How to Prevent Feedback Run Faults on APL Motors with Local Manual Switches in SIMATIC PCS 7

In SIMATIC PCS 7 APL motor blocks (such as MotL), an uncommanded feedback fault (MonStaErr) occurs when a local physical switch starts a motor independently while the DCS has not issued a start command (Start = 0). Under standard operating philosophy, this is handled using APL Local Mode (LocalSetting = 2 or 4) triggered by a local selector switch signal. If no electrical local mode feedback signal exists to inform the DCS that the motor was started by hand, engineers must either place the motor into 'Out of Service' (OosOp) prior to manual operation or utilize APL Internal Simulation (SimOnLi) to suppress monitoring alarms without breaking channel driver signal quality.

APL Motor Runtime Monitoring and Uncommanded Feedback Faults

In continuous process automation, motor drive blocks (MotL, MotRevL, MotSpdCL) enforce strict correlation between supervisory control commands and physical feedback telemetry:
* When the block commands a start (Start = 1), it expects the running feedback contact (FbkRun = 1) to close within the dynamic monitoring time (MonTiDynamic).
* When the block commands a stop (Start = 0), it expects FbkRun = 0.

The Uncommanded Run Anomaly

In many industrial plants, mechanical equipment features a local manual hand switch or emergency bypass push button located directly adjacent to the physical motor for maintenance or local unjamming.

If an operator turns the manual switch locally:
1. The contactor energizes and the motor starts running.
2. The auxiliary running contact closes, sending FbkRun = 1 into the PCS 7 controller.
3. The DCS block detects that Start = 0 while FbkRun = 1.
4. The block's static monitoring algorithm evaluates this as a dangerous, uncommanded run condition, asserting MonStaErr = 1 and dispatching high-priority process alarms: 'Motor running without command'.

Engineers frequently attempt to place logic gates (e.g., AND gates combining Start and FbkRun) in CFC, but this corrupts APL quality codes (ST), producing bad signal warnings.

Standard Architecture: APL Local Mode Operation

The officially certified Siemens methodology for managing field hand switches in PCS 7 APL is Local Mode:

The Local Mode Mechanism

Local Mode allows an operator or field technician to control the motor locally while the DCS monitors status without asserting feedback alarms:
* LocalSetting Parameter:
* LocalSetting = 1 or 3: Control settings are made by CFC logic; runtime monitoring remains active.
* LocalSetting = 2 or 4: Control settings are derived from internal tracking of the feedback signal (FbkRun). In this mode, runtime monitoring is automatically deactivated. When the motor starts locally, the block sets its internal state to 'Running' without generating a monitoring fault!
* Feature.Bit10: Governs mode transition upon exiting Local Mode:
* 0 = Exits from Local to Manual mode.
* 1 = Returns to the operating mode that was active prior to entering Local Mode (e.g., Automatic).

Engineering Solutions When Local Switch Signals Are Missing

A frequent field complication arises when the physical installation lacks an auxiliary contact on the local switch to inform the DCS that local mode has been engaged:

Workaround Method 1: Operating State 'Out of Service' (`OosOp`)

Before starting the motor by hand:
1. The control room operator opens the MotL faceplate and selects 'Out of service'.
2. In 'Out of service', the block algorithms, interlocks, and feedback monitoring timers are deactivated.
3. The technician operates the motor locally without generating alarms.
4. Once local operation finishes, the operator switches the faceplate back to Manual or Automatic.

Workaround Method 2: Internal Simulation (`SimOnLi` / Faceplate Simulation)

If the motor must remain part of sequential logic without tripping on feedback mismatch:
1. Enable APL Internal Simulation on the motor block.
2. The block internally loops back the control output to feedback signals, ignoring the physical DI state and eliminating monitoring errors.

Workaround Method 3: Upgrading the Electrical Circuit (Recommended)

The robust, code-compliant solution is wiring an auxiliary contact from the local switch (or motor disconnect isolator) to an available digital input module, interconnecting it directly to LocalLi on the MotL block.

APL Motor Operating Modes and LocalSetting Reference Matrix

The following table outlines the operational behavior of APL motor blocks across different operating modes and local configurations:

Operating ModeLocalSettingCommand SourceRuntime Monitoring Active?Behavior on Uncommanded FbkRun = 1Automatic (AutoAct = 1)Not applicableInterconnected CFC / SFC logicYes (MonTiDynamic/Static)Trips with MonStaErr AlarmManual (ManAct = 1)Not applicableOperator Faceplate ButtonsYes (MonTiDynamic/Static)Trips with MonStaErr AlarmLocal Mode (LocalAct = 1)2 or 4Local Field Pushbutton / SwitchDeactivatedClean tracking; Zero AlarmsLocal Mode (LocalAct = 1)1 or 3Local Signals via CFCActiveGenerates fault if logic disagreesOut of Service (OosAct = 1)Not applicableNone (Block bypassed)DeactivatedDisables all monitoring

Configuring LocalSetting = 2 provides the cleanest operational tracking when field switches are integrated.

Verification

To verify proper motor operation and alarm suppression:
1. In CFC Editor, verify that LocalSetting = 2 and Feature.Bit10 = 1.
2. Connect the local switch feedback to LocalLi.
3. In the field, switch the motor to Local mode and turn on the hand switch.
4. Verify on the WinCC faceplate:
* The operating mode displays 'Local'.
* The motor icon indicates 'Running' (green).
* Confirm that zero MonStaErr alarms appear in the WinCC AlarmList.
5. Turn off the hand switch and return to Remote/Auto; confirm that the block returns smoothly to Automatic mode.

Limitations and Operational Precautions

Never place raw boolean AND gates between channel drivers and technological feedback inputs (FbkRun); doing so discards the APL signal status byte (ST), triggering bad quality errors.
Running a motor locally while the block is in Automatic mode without a local mode interlock can cause process safety hazards if sequential software assumes the motor is stopped.
Operating in 'Out of service' mode disables emergency interlocks; ensure local physical emergency stop pushbuttons are hardwired directly into the motor contactor coil circuit.
If using internal simulation, ensure clear administrative procedures are established so simulation is removed prior to normal production.

Engineer FAQ

Why did placing an `AND` block in CFC cause the motor block to report a bad signal?

APL blocks use structured signals containing both value (BOOL) and signal status (BYTE). A generic Step 7 AND block strips the signal status or passes undefined quality, causing the motor block to flag Bad Quality.

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

In LocalSetting = 2, switching to Local mode is only possible from Manual mode. In LocalSetting = 4, switching to Local mode is permitted from both Manual and Automatic modes.

Can an operator switch to Local mode directly from the WinCC faceplate?

By default, Local mode is selected via field signals (LocalLi). However, if LocalSetting = 1 or 3 and Feature.Bit0 = 1 is configured, button selection on the faceplate can be enabled.

Technical References

Related Tags

MotL
Local_Mode
LocalSetting
FbkRun
MonStaErr

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How to Prevent Feedback Run Faults on APL Motors with Local Manual Switches in SIMATIC PCS 7