1. Introduction
The AK-PC 551 controller is designed for capacity regulation of compressors and condensers in small refrigeration applications. It can manage up to 8 compressors and one condenser.
Application Scenarios:
- One suction group + one condenser group
- Two suction groups + one shared condenser (max. 4 + 4 steps)
- One compressor group, max. 8 steps
- One condenser group, max. 8 steps
Advantages:
- Energy savings through optimization of suction pressure, night time increase, floating condensing pressure, and load limitation.
Input and Output Capabilities:
- Analogue inputs: max. 8 (for pressure transmitters, temperature sensors, voltage signals)
- Digital inputs: max. 8 (for safety controls, day/night signals)
- Relay outputs: max. 6 (for compressors, condenser fans)
- Solid state outputs: max. 2 (for capacity valves, unloaders)
- Analogue outputs: max. 4 (for speed control of compressors or condenser fans)
Operation:
Daily operation can be set up directly on the controller or via an external display. The interface adjusts based on the setup. Operation is password-protected with three access levels. The controller supports multiple languages, selectable at start-up.
Data Communication:
Features built-in Modbus data communication, enabling connection to AK-SM 800 system devices.
2. Suction Group
Details the regulation of compressors within a suction group.
Compressor Types Supported:
- Single-step compressors (one can be speed-regulated)
- Compressors with unloaders
- Scroll compressors (including digital scroll)
- Copeland Stream compressor (4 cylinders, one unloader)
- Bitzer CRII compressor (4 cylinders, two unloaders)
Capacity Distribution (FIFO):
Compressors cut in/out based on the "First In First Out" principle to equalize operating hours. The lead compressor is always engaged first, and variable capacity fills gaps.
Timer Restrictions and Safety Cut-Outs:
If a compressor is prevented from starting due to timers or safety cut-outs, it is replaced by another compressor.
Capacity Regulation:
Controlled by pressure/temperature sensor signals and a set reference. A neutral zone is maintained. Cut-in/cut-out occurs when pressure deviates from the reference plus/minus half the neutral zone. Variable capacity compressors adjust output to maintain pressure.
Control Sensor:
Typically controlled by the Po pressure transmitter. For brine control, the S4 sensor is used, with Po for frost protection.
Reference:
Can be set or variable (e.g., for night increase or Po optimisation). Limits can be set for the reference value.
Load Shedding:
Limits maximum compressor capacity to control total electrical load. The threshold cannot be set below the compressor's lowest capacity step.
3. Condenser
Details the control of condenser fans and regulation based on condenser pressure.
Fan Control:
Fans can be controlled incrementally via relays or speed-controlled via analogue output (0-10V), potentially using a VLT-type transformer. EC motor fans can use the 0-10V signal directly. Simultaneous step and speed control is supported. Night operation limits fan noise by reducing capacity.
VSD Max Speed Limitation:
Limits fan speed to reduce noise. This is bypassed by safety functions like Sd max. and Pc max.
Control:
Regulation is based on Pc pressure transmitter or S7 media temperature sensor, compared against a regulation reference. The reference can be fixed or variable (following outdoor temperature via Sc3 sensor). Min/max references are required. Heat recovery can increase the reference via a DI input.
Media Temperature:
If controlling media temperature, the S7 sensor is used. The Pc pressure transmitter is also required for high-pressure monitoring.
4. Safety Functions
Details various safety limits and sensor failure handling.
Pressure Safety:
- Min./max. suction pressure (Po): Triggers compressor cut-out or alarm.
- Max. condensing pressure (Pc): Connects all condenser fans, reduces compressor capacity, or cuts out all compressors if exceeded.
Switch Inputs:
- LP switch (DI): Stops all compressors.
- HP switch (DI): Stops all compressors. Fan capacity increases.
Superheating and Temperature Safety:
- Min./max. superheating (Ss): Triggers alarm.
- Max. discharge gas temperature (Sd): Reduces compressor capacity or stops compressors. For specific compressor types (digital scroll, Stream, CRII), capacity may increase for self-cooling.
