Pack Controller FAQ
Pack Controllers
Communication
AK-PC 551
Condenser
Condenser control
Condenser control using P- or PI-control
If the condenser control type is set to “P-control” (Proportional control), this control will allow some deviation from the reference, because it is just a P-controller with a P-band allowing a difference between the reference and the controlled value where the size of the difference is depending on the output. P-control is fast but not precise, whereas PI-control (Proportional Integration control) is precise. With PI-control the integration function will, so to speak, integrate the fault away.
The Control Type can be set in the controller. Below it is shown how the control type is changed from P-band control to PI-control using the AK-PT 50 PC software:
Capacity control
Bitzer CRII control
When the requested capacity reaches the setting of the “Start capacity”, the compressor will start. Start capacity setting for Scroll and CRII is by default set to:
Min: 10%
Max: 60%
Fac: 30%
When the unloader valve goes ON/loads (power to the coil is OFF), the cylinders will deliver capacity.
Example with requested capacity of 10 % on a 4-cylinder Bitzer CRII:
2 seconds is 10 % of a 20 second period time and that is for each of the valves controlling 2 cylinders.
The first valve will go ON for 10 % (10 % of 50 % capacity – 2 cylinders out of 4) of the period time and OFF for 90 % of the time. A half period after the first valve went ON, the second valve will also go ON for 10 % (10 % of 50 % capacity) of the Period time. In total the capacity will be 10 % (10 % of 50 % + 10 % of 50 % = 10 % of 100 % capacity).
For fig. 1
- The minimum period time you can set is 10 seconds (nominal is 20 seconds and maximum is 60 seconds)
- The minimum time an unloader valve should be operated in loaded or unloaded state is 2 seconds
With this configuration it is only possible to go down to 20 % capacity. If it is required to go down to 10 % the period time will have to be increased to 20 seconds.
However, the initial start of compressor capacity is a bit more complicated. It is easier to make a sketch (See fig 2)
The unloaders are inversed:
Please be aware that since software version 1.20 of the AK-PC 551 controller (with Packman 2 capacity control) is taking care of the inversion of the unloading valve outputs, the digital outputs should not be inversed for the unloading valves – the controller is taking care of the inversion.
Secondly, please also be aware that whenever a compressor is at standstill, the unloading valve will be without power/loaded. Just before the compressor starts, the unloading valve is powered and the compressor is started (1-2 seconds between these two actions)
Generic strategy for capacity control of compressors
In the neutral zone, the regulating compressor controls the capacity so that pressure can be maintained. When it can no longer maintain the pressure within the neutral zone, the controller will cut-out or cut-in the next compressor in the sequence. In principle, unloaders are treated as compressors, but unloaders can be switched inside the neutral zone.
The capacity distribution is cyclic where operation time is equalized – sequential control is not possible.
A lead compressor can be defined which then will be the first compressor to start and the last to stop.
The following is valid for Packman2 which now has been implemented in all our Pack controllers.
Examples:
In the following examples, the Multi compressor setting (several compressors with same compressor configuration without a lead compressor - comp. with same capacity and number of unloaders) is used. See the three tables below.
In an another example it is explained how the DO switches (no shift between running hours). “Multi” is selected with 3 compressors with one unloader each. This will show the principles:
- All outputs are OFF when the controller is in stand-by with Main switch OFF or when requested capacity is not calling for capacity (Requested capacity = 0 and Running capacity = 0).
- This means that all 3 compressors are OFF and all 3 unloader coils are powered OFF to save energy (no need to activate a coil burning power when it is not needed). However, the unloaders are actually “loaded”, asking for capacity.
- When the requested capacity reaches 17 %, the unloader coil of the compressor with least running hours will switch ON (unloading). Approximately one second later the compressor with least running hours will switch ON giving 17 % capacity.
- When the requested capacity goes up to 33 %, the unloader coil of the compressor with least running hours will switch OFF (loading). The capacity is now 33 %.
- When the requested capacity goes up to 50 %, the unloader coils of the compressor with second least running hours will switch ON (unloading). Approximately one second later the compressor with second least running hours will switch ON.
- When the requested capacity goes up to 67 %, the second unloader coil of the compressor with most running hours will switch ON (unloading). Approximately one second later the compressor with most running hours will switch ON.
- Finally, when the requested capacity goes up to 100 %, the unloader coil of the compressor with most running hours will switch OFF (loading).
- In the opposite direction, the compressor with most running hours will be the first to reduce capacity at 83 % where an unloader coil goes ON (unloading).
