PLC Simulator
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30 min

Motor Start/Stop & Fault Monitoring

motorsafetylatching

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Motor Start/Stop & Fault Monitoring scenario preview

Briefing

Build the command and diagnostic logic for a three-wire motor starter. A momentary START must latch the run command; STOP, E-stop status, and thermal-overload status must drop it. Fault indications stay latched until the cause is clear and the operator acknowledges with STOP. A 500 ms command/feedback mismatch check detects an auxiliary contact that remains on after the contactor command turns off. This is a training simulation of standard PLC logic, not a design for a safety-rated emergency-stop circuit.

Objectives

  • Start button pulse energises the contactor and RUN_LAMP, and the motor seals in
  • Stop button drops the contactor and RUN_LAMP
  • E-stop drops the contactor and latches a fault until reset
  • Thermal overload drops the contactor and latches a fault until reset
  • Auxiliary feedback remaining on while the contactor command is off latches a fault after 500 ms
  • A reset acknowledges a cleared fault but never restarts the motor by itself

Hints

  • Your editor opens blank. To load the lesson scaffold (the START latch and the three output rungs are already built), open Workbench options → Start lesson over → Restore lesson starter, then finish the three TODOs.
  • Build in four checkpoints: feedback timer → fault latch/reset → run reset → outputs. Check after each checkpoint so a failure points to a smaller area.
  • The first check needs both MOTOR_CONTACTOR and RUN_LAMP on after a START pulse and off after STOP. RUN_LAMP is its own output and needs its own rung; only a rung that drives MOTOR_CONTACTOR leaves the lamp dark.
  • Use a TON with IN := MOTOR_AUX AND NOT MOTOR_CONTACTOR and PT := 500 ms. The 500 ms is this exercise’s diagnostic threshold, not a universal safety value.
  • Latch FAULT_BIT on ESTOP, THERMAL_OL, or T_STUCK.Q. Clear it only when STOP_PB is pressed and every fault cause has cleared.
  • Latch RUN_BIT from START_PB while no fault is active; reset it from STOP_PB, ESTOP, or THERMAL_OL. SET/RESET or a behaviorally equivalent seal-in is accepted.
  • Drive MOTOR_CONTACTOR and RUN_LAMP from RUN_BIT AND NOT FAULT_BIT; drive FAULT_LAMP from FAULT_BIT.

I/O Table

Inputs

START_PB

Start push-button (momentary)

BOOL · %I0.0

STOP_PB

Stop push-button (momentary)

BOOL · %I0.1

ESTOP

Emergency stop (asserted when hit)

BOOL · %I0.2

THERMAL_OL

Thermal overload relay trip

BOOL · %I0.3

MOTOR_AUX

Contactor auxiliary feedback contact

BOOL · %I0.4

Outputs

MOTOR_CONTACTOR

Main motor contactor coil

BOOL · %Q0.0

RUN_LAMP

Motor running indicator lamp

BOOL · %Q0.1

FAULT_LAMP

Fault indicator lamp

BOOL · %Q0.2

Your program will be tested against:

All test cases run automatically when you submit. Assertions are hidden until you pass.

  1. #1Start energises contactor and run lamp, stop drops both

    Press start -> contactor and RUN_LAMP on; press stop -> both off

  2. #2Start pulse seals in via latch

    Momentary start-pulse — contactor stays on after button release

  3. #3E-stop latches fault, clears only via stop-pb

    E-stop drops contactor + raises fault; releasing E-stop does not clear; stop-pb clears

  4. #4Thermal overload latches fault, needs reset + stop-pb to clear

    Thermal trip drops contactor + fault; clears only after thermal resets AND stop-pb pressed

  5. #5Stuck auxiliary feedback latches fault

    MOTOR_AUX asserted while MOTOR_CONTACTOR is off for >500ms -> fault

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Runnable simulator field guide

Motor start-stop PLC scenario: test checklist

Direct answer

The learner can predict every state, run the normal cycle, detect missing proof and restore operation without bypassing the modeled fault.

Written for beginners and maintenance learners practicing three-wire behavior, PLC command ownership, feedback timing, overload response and recovery.

an operator and instructor reviewing alarm, trend, machine-state and recovery evidence in a simulation control room while studying motor start-stop, run proof and fault-monitoring scenario
System map / 02

NODE 01observable

Scenario setup

Initial state, start and stop inputs, permissive, run command, starter state, motor response, proof timer, trip, reset and restart.

NODE 02observable

I/O and state map

Operator input through PLC logic and output to modeled starter, motor motion, auxiliary or speed feedback and alarm state.

NODE 03observable

Normal run

Start, confirmed run, stop, second start and normal reset executed from clean conditions.

NODE 04observable

Boundary run

Held start, simultaneous commands, missing feedback, overload trip, output stuck, stop during start and restart after reset.

NODE 05observable

Injected fault

An input, rung-state, output, starter, motor, proof, timer, alarm or recovery mismatch.

NODE 06observable

Target check

The behavior recreated with site drawings, target instructions, actual starter or drive and supervised acceptance tests.

Answer surface / 07

What should a PLC motor start-stop exercise include?

It should include stop priority, start request, maintained state, permissives, output command, independent run proof, timeout, trip, reset and restart cases.

Why use motor feedback instead of the PLC output bit?

The output bit proves only controller intent. Auxiliary, speed or process feedback provides independent evidence that the downstream equipment responded.

Real plc motor start stop ladder logic footage

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PLC Motor Start/Stop — Build and Test the Classic Circuit