Sensor Failure:
Alarms are generated for missing sensor signals. Regulation continues with reduced capacity or specific fallback logic for Po, Pc, Sd, Ss, Sc3, S4, and Saux sensors. A faulty sensor must be OK for 10 minutes to clear the alarm. Manual reset is possible.
General DI Alarms:
Three general alarm inputs can be configured with text and delay times.
General Thermostat:
Can be configured if a relay output and analogue input are available.
Injection ON:
Interrupts refrigerant injection to evaporators when compressors cannot start. The signal is maintained while compressors are running or can be started. It is switched OFF if compressors cannot start due to timers, safety cut-outs, or high suction saturation temperature for an extended period.
5. Display Overview
Illustrates the user interface and how it displays regulated parameters based on the system configuration (e.g., one or two suction groups, condenser group).
6. Set-up Overview
Describes three methods for controller setup: "Wizard", "Quick settings", and "All parameters".
Setup Methods:
- Wizard: Guides the user through a series of settings for initial setup.
- Quick Settings: Allows selection from pre-defined combinations of compressors and fans.
- Parametric Setup: Review of all parameters (detailed in later sections).
The initial screen upon delivery shows "No application configured". A default password (300) is used. Setup involves selecting language, plant type, and then configuring specific parameters.
Suction A Settings
Detailed parameters for Suction Group A control.
Control Status:
Provides real-time status of the control circuit (e.g., No comp., Normal ctrl, Alarm Comp., Sensor error, Load shed).
Actuel zone:
Indicates the current pressure relative to the reference (P0 error, Zone, NZ, + Zone).
Control temp., Reference, Running capacity, Requested capacity, No. of running comp.:
Readings for current temperature, reference value, connected capacity (%), and number of active compressors.
Pressure and Temperature Readings:
PoA Pressure, ToA Saturated temp., Pc Pressure, Tc Saturated temp.
Status Indicators:
Day / Night status, Load shed status, Injection ON A status, MC Load Shedding status, MC Night Setback status.
Control Settings:
| Parameter | Description | Range/Options |
|---|---|---|
| Control mode | Regulation type | MAN / OFF / AUTO (Default: AUTO) |
| Setpoint | Regulation reference | Min: -80°C (-1.0 bar), Max: 30°C (50 bar) |
| Neutral zone | Neutral zone around reference | Min: 0.1 K (0.1 bar), Max: 20 K (5.0 bar) |
| Night offset | Reference adjustment for night | Min: -25 K (-5.0 bar), Max: 25 K (5.0 bar) |
| Max Reference | Highest permissible regulation reference | Min: -50°C (-1.0 bar), Max: 80°C (50.0 bar) |
| Min Reference | Lowest permissible regulation reference | Min: -80°C (-1.0 bar), Max: 25°C (40.0 bar) |
| PI control selection | PI regulation reaction speed | 1 (slow) to 10 (quick). Custom option (0) for trained staff. |
| Gain factor Kp | Amplification factor | (Visible/set when PI control is "0") |
| Integration time Tn | Integration time | (See above) |
| + Zone rate of change | Change coefficient for + zone | (See above) |
| - Zone rate of change | Change coefficient for - zone | (See above) |
Compressor Settings
Configuration parameters for compressor operation.
