Model Forum / General / Models / April 2009

That's good advice.  The heatsink can somewhat mislead.  In my
experience extruded alumninium heatsinks have a far better thermal
conductivity than pressure cast alloy ones.  If it's a cast heatsink
then it might be substantially hotter under the IGBT/Rectifier pack.
Most thermal models look at trends, or rather the rate of change of the
temperature, such that the management software can say, 'hold on guys,
they're pulling a a lot of current, turn the fans on'.

To be honest some thermal models are cleverer than others.  On the drive
I worked on, the thermal sensor was in the IGBT pack, and there was non
in the rectifier pack.  This was a 20hp/15kW drive and the semiconductor
tech in those days was'nt man enough to get a 20hp solution in to a
single pack.

What I had to do was to look at the magetising current in the inverter,
because that would still create losses in the IGBT stack, but bugger all
in the rectifier.  You get high mag current when you're running the
motor no load.  Once you start asking the motor to begin supplying
Torque, obviously the torque producing current part begins to climb
which will then start to put up losses in the rectifier and so on.  We
all sorts of frig factors in there but it did work.  We had to do it
this way because one day I decided to abuse the driver by running at
12kHz and full load current...it did'nt take long for it to blow up
because the management software had'nt been designed then.

Anyway I digress....sorry.  Thought you might be interested.

Rob.

As I mentioned in my first post, when initially installed the fan
operated only when the motor was running. I haven't changed any
parameters since then.
Now, for the first few minutes or so after switching on, the fan runs
only when the motor runs, but after using the lathe lightly it runs for
quite a while (4 or minutes at least) when the motor is off. Towards the
end of the 4/5 minutes, the fan switches off and on intermittently,
sometimes staying off for several seconds before starting again. 10 of
15 minutes later, as long as I don't start the motor, the fan stops.
As also mentioned, I can detect no rise in temperature anywhere on the case.