Implementing Duty-Standby Motor Switchover in SIMATIC PCS 7 - Industry Library Aggr08 vs Custom APL Logic

In SIMATIC PCS 7 systems requiring automated 1+1 duty-standby motor changeover with fault switchover and operating hours equalization, the standardized manufacturer solution is the aggregate control block Aggr08 (or Aggr16) from the optional SIMATIC PCS 7 Industry Library (IL). If the Industry Library add-on is unavailable, engineers can implement equivalent autonomous switchover using standard Advanced Process Library (APL) MotL blocks coupled with runtime accumulation (RunTime) and interlock logic (Intlk02/Intlk04). The Industry Library requires separate engineering and runtime licensing.

Duty-Standby (1+1) Operational Requirements in Process Plants

Critical process systems—such as cooling water circulation pumps, boiler feedwater systems, and lubrication oil units—frequently operate in a 1+1 redundant configuration (one unit actively running while the second unit remains in hot standby).

An automated duty-standby control scheme must fulfill three operational requirements:
1. Bumpless Fault Switchover: If the active motor experiences an electrical trip, thermal overload, or feedback discrepancy, the standby motor must start automatically without requiring operator intervention.
2. Maintenance / Manual Changeover: Operators must be able to swap duty and standby roles seamlessly from the supervisory console.
3. Runtime Equalization: The automation system should alternate duty roles periodically based on accumulated operating hours to ensure uniform mechanical wear.

Industry Library Aggr08 Aggregate Control Architecture

The dedicated Siemens solution for multi-unit rotation is the SIMATIC PCS 7 Industry Library (IL):
- Block Aggr08 / Aggr16: Controls the coordinated sequencing of up to 8 (or 16) identical technological units.
- Operating Modes:
- Automatic rotation based on configured run time intervals.
- Automatic changeover to available standby units upon fault detection (Trip).
- Priority management (assigning specific units to base load, peak load, or standby).
- HMI Integration: Provides dedicated WinCC faceplates displaying aggregate status, unit availability, and manual switchover triggers.
- Procurement Requirement: The Industry Library is a commercial add-on product requiring an Engineering License for the ES and an AS Runtime License for each target automation system.

Engineering Autonomous 1+1 Switchover Using Standard APL Blocks

Where procuring the Industry Library is impractical, an equivalent 1+1 duty-standby scheme can be constructed using standard APL components:

Instantiate Motor Blocks: Place two standard APL motor blocks (MotL, FB 1850) for Motor A and Motor B.
Operating Hours Tracking: Connect the run feedback of each motor to an APL runtime accumulation block (RunTime).
Fault Detection Logic: Interconnect the fault output of Motor A (Fault = 1 or Trip = 1) to the automatic start input of Motor B (StartAut via an edge trigger R_TRIG), and vice versa.
Interlocking Supervision: Route mutual exclusion interlocks through Intlk02 blocks wired to the Permit or Intlock inputs of both motors to prevent both motors from running concurrently unless an overlap transition is explicitly desired.
Mode Management: Configure Feature.Bit0 = 1 on both MotL instances so that upper-level CFC logic retains mastership over operating modes.

Architectural Comparison: Industry Library vs Custom APL

The following table compares the Industry Library aggregate approach against a custom APL implementation:

Evaluation MetricIndustry Library (Aggr08)Custom APL Logic (MotL + Logic)Licensing RequirementSeparate Add-On License (ES + AS RT)Included in standard PCS 7 / APL POsScalabilityEasily scales up to 8 or 16 unitsScales well for 2 units; complex for >3 unitsEqualization AlgorithmNative operating hours balancing built-inRequires custom comparison logic in SCL/CFCWinCC FaceplateDedicated aggregate overview faceplateRequires navigating two separate motor faceplatesEngineering TimeMinimal (Pre-assembled block)Moderate (Requires custom logic and interlocks)

Procurement Disclaimer: Industry Library order numbers and licensing prerequisites vary across PCS 7 versions. Integrators must confirm software entitlements with an authorized Siemens distributor.

Verification

Verify 1+1 duty-standby switchover functionality:
1. In WinCC Runtime, place both motors in Automatic mode and start Motor A as duty.
2. Simulate a motor fault on Motor A (e.g. drop feedback signal FbkRun); verify that Motor A trips immediately.
3. Confirm that Motor B starts automatically within 1 to 2 seconds without operator action.
4. Verify that an alarm 'Duty Motor Fault - Standby Started' generates in the WinCC alarm banner.
5. Test manual changeover: Trigger a manual swap command; confirm Motor B starts before Motor A stops (or stops Motor A and starts Motor B per operational sequencing).

Limitations and Operational Precautions

Essential precautions for duty-standby automation:
- Prevent pump cavitation or pressure surges: Determine whether the process requires 'Make-Before-Break' (standby starts before duty stops) or 'Break-Before-Make' (duty stops before standby starts) transition timing.
- If both motors are in fault or in Manual mode, the switchover sequence must generate a high-priority 'Aggregate Unavailable' alarm to warn operators.
- Ensure electrical power supplies and MCC breakers are fed from diverse distribution boards so an upstream bus fault does not take down both redundant motors simultaneously.

Engineer FAQ

Is the Industry Library included on the standard PCS 7 installation media?

The software setup files may be included on optional disks or downloadable from SIOS, but executing the blocks requires purchasing a valid Industry Library license key.

Can Aggr08 balance run time across three pumps where two are running and one is standby?

Yes. Aggr08 supports configurable operational requirements, such as '2 of 3 running' with cyclic rotation of the idle unit based on accumulated hours.

Can I use standard MotL blocks with Aggr08?

Yes. Aggr08 acts as a supervisory coordinator, issuing commands to standard APL MotL blocks via their AutModLi, StartAut, and StopAut interconnection pins.