Siemens LOGO! Soft Comfort V7 can combine fault changeover and runtime alternation cleanly when pump selection is stored as a duty state. A repaired pump must become available without automatically reclaiming duty.
Control approach comparison
| Approach | Behavior | Main limitation | Use |
|---|---|---|---|
| Pulse generator and counter | Counts pulses while the selected pump runs | The pulse period, counter retention, reset logic, and conversion to hours add failure points | Use only when the available blocks cannot accumulate runtime directly |
| Retentive runtime accumulator | Accumulates actual pump operation and compares it with an operator-entered limit | Requires a deliberate reset and retention policy | Preferred for runtime-based changeover |
| Total-hours comparison | Selects the pump with fewer accumulated hours | A repaired pump may immediately reclaim duty, contrary to the required sequence | Balancing lifetime hours, not enforcing a complete duty interval |
Use a retentive runtime accumulator with a latched duty selector. Do not use a direct comparison of the two total-hour values as the sole selector. That comparison can transfer duty as soon as the failed pump returns, while the required behavior keeps the replacement pump active until its interval expires or it faults.
Recommended control architecture
Divide the logic into four functions: permissives, pump availability, duty selection, and output commands. The duty selector stores either Pump 1 or Pump 2 as the active unit. Faults and completed runtime intervals request a transfer; they do not drive the opposite output directly.
Pump 1 available = NOT Pump 1 fault
Pump 2 available = NOT Pump 2 fault
Pump 1 command = Run demand AND Supply healthy
AND Pump 1 selected AND Pump 1 available
Pump 2 command = Run demand AND Supply healthy
AND Pump 2 selected AND Pump 2 availableCount runtime from confirmed operation when auxiliary run feedback is available. Counting only the command can record hours while a contactor, motor, or process interlock prevents operation. If the installation has no run feedback, document that the displayed value represents commanded hours.
Commissioning prerequisites
| Input or setting | Required function | Check before proceeding |
|---|---|---|
| Supply healthy | Blocks both start commands when control or motor power is unavailable | Both outputs remain off when the signal is false |
| Run demand | Represents the external request for pumping | No pump starts without demand |
| Pump 1 and Pump 2 faults | Remove the affected pump from service | Each fault independently blocks its matching output |
| Duty memory | Retains which pump owns the current interval | A restart produces the selected startup behavior |
Before anything else, test every permissive and fault input online. Do not move on until no combination can energize both pump commands simultaneously.
Duty-changeover procedure
- Create one selected-pump state. Use mutually exclusive stored states for Pump 1 selected and Pump 2 selected. Define a startup selection instead of allowing both states to default independently.
- Gate each output. Combine run demand, supply healthy, the matching selected state, and the matching availability signal. Confirm that forcing either fault false-to-true removes only the affected command.
- Accumulate selected-pump runtime. Enable the active accumulator only while its pump is confirmed running. Stop accumulation when demand disappears. Decide whether a normal demand stop pauses the interval or resets it; pausing preserves actual operating time.
- Prioritize fault transfer. If the selected pump faults and the standby pump is available, clear the current selection, select the standby pump, and initialize the standby duty interval according to the chosen policy. The recommended policy gives the replacement pump a new full interval.
- Block automatic return after repair. When the failed pump becomes healthy, change only its availability status. Keep the current duty latch unchanged. The repaired pump waits until the active pump reaches its runtime limit or develops a fault.
- Transfer without overlap. Remove the outgoing run command before applying the incoming command. If the process requires a stop delay or confirmation of contactor dropout, obtain that requirement from the motor-control design and configure it explicitly.
- Reset only the completed interval. After a successful runtime transfer, reset the outgoing interval accumulator. Keep permanent motor-hour totals separate if maintenance personnel need lifetime values.
Fault recovery and power restoration
Pump 2 takes duty immediately if it is available. Repairing Pump 1 must not switch the selector back. Pump 2 continues until it completes its own duty interval or faults.
If Pump 1 starts when power returns, the duty state was either not retained or was reinitialized to Pump 1 during startup. Store the selected-pump state and the active interval if operation must resume exactly where it stopped. When retention is not desired, implement an explicit restart policy such as alternating the startup choice or selecting the first available pump. Do not let block initialization accidentally define plant behavior.
A total-hours comparator answers a different question: which motor has run less over its life? It does not represent ownership of the current duty interval. Use total hours for maintenance and long-term balancing, while the latched selector controls immediate operation.
Functional verification and recurring pitfalls
- Start with both pumps healthy, apply supply healthy and run demand, and confirm that only the selected pump runs.
- Fault the active pump while the standby pump is unavailable. Confirm that both commands remain off and that the control exposes an unresolved pumping fault rather than repeatedly commanding a failed unit.
- Remove run demand and verify that runtime does not increase. Restore demand and confirm the chosen pause-or-reset policy.
- Change the runtime setting from the operator panel and test values below, equal to, and above the accumulated time. Define whether a new setting below elapsed runtime causes an immediate transfer; immediate comparison is usually the simplest predictable behavior.
- Cycle control power during an active interval. Confirm that both duty ownership and elapsed runtime match the documented retention policy.
Common faults include using a timer that resets with every demand interruption, counting clock pulses while the motor is stopped, resetting both accumulators during any transfer, allowing a repaired pump to overwrite the duty latch, and using separate output branches that can become true in the same scan.
FAQ
The controller selects the other pump when it is available. Under the recommended policy, the replacement starts a new 6-hour duty interval.
What happens if the failed pump is repaired while the standby pump runs?
The repaired pump becomes available but remains off. It waits until the active pump reaches its runtime limit or faults.
What happens if both pumps are in fault?
Both run commands stay off because neither pump is available. Expose a common pumping-unavailable alarm and require at least one cleared fault before restarting.
What happens if the operator reduces the runtime setpoint below elapsed time?
A direct runtime comparison requests an immediate transfer. Test this panel action during commissioning and restrict the permitted entry range to values accepted by the operating procedure.
What happens if Siemens LOGO! loses power during a duty interval?
The result depends on retention of the duty latch and runtime accumulator. Perform the final verification by cycling power during an active interval and confirming that the intended pump selection and elapsed runtime return.