C135 PCS 7 (PCS7)
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Suppressing Low Motor Current Alarms When Stopped Using APL MotL and MonAnL in PCS 7

Suppressing Low Motor Current Alarms When Stopped Using APL MotL and MonAnL in PCS 7

In SIMATIC PCS 7 APL, monitoring motor current with an analog monitoring block (MonAnL or the auxiliary AV interface of MotL) generates nuisance low-current alarms whenever the motor is stopped. To suppress these alarms while preserving operator faceplate accessibility to view and edit alarm setpoints: 1) Interconnect the motor stopped state (NOT MotL.FbkRun.Value) to the message lock pin MonAnL.MsgLock, which suppresses alarm generation and acoustic annunciation while keeping all limit values visible and editable in the faceplate; 2) Utilize APL Alarm Hiding by connecting NOT MotL.FbkRun.Value to MonAnL.HideEn to dynamically filter the alarm from the OS alarm banner; or 3) Use Feature Bit 23 (Limits evaluation inactive if MsgLock active) to cleanly inhibit threshold checking during stopped states.

Table of Contents

Executive Summary

In process applications such as centrifugal pumps, cooling tower fans, and slurry agitators, continuous monitoring of motor operating current provides vital mechanical protection against broken drive belts, decoupled couplings, and impeller cavitation (which manifest as abnormal drops in operating current). In the PCS 7 Advanced Process Library (APL), engineers typically measure motor current using MonAnL (or the auxiliary value AV channel on MotL). However, when the motor stops, current drops to zero, triggering persistent Low-Low (AH_Act / AL_Act) alarms. Standard methods like connecting running feedback to AV_AL_En or AV_AL_MsgEn disable operator editing or remove limit rows from the faceplate. This guide details the proper APL engineering methodologies to suppress nuisance stopped alarms while maintaining full operator visibility and setpoint tunability.

Architecture and Fundamental Mechanics

Alarm Management Architecture: MotL and MonAnL Coupling

+-------------------------------------------------------------------------+
|                         APL Motor Block (MotL)                          |
|                                                                         |
|   - FbkRun.Value (1 = Motor Running, 0 = Motor Stopped)                 |
+-----------------------------------|-------------------------------------+
                                    |
                                    v Inverter Logic (NOT)
                                    |
                                    v Stop Signal (1 = Stopped)
+-----------------------------------|-------------------------------------+
|                   Analog Monitoring Block (MonAnL)                      |
|                                                                         |
|   - PV_In: Motor Current (0 - 150 A)                                    |
|   - MsgLock: Connected to Stop Signal                                   |
|   - Feature Bit 23 = True (Evaluate limits only when MsgLock = False)   |
|                                                                         |
|   +-----------------------------------------------------------------+   |
|   | OPERATING BEHAVIOR:                                             |   |
|   |                                                                 |   |
|   | Motor Running (FbkRun = 1):                                     |   |
|   | - MsgLock = 0                                                   |   |
|   | - Current < Limit -> Triggers Low Current Alarm (Belt Break)    |   |
|   |                                                                 |   |
|   | Motor Stopped (FbkRun = 0):                                     |   |
|   | - MsgLock = 1                                                   |   |
|   | - Alarm Generation SUPPRESSED                                   |   |
|   | - Faceplate Limits REMAIN FULLY VISIBLE & EDITABLE BY OPERATORS |   |
|   +-----------------------------------------------------------------+   |
+-------------------------------------------------------------------------+

Evaluation of Interconnection Pins

  1. AV_AL_En (Alarm Enable): If interconnected to MotL.FbkRun, setting it to 0 when stopped completely removes the alarm threshold configuration line from the OS faceplate. Operators cannot inspect or edit the trip setpoint while the motor is stopped.
  2. AV_AL_MsgEn (Message Enable): Interconnecting this input locks the parameter, preventing the operator from enabling or disabling messages from the faceplate.
  3. MsgLock (Message Lock - Recommended): Interconnecting NOT MotL.FbkRun to MsgLock suppresses the generation of outgoing WinCC alarm messages. The limit values remain completely visible on the OS faceplate, operators can adjust trip thresholds at any time, and the alarm list remains clean during motor stops.
  4. APL Alarm Hiding (HideEn): Dynamically hides the message from the active alarm list in WinCC while maintaining logged state.

Step-by-Step Implementation Procedure

Step 1: CFC Chart Interconnection Using MonAnL

  1. Open the CFC chart containing the motor control block MotL.
  2. Insert an analog monitoring block MonAnL (FB1845) to process the motor current signal.
  3. Connect the analog current input driver (Pcs7AnIn.Out) to MonAnL.PV_In.
  4. Insert a digital NOT logic block (Inv01 or bitwise NOT).
  5. Connect MotL.FbkRun.Value to the input of the inverter.
  6. Connect the inverted output to MonAnL.MsgLock.

