Detecting the operating status of a compressor based on the current consumption
Use Case
The correct functioning of a compressor is essential for the production processes. The availability of a compressor has an enormous impact on many production-relevant systems and processes. Important key figures for this are provided by detecting the actual operating hours of the compressor via the power consumption.
As a rule, maintenance is carried out at regular intervals. Thus, progressive damage is detected too late and results in unplanned downtimes and high repair costs.
The starting position
No needs-based maintenance of the compressor
Until now, the operating hours were only detected via the rotational speed of the compressor drive and not via the actual use. Compressors can work in different operating statuses. Due to the differences in load, the individual components of the compressor are subjected to different levels of stress over time. In the NO-LOAD mode, the compressor is at rest and does not wear out.
The operating hours of the compressor are currently only detected via the speed of the drive motor. This method cannot distinguish between NO-LOAD and LOAD. Therefore it is not possible to detect the operating hours per operating range with regard to NO-LOAD / LOAD.
Maintenance was scheduled at regular intervals, regardless of how much the system had actually been used during this period.
Goal of the project
Detecting the current consumption of the compressor and further processing in moneo as a basis for needs-based maintenance
The aim is to organise needs-based maintenance by means of data acquisition.
The value of the current consumption is to be measured using a current converter. Moreover, the different operating ranges of the compressor are to be determined, visualised and their respective times calculated. Useful life and associated wear and tear are detected.
Implementation
moneo RTM is centrally installed on a server. The IO-Link masters are connected to the server via an internal VLAN.
ifm has a wide range of automation components. The ZJF056 current converter and the DP2200 evaluation system and display for analogue signals were chosen for this application.
The current converter is used for any phase of the three-phase supply lines to the compressor. The measured value of the converter is available as a 4...20mA analogue signal on the signal output. This value is converted from 4...20 mA to an IO-Link signal via the DP2200. It has to be taken into account that the “internal load” of the DP2200 is activated.
The data is provided to moneo RTM via an IO-Link master of the AL1352 series.
To obtain useful process values, the measured value of the current converter must be converted into the actual current value (4 mA ≙ 0 A, 20 mA ≙ 50 A*) of the converter. This is done in moneo RTM via the function “Calculated values”.
In the analysis, the calculated current value is then used to recognise the respective operating range of the compressor.
* Factory setting of the ZJF056 = 100 A. To increase the measuring accuracy, the measuring range was adjusted to 50 A via the DIP switch on the converter.
- Operating range: off / current consumption in A: <1
- Operating range: no-load / current consumption in A: 1 – 40
- Operating range: load / current consumption in A: >40
Success
Organisation of needs-based maintenance
By detecting the consumed current it is possible to determine the current operating range. This allows the operating hours in the respective operating range of the compressor to be counted. Based on this data, maintenance of the compressor can be planned based on the needs.
Assessing the plant capacity utilisation
In addition, information on plant capacity utilisation can also be calculated. If the production plant is being expanded, this value can be used, for example, to determine whether the capacity of the existing compressor or compressors is still sufficient. It is also possible to assess the available compressed air capacity with regard to possible energy cost savings when using different aggregate units.
System structure
- Current converter
- Signal converter for analogue 4..20 mA to IO-Link(DP2200)
- IO-Link master
Dashboard
Get the big picture on the moneo dashboard. The dashboard provides the user with an overview of the relevant process values for this plant.
- Current value measured at the sensor in amperes
- Operating hours counter OFF
- Operating hours counter NO-LOAD
- Operating hours counter LOAD
- Ratio of NO-LOAD time to total runtime
- Ratio of LOAD time to total runtime
Analysis
The analysis function can be used to access historical data and compare different process values. The diagram shows a typical characteristic curve for OFF (a), LOAD (b) and NO-LOAD (c).
It can be observed that the speed (violet) is the same for LOAD and NO-LOAD. However, the current consumption (blue) and the energetic work thereby converted differs significantly in the two operating ranges.
- Speed reaches maximum value, current consumption for OFF and NO-LOAD range
- Current consumption in the LOAD range
Settings & rules: Manage thresholds
Static thresholds
The operating hours of the individual operating statuses can be monitored statically. For example, corresponding tickets can be generated after a certain number of hours in the load range or a certain number of hours of the total runtime. Warning and alarm tickets are available for this purpose, which can be used for timely planning.
- Warning after reaching 950 operating hours in the load range
- Alarm after reaching 1,000 operating hours in the load range
Ticket processing rules
This function can be used to easily define what should happen after a warning or alarm has been triggered, e.g.:
- Email notification
- SAP integration
For applications where maintenance measures are necessary, it is advisable to plan the service call in good time.
Calculated values
The “Calculated values” function is used to further process the process data.
Conversion from an analogue value to a current value
In this use case, the analogue current signal provided by the sensor is converted into the value of the current flow through the conductor, in the unit ampere.
Motor current = (AIN - 4) * ((AEP - ASP)/16)
- Analogue current value of the current converter (4...20 mA)
- Analogue start point (0 A = 4 mA)
- Analogue end point (50 A= 20 mA)
- Current span analogue value (20 mA - 4 mA = 16 mA)
- Offset analogue value (4...20 mA to 0...16 mA)
- Calculation of the delta of start point to end point (AEP – ASP = ∆A)
- Calculation of the factor for current to pascal (∆A / 16 mA = factor)
- Multiplication of the current value (0...16 mA) with the factor
- Rounding the result to one decimal place
- Result of current flow in A
Calculation of the operating hours
The values of the current consumption of the respective operating ranges determined via the analysis function can be used to create different operating hour counters. These calculate the time spent in the respective operating status.
- Minimum current consumption for the operating range in A
- Current consumption in A
- Maximum current consumption for the operating range in A
- Comparison of current consumption with minimum current consumption
- Comparison of current consumption with maximum current consumption
- Logical conjuntion (AND) of maximum and minimum current consumption
- Time counter
- Output of the time in h
Determining the total runtime
The total runtime of the compressor (OFF + NO-LOAD + LOAD) can be determined by simply adding the respective operating hour counters.
Total Time = Off + No-Load + Load
- Operating hours counter OFF in h
- Operating hours counter NO-LOAD in h
- Operating hours counter LOAD in h
- Addition of operating hours NO-LOAD + LOAD in h
- Result of NO-LOAD + LOAD added with operating hours OFF in h
- Rounding the result to one decimal place
- Output total runtime in h
Ratio calculation of the individual operating statuses
In a further calculation, the hours of the respective operating ranges are set in relation to the total time. This provides a good overview of the compressor utilisation.
- Operating hours LOAD
- Total runtime of the compressor
- Constant 100%
- Ratio of operating hours LOAD to total runtime
- Ratio multiplied by 100%
- Output ratio in percent
