Danfoss AK-PC 351 Capacity Controller
USER GUIDE
MODEL: AK-PC 351
Introduction
The daily operation of the AK-PC 351 can be set up directly on the controller. The display images will guide you through the setup process, allowing you to adjust relevant settings. The operation is password protected with three levels of access. Choose your preferred language during start-up.
Application
The controller is used for capacity regulation of compressors and condensers in small refrigeration applications. A maximum of 4 compressors and one condenser can be regulated. For example:
- One suction group + one condenser group, max. 6 steps total
- One compressor group, max. 4 steps
- One condenser group, max. 4 steps
Advantages
Energy savings via:
– Optimisation of suction pressure
– Night set back
– Floating condensing pressure
Input and output
There are a limited number of available inputs and outputs.
For each signal type, though, the following can be connected:
- Analogue inputs, max. 4 pcs.
Signal from 2 pressure transmitters and 2 temperature sensors
- Digital inputs, max. 8 pcs.
Signal from automatic safety control, external start stop, night signal, general alarm.
- Relay outputs, max. 5 pcs.
Connection of compressors, condenser fans, alarm relay
- Solid state outputs, max. 1 pcs.
Control of bypass on a digital scroll or for controlling unloader on a stream compressor. If the output is not used for this function, it can be used as ordinary relay output
- Analogue outputs, max. 2 pcs.
Speed control of compressors and condenser fans.
Operation
The daily operation can be set up directly on the controller.
During set-up, the display images will be adjusted so that only the relevant images are opened for additional setting and end-user operation.
The operation is password protected, and three levels of access can be granted.
The controller contains several languages. Select the preferred language at start-up.
Data communication
The controller has built-in Modbus data communication, and it can be connected to an AK-SM 800 type system device.
Suction Group
Compressor types
The following types of compressor combinations can be used for regulation:
- Single-step compressors
- Speed controlled compressor together with single-step
- Digital scroll compressor together with single-step
- Stream 4 cylinder compressor together with single-step
- Compressors with an equal number of unloaders.
Capacity regulation
The cut-in capacity is controlled by signals from the connected pressure transmitter/temperature sensor and the set reference.
Set a neutral zone around the reference .
In the neutral zone the pressure is controlled by the regulating compressor. 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.
When further capacity is either cut out or cut in, the capacity from the regulating compressor will be modified accordingly to maintain the pressure within the neutral zone (only where the compressor has variable capacity).
– When the pressure is higher than the “reference + a half neutral zone”, cut-in of the next compressor (arrow up) is permitted.
– When the pressure is lower than the “reference – a half neutral zone”, cut-out of a compressor (arrow down) is permitted.
– When the pressure is within the neutral zone, the process will continue with the currently activated compressors.
Control sensor
Normally, a suction group is controlled based on a signal from the Po pressure transmitter.
If control on a brine, the S4 sensor must be the control sensor.
An external, low-pressure switch can be connected to DI7 for frost protection.
The reference
At set or variable reference can be used for regulation. For example, the variable reference can be used for a night time increase or Po optimisation. Enter a set point here so that a contribution from the Po optimisation or night time increase is added. This contribution can raise or lower the reference, as determined by the momentary cooling need.
To limit the reference from values that are too high or too low, set a max. and min. limit.
Condenser
Fan control
The fans can be controlled incrementally using the controller’s relays, or they can be speed-controlled via the controller’s analogue output.
Speed control can be via a frequency VLT-type transformer.
If the fans have EC motors, the 0-10 V signal can be used directly.
Control
Regulation is carried out based on a signal from the Pc pressure transmitter or an S7 media temperature sensor. The signal is compared with the regulation reference.
The regulation reference can originate from one or more of the following functions:
- Fixed reference
- Variable reference, which follows the outdoor temperature.
When the outdoor temperature drops, the reference will drop by a corresponding amount.
This variable reference requires the installation of an Sc3 outdoor temperature sensor. The sensor must be positioned so that it registers the correct outdoor temperature. In other words, it must be shielded from direct sunlight and located near the airway of the condenser.
This regulation requires setting a min. and max. reference, so that the regulation process is kept within the given limits.
Media temperature
If controlling a media temperature, the control sensor must be set to S7. This temperature sensor must be located in the desired medium.