| Parameter | Description | Range/Options |
|---|---|---|
| First step runtime | Time before next compressor can start. | Min: 0 s, Max: 300 s |
| Pump down | Pump-down function for last compressor. | Min: -80°C (-1.0 bar), Max: 30°C (50.0 bar) |
| Load shed limit | Capacity limit during load shed signal. | Min: 0 %, Max: 100 % |
| Emergency cap. day | Capacity during daytime operation (sensor failure). | Min: 0 %, Max: 100 % (Default: 50%) |
| Emergency cap. night | Capacity during night operation (sensor failure). | Min: 0 %, Max: 100 % (Default: 25%) |
| Comp. start delay | Delay after forced closing of expansion valves. | Min: 0 s, Max: 180 s |
| Injection OFF delay | Delay of forced closing of expansion valves. | Min: 0 s, Max: 300 s |
| Control sensor | Regulation sensor for suction circuit. | Po (Pressure) or S4 (Media temp. for brine). |
| Compressor mode | Type of compressor for regulation. | Multi all, Multi+Single, Speed+Multi, Speed+Single, CRII4+Multi, CRII4+Single, Stream 4+Multi, Stream 4+Single, Digital scroll, Single-step only, None. |
| No. of compressors | Total number of compressors. | Min: 1, Max: 8. |
| Lead comp. size | Nominal capacity of the first compressor. | Min: 1 kW, Max: 100 kW. |
| Comp. size | Nominal capacity of other compressors. | Min: 1 kW, Max: 100 kW. |
| VSD Min. speed | Minimum speed for VSD compressors. | Min: 10 Hz, Max: 60 Hz. |
| VSD Start speed | Minimum start speed for VSD compressors. | Min: 20 Hz, Max: 60 Hz. |
| VSD Max speed | Highest permitted speed for VSD compressors. | Min: 40 Hz, Max: 120 Hz. |
| PWM period time | Period time for unloader valve (Scroll/Stream). | Min: 10 s, Max: 20 s. |
| CRII Period time | Period time for unloader valve (CRII). | Min: 10 s, Max: 20 s. |
| Comp. 1 min cap. | Minimum capacity for compressor 1. | Min: 10%, Max: 50%. |
| Comp. 1 start cap | Start capacity for compressor 1. | Min: 10%, Max: 60%. |
| Comp. 1 Sd temp. | Monitor discharge gas temp. (Sd) for compressor 1. | Yes/No. |
| Comp. 1 Sd max. | Maximum Sd temperature for compressor 1. | Min: 0°C, Max: 195°C. |
| No. of unloaders | Number of unloaders for compressor. | Min: 1, Max: 3. |
| Comp. safety input | Compressor safety circuit input. | Yes/No. |
| LP switch via DI | Low pressure safety circuit input. | Yes/No. |
| Load shedding via DI | Load limitation input. | DI or Night Mode. |
| Sd disch. gas temp. | Shared discharge temperature signal. | Yes/No. |
| Ss suction superheat | Monitor suction superheat signal. | Yes/No. |
| Injection ON function | Stop injection into evaporators. | No/Network/Relay. |
| Lead comp. Min ON | Min. On-time for first compressor. | Min: 0 min, Max: 60 min. |
| Lead comp. Min OFF | Min. Off-time for first compressor. | Min: 0 min, Max: 30 min. |
| Lead comp. Restart | Min. restart time for first compressor. | Min: 1 min, Max: 60 min. |
| Lead comp. Safety delay | Delay before compressor 1 cuts out for safety. | Min: 1 min, Max: 10 min. |
| Comp. Min ON | Min. On-time for remaining compressors. | Min: 0 min, Max: 60 min. |
| Comp. Min OFF | Min. Off-time for remaining compressors. | Min: 0 min, Max: 30 min. |
| Comp. Restart | Min. restart time for remaining compressors. | Min: 1 min, Max: 60 min. |
| Comp. Safety delay | Delay before compressors cut out for safety. | Min: 1 min, Max: 10 min. |
| Compressor status | Operating status of compressors. | (e.g., Alarm, Ready, Running, Disabled) |
| Comp. 1 Sd gas | Sd temperature of compressor 1. | Read only. |
| Comp. 1 cap | Connected capacity of compressor 1. | 0-100%. |
| Compressor runhours | Total operating time of compressors. | Read only. |
| Compressor cycles | Number of compressor starts. | Read only. |
| Comp. 1 out of service | Service mode for compressor 1. | Yes/No. |
Suction B Settings
For Suction Group B, refer to the descriptions for Suction Group A. Note: Bitzer CRII cannot be used in Suction Group B.
Condenser Control Settings
Parameters for condenser regulation.
Control Status:
Status of the condenser circuit (e.g., Main Sw. off, Ready, Running, Manual ctrl, High Pc/Sd).
Readings:
Control temp., Reference, Running capacity, Requested capacity, No. of running fans, Tc Saturated temp., Pc Pressure, S7 Media.
