Compressor Sequencing: Priority Logic, Not Fixed Outputs

Production stays online after the fix because the PLC selects compressors by assigned rank, proves that the selected machine started, and adds capacity only when pressure continues to fall. Keep the HMI assignment, demand calculation, physical outputs, and run proof as separate functions.

Reject the quick fixes that hide the fault

Separate demand, selection, proof, and capacity

Build four distinct layers. The demand layer compares measured pressure with 95 psi. The selection layer converts priority ranks into physical compressor numbers. The command layer operates the correct outputs. The proof layer decides whether the commanded machine actually joined the air system.

Do not let an HMI number energize an output by itself. In this arrangement, DS1, DS2, and DS3 hold the assigned ranks for compressors 1, 2, and 3. DS4 can hold the currently requested rank. For example, when DS4 = 2, the logic must find which of DS1 through DS3 equals 2 and call that compressor.

For requested rank 1, 2, then 3:
    Find the compressor whose DS value equals requested rank
    If that compressor is available, command it
    Wait for run proof
    If proof fails, mark it unavailable and try the next rank
    If proof succeeds, evaluate the pressure response
    Add the next rank only if pressure continues to fall

This separation also handles the stated temperature-shutdown case. Loss of run proof makes the selected unit unavailable to the sequence, regardless of which physical compressor currently owns priority 1.

Scale and validate the pressure signal

The pressure transmitter is a ProSense with a 0-150 psi, 4-20 mA signal connected to an analog input card. After converting the card's raw counts to milliamps, use linear scaling:

Pressure psi = (Measured mA - 4 mA) × 150 psi / 16 mA

The 95 psi staging threshold corresponds to approximately 14.13 mA, derived as 4 mA + (95/150 × 16 mA). Use the analog card manual to enter its actual raw-count endpoints; those values are card-specific and are not supplied here.

Qualify the signal before using it for automatic starts. Treat a diagnosed broken loop, overrange, underrange, or analog-module fault as a bad process value. A frozen but electrically valid signal requires a plausibility check: compare pressure movement with compressor commands and plant air consumption. Put the sequence in a defined fallback state rather than repeatedly starting machines from an invalid reading.

A single threshold is not a complete stop strategy. Configure a separate higher pressure at which staged demand clears, plus the permitted minimum run, unload, and restart restrictions from each compressor manual. Without separation between start and release pressures, normal transmitter noise can chatter the demand state.

Validate every HMI priority assignment

Accept automatic operation only when DS1, DS2, and DS3 form a valid permutation of 1, 2, and 3. Reject zero, values outside that range, and duplicate ranks. A duplicate leaves one rank unassigned and makes failover behavior ambiguous.

1. Range-check each HMI value before copying it into the active sequence.
2. Compare all three pairs: DS1 versus DS2, DS1 versus DS3, and DS2 versus DS3.
3. If any pair matches, inhibit automatic sequencing and display a priority-configuration fault.
4. Latch a validated copy for the active run sequence. Apply later HMI changes when the sequence is idle, or deliberately restart selection under a documented transition rule.

Do not rewrite priorities automatically merely to remove a duplicate. That hides an operator-entry error and may start a machine the operator intended to leave at another rank.

Run the sequence as explicit states

State logic prevents timers, commands, and failures from fighting across separate rungs. Use the following operating progression:

1. Idle: Monitor the valid pressure and priority configuration. Clear transient sequence timers.
2. Call rank 1: When pressure falls below 95 psi, set DS4 to 1, resolve the physical compressor, and issue its start command.
3. Wait for proof: Start a feedback timer with the command. A suggested implementation considered 1000 ms or 5000 ms, but the commissioned setting must exceed the machine's normal contactor pickup and controller start sequence while still detecting a failed start promptly.
4. Evaluate capacity: After run proof, capture pressure and monitor its direction. The proposed operating check uses about one minute. If pressure remains steady or rises, hold the current stage; if it continues falling, request the next rank.
5. Call rank 2 or 3: Repeat selection, start, proof, and pressure-response checks. Skip any unavailable rank without changing the operator's stored priorities.
6. Release demand: When pressure reaches the configured upper limit, unload or stop machines according to their interface and manufacturer restrictions. Remove capacity in a defined order rather than dropping every command simultaneously.

Use separate timers for start proof and pressure response. Start proof answers whether the commanded unit changed to its running state. Pressure response answers whether confirmed equipment supplies enough air to arrest the loss.

Distinguish contactor proof from useful production

A current sensor keeps motor-current feedback isolated from contactor line-voltage wiring and can distinguish contactor closure from actual load. Establish its running-load band from measured healthy operation; no universal current threshold is supplied. Pressure response remains the final system-level proof because a turning motor does not necessarily produce air.

Match commands to each compressor interface

Keep the supervisory request identical, but translate it differently for each machine. Compressor 1 uses y1 and y2 in its start/stop circuit. Compressor 2 requires one dry contact for start and stop, with feedback taken from its fan contactor. Compressor 3 uses y4 and y5 in the same general manner as compressor 1.

Confirm whether the two-output interfaces require momentary start/stop pulses or maintained signals from the actual compressor schematics. Confirm the normal state and required behavior of compressor 2's dry contact. Keep the compressor's native temperature, overload, pressure, and safety controls in authority; the PLC supplies supervisory requests and does not bypass local protection.

Compressor type changes the acceptable stopping strategy. Reciprocating machines can generally tolerate more start-stop operation, although contactor wear and motor heating still limit cycling. The operating notes identify a possible limit of only 3 or 4 starts per hour for screw compressors; read the actual machine manual and program its stated start limit, unload behavior, and minimum run time rather than applying that figure universally.

Prove the failover before releasing automatic mode

1. Force or simulate pressure just below 95 psi and confirm that only the machine assigned priority 1 receives a request.
2. Withhold its auxiliary feedback. Confirm that the start-proof timer expires, the unit is marked unavailable, and the machine assigned priority 2 starts.
3. Return valid feedback but simulate pressure continuing to fall. Confirm that the pressure-response interval calls the next rank without falsely declaring the first unit failed to start.
4. Simulate pressure steady and then rising. Confirm that no additional compressor starts.
5. Test every permutation of DS1, DS2, and DS3. Confirm that physical outputs follow the assigned rank rather than a fixed compressor number.
6. Enter duplicate and out-of-range priorities. Confirm that automatic mode is inhibited and no ambiguous output is energized.
7. Disconnect or fault the analog input through the approved test method. Confirm that invalid pressure cannot trigger uncontrolled staging.
8. Test each interface separately: y1/y2, the compressor 2 dry contact, and y4/y5. Confirm local stop, overload, and temperature shutdowns remain effective.
9. Restore a failed unit and verify that reset behavior is deliberate. Do not let intermittent feedback repeatedly reinsert and restart it during the same demand event.

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