Selecting a PLC for a 14-Pump Control Application Guide

The 14-pump project needs a PLC with enough discrete I/O for float switches, manual controls, pump commands, and AC-pump feedback, plus an HMI arrangement that works at two locations. Select the controller only after separating command logic from actual pump-status detection.

How many signals must the controller handle?

Count signals before comparing PLC models. The five 24 VDC pumps and five 120 VAC pumps form the ten float-controlled pumps; four additional 24 VDC pumps need daily timed operation and manual control. The pump motor ratings do not determine the PLC output rating because contactors and relays switch the pump loads.

This is a planning baseline, not a final module selection. It assumes two independently wired float switches per float-controlled pump, an individual physical button for every pump, and one command output per pump. HMI commands normally arrive as controller tags over a communications link rather than as discrete inputs. Confirm whether the physical button count, switch wiring, and status feedback match the intended installation before sizing I/O.

Do the float-switch readings mean start or stop?

The requested behavior is that either of a pump’s two float switches going off triggers the pump. In Boolean terms, that is an OR condition: either input in its triggering state can request operation. The electrical state read by the PLC may be ON or OFF depending on the switch contact arrangement and wiring, so map the actual input indication to the intended process meaning rather than assuming an energized input means “float active.”

For each pump, verify the two switch states individually, then check both together. Confirm that either triggering condition asserts the pump request and that neither triggering condition removes it, unless the process requires a different stop rule. This matters because a simple OR request can keep a pump running while either switch remains in its triggering state; hysteresis, latching, or separate start/stop levels may be needed to prevent frequent cycling, but the required process behavior must determine that logic.

Does the AC-pump relay need command state or run feedback?

“If any of the five AC pumps goes off” can mean either that the PLC removes a pump command or that a pump stops running. Those are different signals. The command tag tells the PLC what it requested; it does not prove that the contactor closed or that the pump continued running.

If the aggregate relay should remain active for as long as any pump is off, use a maintained Boolean condition. If it should indicate only the moment a pump changes from running to stopped, use transition detection and specify whether the event must latch until acknowledged. The project description does not choose between those behaviors; settle that before writing the output logic.

Can the PLC outputs switch the pumps directly?

No: the stated design uses contactors and relays, so the PLC outputs only provide control signals. Select output hardware for the control-circuit voltage, current, output type, and the actual coil or interposing-relay load—not for the pump’s 15 A, 10 A, or 5 A rating. Check the output module and coil documentation together, including any required suppression for inductive coils.

Keep the PLC command state distinct from field status. A PLC output turning on confirms an electrical command at the output point; it does not confirm contactor operation or motor operation. If the operator needs proof that an AC pump is running, wire suitable feedback to an input and display that feedback separately from the command tag.

How should the two HMI locations connect?

Both operator locations need access to the same pump-command, schedule, and status data. Choose an HMI and PLC combination that supports the required connection to one controller, then verify that both HMIs can display and write the intended tags without conflicting command behavior. The source lists a compatible HMI alongside a Wago PLC and also mentions Exor screens with an integrated CODESYS PLC; if using an integrated PLC, check whether it can handle the full I/O count and both HMI locations.

Define command priority explicitly. A float request, HMI command, and physical button can all request operation; decide whether each is momentary or maintained and how an operator stops a pump when a float still requests it. Show command state and, where fitted, run feedback as separate HMI values. Confirm what each screen displays if its communication with the controller is lost.

Which controller candidates fit the project?

The suggestions in the project discussion identify candidates, not a completed fit check. Compare each current model against the calculated I/O, HMI communications, programming environment, and availability of software and licenses. The project owner has no PLC software or licenses, so include setup and programming access in the selection rather than treating hardware cost as the only constraint.

Loxone was also considered, but no detailed I/O or integration fit was provided. Evaluate it by the same checklist rather than selecting it based on the name: controller I/O, two HMI endpoints, schedule capability, output interface, programming access, and maintainability.

What is the selection and checkout sequence?

Use the following decision path to turn the requirements into a purchasing and commissioning checklist:

1. Inventory every field signal: two float inputs for each of ten pumps, each physical button, fourteen pump commands, and any AC-pump run feedback. Resolve whether each button is required and whether each pump needs feedback.
2. Choose the aggregate relay meaning: commanded off, not running, or faulted. Decide whether the output is maintained or event-based, and define its behavior for intentional stops.
3. Compare exact controller models against the resulting I/O count, expansion capacity, output electrical compatibility, timers/schedules, and communications for two HMIs. Check current hardware and software documentation before ordering.
4. Confirm programming access, required software, and license costs. Select a platform the project owner can configure and maintain.
5. Wire and test each input/output point separately. Verify both float inputs per pump, physical buttons, output-to-relay interfaces, and AC-pump feedback before enabling automatic control.
6. Test priority and abnormal conditions: manual request with float inactive, float request with manual control inactive, multiple requests, loss of a run-feedback signal, and loss of HMI communications. Confirm the aggregate relay response against the chosen definition.
7. Run the daily schedule for each of the four additional pumps and verify the time basis and schedule settings in the controller/HMI. Confirm that manual operation behaves as specified alongside the schedule.

For final verification, compare each HMI’s command and run-status indications against the controller’s live tags and field input/output states while exercising each pump’s float, manual, schedule, and feedback paths.

Frequently asked questions