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VLT® drives lasted 30 years - at least

03 December 2008
When, in 2006, Preben Thøgersen changed its VLT® 5 drive for a new and more powerful VLT®drive, it had been in continuous operation for 28 years, only interrupted by a fuse that blew one Friday afternoon.

Back then Poul Thøgersen called Danfoss Drives service organization, and a half hour later the pump started again.
"Except for that, nothing has ever gone wrong. I certainly would tell if there had been problems, but that’s not the case" he says.

The reason to change the old drives was not that they were obsolete, but that the residential area the pumps supplied, was extended from 30 houses to approx. 750, so greater capacity was needed.
The VLT® 5 Preben Thøgersen bought 30 years ago was installed at a water reservoir where it operated a 5-hp water pump. Beside it, he installed a similar VLT® 5 as a back-up. When it was removed 28 years later, it had never been in operation.
The old drives are the 2nd generation of the world’s first series-produced frequency converters. In principle they were decentral drives intended to be mounted close to the motor.

The first Danfoss drives had the power electronics immersed in oil, partly for insolation and partly to take the heat off to the radiator-like outside cooling ribs.
The drives have no moving parts and the enclosure class is IP 54 - and they weigh 60 kg.

Now Preben Thøgersen has donated the two drives back to Danfoss Drives factory in Graasten where it all began and which since 1968 has evolved into the world’s leading supplier of drives up to 90 kW. Danfoss recently expanded its product range, so that it now covers up to 1.2 megawatts - and up to 690 V.

A modern VLT® drive in the same power range as the old one is now only one tenth of the weight and considerably smaller. With a similar enclosure - IP 55 - it weighs 13.5 kg. Preben Thøgersen still buy VLT ® drives for his water pumps because of their reliability.

"But now I feel quite sad," he says, petting the two fully functional, 30 year old, VLT® drives before they leave.

There are still other original VLT® drives in operation elsewhere so it is still not clear just how long a VLT® drive can last.
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