Automating Start Command Reset on Motor Blocks After Power Loss and Trip Faults

In SIMATIC PCS 7 Advanced Process Library (APL), preventing motors from unexpectedly restarting upon voltage restoration or interlock clearing requires parameterizing block startup and trip-reset feature bits on MotL (FB 1850) and MotRevL (FB 1851). By setting Feature.Bit0 = 0 (Set startup characteristics to safe state), Feature.Bit1 = 0 (Reset commands on trip), and Feature.Bit30 = 1 (Resetting independent of operating mode), the motor block drops its start demand immediately upon any power interruption or trip, latching in safe de-energized state until an operator or sequence issues a validated reset.

Process Safety Requirements for Automated Motor Restarts

In industrial plants, an unexpected automatic restart of rotating machinery (pumps, conveyor belts, crushers, agitators) following a power dip, auxiliary voltage loss, or safety trip represents an extreme physical hazard to personnel and machinery:
* International safety standards (IEC 60204-1 'Safety of machinery - Electrical equipment of machines') strictly dictate:
- 'An automatic restart of a machine following restoration of power or clearing of an interlock shall be prevented if such restart can cause a hazardous condition.'
* In SIMATIC PCS 7, when an Automation Station reboots or when an auxiliary 24 V DC power bus experiences a transient brownout, motor blocks in the Advanced Process Library (APL) must deterministically discard prior start commands and enforce a deliberate, controlled restart sequence.

APL Motor Block Behavior on Power Failure and Interlock Trips

The APL motor family (MotL, MotRevL, MotSpdCL) evaluates multiple conditions that terminate active operation:
1. CPU Restart / Power Restoration: Handled in initialization Organization Blocks (OB100).
2. Interlock Trips (Intlock, Protect): Immediate de-energization of output Start (0).
3. Trip Faults (Trip = 0): Thermal overload relay or emergency trip switch opens.
4. Monitoring Errors (MonDynErr, MonStaErr): Contactor feedback timeout.

By default, without explicit feature bit parameterization, a motor operating in Automatic mode commanded by continuous step logic (StartAut = 1) could theoretically resume operation the instant the interlock condition heals, unless the engineering logic incorporates command latching and reset protection.

Configuring Startup Feature Bits (Feature.Bit0 and Feature.Bit1)

To enforce rigorous safety behavior, APL provides specialized feature bits:
* Feature.Bit0 (Set startup characteristics):
- 0 (Default / Safe): Upon CPU startup or restart, the block initializes in the safe de-energized state (Start = 0). All automatic commands are reset.
- 1: The block attempts to restore its pre-shutdown operating state (discouraged for rotating machinery).
* Feature.Bit1 (Reaction to power failure / auxiliary voltage trip):
- 0: All active start commands (StartAut, StartMan) are discarded upon a trip or power loss.
* Feature.Bit9 (Resetting via input signals):
- When enabled, allows resetting trip states via an external input signal (RstLi).
* Feature.Bit30 (Resetting depending on operating mode):
- Controls whether faults can be reset only in Manual mode (0) or in any operating mode (1).

Motor Block Safety and Command Reset Feature Matrix

The following matrix outlines safety configuration settings on APL motor blocks:

Parameter / Feature BitRecommended SettingOperational Function / Safety BenefitFeature.Bit00 (Safe State)Enforces zero output on CPU restart / power recoveryFeature.Bit10 (Discard Commands)Drops start demands immediately upon interlock tripFeature.Bit301 (Reset in Any Mode)Allows automated reset via SFC sequence or HMITripConnected to Pcs7DiIn.OutThermal overload / breaker auxiliary contact (0 = Trip)RstLiInterconnected to Reset LogicReceives positive edge pulse (0 -> 1 -> 0) to acknowledge faultRstOpConnected to FaceplateManual operator reset pushbutton from WinCC OSAutModLi / ManModLiInterconnected to Step LogicCoordinates operating mode transitions safely

Step-by-Step Procedure to Implement Safe Command Resetting

Follow this procedure to configure safe command resetting on MotL:
1. Configure Feature Bits in CFC:
- In the CFC chart, open the properties of MotL.
- Set Feature.Bit0 = 0 (Startup in safe state).
- Set Feature.Bit1 = 0 (Discard start commands on trip).
- Set Feature.Bit30 = 1 (Allow reset in Automatic mode via RstLi).
2. Wire the Trip and Interlock Inputs:
- Connect the motor breaker auxiliary contact (thermal overload) to input Trip.
- Connect protective safety interlocks to input Protect.
3. Implement Pulse-Edge Reset Logic:
- In Automatic mode, ensure step sequences (SFC) do NOT supply a continuous static 1 to StartAut.
- Use an edge-triggered start command, or ensure the sequence checks that MotL.Out = 1 before advancing.
- When a trip clears, require an explicit operator reset pulse routed to RstLi or RstOp.
4. Compile and Download:
- Compile CFC charts with Scope: Changes only.
- Download changes to the AS controller in RUN mode.
5. Verify Runtime Safety:
- Start the motor in runtime.
- Simulate a power dip or toggle the Trip input from 1 to 0.
- Confirm the motor stops instantly and does not restart when Trip restores to 1 until the reset pulse is executed.

Verification

Verify safe shutdown and unlatched restart prevention:
1. Start the motor in Automatic or Manual mode: Confirm output Start = 1 and FbkRun = 1.
2. Simulate a Voltage Dip / Trip Fault: Toggle input Trip from 1 to 0:
- Confirm output Start immediately de-energizes to 0.
- Confirm motor status transitions to Tripped / Faulted.
3. Restore Healthy Voltage: Toggle Trip back from 0 to 1:
- Crucial Safety Test: Confirm output Start REMAINS AT 0! The motor must NOT restart automatically.
4. Execute Reset Command: Pulse input RstOp or RstLi (0 -> 1 -> 0):
- Confirm the trip fault clears to healthy standby.
5. Issue a new Start command: Confirm the motor starts cleanly under validated operator control.

Limitations and Operational Precautions

Never wire a static constant 1 to RstLi or RstOp; permanently holding the reset input high completely defeats unlatched fault protection and violates IEC 60204-1 safety standards.
In safety-instrumented functions (SIF / SIL applications), motor emergency trips must be executed through certified fail-safe hardware (S7 F Systems / Safety Matrix) disconnecting main contactor power, not through standard APL software blocks alone.
Parameterize motor acceleration ramp times appropriately to prevent mechanical shaft shear during rapid restart sequences.

Engineer FAQ

Why did my motor restart automatically after an interlock cleared?

If the sequence logic maintains a continuous static 1 on StartAut without monitoring interlock state, and Feature.Bit1 is set to 1, the motor block will re-energize as soon as the interlock input restores to healthy. Set Feature.Bit1 = 0 and enforce pulse-based starting in SFC.

What does the Trip input do compared to Intlock?

Trip specifically monitors motor electrical protection devices (bimetallic thermal relays, circuit breaker fault contacts). When Trip = 0, the motor shuts down immediately and logs an electrical trip alarm.

Can I reset all tripped motors in a plant section simultaneously?

Yes. By interconnecting a central 'Plant Reset' operator tag to input RstLi across all motor blocks in that area, an operator can clear healthy cleared trips with a single authorized command.