Danfoss ADAP-KOOL AK-HP 780 Design Manual page 86

Two speed-regulated compressors
The controller is capable of regulating the speed of two compres-
sors of the same or different sizes. The compressors can be com-
bined with one-step compressors of the same or different sizes,
depending on the choice of coupling pattern.
General regarding handling:
Generally, the two speed-regulated compressors are managed
according to the same principle as for one speed-regulated com-
pressor. The advantage of using two speed-regulated compres-
sors is that it allows for a very low capacity, which is an advantage
for low loads. At the same time, it produces a very large, variable
regulating area.
Compressor 1 and 2 both have their own relay outlets to start/
stop separate frequency converters, for example of type AKD.
Both frequency converters use the same analog output signal AO
which is connected to the frequency converters' analog signal in-
put. The relay outputs will start and stop the frequency converter
and the analog signal will indicate the speed.
The precondition for using this regulating method is that both
compressors have the same frequency range.
The speed-regulated compressors will always be the first to start
and the last to stop.
Cut-in
The first speed-regulated compressor will be started when there is
a capacity requirement which matches the setting.
The "Start speed" (relay outlet changes to on and the analog out-
let is supplied with a voltage that matches this speed). It is now
up to the frequency converter to bring the speed up to the "Start
speed".
The capacity step will now be cut in and the desired capacity
determined by the controller.
The start speed should always be set so high that a good lubrica-
tion of the compressor is quickly reached during start-up.
For a cyclical coupling pattern, the subsequent speed-regulated
compressor will be cut in when the first compressor runs at max.
speed and the desired capacity has reached a value that allows
the cut-in of the next speed-regulated compressor at start speed.
Afterwards, both compressors will be cut in together and will run
in parallel. The following one-step compressors will be cut in and
out in accordance with the selected coupling pattern.
86 Heat pump controller RS8GF202 © Danfoss 2015-08
Controlling – decreasing capacity
The speed-regulated compressors will always be the last compres-
sors running.
When the capacity requirement during cyclical operations be-
comes less than "Min. speed" for both compressors, the speed-
regulated compressor with the most operating hours will be
cut-out. At the same time, the speed of the last speed-regulated
compressor increases so that the capacity is increased to the level
that matches the cut-out compressor's step.
Cutout
The last speed-regulated compressor will be cut-out when the
compressor has reached "Min. speed" and the capacity require-
ment (desired capacity) has decreased to under 1% (see however
the section on the pump down function).
Timer restriction and safety cut-outs
Timer limits and safety cut-outs on speed-regulated compres-
sors should be managed in accordance with the general rules for
individual coupling patterns.
Short descriptions and examples are given below of the handling
of two speed-regulated compressors for the individual coupling
patterns. For a more detailed description, refer to the appendix at
the end of the chapter.
Sequential operation
During sequential operations, the first speed-regulated compres-
sor will always start first. The following speed-regulated compres-
sor will be cut in when the first compressor runs at max. speed
and the desired capacity has reached a level that allows the cut-in
of the next speed-regulated compressor at start speed. After-
wards, both compressors will be cut in together and they will run
in parallel. The following one-step compressors will be cut in and
out in accordance with The First-In-Last-Out principle.
AK-HP 780