Control Settings:
| Parameter | Description | Range/Options |
|---|---|---|
| Control mode | Regulation type | MAN / OFF / AUTO (Default: AUTO) |
| Setpoint | Condenser regulation set point. | Min: -25°C (-1.0 bar), Max: 90°C (159 bar) |
| Sc3 offset | Temperature offset for fluid reference. | Min: 0 K, Max: 20 K |
| Min. reference | Lowest permissible regulation reference. | Min: -25°C (-1.0 bar), Max: 100°C (159 bar) |
| Max. reference | Highest permissible regulation reference. | Min: -25°C (-1.0 bar), Max: 100°C (159 bar) |
| Heat recovery SP | Temperature set point for heat recovery. | Min: 20°C (-1.0 bar), Max: 90°C (159 bar) |
| Heat rec. ramp down | Ramp-down speed after heat recovery. | Min: 0.1 K, Max: 100 K (per minute). |
| Capacity limit night | Fan capacity limitation at night. | Min: 0%, Max: 100%. |
| Gain factor Kp | Amplification factor for PI regulation. | Min: 0.5, Max: 50. |
| Integration time Tn | Integration time for PI regulation. | Min: 10 s, Max: 900 s. |
| Fan configuration | Selection of regulation sensor. | Pc pressure transmitter or S7 media temperature sensor. |
| Reference mode | Reference setting. | Setpoint or Floating (based on outdoor temp.). |
| Capacity ctrl. mode | Fan control mode. | Variable, Step, or Variable + Step. |
| No. of fans | Number of fans. | Min: 0, Max: 8. |
| Control type | Regulation type. | P/PI (Default: PI). |
| VSD Start speed | Start value for frequency converter. | Min: 0%, Max: 60%. |
| VSD Min. speed | Minimum speed for frequency converter. | Min: 0%, Max: 40%. |
| VSD Max. speed | Maximum speed for frequency converter. | Min: 61%, Max: 100%. |
Fan Safety Monitoring
Configuration and monitoring of fan safety functions.
| Parameter | Description | Options |
|---|---|---|
| VSD Start via DO | Relay connection for frequency converter start/stop. | Yes/No. |
| Monitor fan safety | Safety monitoring of condenser fans. | None, Individual, Common. |
| Fan at comp. OFF | Fan control when compressors are stopped. | Normal or Energy-optimised. |
| Heat recovery via DI | Heat recovery cycle activation via DI input. | Yes/No. |
| Fan status | Fan speed and operation status. | Read only. |
| Fan Runhours | Total fan operating hours. | Read only. |
| Fan cycles | Number of fan starts. | Read only. |
Safety Limits
Defines safety limits for various parameters and associated alarms.
| Parameter | Description | Range/Options |
|---|---|---|
| PoA Min limit | Safety limit for minimum PoA. | Min: -120°C (-1.0 bar), Max: 30°C (159 bar) |
| PoA Max alarm | Alarm limit for high PoA. | Min: -30°C (-1.0 bar), Max: 100°C (159 bar) |
| PoA Max delay | Delay time for PoA max alarm. | Min: 0 min, Max: 240 min. |
| Superheat Min lim A | Alarm limit for insufficient superheating (Suction line A). | Min: 0 K, Max: 20 K. |
| Superheat Max lim A | Alarm limit for excess superheating (Suction line A). | Min: 20 K, Max: 80 K. |
| Superheat delay A | Delay time for superheat alarm (A). | Min: 0 min, Max: 60 min. |
| SdA Max limit | Safety limit for max. SdA. | Min: 0°C, Max: 195°C. |
| PoB Min limit | Safety limit for minimum PoB. | Same settings as Suction Group B. |
| PoB Max alarm | Alarm limit for high PoB. | Same settings as Suction Group B. |
| PoB Max delay | Delay time for PoB max alarm. | Same settings as Suction Group B. |
| Superheat Min lim B | Alarm limit for insufficient superheating (Suction line B). | Same settings as Suction Group B. |
| Superheat Max lim B | Alarm limit for excess superheating (Suction line B). | Same settings as Suction Group B. |
| Superheat delay B | Delay time for superheat alarm (B). | Same settings as Suction Group B. |
| SdB Max limit | Safety limit for max. SdB. | Same settings as Suction Group B. |
| Pc max limit | Safety limit for max. Pc. | Min: -1 bar, Max: 159 bar. |
| Tc Max limit | Safety limit for max. Tc. | Read only. |
| Pc Max delay | Time delay for Pc max. alarm. | Min: 0 min, Max: 240 min. |
| HP switch via DI | Signal from HP switch. | Yes/No. |
| Safety restart time | Delayed start-up following safety cut-out. | Min: 0 min, Max: 60 min. |
| Sensor alarm reset | Reset alarm after sensor error. | Min: 0 min, Max: 30 min. |
General Functions
Configuration for general alarms, thermostats, and other system-wide functions.
