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Understanding and Configuring SWT Switching Time Monitoring in SIMATIC PCS 7 SIMOCODE Pro Drivers

Understanding and Configuring SWT Switching Time Monitoring in SIMATIC PCS 7 SIMOCODE Pro Drivers

In SIMATIC PCS 7 SIMOCODE Pro driver blocks (such as SIMO_MD FB620, MM_Drv, and MM_Diag), SWT (Switching Time / Schaltzeitüberwachung) monitoring supervises the dynamic transition time between issuing a motor start, stop, or reversal command and receiving valid feedback confirmation from the SIMOCODE Pro motor management relay. When SWT monitoring is active, if the contactor auxiliary contacts or motor current threshold (I > 0) fail to confirm the commanded operational state before the configured SWT timer expires, the driver block trips the motor (MonErr = 1), asserts a switching fault alarm, and blocks further start commands until acknowledged. For reversing and multi-speed starters, SWT also enforces mandatory interlock dead-times to prevent line-to-line short circuits during contactor changeover.

Table of Contents

Functional Architecture of SWT Monitoring in SIMO_MD

The SIMATIC PCS 7 SIMOCODE Pro library integrates intelligent motor management relays (SIMOCODE Pro C, Pro S, and Pro V) into the PCS 7 process control architecture. Motor control blocks such as SIMO_MD (FB620) or Advanced Process Library (APL) style drivers (MM_Drv and MM_Diag) coordinate control commands, cyclic I/O data, acyclic diagnostics, and faceplate visualization.

Within these driver blocks, SWT Monitoring (Switching Time Monitoring, derived from the German Schaltzeitüberwachung) serves two primary mechanical and electrical protection roles:

  1. Dynamic Feedback Supervision: When the automation system issues a command (such as Fwd or Rev), mechanical contactors require a discrete interval to energize and pull in, and motor current requires time to establish. SWT monitoring tracks this transition window. If the feedback state fails to arrive within the configured SWT_Time (or MonTiDynamic), the block concludes that the contactor failed to pull in, mechanical linkage jammed, or control power was lost.
  2. Reversing Interlock Dead-Time (Umschaltpause): When reversing a motor's direction of rotation (transitioning from Forward to Reverse), energizing the reverse contactor while the forward contactor arc is still extinguishing causes a catastrophic phase-to-phase dead short. SWT monitoring enforces a programmable dead-time pause during which both contactors remain de-energized, allowing the electrical arc to quench and the motor shaft to decelerate before the opposing contactor is commanded.

Operating Sequences and Timeout Consequences

When SWT monitoring is activated on a motor feeder, the internal state machine executes strict supervisory checks across all operational states:

Start Sequence Supervision

  1. An operator or automated sequence issues a Start command (FwdAut = 1 or FwdMan = 1).
  2. The block starts the dynamic switching timer (SWT_Time).
  3. The block monitors the SIMOCODE cyclic feedback byte: auxiliary contact input (FbkFwd) and motor current threshold (I > 0).
  4. Normal Operation: If valid feedback arrives before the timer expires, the timer resets, the motor state transitions to Running, and static feedback monitoring (MonTiStatic) takes over.
  5. Timeout Fault: If the timer reaches zero without confirmed feedback:
    - The block immediately asserts MonErr = 1 (Monitoring Error).
    - An emergency stop command is sent to SIMOCODE (Stop = 1).
    - An ALARM_8P message is transmitted to WinCC: 'Motor Switching Time Error / Feedback Fault'.
    - The WinCC faceplate displays a yellow or red monitoring fault symbol, and automatic re-starting is locked out until acknowledged by an operator (AckOp = 1).

Direction Reversal Sequence

  1. While running Forward (FbkFwd = 1), a command to run Reverse (RevAut = 1) is received.
  2. The block commands Forward contactor open (Fwd = 0).
  3. The block enters the Interlock Dead-Time phase (SWT_Pause). Even though Reverse is commanded, Rev output remains 0.
  4. Once the pause timer expires and FbkFwd drops to 0, the block closes the Reverse contactor (Rev = 1) and starts the reverse switching timer.
  5. If reverse feedback is not established within SWT_Time, MonErr trips the drive.

Configuration Guidelines for SWT Parameters

To configure SWT monitoring correctly without causing nuisance trips:

Step 1: Determine Mechanical Contactor Pull-in Time

Review manufacturer datasheets for the motor contactors (e.g., SIRIUS 3RT contactors). Standard low-voltage contactors pull in within $30\text{ ms}$ to $80\text{ ms}$, while large vacuum contactors or circuit breakers may require $150\text{ ms}$ to $300\text{ ms}$.

Step 2: Factor in PROFIBUS / PROFINET Update Latency

The cyclic communication scan between the S7-400 CPU, communication processor, and SIMOCODE Pro introduces bus latency (typically $20\text{ ms}$ to $100\text{ ms}$ depending on baud rate and node count).

