Operating Principle of WarnAct and Pre-Warning Times in APL MotorL - Manual/Auto vs Local Mode

In SIMATIC PCS 7 Advanced Process Library (APL), the pre-warning output WarnAct on motor control block MotL (FB1850) operates exclusively in Manual mode (governed by parameter WarnTiMa) and Automatic mode (governed by WarnTiAu). In Local mode (LocalAct = 1), WarnAct intentionally remains 0 and software pre-warning is disabled by design. In industrial control architecture, Local mode represents direct field operation where an operator is physically present at the motor control station (LCS); software start delays are omitted to prevent hazardous discrepancy during local testing. If a warning horn is required for local starting, engineers must implement external CFC timer logic (e.g. TimerP) feeding ExtMsg1 and physical sirens.

Architecture of Pre-Warning Signaling (WarnAct) in APL MotL

In industrial process facilities, large rotating machinery (conveyors, crushers, agitators, heavy slurry pumps) presents severe hazards if started without prior acoustic or optical warning. The SIMATIC PCS 7 Advanced Process Library (APL) provides built-in start pre-warning functionality on 'Large' drive blocks, including MotL (FB1850), MotRevL (FB1851), and MotSpdCl (FB1856).

When a start command is issued:

The controller asserts boolean output WarnAct = 1.
WarnAct energizes an acoustic siren or flashing beacon in the plant.
The controller delays energizing the motor contactor output (Fwd or Rev) for the duration of the configured pre-warning timer:
- WarnTiMa: Pre-warning time in seconds when starting in Manual mode.
- WarnTiAu: Pre-warning time in seconds when starting in Automatic mode.
Once the timer elapses, WarnAct drops to 0 and Fwd energizes, running the motor.

Why Local Mode Bypasses Internal Software Pre-Warning

Engineers frequently notice that despite parameterizing WarnTiMa and WarnTiAu, issuing a start command while the motor is switched to Local mode (LocalAct = 1) results in WarnAct remaining stubbornly at 0, with the motor either starting immediately or failing to sound the siren.

This behavior is governed by intentional APL safety design principles:

Operating ModeStart Command SourceOperator Physical LocationPre-Warning Behavior (WarnAct)Automatic ModePLC / CFC Sequencing logicRemote / AutonomousActive (Delays start by WarnTiAu seconds)Manual ModeWinCC OS FaceplateRemote Control RoomActive (Delays start by WarnTiMa seconds)Local ModeField Local Control Station (LCS)Physically at the MachineInactive (WarnAct = 0; no software delay)

In Local mode, the maintenance technician or operator is physically standing directly beside the machinery, observing the drive and holding a field control switch. Introducing an unexpected DCS software timer delay between pressing the physical pushbutton and contactor pull-in causes operator confusion, often leading to repeated button cycling or mistaken troubleshooting.

Engineering Workaround for Field Pre-Warning via External Timers

If plant operating regulations or safety risk assessments mandate an acoustic horn before local starting, engineers can implement external CFC logic using standard library blocks:

Capture Local Start Request: Connect the field local start pushbutton signal to an external input, or monitor MotL.LocalSetting.
Insert Timer Block (TimerP / TON): Place a TimerP (pulse/delay) block in the CFC chart. Parameterize the delay time (e.g., 5.0 seconds).
Drive Field Siren and ExtMsg1: Connect the timer's active output to the digital output driving the physical warning horn. Simultaneously, connect this signal to MotL.ExtMsg1 to log a WinCC notification: 'Local Start Pre-Warning Active'.
Pass Delayed Signal to Contactor: Connect the timer's elapsed output (Q) to the local start input (LocalSetting or hardware contactor permissive), ensuring the motor only starts after the acoustic warning completes.
Compile and Download: Execute Chart -> Compile -> Chart as Program -> Changes Only and download to the AS controller.

Verification

Verify pre-warning execution across all operating modes through these operational tests:

Manual Mode Verification: Switch MotL to Manual mode on the WinCC faceplate. Click Start. Confirm WarnAct = 1 for exactly WarnTiMa seconds while Fwd remains 0. Once the timer expires, confirm WarnAct clears and Fwd turns 1.
Automatic Mode Verification: Trigger an automatic start command. Confirm that WarnAct activates for WarnTiAu seconds prior to motor start.
Local Mode Custom Logic Check: Switch to Local mode. Actuate the local start pushbutton. Verify that your external TimerP energizes the siren for the specified duration before the motor contactor engages.

Limitations and Operational Precautions

When engineering pre-warning circuits, adhere to these industrial safety standards:

Distinguish Pre-Warning from Emergency Stops: Pre-warning sirens do not replace hardwired Emergency Stop pushbuttons or pull-cord switches. Safety circuits must trip drives independently of CFC software blocks.
Audible vs Visual Warnings: In high-noise industrial areas (e.g., crushing mills, turbine halls), combine acoustic horns with synchronized rotating strobe lights connected to the warning output.
APL Block Variants: Note that small block variants (such as MotS) do not contain pre-warning logic; only standard 'Large' blocks (MotL, MotRevL, MotSpdCl) feature WarnAct.

Engineer FAQ

Why does MotS not have WarnAct?

MotS is the 'Small' motor block optimized for minimal memory and CPU execution overhead. Advanced features like pre-warning, rapid stop, and multiple interlocks are exclusive to MotL.

Can I abort a motor start during the pre-warning phase?

Yes. In Manual mode, clicking Stop on the faceplate while WarnAct is active immediately aborts the timer and cancels the start sequence.

Does WarnTiMa accept fractional seconds?

Yes. WarnTiMa and WarnTiAu are REAL data types accepting fractional seconds (e.g. 3.5 s), though whole integer seconds (5.0 s, 10.0 s) are typical industry practice.