Digital Input Alarms:
Up to 3 general alarms can be defined, not directly related to regulation. These can be configured with alarm text, delay time, and active on/off signal. The number of general alarm functions can be set.
DI Alarm Text:
Selectable alarm texts include: General alarm, High/Low pressure, High/Low temperature, Oil level/temperature, Liquid level, Leak detection, Inverter fault.
DI Alarm Delay:
Configurable delay time for DI1 alarm (and DI2, DI3).
Thermostat:
One general thermostat can be defined, requiring an analogue input (Saux) and a relay output. Settings for cut-in temperature, cut-out temperature, and current temperature are available.
System Settings
Configuration options for display, password, time, network, and factory reset.
| Category | Setting | Options |
|---|---|---|
| Display | Language | English, German, French, Spanish, Italian, Portuguese, Dutch, Russian, Polish, Czech, Turkish, Hungarian, Croatian, Serbian, Romanian (Default: UK English) |
| Engineering units | SI / Imperial | |
| Pressure units | bar / PSIG | |
| Temperature units | °C / °F | |
| Time format | 12-hour or 24-hour | |
| Screen saver time | Min: 1 min, Max: 60 min. | |
| User logout time | Min: 1 min, Max: 60 min. | |
| Display contrast | Min: 0, Max: 100. | |
| Password | Password levels 1, 2, 3 | Access codes for end-user, installer, and system configuration. |
| Real time clock | Date and time | Year, month, date, hours, minutes. |
| Weekly schedule | Weekly plan | Opening/closing hours for regulation reference and fan speed adjustment. |
| Network | Modbus Address | Min: 0, Max: 120. |
| Baudrate | 38.4 (or 19.2 for SLV mode). | |
| Serial mode | 8E1 (default). | |
| Reset to factory | Return to factory settings | Yes/No. Resets all settings and clears alarm list. |
I/O Configuration
Allows selection and assignment of functions to individual inputs and outputs. Functions can be selected for digital outputs (DO), digital inputs (DI), analogue outputs (AO), and analogue inputs (AI).
Digital Outputs (DO1-DO8):
Functions include pulse output for capacity valves, compressor start/unloaders, fans, Injection ON, and Alarm.
Digital Inputs (DI1-DI8):
Functions include compressor safety inputs, fan safety input, external main switch, HP/LP safety switches, night status, heat recovery, load shedding, and general alarm inputs.
Analogue Outputs (AO1-AO4):
Used for compressor and condenser speed control.
Analogue Inputs (AI1-AI8):
Used for pressure transmitters (PoA, PoB, Pc), temperature sensors (Sc3, S4, S7), discharge gas temperature (Sd), suction gas temperature (Ss), and Saux for general thermostat.
I/O Status and Manual Control
Provides status of I/O signals and allows manual override control.
I/O Status:
Displays the current status of digital outputs, digital inputs, analogue outputs, and analogue inputs.
I/O Summary:
Shows the maximum available and currently used inputs/outputs.
I/O Manual Control:
Allows manual control of relay outputs (Auto/On/Off) and analogue outputs (Auto/Man, 0-100%). Remember to switch back to "Auto" after manual control.
Alarm Priorities
Defines priority levels for alarms: Critical, Severe, Normal, and Disable. Each incident can be assigned a priority to modify its importance.
Alarm List:
A comprehensive list of alarms, their reasons, priority settings, and default values is provided, categorized by General Alarms, Suction A Alarms, Suction B Alarms, and Condenser Alarms.
