP for Control Techniques, it becomes easy to understand the hesitance to proceed until all the financial and technical risks are clearly understood and engineering CAD resource is committed.

Control Techniques' first steps consisted of feasibility studies, with up-front "crystal-ball", sketched visions of what the support chassis might look like.

This gave the estimated weights and the data to make a sensible approach to a whole host of potential suppliers.

Budgetary part and tooling costs for the expandable tool concept covered comprehensive searches in the UK, Spain, Germany, Czech Republic, Hungary, Italy and France.

This yielded the necessary financial confidence to proceed.

Detailed analytical studies of large mouldings gave Control Techniques the necessary confidence that the concept had the necessary strength and the project was given the "green light".

At key stages of the 3D design (Pro/Engineer), the structure was analysed using finite element analysis methods.

Boundary conditions for particular attention were the bending and twisting moments transmitted through the terminal bolt, 350A cables, and the support points for heavy choke transformer assemblies and wall mounted support interfaces.

Control Techniques insisted that certain parameters, established during the part qualification stages, are monitored and controlled at the production press.

The problem, if anything, with this process is consistency of glass flow with the carrier resin - part strength here being the concern.

Prime factors for consistent mould quality are; charge pattern, tool temperatures, vacuum level, cycle time and material quality.

"SMC proved to be the solution in every aspect", says Fleming.

"It has been proved to be exceptionally strong - better in many ways than the steel equivalent - with a high dielectric strength, a zero UL rating - the highest possible electrical insulation performance - is halogen-free with the highest possible high flame retardant rating - and what's more, it cuts the cost of manufacturing so the end product becomes very competitive".

The result is a range of drives with the highest power to size ratio on the market today, with the highest safety and fire retardant performances.

"Our objectives, as design engineers, are to reduce the number of parts, simplify manufacture and produce a product that's better than the competition", concludes Fleming.

"The use of advanced composites has been the key that has unlocked these objectives with the latest range of large Unidrive SP AC drives". Request a free brochure from Control Techniques ...

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