In the May supplement this year, we invited readers to seek out any
working Danfoss VLT drive to find out whether it could be among the
oldest units still in service in this country. Well, some months later
and a lot of cobweb clearing out of the way, you have managed to locate
one! Here are the results, plus a bit of background to the competition
In May of this year, DPA got together with Danfoss in the UK to run a
competition to see if we could find the oldest working Danfoss VLT
(VeLoTrol) series drive. Four categories were stipulated, relating to the
age of each drive series, and in descending order these were: VLT5 (the
oldest in the series), VLT100, VLT200 and VLT3000.
Danfoss introduced its VLT5 - the first series-produced frequency
converter for general use with ac induction motors - back in 1968. It was
the result of a breakthrough in rectifier design, which made use of a new
type of power semiconductor that was then only just coming on to the
market. This short circuit proof rectifier - the ‘Forese’ rectifier, as
it became known - was capable of large output powers and it became a core
circuit component of the VLT series for many years to come.
With the early success of the VLT5 came the inevitable competition from
companies such as General Electric and Stromberg, but Danfoss had a
two-year lead, thanks to the Forese circuit (patented by this time) and a
special oil cooling method that was used in the VLT5 and subsequent
drives right up until 1977. The 20 litre oil reservoir, pump and solenoid
valves made up a third of the VLT’s overall weight, but by the early
1980s much lighter forced air cooling was introduced with the new VLT200
series. The VLT3000 was launched at the end of the 1980s and this was the
first to feature Danfoss’ VVC sensorless vector technology.
The very latest drive in the VLT series - the VLT5000 - was introduced in
1997 and how things have changed! In 1968, the VLT5 weighed 54kg; a
corresponding VLT5000 weighs just 14kg. Modern VLT drives are far easier
to set up and have time-saving facilities such as Adaptive Motor Tuning
that tunes the drive to the motor without having to disconnect them. But,
as we have discovered, those early drives are as important to their
applications today as they were decades ago when they were first
installed - so on to our winner and runners up!
We didn’t think we’d get one but sure enough, there is a VLT5 still in
service here in the UK and our thanks and congratulations go to Brian
Lewis of Greenbank Machinery in Cheshire, for bringing it to our
attention. Mr Lewis’s VLT5 was installed back in 1974 and is still in use
on a test bed for tachgenerators. He wins First Prize for spotting this
26 year-old installation - a 28in flat screen TV worth £500.
John Harrison of Pasta Foods in Great Yarmouth is our winner in the
VLT100 category. Mr Harris’s drive was installed in 1984 and controls
dryers on a manufacturing line. A VLT200 was located by Gary Hollins at
the Marston Brewery, where it is used to control a pumping application.
This was installed at the beginning of 1988. Finally, the earliest
VLT3000 was discovered by Paul Wareham of British Aluminium. This
controls a cutting saw and replaced an earlier VLT back in 1991. Our
congratulations go to Mr Harrison, Mr Hollins and Mr Wareham who each
receive midi hi-fi systems worth £300. We would like to thank all those
who took part in the competition, including all those with worthy entries
who were nonetheless pipped to the post by our winners.
For more information about Danfoss VLT drives: