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How to Configure Local Mode Tracking in PCS 7 APL MotL Without Physical Remote-Local Feedback

How to Configure Local Mode Tracking in PCS 7 APL MotL Without Physical Remote-Local Feedback

To configure local mode tracking on the SIMATIC PCS 7 Advanced Process Library (APL) motor block MotL (FB1850) when the physical field equipment lacks a dedicated "Local / Remote" selector switch input, set the parameter LocalSetting = 2 or LocalSetting = 4. When a motor is started from a local field push-button station without notifying the DCS, the sudden arrival of running feedback (FbkRun) without an active DCS command (Start) normally generates a "Feedback Error" alarm. By creating logic that asserts LocalLi = 1 and StartLocal = 1 upon detecting unexpected running feedback and configuring LocalSetting = 2 (Local mode with tracking), MotL transitions seamlessly to Local mode, tracking the physical contactor state on WinCC faceplates without asserting its own digital output (Start).

Table of Contents

The Dilemma of Missing Field Remote/Local Switch Contacts

In legacy electrical substations, MCCs, and package skids, field motor starters frequently lack auxiliary dry contacts for control location:
- Field Setup: A local start/stop pushbutton station is wired directly to the motor starter coil in the Motor Control Center (MCC).
- DCS Interface: The DCS only receives a single digital input: the Running Feedback auxiliary contact (FbkRun), along with a DCS digital output for Remote Start (Start).
- The Operational Challenge: If a field operator presses the local Start button:
1. The contactor energizes, and FbkRun asserts to 1.
2. In the DCS, MotL is currently in Auto or Manual mode with Start = 0.
3. MotL detects running feedback without a command and raises a high-priority alarm: Feedback error entering state.
4. If an engineer attempts to force Local mode blindly, MotL might activate its physical Start output, creating an unwanted electrical latch.

Understanding MotL LocalSetting Operating Modes (1, 2, 3, 4)

Parameter LocalSetting defines how MotL evaluates commands and outputs in Local mode:

LocalSetting ValueDescriptionOutput Behavior (Start)Feedback EvaluationLocalSetting = 1Local control via faceplateControlled by WinCC operator faceplate buttons in Local mode.Standard feedback monitoring.LocalSetting = 2Local control via signals with trackingOutput Start is NOT asserted; MotL tracks the state of FbkRun.Suppresses Feedback Faults: The block accepts field running state without raising discrepancy alarms.LocalSetting = 3Local control via signals without trackingOutput Start asserts based on StartLocal signal.Standard feedback monitoring.LocalSetting = 4Local control via signals, tracking, and bumpless returnOutput Start is NOT asserted; tracks FbkRun; returns to Manual mode with tracking upon local stop.Suppresses feedback faults; prevents process shocks when local run ends.

For installations lacking a physical switch, LocalSetting = 2 or 4 is mandatory because it decouples the PLC's Start output entirely.

Why Naive Logic Causes Output Command Conflicts and Feedback Faults

When engineers write custom CFC logic to detect local running:

IF (FbkRun = 1) AND (Start = 0) THEN
    LocalLi := 1;
    StartLocal := 1;
END_IF;

If LocalSetting is left at default 0 or set to 3, asserting StartLocal causes MotL to assert output pin Start = 1. In the electrical MCC, the DCS start relay energizes. If the field operator subsequently presses the physical Stop button, the motor stops momentarily, but the latched DCS Start output immediately re-starts the motor! To prevent this catastrophic safety hazard, Start must remain strictly de-asserted.

Method: Step-by-Step Implementation of Local Detection Logic

Follow this procedure to configure safe local tracking without output command interference:

Step 1: Configure MotL Block Parameters

  1. Open the CFC chart containing the MotL block.
  2. Set parameter LocalSetting = 2 (or 4).
  3. Set parameter ModLiOp = 1 (enables mode switching via interconnection lines).
  4. Verify parameter Feature.Bit27 (Interlock display in local mode) matches plant safety standards.

