New drives installed inside oil-cooled drive casings

Twenty-five years ago drives were in their infancy and very much an emerging new technology. As such, many of products from that era now seem to come from the 'weird and wonderful' school of design, and all were inefficient by today's standards, with 70-75 per cent being typical then, compared to 98 per cent or more now.

However, many of these drives have survived to this day, faithfully and constantly executing their duties and, as such, are worthy of high praise. Most have been used in very aggressive environments throughout their long working lives and had the advantage of being completely water- and dust-proof.

When George Newton of Newton Tesla in Warrington was asked to overhaul some oil-cooled inverter drives for a local cement works, he was surprised that the 25-year-old units had stood up to the rigours of such an arduous industry for so long.

He says: "They were so ruggedly built that from the outside they looked rather like nuclear containment flasks. They were covered in cast cooling fins and huge compared to today's drives of similar capacity." Their functionality, however, was best described as basic, as they pre-dated the digital era.

Easy installation

Newton Tesla's brief was to bring the performance up to contemporary standards without making the drives difficult to refit back into their original installations.

Newton comments: "At first I was thinking that we would put a modern Mitsubishi E540 drive into a new sheet steel cabinet and make bespoke interfaces so that they could be reinstalled back on site. But then I had a brain wave and wondered about stripping out the original electronics and putting a Mitsubishi inverter in the oil-filled case. Doing so would maintain the same external appearance and connections, making it possible to refit on site in a matter of minutes."

The drives were opened and all internal components, heatsinks, reactors and printed circuits were discarded, leaving an internal steel frame into which the new Mitsubishi E540 fitted quite conveniently. The E540's inputs and outputs were connected to the original connection socket, giving the same control functions as the original. The inverter was mounted in such a way that when the steel frame was lowered back in to the oil tank it would be completely submerged in the oil. The oil is a high-grade insulating oil widely used in high-voltage switchgear and Newton reasoned that there would be no detrimental effects on the Mitsubishi unit. However, he did remove its air-cooling fan, as it would be superfluous.

An FR-PU04 parameter keypad was mounted under the outer lid to allow setting of the drive within the oil tank.

Newton recalls: "This was such a radical solution and, as far as we know, the first time it had been done, that we did extensive load testing before returning the drives to the cement works. They ran perfectly no matter how we loaded them. In fact the oil cooling proved very effective; the outer tank surface topped out at only 31degC, with a load of 5.5kW for eight hours, whereas with air cooling we would expect the heatsink to reach 60 - 75degC."

The maintenance engineers at the cement works were delighted with the results, not least because the drives were reinstalled with absolutely no fuss. Running tests have shown that the new Mitsubishi's have improved overall plant performance by a measurable margin, and Newton is now confidently presenting his refurbishment idea to the many water companies who have older oil-cooled drives in often critical roles.