High-pressure monitoring can occur with an external, high-pressure switch on DI8.
Safety functions
Min./max. suction pressure Po
The suction pressure is recorded continuously.
If the measured value falls below the set minimum limit, the compressors will immediately cut out.
If it exceeds the max. value, an alarm will be generated once the time delay has elapsed.
Max. condensing pressure Pc
If the condensing pressure reaches the upper permissible value, the controller will connect all condenser fans to keep the pressure down. At the same time, a portion of the compressor capacity will be disconnected. If the pressure remains near the threshold value, even more compressors will be disconnected.
All compressors will be disconnected immediately if the threshold value is exceeded.
LP switch
On/off signal on a DI7 input
If a signal is received, all compressors will immediately be stopped. When the signal is removed again, the capacity is regulated up again.
HP switch
On/off signal on a DI8 input
If a signal is received, all compressors will immediately be stopped. Fan capacity will increase depending on how much the
Pc measurement exceeds the reference.
When the signal is removed again, the capacity is regulated up again.
Max. discharge gas temperature Sd for digital scroll / stream
Temperature sensor on an AI input.
A signal can be received from a Pt 1000 Ohm sensor on the pres-sure pipe.
If the temperature nears the set max. temperature, the capacity of the compressor will be increased so that the compressor can cool down itself.
The compressors will be stopped if the temperature reaches up to the set max. temperature value.
Sensor failure
If lack of signal from one of the connected temperature sensors or pressure transmitters is registered an alarm will be given.
- In the event of a Po error (S4 error), regulation will continue with a set capacity in daytime operation (e.g. 50%), and a set capacity in night operation (e.g. 25%), but with a minimum of one step.
- In the event of a Pc error, the condenser capacity that corresponds to how much compressor capacity is connected will cut in. Compressor regulation will remain normal.
- When there is an error on the Sd sensor the safety monitoring of the discharge gas temperature will be discontinued.
- In the event of an error on the outdoor temperature sensor, Sc3, the permanent setting value will be used as a reference.
NB: A faulty sensor must be OK within 10 minutes before a sensor alarm is cancelled.
General DI alarm
On/off signal on a DI8 input
If the input is used as general alarm input alarm text and delay times can be connected.
Alarm and text will appear when the delay time has elapsed.
Display overview
End-user overview
The images in this daily user interface will depend on how the set-up is made. They will illustrate what is regulated. For example: One suction group, one condenser group, or a combination. See examples below:
Set-up overview
There are two ways in which the controller can be set up. Select the one that is easiest for you: either “Wizard” or a review of “all parameters”.
Alarm list
* The alarm “IO configuration error” is activated if not all IO functions have been assigned to a hardware Input or output.
The reason is that too many functions have been selected via the configuration of the controller.
Go to the menu point “Main menu => IO status => IO summary”.
In this screen you can see if you have configured too many functions of a certain type – indicated by an exclamation mark ” ! ” Please refer to the screen example, were too many DO functions have been configured.
Solve the problem by adapting the DO functions to the max. No of DO outputs.
Sensor alarms
Sensor alarms shut off automatically when the sensor has been O.K. for 10 minutes.
If you have corrected the sensor error and want to perform a manual, forced removal of the alarm, go to the “Alarm detail display” Press and hold the “X” key for 2 seconds here.
Connections when using Setup Wizard
Digital outputs (DO1-DO6):
If you have used the Setup Wizard for the configuration, the control-ler will automatically assign the outputs in accordance with the following prioritised order:
- PWM outputs for digital scroll or Stream compressor will be lo-cated on solid state outputs DO6
- Compressor start and unloaders
- Fans
- Alarm (primarily DO6, but if DO6 is occupied then the first vacant DO).
Digital inputs (DI1-DI8) are estab-lished on the following inputs:
DI1-4: Compressor safety input DI5: Fan safety input
DI6: External Main switch (Start/Stop)
DI7: LP safety switch or Night status
DI8: HP safety switch or General alarm input
Analog outputs (AO1-AO2) are es-tablished on the following outputs:
AO1: Compressor speed control AO2: Condenser speed control
Analog inputs (AI1-AI4) are estab-lished on the following inputs:
AI1: Sc3 outside temperature AI2: Sd discharge gas temperature AI3: Po suction pressure or S4 media temperature
AI4: Pc condensation pressure or S7 media temperature
The assignment of functions on the respective inputs and outputs can be regulated in “IO configuration”. Here is an example of 3 compressors and 2 fans:
Connections
Supply Voltage. 24 V a.c. or 24 V d.c.