Step 2: Implement Local State Detection in CFC

  1. Insert an AND logic block (And02) and a NOT block from the PCS 7 Basic Library.
  2. Invert the DCS command state: connect MotL.Start to the input of the NOT block.
  3. Wire the AND block:
    - Input 1: Physical running feedback (Pcs7DiIn.Out.Value from the contactor).
    - Input 2: Output of the NOT block (confirming DCS did not issue the start command).
  4. The output of the AND block represents: Field_Local_Active.

Step 3: Wire Mode Signals to MotL

  1. Connect Field_Local_Active directly to MotL.LocalLi (Local mode command via interconnection).
  2. Connect Field_Local_Active to MotL.StartLocal (Local run command).
  3. Connect Pcs7DiIn.Out to the standard feedback input MotL.FbkRun.
  Contactor FbkRun ──────┬────────────────────────► MotL.FbkRun
                         │
                         ▼
                   ┌───────────┐
                   │   AND     │──────► MotL.LocalLi
  MotL.Start ──[NOT]──►│           │──────► MotL.StartLocal
                   └───────────┘

Step 4: Operational Result

  • When started locally from the field: Field_Local_Active goes TRUE. MotL switches to Local mode. Because LocalSetting = 2, MotL.Start remains 0.
  • The WinCC faceplate updates to show the motor icon spinning in Green with the yellow "Local" mode banner.
  • Zero feedback error alarms are generated.
  • When the field motor stops: FbkRun drops to 0. Field_Local_Active goes FALSE. MotL returns cleanly to Automatic or Manual mode.

Verification

To verify that local mode tracking functions safely:
- Simulate field running feedback (FbkRun = 1) while the DCS is idle in Manual mode.
- Verify in CFC online Test Mode:
- MotL.LocalAct asserts to 1.
- MotL.Start output remains strictly 0 (contactor coil output remains off).
- Parameter FbkRunOut reflects the active running state.
- Check WinCC AlarmControl: confirm that no "Feedback error" alarm appears.
- Clear the feedback (FbkRun = 0): verify that MotL exits Local mode and returns to idle state without latching.

Limitations and Operational Precautions

  • Permit and Interlock Bypassing in Local Mode: By default, standard APL blocks do not enforce software interlocks while in Local mode (LocalSetting = 2). Ensure that physical field emergency stop buttons are hardwired directly into the motor starter control circuit.
  • Feature.Bit24 (Local mode enabling): Ensure that Feature.Bit24 is not configured to lock out local operation.
  • Bumpless Transition to Auto: If LocalSetting = 2 is used, the motor will return to Auto mode when field running stops. If the Auto sequence is currently asserting start, the motor will immediately restart. If this is undesirable, use LocalSetting = 4, which forces the block into Manual mode upon exiting Local.

Engineer FAQ

What is the primary difference between LocalSetting = 2 and LocalSetting = 4?

Both settings decouple the Start output and track feedback. However, LocalSetting = 2 returns the block to its previous mode (Auto or Manual), whereas LocalSetting = 4 forces the block into safe Manual mode when local running ceases, requiring operator intervention before automated restarts.

Can an operator start the motor from the WinCC faceplate while in Local mode?

No. While in Local mode (LocalAct = 1), operator Start/Stop commands on the WinCC faceplate are locked out to protect field personnel working near the physical equipment.

Why does MotL generate a feedback error if LocalSetting = 1 is used?

LocalSetting = 1 configures Local mode for control via WinCC faceplate buttons. Because the block expects the command to originate from the faceplate, external contactor feedback without an operator command triggers a discrepancy fault.

Does this tracking logic work with bidirectional motors (MotRevL)?

Yes. The identical logic structure applies to MotRevL (FB1851), wiring separate detection branches for forward feedback (FbkFwd) and reverse feedback (FbkRev).

Technical References

  • Siemens Industry Online Support: SIMATIC PCS 7 Advanced Process Library (APL) Manual.
  • SIOS Entry 109742323: PCS 7 Compendium Part A - Configuration Guidelines (Section: Motor and Drive Engineering).
  • SIOS Entry 109809264: SIMATIC Process Control System PCS 7 Function Manual.
  • Siemens Support FAQ: Operating Modes and Local Setting Parameters in SIMATIC APL.

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Related Tags

  • MotL
  • Local Mode
  • APL
  • LocalSetting
  • Feedback Error
  • Motor Control
  • Industrial Automation
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