Converting Motor Current Percentage Values to Absolute Physical Units in PCS 7 CEMAT

To convert motor current values displayed as relative percentages (0...100%) on WinCC faceplates into absolute physical engineering units (0...90 Amperes) across plant motors in SIMATIC PCS 7 with CEMAT, configure the scaling parameters directly inside the AS controller CFC logic rather than modifying individual HMI graphics. On the technological motor block (C_DRIVE_1D) or analog measurement block (C_MEASUR), configure the scale limits (Scale.Low = 0.0, Scale.High = Nominal_Current) and assign the unit string to Amperes (A). Alternatively, if the transducer delivers percentage, multiply the percentage by the motor rated current using a MUL_R block before feeding the block input.

Architecture of Motor Current Processing in CEMAT and APL

In industrial plants running SIMATIC PCS 7 with the CEMAT library (such as cement mills, crushers, and kilns), motor operating current is monitored for thermal protection, overload alarms, and power calculation.

In legacy configurations, field analog transducers (0...20 mA / 4...20 mA) or SIMOCODE units frequently deliver current as a relative percentage of full scale (0.0% to 100.0%). On the operator faceplate, operators prefer reading absolute current in Amperes (e.g., 45.2 A) to verify motor load against physical nameplate ratings.

Why Scaling Must Be Handled at the Controller Level (AS)

When a plant operates over 50 motors, modifying 50 individual WinCC faceplates or attempting graphic-level unit conversions is poor engineering practice:

Standard CEMAT faceplates (@PG_C_DRIVE_1D.pdl, @PG_C_MEASUR.pdl) dynamically read their units (Unit) and scaling (Scale.High, Scale.Low) directly from the AS controller block.
If scaling is performed only in WinCC Tag Management, the controller's internal alarm thresholds (Overload limit, Undercurrent trip) remain misaligned in percentage, causing trip logic divergence.
Implementing the conversion at the AS controller level automatically harmonizes OS faceplates, alarm thresholds, historical archives, and interlock comparisons.

Method 1: Direct Engineering Unit Scaling on C_MEASUR / C_DRIVE_1D

In the CEMAT library, analog currents are processed by the block C_MEASUR (or modern APL MonAnL):

Input Signal Assignment: Connect the raw transmitter signal to input MV_In.
Configure Physical Scale Range:
- Set parameter SCB (Scale Bottom / Low Limit) = 0.0.
- Set parameter SCE (Scale End / High Limit) = Motor Rated Current in Amperes (e.g. 90.0).
Assign Engineering Unit:
- Set parameter UNIT = 'A' (or unit code 1120).
OS Compile: Compile the OS; the faceplate automatically reads 0...90 A with unit A.

Method 2: Mathematical Scaling via CFC Arithmetic Blocks (MUL_R / DIV_R)

If an upstream block or smart relay delivers a pre-calculated percentage (0...100%):

Calculate absolute current using the linear formula:
$$\text{Current (A)} = \frac{\text{Relative Value (\%)} \times I_{\text{nominal}}}{100.0}$$
In CFC, instantiate MUL_R (FC 63) to multiply the percentage by rated current ($I_{\text{nom}}$), and DIV_R (FC 64) to divide by 100.0.
Connect the output to the motor block's current input.

Why WinCC Tag Linear Scaling is Discouraged for Technological Faceplates

WinCC Tag Management supports Linear Scaling (e.g. AS value 0...100 -> OS value 0...90). However, on standard PCS 7 faceplates, this creates severe anomalies:

The faceplate header and limit bars continue displaying the unit string and limit boundaries (Limit.High, Limit.Low) compiled from the AS block.
Operators will see an absolute current value of 75 plotted against a percentage scale labeled %, causing confusion.

Comparison of Current Scaling Methodologies

The table below contrasts scaling approaches for multi-motor installations:

Engineering MethodImplementation LevelPlant-Wide ScalabilityFaceplate Unit AlignmentInterlock Safety ConsistencyC_MEASUR Block ScalingAS Controller (CFC)Highest (Template based)100% AutomatedFull alignment across AS and OSCFC Arithmetic (MUL_R)AS Controller (CFC)High (Modular logic)Manual unit assignmentConsistent AS logicWinCC Linear ScalingOS Tag ManagementLow (50 tags to edit)Broken unit labelsSevere risk (AS trips remain in %)Custom Faceplate ScriptWinCC Graphics (PDL)Worst (Overwritten on update)Screen specificZero AS integration

Verification

Verify scaled current rendering in runtime:

Open the CFC chart online; inject 50% current signal.
Confirm the block output evaluates to 45.0 A (for a 90 A motor).
In WinCC OS Runtime, click the motor block icon to open @PG_C_DRIVE_1D.pdl.
Confirm that the current field displays 45.0 with unit label A.
Verify that high-current alarm limits display correctly in Amperes on the faceplate limit tab.

Limitations and Operational Precautions

Review the following operational precautions regarding current scaling:

Nominal Current Verification: Always cross-check the motor nameplate rated current ($I_n$) from the electrical switchgear schedule before assigning SCE limits.
CT Ratio Calibration: If current transformers (CTs, e.g. 100/5 A or 200/1 A) are used, ensure transducer scaling accounts for the physical transformer turn ratio.
Alarm Re-parameterization: When switching from percentage to absolute Amperes, all related alarm limits (I_AH, I_WH) must be updated to Amperes to prevent immediate false trips.

Engineer FAQ

Can I change the display from percent to Amperes across all 50 motors in one place?

If the motors use a common Control Module Type (CMT) or Process Tag Type (PTT), updating the Scale parameters and Unit in the template updates all 50 instances simultaneously upon synchronization.

Why shouldn't I use WinCC Tag Management Linear Scaling to change current to Amperes?

Linear Scaling in WinCC only scales the raw numerical value. The faceplate unit label, bar graph limits, and controller trip thresholds in the AS remain in percent, causing dangerous operational confusion.