Configuring Single-Speed Motor Block MotS (FB1910) and Resolving Motor Feedback Errors in SIMATIC PCS 7 APL

In SIMATIC PCS 7 Advanced Process Library (APL), when starting a direct-on-line (DOL) motor using the compact single-speed motor function block MotS (FB1910) in Manual mode, a 'Motor Feedback Error' occurring after 2 to 3 seconds indicates that the run feedback input (FbkRun) did not transition to TRUE before the dynamic monitoring timer (MonTiDynamic, default 3.0 s) expired. To resolve this error, connect the motor contactor's physical auxiliary contact through a Pcs7DiIn driver block to MotS.FbkRun. To control the physical contactor coil, connect MotS.Start to a Pcs7DiOu channel driver. For bench testing or offline simulation where field hardware is not present, connect MotS.Start directly back to MotS.FbkRun or activate internal software simulation by setting MotS.SimOn = 1.

Operational Architecture of the MotS (FB1910) Function Block

The Advanced Process Library (APL) provides standardized function blocks for motor and drive control. While MotL (FB1850) is the feature-rich motor block supporting multiple interlocks, local modes, and detailed monitoring, MotS (FB1910) is the lightweight, compact single-speed, single-direction motor function block designed to optimize CPU memory and execution time in standard applications.

The basic operational cycle of MotS involves three coordinated phases:
1. Command Generation: When commanded to start via operator manual action (StartMan = 1) or automated sequence logic (StartAut = 1), the block validates safety interlocks (Intlock, Permit, Protect). If all interlocks are satisfied, the block asserts the output command:
$$Start.Value = TRUE$$
2. Dynamic Feedback Supervision: Simultaneously, the block starts an internal countdown timer governed by the parameter MonTiDynamic (Dynamic Monitoring Time, default $3.0\text{ s}$).
3. Steady-State Monitoring: The block waits for the field contactor auxiliary contact to confirm that the motor is running by setting input FbkRun.Value = TRUE. If confirmed before MonTiDynamic reaches zero, the block transitions to the Running state and switches to static monitoring (MonTiStatic).

Root Cause of the Motor Feedback Error

When a beginner engineer places MotS in a CFC chart without connecting external I/O blocks, starting the motor triggers an immediate alarm:
$$\text{Alarm Banner: } \text{Motor Feedback Error } (MonErr = 1)$$

This occurs because:
1. The engineer toggled the Manual Start command on the faceplate.
2. MotS asserted Start = 1 and initiated the $3.0\text{ s}$ timer (MonTiDynamic).
3. Because the input parameter FbkRun was left unconnected (floating at its default state of FALSE), the block never received feedback confirmation.
4. When MonTiDynamic expired at $3.0\text{ s}$, the block concluded that the physical contactor failed to pull in or auxiliary contact wiring was severed.
5. MotS immediately de-energizes Start ($Start = 0$), trips the motor, asserts the monitoring error flag MonErr = 1, and raises an ALARM_8P fault in WinCC.

Complete DOL Motor Control Circuit Configuration

To implement a complete direct-on-line (DOL) motor control loop in CFC connecting physical field hardware:

flowchart LR
    A["CFC MotS Block (FB1910)"] -- "Start.Value" --> B["Pcs7DiOu Channel Driver"]
    B -- "QW Output" --> C["Physical Contactor Coil (24VDC / 120VAC)"]
    C -. "Mechanical Closure" .-> D["Contactor Auxiliary NO Contact"]
    D -- "IW Input" --> E["Pcs7DiIn Channel Driver"]
    E -- "PV_Out.Value" --> A["MotS.FbkRun"]

Step 1: Configure Digital Output Channel Driver (Pcs7DiOu)

In CFC, insert the output driver block Pcs7DiOu (FB1873).
Connect MotS.Start to Pcs7DiOu.PV_In.
In the symbol table or HW Config, assign the physical digital output address (e.g., Q 4.0) to Pcs7DiOu.PV_Out.

Step 2: Configure Digital Input Channel Driver (Pcs7DiIn)

In CFC, insert the input driver block Pcs7DiIn (FB1871).
In HW Config, map the contactor auxiliary normally open (NO) contact address (e.g., I 4.0) to Pcs7DiIn.PV_In.
Connect Pcs7DiIn.PV_Out to MotS.FbkRun.