Step 3: Configure SWT_Time

Set SWT_Time (or MonTiDynamic) to at least twice the total expected response delay:
$$SWT_Time \ge 2 \cdot (t_{contactor} + t_{comm_cycle} + t_{current_filter})$$
For standard direct-on-line low-voltage motor starters, a value between 1.5 s and 3.0 s provides robust operation. For high-inertia fans or soft starters, values between 5.0 s and 10.0 s are appropriate.

Step 4: Configure Reversing Pause Time

For reversing starters, set the reversing dead-time pause (Rev_Pause or SWT_Pause) based on the motor frame size:
- Fractional to $15\text{ kW}$: $200\text{ ms} - 500\text{ ms}$.
- $15\text{ kW}$ to $75\text{ kW}$: $500\text{ ms} - 1.0\text{ s}$.
- Above $75\text{ kW}$: $1.0\text{ s} - 2.5\text{ s}$ to prevent counter-torque shear stress on mechanical couplings.

SIMOCODE Pro Monitoring Parameter Matrix

The following parameter matrix details the configuration parameters associated with SIMOCODE motor monitoring:

Parameter NameBlock InterfaceDefault ValueEngineering RangeFunction DescriptionMonEn (Monitor Enable)VAR_INPUT1 (TRUE)0 or 1Enables feedback monitoring (SWT dynamic and static)SWT_Time / MonTiDynamicVAR_INPUT3.0 s0.5 s - 60.0 sMaximum allowable time for contactor/current confirmationMonTiStaticVAR_INPUT2.0 s0.5 s - 30.0 sTime filter for unexpected feedback loss during steady-state runSWT_Pause / Rev_PauseVAR_INPUT1.0 s0.1 s - 10.0 sMandatory dead-time pause during direction changeoverFbkFwd / FbkRevVAR_INPUT0 (FALSE)0 or 1Contactor auxiliary contact binary feedback signalsCurrent_OnVAR_INPUT0 (FALSE)0 or 1Current threshold confirmation ($I > I_{limit}$) from current measuring moduleMonErrVAR_OUTPUT0 (FALSE)0 or 1Active monitoring fault flag tripping motor and triggering alarm

Verification

To verify that SWT monitoring functions correctly on a SIMOCODE motor feeder:
1. In CFC, open the chart containing SIMO_MD (or MM_Drv) and enter online monitoring mode.
2. Confirm that MonEn = 1 and SWT_Time is set to the desired test value (e.g., 3.0 s).
3. Issue a Start command from the WinCC faceplate while the motor disconnect switch is OPEN (or simulate contactor failure by disabling auxiliary contact feedback).
4. Observe the runtime behavior: The motor command will activate, the SWT_Time countdown will elapse, and exactly at $3.0\text{ s}$, MonErr will transition to 1.
5. Verify that the output command drops to 0 immediately.
6. Check the WinCC alarm list to confirm that the 'Switching Time Error' alarm was logged with the exact timestamp.
7. Reset the disconnect, click the Reset button on the faceplate, and verify that MonErr clears to 0.

Limitations and Operational Precautions

Observe the following operational constraints when applying SWT monitoring:
- Never Disable Monitoring in Production: While disabling MonEn may temporarily bypass nuisance trips during commissioning, running without monitoring exposes switchgear to undetected contactor failure, motor stalls, and equipment fires.
- Star-Delta Starters: On star-delta configurations, ensure SWT monitoring accounts for the star-to-delta transition timer. Setting SWT_Time shorter than the star acceleration time causes premature trips during delta changeover.
- Current Feedback vs Auxiliary Contacts: Using auxiliary contacts alone detects contactor mechanical movement but cannot detect a blown fuse or severed motor cable. Using SIMOCODE current threshold ($I > 0$) as feedback provides true process confirmation.

Engineer FAQ

What is the difference between dynamic switching time (SWT) and static monitoring time?

Dynamic switching time (SWT) monitors the transition window between commanding a change of state and receiving confirmation (e.g., 3 seconds to start). Static monitoring time checks that feedback remains stable while running; if contactor feedback drops during steady operation, static monitoring trips the motor.

Why does my motor trip on SWT timeout even though the contactor closed?

This occurs if the SIMOCODE auxiliary contact input is wired to a normally closed (NC) contact instead of normally open (NO), if the digital input assignment in the SIMOCODE parameter file is misconfigured, or if the current measuring module threshold (I > 0) is set higher than actual motor idle current.

Can I adjust SWT_Time directly from the WinCC faceplate?

Yes, if the operator account possesses the required authorization level (typically Authorization Level 6: Higher process control). In the APL/SIMOCODE faceplate Parameter view, engineers can adjust dynamic and static monitoring times.

Technical References

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Related Tags

  • SIMOCODE Pro
  • Motor Control
  • SWT
  • Switching Time
  • SIMATIC PCS 7
  • APL
  • Feedback Monitoring
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