C135 PCS 7 (PCS7)
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Configuring Unidirectional Speed Control and Readback Bypass on MotSpdCL

Configuring Unidirectional Speed Control and Readback Bypass on MotSpdCL

To configure the bidirectional variable-speed motor block MotSpdCL from the PCS 7 Advanced Process Library (APL) for single-direction operation, disable operator reverse commands by setting OS_Perm.Bit6 = 0 ('Operator can start the motor in reverse = False') and leave the reverse start output Rev unconnected in the CFC chart. If the physical drive lacks actual speed feedback, bypass readback monitoring errors (MonDynErr) by connecting the setpoint output directly back to the readback input (MotSpdCL.SP_Out -> MotSpdCL.Rbk) or by disabling speed monitoring via the block parameter SpeedMonEn = 0.

Table of Contents

Executive Summary

When automating variable-frequency drives (VFDs) controlling unidirectional machinery—such as centrifugal slurry pumps, exhaust fans, or single-direction conveyors—process engineers frequently select the APL block MotSpdCL (FB1856, Motor with Controllable Speed). However, MotSpdCL is architected natively for bidirectional rotation (Forward and Reverse). If instantiated without customization, the WinCC faceplate displays reverse control buttons, creating operational confusion and risks of commanding reverse rotation on machinery that could suffer mechanical damage. Furthermore, if the drive installation lacks an encoder or analog speed readback signal, MotSpdCL trips readback monitoring alarms. Configuring operator permission masks (OS_Perm), loopback setpoint routing, and faceplate adjustments cleanly converts MotSpdCL into a robust single-direction drive controller.

Architecture and Fundamental Mechanics

Adapting MotSpdCL for unidirectional, open-loop speed control involves configuring three internal functional layers:

1. Directional Restriction via Operator Permissions (OS_Perm)

MotSpdCL manages operator control permissions dynamically via the 32-bit DWORD parameter OS_Perm:
- OS_Perm.Bit6: 'Operator can start the motor in reverse'. Setting OS_Perm.Bit6 = 0 (False) disables and permanently greys out the 'Reverse' button on the WinCC Standard faceplate view.
- In the CFC chart, the physical digital output driver (Pcs7DiOu) is connected exclusively to Fwd (Forward command). The Rev (Reverse command) output is left unconnected or tied to an alarm latch.
- In automatic mode, ensure the interlocking CFC logic cannot assert RevAut = 1.

2. Readback Monitoring Mechanics (Rbk vs SP_Out)

MotSpdCL features comprehensive speed monitoring:
- When commanding a speed setpoint (SP_Out), the block expects actual rotational feedback at the input Rbk (Readback value, structured REAL).
- If the deviation $|SP_Out - Rbk|$ exceeds the tolerance threshold MonTiDynamic, the block asserts MonDynErr = 1, generating an alarm and tripping the motor.
- In installations without physical tachometers or VFD analog speed feedback, two engineering solutions exist:
1. Setpoint Loopback: Connect MotSpdCL.SP_Out directly to MotSpdCL.Rbk on the CFC chart. The block reads its own commanded setpoint as instantaneous actual feedback, maintaining tracking with zero error.
2. Monitoring Disable: Set parameter SpeedMonEn = 0 (or MonEn = 0), which deactivates static and dynamic speed discrepancy supervision entirely.

Step-by-Step Implementation Procedure

To implement a single-direction variable speed motor with bypassed readback in CFC:

Step 1: Instantiate MotSpdCL in CFC

  1. In SIMATIC Manager, open the target CFC chart.
  2. Drag MotSpdCL from the PCS 7 APL library into the chart.
  3. Connect the forward run command Fwd to the digital output channel driver (Pcs7DiOu.PV_In).
  4. Leave the reverse run command Rev completely unconnected.

Step 2: Configure Unidirectional Permissions

  1. Open the Object Properties of MotSpdCL -> Parameters tab.
  2. Locate the parameter OS_Perm.
  3. Clear Bit 6 ('Operator can start the motor in reverse') by setting the bit value to 0.
  4. Verify that Feature.Bit0 (Startup behavior) and Feature.Bit7 (Enabling local operating mode) are set appropriately for the drive.

Step 3: Configure Speed Readback Bypass

  1. Option A (Setpoint Loopback - Recommended):
    - Connect the block's own setpoint output SP_Out $\rightarrow$ Rbk.
    - This satisfies internal tracking routines and displays the active setpoint as actual speed on the faceplate.
  2. Option B (Disable Speed Monitoring):
    - Set the parameter SpeedMonEn = 0 (or MonEn = 0).
    - Set dynamic monitoring time MonTiDynamic = 0.0.

Step 4: Compile and Verify Faceplate Display

  1. Compile and download CFC changes to the S7-400 / CPU 410 Automation Station.
  2. Execute OS compilation if faceplate text properties were customized.
  3. Open the MotSpdCL faceplate in WinCC Runtime.

Verification

To confirm unidirectional behavior and alarm-free operation:

  1. Faceplate Command Test:
    - Open the MotSpdCL faceplate in WinCC Runtime with operator credentials.
    - Confirm that the 'Forward' button is active and green/selectable.
    - Confirm that the 'Reverse' button is visually greyed out and unclickable.

  2. Speed Setpoint and Readback Test:
    - Enter a speed setpoint of $1200\text{ RPM}$ ($75\%$).
    - Start the motor. Confirm that SP_Out transitions to $1200\text{ RPM}$.
    - Confirm that the actual speed field displays $1200\text{ RPM}$ without generating a MonDynErr monitoring fault.

  3. Alarm Log Inspection:
    - Verify that WinCC Alarm Logging remains free of speed discrepancy warnings.

Limitations and Operational Precautions

Operational limitations and precautions:

  1. Absence of Real Stall Detection: Bypassing readback by looping SP_Out to Rbk blinds the controller to physical motor stalls or broken drive couplings. Electrical overload relays or VFD fault contacts (Trip) must be connected to provide equipment protection.
  2. Custom Faceplate Modifications: While setting OS_Perm.Bit6 = 0 greys out the Reverse button, removing the button entirely requires creating a project-specific faceplate (@PG_MotSpdCL_SingleDir.pdl). Modifying system PDLs directly is not supported.
  3. Ramping Times: If the physical VFD has a slow internal acceleration ramp (e.g., 30 seconds), connecting real feedback without increasing MonTiDynamic will cause nuisance dynamic monitoring faults.
  4. Commercial Disclaimer: Verify all software libraries, engineering licenses, and system packages with an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

How do I grey out the Reverse button on MotSpdCL?

Set OS_Perm.Bit6 = 0 on the MotSpdCL block instance. This disables operator authorization to start the motor in reverse.

What happens if there is no speed encoder connected to Rbk?

If Rbk is left open, MotSpdCL detects a discrepancy between commanded speed and actual speed, raising a dynamic monitoring error (MonDynErr) and tripping the motor.

Why connect SP_Out back to Rbk?

Connecting SP_Out to Rbk makes the block treat its own setpoint output as the actual speed feedback, preventing monitoring errors when no physical speed transmitter exists.

Can I disable speed monitoring entirely instead of looping SP_Out?

Yes. Setting parameter SpeedMonEn = 0 (or MonEn = 0) turns off speed deviation supervision.

Is there a dedicated single-direction variable speed motor block in APL?

No. MotSpdCL is the standard APL block for all variable speed motors; unidirectional control is achieved by configuring OS_Perm and output connections.

Technical References

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Related Tags

  • MotSpdCL
  • Variable Speed Drive
  • OS_Perm
  • Speed Readback
  • APL Motors
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