DO – Digital outputs, 6 pcs. DO1 – DO6
DO6 is solid state relay.
The relays are de-rated to the specified values.
If an alarm relay is defined, it will be driven under normal op-eration and it will drop in the event of alarms and insufficient power to the controller.
AI – Analogue inputs, 4 pcs. AI1 – AI4
AI1- Sc3: Pt 1000 ohm, AKS 11 or AKS 21.
AI2 – Sd compressor 1: NTC 86K ohm @ 25°C, from digital scroll or Pt 1000 ohm
AI3: Presure transmitter Po or temperature sensor S4, Pt 1000 ohm
AI4: Pressure transmitter Pc or temperature sensor S7, Pt 1000 ohm
Pressure transmitters
- Ratiometric: 10-90% of supply, AKS 32R / AKS 2050
- Signal: 1-5 V, AKS 32
- Current: 0-20 mA / 4-20 mA, AKS 33 (supply = 12 V)
DI – Digital switch inputs, 8 pcs. DI1 – DI8
The connection may be a shut-down or interruption function. Select what is to be activated during configuration.
DI1-4: Safety circuits, compressor 1, 2, 3 and 4
DI5: Safety circuits, condenser fans
DI6: External Main Switch
DI7: Night signal or LP switch
DI8 General alarm or HP switch
AO – Analogue output, 2 pcs. AO1 – AO2
Must be used when using a frequency converter or EC motors. Obtain 0-10 volts from terminals COM and AO1 (compressor), respectively COM and AO2 (fans).
Modbus
It is important that the installation of the data communication cable be done correctly. Cf. separate literature No. RC8AC. Remember termination at the bus termination.
The capacity from the digital scroll compressor
The capacity is divided into period times as “PWM per”. 100% capacity is delivered when cooling takes place for the whole period. An off time is required by the bypass 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 capacity control 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.
The capacity can similarly be limited so that the compressor cannot deliver 100% capacity. It is not normally necessary to limit this max. capacity.
Sd monitoring
When regulating with Sd monitoring, the controller will increase capacity if the temperature nears the Sd limit. This will result in better cooling of the digital scroll compressor.
Stream compressor
The PWM signal can also be used to control one stream compressor with one unloader valve.
The compressor capacity is distributed by up to 50% for one relay and the remaining 50-100% for the unloader. The unloader is con-nected to DO6.
Sd can be monitored like a scroll compressor.
Injection off
The electronic expansion valves in the cooling appliances must be closed when all the compressors are prevented from starting. As a result, the evaporators will not be filled with fluid that can be led to a compressor when the regulation process restarts. The function can be prompted via data communication.
Data
Capacitive load
The relays cannot be used for the direct connection of capacitive loads such as LEDs and on/off control of EC motors.
All loads with a switch mode power supply must be connected with a suitable con-tactor or similar.
Mounting /Dimensions
ORDER
List of literature
Installation guide for extended operation RC8AC
Here you can see how a data communication connection to ADAP-KOOL® Refrigeration control systems can be establis-hed.
Specifications:
- Model: AK-PC 351
- Capacity Controller
- Built-in Modbus data communication
- Password protected operation with three levels of access
- Multiple language support
FAQ (Frequently Asked Questions)
Q: How do I set the preferred language on the controller?
A: You can select the preferred language during start-up of the AK-PC 351.
Q: What happens if the suction pressure falls below the set minimum limit?
A: If the measured suction pressure falls below the minimum limit, the compressors will immediately cut out to prevent damage.
Q: How can I control the condenser fans?
A: The condenser fans can be controlled either incrementally using relays or speed-controlled via the controller’s analogue output.
Documents / Resources
 | Danfoss AK-PC 351 Capacity Controller [pdf] User Guide AK-PC 351, AK-PC 351 Capacity Controller, AK-PC 351, Capacity Controller, Controller |