Sensor Alarms:
Sensor alarms automatically shut off when the sensor is OK for 10 minutes. Manual reset is possible.
Troubleshooting: ERR31 Alarm
An "ERR31" alarm on the external display indicates a communication error between the display and the controller. This can be due to incorrect terminations or communication interruptions.
Troubleshooting Steps:
- Inspect terminations.
- Check the external display's software version (must be 1.13 or newer).
- Verify display settings: MCX selection (Clear UI, Autodetect), COM selection (CAN), Start up mode (Remote application), CAN settings (Baudrate: Autobaud, Node ID: 126).
After checks, the display will retrieve data from the controller (may take up to 5 minutes).
10. Connections
Details the electrical connections for the controller.
Connection, Lower Level:
Includes connections for Power (L, N), Modbus (A+, B-), and Digital Outputs (DO1-DO8).
Connection, Upper Level:
Includes connections for External Supply (24V AC/DC), Analogue Outputs (AO1-AO4), Digital Inputs (DI1-DI8), and Analogue Inputs (AI1-AI8).
Electric Noise Considerations:
Signal cables must be kept separate from high voltage cables, using separate cable trays and maintaining a distance of at least 10 cm. Cables longer than 3m at DI input should be avoided.
Supply Voltage:
Available in 24 V AC/DC or 230 V AC.
Modbus:
Important for data communication; refer to separate literature RC8AC. Bus termination is required.
DO - Digital Outputs:
8 pcs. DO5 and DO6 are solid state relays. Alarm relay function is described.
AO - Analogue Output:
4 pcs. Used for frequency converters or EC motors. Requires 24V supply on AO3/AO4 (external). Polarity of N is important.
AI - Analogue Inputs:
8 pcs. Used for pressure transmitters (AKS 32R, AKS 32, AKS 33) and temperature sensors (Pt 1000, NTC 86K). Factory settings map AI1-AI4 to PoA, PoB, Pc, and Sc3.
DI - Digital Switch Inputs:
8 pcs. Used for shut-down or interruption functions.
Compressor Capacity Control
Explains capacity control methods for different compressor types.
Digital Scroll Compressor:
Capacity is divided into "PWM period time". 100% capacity is delivered when cooling is active. The controller varies capacity based on cut-in time. A minimum capacity limit (e.g., 10%) can be set to ensure self-cooling.
Copeland Stream Compressor:
Pulse signal controls one stream compressor with one unloader valve (4 cylinders). Capacity distributed via relay and unloader (connected to DO5 or DO6).
Bitzer CRII:
Pulse signal controls CRII with 2 unloaders (4 cylinders). Capacity controlled from 10-100% based on unloader pulsation (connected to DO5 or DO6).
Sd Monitoring:
When regulating with Sd monitoring, capacity increases if temperature nears the Sd limit, improving cooling of unloaded compressors.
Injection Off:
Ensures electronic expansion valves close when compressors are prevented from starting, preventing evaporator overfilling. Can be controlled via compressor relays or data communication.
11. Data & 12. External Display
Data:
Provides technical specifications for supply voltage, analogue inputs, digital inputs, relay outputs, voltage outputs, display output, data communication, environments, enclosure, weight, mounting, connection terminals, and approvals.
External Display:
An external display (MMIGRS2) is available for front assembly (IP 20) and connects via a cable plug. Ordering information is provided.
13. Ordering & 14. Mounting/Dimensions
Ordering Information:
Lists available product types (AK-PC 551, MMIGRS2), their functions, operation modes, supply voltages, and corresponding code numbers.
Mounting/Dimensions:
Provides dimensional drawings for DIN rail mounting (IP 20).
Installation Considerations:
Emphasizes that proper installation is crucial to prevent malfunctions and plant breakdowns. Electronic controls supplement, but do not replace, normal engineering practice. Danfoss is not responsible for damage due to improper installation. Installers must check installations and fit safety devices. Special attention is noted for compressor stop signals and liquid receivers.
List of Literature:
References the Installation guide for extended operation RC8AC for data communication connections to ADAP-KOOL® systems.
