Compressor Timers; Compressor With Variable Capacity - Danfoss AK-PC 781A User Manual

Example:
- Two speed-regulated compressors with a nominal capacity of
20 kW and frequency range 25-60 Hz
- Two one-step compressors, each of 20 kW
Best fit
During best-fit operations, the speed-regulated compressors can
have different sizes and they will be handled in such a way that
the best possible capacity adjustment is achieved. The smallest
compressor will be started first, then the first will be cut-out and
the second compressor will cut in. Finally, both compressors will
be cut in together and will run in parallel.
The following one-step compressors will, in every case, be
handled in accordance with the best-fit coupling pattern.
Example:
- Two speed-regulated compressors with a nominal capacity of
10 kW and 20 kW respectively
- Frequency range of 25-60 Hz
- Two one-step compressors of 20 and 40 kW respectively
Two independent speed-regulated compressors
If the two speed-regulated compressors need to be controlled
asynchronously, they must each have their own analogue voltage
signal.
The controller first starts one of the speed-regulated compressors.
If more capacity is required, the other speed-regulated compres-
sor is started, and then the single compressors.
Analogue outputs
Max. speed
Nominal
Start speed
Min. speed
The first is run up to maximum speed. Number two is then acti-
vated and run up to nominal speed – and kept there. The speed
of number one is reduced at the same time, so the capacity is ba-
lanced. All variations are now handled by number one. If number
one reaches maximum speed, number two will also be raised.
If number one reaches minimum speed, it will be kept there while
number two takes over the variation below its nominal speed.
When engaging and disengaging, the total hours of operation for
the compressors is compared, so they are run an equal number of
hours.
AK-PC 781A Capacity controller RS8HE202 © Danfoss 2017-07

Compressor timers

Time delays for cutins and cutouts
To protect the compressor against frequent restarts three time
delays can be put in.
- A minimum time to run from a compressor's startup and until it
may be restarted.
- A minimum time (ON-time) for the compressor to operate before
it may be stopped again.
- A minimum OFF time to run from a compressor stops and until it
may be restarted
When unloaders are cut in and out, the time delays will not be
used.
Timer
The operating time of a compressor motor is registered continu-
ously. You can read out:
- operating time for the previous 24-hour period
- total operating time since the timer was last set to zero-set.
Equalizing operating hours
Operating hours are also summed in the "Equalization time" field.
During cyclical operation, this field is used for equalizing opera-
ting hours.
Coupling counter
The number of relay cutins and cutouts is registered continuously.
The number of starts can be read out here:
- Number during the previous 24-hour period
- Total number since the counter was last set to zero-set.

Compressor with variable capacity

Digital scroll compressor
The capacity is divided into period times as "PWM per". 100% ca-
pacity is delivered when cooling takes place for the whole period.
An off time is required by the by-pass valve within the period and
an on time is also permitted. There is "no cooling" when the valve
is on.
The controller itself calculates the capacity needed and will then
vary it according to the cut-in time of the by-pass valve.
A limit is introduced if low capacity is needed so that the cooling
does not go below 10%. This is because the compressor can cool
itself. This value can be increased if necessary.
Refrigeration
No refrigeration
Period time
Min. capacity
101