| When the Profibus control word is used to
start or stop the drive, please note that it is not possible to
have a totally parallel method of control such as digital inputs
onto the control card in the VLT3000. This is because the control
word uses a non-latch type of control, ie. you set a bit to one and
the drive runs, you set a bit to zero and the drive stops. There
are combinations which allow alternative control points but the
best we could come up with (after lengthy consultation with
Danfoss), was a system which meant that wherever you stopped the
drive from, you had to restart it from. This was using Profibus
control and digital inputs onto the card which came from
pushbuttons in the field. If two points of control are required, I
recommend that the start/stop control be performed through a PLC.
Also we came across the problem of changing speed setpoints,
which is done through Profibus by changing the bus reference for
each drive. They had a field setup which involved turning a switch
to the right to jog up the speed and to the left to jog down the
speed - the way that this was achieved in the VLT3000 was to
configure 3 digital inputs - one for jog up, one for jog down and
one for freeze reference. The freeze reference input holds the bus
reference so that the jog up & down functions are activated -
as soon as the freeze reference is turned off, the 'jogged' speed
reverts to the bus reference. Thus in this case they needed to keep
the freeze reference turned on to retain any field changes made to
the speed. This also precluded Citect from changing the speed,
since the bus reference was frozen with the freeze reference input.
We could not work around this so it was decided to only have the
operators able to change the speeds in the field, from the
pushbuttons. This is a limitation of the Danfoss VLT setup. There
may be other ways of configuring the drive for external speed
changes but this particular way is not very useful if Citect needs
to change the speeds.
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