Modernize the existing relay-based panel by selecting an expandable PLC around the complete signal list, not the 19 motor outputs alone. The PLC must sequence the contactor commands, confirm each preceding motor has started, and reduce the filling conveyor speed when the hopper overflow signal becomes active.
Define the Control Requirement
One start command initiates the sequence. The PLC commands the first motor, waits for confirmed operation, applies the configured delay, and then commands the next motor. Repeat this logic through all 19 motors. Do not advance merely because a timer expires: the stated requirement also calls for verification that the preceding motor has started.
The evidence does not define the required behavior after a failed start. Specify whether the sequence must pause, stop all motors, generate an alarm, or permit an operator retry before writing the program.
Build the I/O Count Before Selecting Hardware
| Signal group | Supported requirement | Sizing implication |
|---|---|---|
| Motor commands | 19 motors require individual start commands. | Provide at least 19 suitably assigned digital outputs if each contactor has a separate PLC command. |
| Motor confirmation | Each next start depends on confirmation of the preceding motor. | Provide one input for every independently monitored motor. If all 19 motors require confirmation, that implies 19 feedback inputs. |
| Sequence start | One pushbutton initiates the sequence. | Provide a digital input for the start command. |
| Hopper overflow | A probe requests reduced conveyor speed. | Provide a compatible digital input when the probe or its controller supplies a discrete signal. |
| Inverter control | The conveyor must change speed. | The output requirement depends on whether speed selection is discrete, analog, or communicated over a bus. |
Under the explicit assumption of 19 individual motor confirmations, the described functions account for 21 digital inputs: 19 feedback signals, one sequence-start signal, and one hopper signal. This is not a final panel count because stop circuits, resets, fault feedback, alarms, and spare capacity are unspecified. The filling conveyor may also be one of the 19 motors, so do not add another motor output until the machine layout confirms it.
Select the Inverter Control Method
| Method | Capability | Decision |
|---|---|---|
| Discrete preset speeds | PLC outputs select inverter-programmed fixed frequencies. | Matches a simple normal-speed/reduced-speed command from an overflow switch, subject to the inverter supporting preset selection. |
| Manual potentiometer | An operator sets the speed directly at the inverter. | Does not provide automatic PLC speed regulation; the PLC can remain limited to start and stop commands. |
| PLC analog reference | An analog PLC output supplies a continuously variable frequency reference. | Requires analog-output capability or an expansion module plus program logic that determines the requested reference. |
| Communication bus | The PLC transmits commands and data digitally. | Requires compatible PLC and inverter interfaces plus protocol configuration knowledge. |
An overflow switch supports threshold-based speed reduction, not true constant-level control. Maintaining a constant hopper level requires a level measurement that represents changing level and control logic that continuously adjusts the inverter reference. If the real requirement is only to slow the conveyor at the overflow threshold, discrete preset-speed selection is the most direct supported architecture.
Implement the Sequential Logic
- Create one sequence state for each motor command. Start with all commands in a defined off state.
- When the start pushbutton is accepted, energize the first motor command.
- Require valid running confirmation from that motor before enabling the delay for the next state.
- After the configured delay expires, command the next motor and repeat the feedback-and-delay condition through the sequence.
- Read the hopper overflow signal independently of the startup sequence. When active, request the inverter's reduced-speed state; when inactive, request its normal operating state, provided that this behavior matches the process risk assessment.
- Define explicit handling for missing motor feedback so a failed start cannot silently release the next motor.
The existing relay-consent arrangement may provide a reusable feedback concept, but confirm what each signal proves. A contactor or relay state does not necessarily prove shaft rotation. The evidence identifies an encoder or an inductive sensor detecting a metal target on the rotating component as possible methods when physical rotation must be verified.
Commission and Verify the Retrofit
Test the logic without permitting unintended machine motion, then commission each command and feedback pair individually. Confirm that every motor maps to the intended output, that its feedback reaches the correct input, and that the next motor remains inhibited until both feedback and delay conditions are satisfied.
Interrupt one feedback signal during testing and verify the defined failure response. Activate the hopper probe and confirm that the inverter changes to the reduced-speed command, then clears or returns to normal speed according to the approved operating sequence. Finally, retain the PLC project, hardware configuration, I/O list, inverter settings, and an electrical drawing so on-site maintenance does not depend on undocumented programming knowledge.
FAQ
How many PLC outputs are needed for 19 motors?
Individual control requires at least 19 digital outputs for the 19 motor commands. Add inverter speed-selection, alarm, or other outputs only after confirming whether the conveyor is already included among those motors.
Can an overflow switch maintain a constant hopper level?
No. A discrete overflow signal can select normal or reduced conveyor speed at one threshold, but constant-level control requires a changing level measurement and continuously adjustable control logic.
How should a PLC verify that the previous motor started?
Use a feedback signal that proves the condition required by the process. If actual rotation must be verified, the evidence supports an encoder or an inductive sensor detecting a metal target; do not treat elapsed time alone as proof of operation.