Step 3: Configure Safety Interlocks (Optional but Recommended)

Connect a field emergency stop push-button or motor overload relay (Pcs7DiIn) to MotS.Intlock or MotS.Trip.

Simulating Motor Operation Without Physical Hardware

When commissioning control logic on an Engineering Station or training simulator without physical I/O hardware connected, use either of two simulation techniques:

Method 1: Hardware-Free Loopback Interconnection

In CFC, draw a signal connection from the output parameter MotS.Start directly back to the input parameter MotS.FbkRun.
When the block commands Start = 1, FbkRun instantly becomes 1 on the same cycle, satisfying dynamic monitoring and keeping the motor in Running state without feedback faults.

Method 2: Native APL Software Simulation Mode

In CFC, double-click MotS and locate parameter SimOn.
Set SimOn = 1 (TRUE).
The APL block enters internal simulation mode:
- It decouples internal logic from external I/O pins.
- When commanded to start, it automatically simulates internal feedback after an internal delay ($1.0\text{ s}$).
- The faceplate displays the standard simulation icon (hand symbol), alerting operators that live process equipment is not engaged.

MotS Operating Parameter and Monitoring Matrix

The following matrix details the critical parameters of MotS (FB1910):

Parameter NameInterface TypeDefault ValueOperational PurposeStartVAR_OUTPUT0 (FALSE)Digital output command driving contactor coil via Pcs7DiOuFbkRunVAR_INPUT0 (FALSE)Contactor auxiliary feedback signal from Pcs7DiInMonEnVAR_INPUT1 (TRUE)Enables/disables feedback monitoring (1 = active, 0 = bypassed)MonTiDynamicVAR_INPUT3.0 sMaximum allowable time for FbkRun to confirm start commandMonTiStaticVAR_INPUT2.0 sDelay filter before tripping if FbkRun drops during steady runMonErrVAR_OUTPUT0 (FALSE)Asserted 1 when feedback times out; trips Start outputSimOnVAR_INPUT0 (FALSE)Enables internal APL software simulation modeTripVAR_INPUT0 (FALSE)Thermal overload relay fault input (trips motor immediately)

Verification

To confirm that MotS functions correctly without feedback errors:
1. In CFC, open the chart containing MotS in online test mode.
2. Confirm that FbkRun is properly interconnected (either to Pcs7DiIn or loopback).
3. Open the WinCC faceplate for the motor in runtime.
4. Select Manual mode and click the Start button.
5. Verify that Start.Value transitions to TRUE.
6. Verify that FbkRun transitions to TRUE before MonTiDynamic ($3.0\text{ s}$) expires.
7. Confirm that the faceplate status changes to solid green (Running) and no 'Motor Feedback Error' alarm appears in the alarm list.
8. Click Stop: Verify that Start drops to 0 and FbkRun clears cleanly.

Limitations and Operational Precautions

Observe the following engineering precautions:
- Never Leave Loopbacks in Production: Software loopback connections (connecting Start directly to FbkRun) must be completely removed before plant startup. Running with loopbacks disables contactor trip detection, allowing a stalled or burned motor to appear 'Running' on the HMI.
- Overload Relay Integration: Always connect the physical thermal overload relay or circuit breaker auxiliary contact to MotS.Trip (or via interlock inputs). Disconnecting feedback monitoring does not protect against electrical overcurrent.
- Contactor Bounce: If high-power contactors experience mechanical contact bounce, increase MonTiDynamic to $4.0\text{ s}$ or configure input debounce filtering on Pcs7DiIn (FltrTi = 50 ms).

Engineer FAQ

What is the difference between MotS (FB1910) and MotL (FB1850)?

MotS is a compact, single-speed motor block with a reduced memory footprint and simplified faceplate. MotL is an advanced motor block supporting local control modes, monitoring channels for warning/alarm thresholds, extensive interlock types, and rapid stop inputs.

Can I disable feedback monitoring completely?

Yes. Setting parameter 'MonEn = 0' disables both dynamic and static feedback monitoring. The block will assert Start without ever checking FbkRun. However, this is dangerous and should only be used during temporary commissioning.

Why did the feedback error occur even though FbkRun is wired?

Common causes include: the physical contactor auxiliary contact is normally closed (NC) instead of normally open (NO); the 24VDC control voltage was lost; or the contactor pull-in time exceeded the configured MonTiDynamic value.