Displaying Motor Current Percentage in CEMAT C_DRV_1D Faceplates: Connecting C_MEASUR and OS Compilation Rules

To display motor current percentage in a CEMAT C_DRV_1D faceplate, interconnecting the MV_PERC output of C_MEASUR to the AV_PERC (or MV_PERC) input of C_DRV_1D is only the first step. For the measured current to appear in WinCC Runtime, you must set parameter REL_AV = 1 (Release Analog Value) on the C_DRV_1D block in CFC to activate the current bar display, and you must perform a WinCC OS Compilation (Changes Only). Downloading changes only to the PLC updates AS execution, but WinCC cannot visualize the current bar until the OS Compiler generates the required tag connections and faceplate structures.

CEMAT Drive and Measured Value Architecture Overview

In industrial mineral processing, mining, and cement plants running SIMATIC PCS 7 with the CEMAT library, electric drives are controlled via the standard block C_DRV_1D (Unidirectional Drive). Operating personnel rely heavily on the drive faceplate to monitor motor current as a percentage of nominal full-load current (FLC) to detect conveyor belt slippage, crusher blockages, and motor overload conditions.

Analog current signals are processed by the dedicated measurement block C_MEASUR. The C_MEASUR block converts 4–20 mA field signals or fieldbus process values into scaled engineering units and calculates the current as a percentage (MV_PERC) based on motor rating. Displaying this percentage within the C_DRV_1D faceplate requires coordinating block parameterization in CFC with OS tag compilation.

Connecting C_MEASUR to C_DRV_1D in Continuous Function Charts

To interconnect the current measurement to the drive block in Continuous Function Chart (CFC):

Open the CFC chart containing the motor drive C_DRV_1D and current transmitter C_MEASUR.
Locate the output parameter MV_PERC (Measured Value in Percent) on C_MEASUR.
Interconnect C_MEASUR.MV_PERC to input parameter AV_PERC (Analog Value in Percent, or MV_PERC depending on CEMAT version) on C_DRV_1D.
Alternatively, in advanced CEMAT architectures, the block pointer input LNK_MEAS or MS_CURR can be configured to point to the C_MEASUR instance, allowing direct one-click navigation from the drive faceplate to the full measurement diagnostic faceplate.

The Critical Role of Parameter REL_AV (Release Analog Value)

A frequent pitfall for automation engineers is that even after wiring MV_PERC to AV_PERC, the faceplate displays a blank space where the current gauge should appear. This is governed by the display release parameter:

Parameter REL_AV (Release Analog Value): On C_DRV_1D, input parameter REL_AV is an authorization flag (default = 0 / False).
Display Suppression: Because many small auxiliary drives (such as cooling fans or lube pumps) do not have current transformers, CEMAT hides the current gauge by default to prevent visual clutter.
Activation: You must explicitly set REL_AV = 1 (or interconnect an enable signal) on C_DRV_1D. This instructs the drive algorithm to process the analog value and command the WinCC faceplate to unhide the current percentage bar and numerical readout.

Why Downloading to the PLC Alone Leaves Faceplates Unchanged

When engineers encounter this issue, they often note: 'I downloaded the changes to my PLC, but the faceplate didn't change!'

Understanding the PCS 7 execution pipeline clarifies this behavior:

PLC Scope: Downloading changes to the AS controller updates the data block execution in controller RAM. The CPU now transfers MV_PERC into C_DRV_1D.AV_PERC internally.
HMI Scope: WinCC faceplates do not interrogate the AS directly to discover new chart links. WinCC reads pre-configured tag structures stored in the OS project database.
Mandatory OS Compile: An OS Compilation (Changes Only) is required. The OS compiler detects the modified block parameters, updates the WinCC tag variables, and registers the analog value link. Without an OS Compile, the WinCC faceplate script sees zero tags associated with the analog input and suppresses rendering.

Signal Interconnection and Parameter Mapping Matrix

The following table details the mandatory parameter connections between CEMAT measurement and drive blocks:

BlockParameter NameData TypeMandatory Value / SourceFunctional RoleC_MEASURMV_PERCREALOutput from calculationCurrent value scaled from 0.0 to 100.0% (or overload %)C_DRV_1DAV_PERCREALConnected to C_MEASUR.MV_PERCReceives percentage current value for faceplate displayC_DRV_1DREL_AVBOOL1 (True)Enables current bar and analog display on faceplateC_DRV_1DLNK_MEASSTRUCT / DWORDPointer to C_MEASUREnables faceplate direct button jump to measurement windowC_DRV_1DAV_LIMITREALMotor FLC limit valueTriggers high current warning bar color change (Yellow/Red)

Step-by-Step Configuration and Commissioning Workflow

Follow this complete sequence to enable motor current percentage display:

In CFC, interconnect C_MEASUR.MV_PERC to C_DRV_1D.AV_PERC.
Open the object properties of C_DRV_1D, select parameter REL_AV, and set its value to 1.
Interconnect the block pointer C_MEASUR instance ID to C_DRV_1D.LNK_MEAS if faceplate navigation is desired.
Compile and download CFC changes to the AS (Charts -> Compile -> Changes Only then Download -> Changes Only).
In SIMATIC Manager, right-click the OS project and select Compile -> Changes Only.
After OS compile finishes, reload server packages if operating a multi-client architecture.
Open the motor faceplate in WinCC Runtime and verify the live current gauge.

Verification

Verify live motor current visualization in WinCC Runtime:

Open the WinCC Runtime screen containing the drive icon.
Click the drive icon to open the C_DRV_1D standard faceplate.
Confirm that the Current (%) bar is visible alongside the numerical display.
In CFC test mode, force C_MEASUR.MV_PERC to 75.0%; confirm that the WinCC faceplate bar immediately moves to 75%.
Click the analog value icon inside the faceplate; verify that it jumps directly to the diagnostic faceplate of C_MEASUR.

Limitations and Operational Precautions

Adhere to the following operational precautions:

Overcurrent Scaling: Ensure C_MEASUR scaling parameters (SCB - Scale Beginning, SCE - Scale End) match the field current transformer (CT) ratio accurately; misconfigured scaling presents false percentage data to operators.
Dynamic Overload Range: CEMAT faceplates support display values above 100% (e.g. up to 150% during motor acceleration); verify that faceplate high limit markers match the motor thermal overload curve.
OS Compile Target: In redundant server systems, ensure the OS compile is downloaded to both servers to avoid asymmetric tag displays.

Engineer FAQ

Can I click on the motor current in the C_DRV_1D faceplate to open the measurement block?

Yes. By interconnecting the measurement block pointer to parameter LNK_MEAS on C_DRV_1D, clicking the analog value jumps directly to the C_MEASUR faceplate.

What happens if the current measurement signal fails (bad quality code)?

If C_MEASUR detects wire break or sensor fault, it asserts quality code 16#00 or 16#28. C_DRV_1D displays a fault badge and changes the bar color to indicate invalid measurement.

Can C_DRV_1D display motor current in Amperes instead of Percent?

Yes. CEMAT supports displaying both physical engineering units (Amperes via parameter AV) and relative percentage (AV_PERC) simultaneously on the extended faceplate view.