Danfoss ADAP-KOOL AK-HP 780 Design Manual page 84

Speed control compressors:
The controller is able to use speed control on the leading
compressor in different compressor combinations. The variable
part of the speed controlled compressor is used to fill in capacity
gaps of the following compressor steps.
General regarding handling:
One of the defined capacity steps for the compressor regulation
may be connected to a speed control unit that may be a
frequency converter type AKD, for example.
An output is connected to the frequency converter's ON/OFF
input and at the same time an analog output "AO" is connected to
the frequency converter's analog input.
The ON/OFF signal will start and stop the frequency converter and
the analog signal will indicate the speed.
It is only the compressor defined as compressor 1 (1+2) that can
be speed controlled.
When the step is in operation it will consist of a fixed capacity
and a variable capacity. The fixed capacity will be the one that
corresponding to the mentioned min. speed and the variable
one will lie between the min. and max. speed. To obtain the
best regulation the variable capacity must be bigger than the
subsequent capacity steps it has to cover during the regulation.
If there are major short-term variations in the plant's capacity
requirement it will increase the demand for variable capacity.
This is how you cut the step in and out:
Cutin
The speed-controlled compressor will always be the first to start
and the last to stop. The frequency converter will be started when
a capacity requirement corresponding to the mentioned "Start
speed" arises (the relay output changes to ON and the analog
output is supplied with a voltage corresponding to this speed).
It is now up to the frequency converter to bring the speed up to
"Start speed".
The capacity step will now be cut in and the required capacity
determined by the controller.
The start speed always ought to be set so high that a fast
lubrication of the compressor is obtained during the start.
84 Heat pump controller RS8GF202 © Danfoss 2015-08
Controlling – increasing capacity
If the need for capacity becomes larger than "Max. Speed" then
the subsequent compressor step will be cut-in. At the same time,
the speed on the capacity step will be reduced so the capacity
is reduced with a size that corresponds to exactly the cut-in
compressor step. Thereby a completely "frictionless" transition is
achieved without capacity holes (refer also to sketch).
Controlling – decreasing capacity
If the capacity requirement becomes less than "Min. speed" then
the subsequent compressor step will be cut-out. At the same
time, the speed on the capacity step is increased so the capacity
is increased with a size that corresponds to exactly the cut-out
compressor step.
Cut-out
The capacity step will be cut-out when the compressor has
reached "Min. Speed" and the requested capacity has dropped to
1%.
Timer restriction on speed controlled compressor
If a speed controlled compressor is not allowed to start due to a
timer restriction, no other compressor is allowed to start. When
the timer restriction has expired the speed controlled compressor
will start.
Safety cutout on speed controlled compressor
If the speed controlled compressor is cutout on safety other
compressors are allowed to start. As soon as the speed controlled
compressor is ready to start it will be the first compressor to start.
As mentioned before the variable part of the speed capacity
should be bigger than the capacity of the following compressor
steps in order to achieve a capacity curve without "holes". In order
to illustrate how the speed control will react at different pack
combinations a couple of examples will be given here:
AK-HP 780