Step 2: Configure Feature Bits on MonAnL

  1. In the Object Properties of MonAnL, open the Feature parameter (or configure via CFC pin Feature).
  2. Set Feature Bit 23 (Limits evaluation inactive if MsgLock is active) to 1 (TRUE).
    - This prevents internal limit comparison flags from toggling while the motor is stopped, ensuring that when the motor restarts, the block does not produce a false edge trip during motor acceleration.
  3. Configure Feature Bit 29 (Output substitute value on signal error) if desired during current sensor faults.

Step 3: Implement Current Inrush Delay

  1. When an AC motor starts across-the-line or via star-delta, current fluctuates rapidly before settling.
  2. In MonAnL, parameterize the alarm delay timer: AL_Lim_Ti (Low Alarm Delay Time) and AL_Lim_Ti to 3.0 to 5.0 seconds.
  3. This provides sufficient time for the motor to accelerate and establish steady-state running current before low-current monitoring engages.

Step 4: Associate MonAnL with MotL Faceplate (Optional)

  1. If you wish to display the current value directly within the MotL faceplate window:
  2. Connect MonAnL.PV_Out to MotL.AV (Auxiliary Value).
  3. In the CFC chart, configure the OS structure tags so the auxiliary trend displays motor current dynamically within the motor faceplate view.
  4. Compile the CFC charts via Chart > Compile > Changes Only and download changes to the AS.

Verification

  1. Stopped Motor Test: With the motor stopped (FbkRun = 0), verify that MonAnL.MsgLock = 1. Check the WinCC Alarm List and confirm that no low current alarm is active.
  2. Faceplate Accessibility Test: Open the MonAnL (or MotL) faceplate in WinCC Runtime while the motor is stopped. Navigate to the Limit view and verify that operators can view, modify, and save low current alarm limits.
  3. Running Trip Test: Start the motor in runtime (FbkRun = 1). Confirm MsgLock = 0. Manually lower the simulated current below the low limit setpoint. Confirm that after the configured delay timer expires, the 'Motor Current Low' alarm triggers properly.

Limitations and Operational Precautions

  1. VFD Speed Compensation: On variable frequency drives (VFDs), motor current varies proportionally with operating speed and load. A static low current threshold may trigger false trips at low speeds; consider utilizing an analog calculation block (Mul04 or Polygon) to scale current trip limits based on speed setpoint.
  2. Feature Bit Availability: Feature Bit 23 behavior is standard in APL V8.1 and higher; verify exact feature bit mappings if running legacy APL versions.

Engineer FAQ

Why does AV_AL_En make the faceplate line disappear?

AV_AL_En completely deactivates the alarm configuration in the faceplate, hiding the limit input row from the operator.

Why is MsgLock the best approach for low current monitoring?

MsgLock suppresses alarm generation while keeping all limit values visible and editable for operators on the HMI faceplate.

What does Feature Bit 23 do on MonAnL?

It stops evaluating limit thresholds when MsgLock is active, preventing false alarm edges during motor startup.

Can I use APL Alarm Hiding instead?

Yes, interconnecting the stopped state to HideEn hides the alarm from the active alarm banner while logging it internally.

How do I prevent alarms during motor inrush and acceleration?

Configure the alarm delay time parameters AL_Lim_Ti and AH_Lim_Ti to cover the motor acceleration duration (e.g. 3-5 seconds).

Can MonAnL current be displayed inside the MotL faceplate?

Yes, by connecting the current signal to the auxiliary input pin AV on the MotL block.

Does connecting MsgLock disable High-High overcurrent alarms?

Yes, if MsgLock is active, all alarms on that MonAnL block are locked. For high-current protection on stopped motors (e.g., short circuit), use a separate block or hardwired protection.

Can an operator manually lock alarms from the faceplate?

Yes, if the MsgLock input is not interconnected, operators with appropriate rights can toggle message lock directly on the faceplate.

Does this solution require downloading in CPU STOP?

No, interconnecting MsgLock and changing Feature bits can be loaded online via delta download in RUN.

What is the difference between MsgLock and MsgEn?

MsgEn enables or disables the message block permanently; MsgLock temporarily suppresses message dispatch during operational states.

Technical References

Related Articles

Related Tags

  • MotL
  • MonAnL
  • Motor Current
  • MsgLock
  • Alarm Suppression
  • APL
  • Feature Bit 23
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