Conveyor belt jamming? Lift out of control? An article to eat through the motor forward and reverse PLC control (with ladder diagram + avoid the pit guide)

I. Three fatal scenarios in the industrial field: why forward and reverse control is just needed?

  1. ​Conveyor belt reverse blockage: A food factory lost ￥20,000 per hour due to the delayed reversal of the signal, which led to the accumulation of materials.
  2. ​Lift emergency stop failure: Hardware interlocks were not installed during maintenance, and workers mistakenly touched the buttons to cause fall accidents
  3. ​Machine tool spindle boring: Forward and reverse switching without buffer, gearbox impact scrapped (maintenance cost ¥ 80,000 +)

​central contradiction: Traditional relay control is slow to respond and has a high failure rate, while PLC retrofits can reduce the 90% incident rate

Second, the underlying principle illustration: three-phase motor how to achieve positive and negative rotation?

​Phase sequence switching is central: If any two phases of the three-phase power supply are switched (e.g., phase A/C is exchanged), the direction of the rotating magnetic field is reversed. Three conditions need to be met:

  • ​contactor set: KM1 (forward), KM2 (reverse) main contact switching phase sequence
  • ​electrical isolation: Strong and weak power wiring in separate slots (PLC 24V control vs. 380V power for motors)
  • ​commutation deadband: Forward and reverse switching requires ≥ 0.5 seconds delay (anti-arc short circuit)
Image Code
graph LR
A[三相电源] --> B{KM1闭合}
B -->|A-B-C相序| C[电机正转]
A --> D{KM2闭合}
D -->|C-B-A相序| E[电机反转]


#svgGraph88626311115323{font-family: "trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge- animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#svgGraph88626311115323 .edge-animation-slow {stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;} #svgGraph88626311115323 .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#svgGraph88626311115323 .error-icon{fill:#552222;}#svgGraph88626311115323 .error-text{ fill:#552222;stroke:#552222;}#svgGraph88626311115323 .edge-thickness-normal{stroke-width:1px;} #svgGraph88626311115323 .edge-thickness-thick{stroke-width:3.5px;}#svgGraph88626311115323 .edge-pattern-solid{stroke- dasharray:0;}#svgGraph88626311115323 .edge-thickness-invisible{stroke-width:0;fill:none;}#svgGraph88626311115323 .edge- pattern-dashed{stroke-dasharray:3;}#svgGraph88626311115323 .edge-pattern-dotted{stroke-dasharray:2;}#svgGraph88626311115323 .marker{fill:#333333;stroke:#333333;}#svgGraph88626311115323 .marker.cross{stroke:#333333;} #svgGraph88626311115323 svg{font-family: "trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#svgGraph88626311115323 p{ margin:0;}#svgGraph88626311115323 .label{font-family: "trebuchet ms",verdana,arial,sans-serif;color:#333;} #svgGraph88626311115323 .cluster-label text{fill:#333;}#svgGraph88626311115323 .cluster-label span{color:#333;} #svgGraph88626311115323 .cluster-label span p{background-colour:transparent;}#svgGraph88626311115323 .label text, #svgGraph88626311115323 span{fill:#333;color:#333;}#svgGraph88626311115323 .node rect,#svgGraph88626311115323 . node circle,#svgGraph88626311115323 .node ellipse,#svgGraph88626311115323 .node polygon,#svgGraph88626311115323 .node path{ fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#svgGraph88626311115323 .rough-node .label text,#svgGraph88626311115323 .node .label text,#svgGraph88626311115323 .image-shape .label,#svgGraph88626311115323 .icon-shape .label{text-anchor:middle;} #svgGraph88626311115323 .node .katex path{fill:#000;stroke:#000;stroke-width:1px;}#svgGraph88626311115323 .rough-node . label,#svgGraph88626311115323 .node .label,#svgGraph88626311115323 .image-shape .label,#svgGraph88626311115323 .icon-shape . label{text-align:centre;}#svgGraph88626311115323 .node.clickable{cursor:pointer;}#svgGraph88626311115323 .root .anchor path{ fill:#333333!important;stroke-width:0;stroke:#333333;}#svgGraph88626311115323 .arrowheadPath{fill:#333333;} #svgGraph88626311115323 .edgePath .path{stroke:#333333;stroke-width:2.0px;}#svgGraph88626311115323 .flowchart-link{stroke :#333333;fill:none;}#svgGraph88626311115323 .edgeLabel{background-color:rgba(232,232,232, 0.8);text-align:centre;} #svgGraph88626311115323 .edgeLabel p{background-color:rgba(232,232,232, 0.8);}#svgGraph88626311115323 .edgeLabel rect{opacity: 0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#svgGraph88626311115323 .labelBkg{background-color:rgba( 232, 232, 232, 0.5);}#svgGraph88626311115323 .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;} #svgGraph88626311115323 .cluster text{fill:#333;}#svgGraph88626311115323 .cluster span{color:#333;} #svgGraph88626311115323 div.mermaidTooltip{position:absolute;text-align:centre;max-width:200px;padding:2px;font-family:" trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33; border-radius:2px;pointer-events:none;z-index:100;}#svgGraph88626311115323 .flowchartTitleText{text-anchor:middle;font-size. 18px;fill:#333;}#svgGraph88626311115323 rect.text{fill:none;stroke-width:0;}#svgGraph88626311115323 .icon-shape, #svgGraph88626311115323 .image-shape{background-color:rgba(232,232,232, 0.8);text-align:centre;}#svgGraph88626311115323 .icon -shape p,#svgGraph88626311115323 .image-shape p{background-color:rgba(232,232,232, 0.8);padding:2px;}#svgGraph88626311115323 . icon-shape rect,#svgGraph88626311115323 .image-shape rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232, 232, 0.8);}#svgGraph88626311115323 :root{-mermaid-font-family: "trebuchet ms",verdana,arial,sans-serif;}


A-B-C phase sequence




C-B-A phase sequence




three-phase power supply




KM1 closed




motor positive rotation




KM2 closed




motor reversal






Three, seven-step practical tutorial: from hardware configuration to stress testing


Step 1: Hardware Selection Pit Avoidance Checklist






appliances
Recommended Specifications
pitfall case






PLC Output Modules
Relay type (non-transistorised)
Transistor type burnt contactor coil




contactors
Category AC-3 (e.g., LC1D09)
Phase-to-phase short circuit due to sticking of common contactors




thermal relay
Rectified current = motor rated current x 1.1
Overload inactivity burns the winding






Step 2: The Golden Rule of I/O Allocation


pythonmake a copy of
# Mitsubishi FX Series Configuration (Conveyor Case)
inputs = {'X0': 'Positive start', 'X1': 'Reverse start', 'X2': 'Stop (normally closed)', 'X3': 'Thermal protection'}
outputs = {'Y0': 'KM1 positive rotation', 'Y1': 'KM2 Inversion', 'Y2': 'Fault indicator'}




​Key Tips: The emergency stop button (X2) must be used with theNormally Closed ContactsPower off and shut down




Step 3: Dual brand ladder diagram analysis (with interlock + delay)


​Siemens S7-200 example (Protherm V18)::


laddermake a copy of
// Network 1: Positive Rotation Control
|--[I0.0]--[I0.2]--[Q0.1NC]--[T37]--(Q0.0)--|  // I0.0 = start, I0.2 = stop
|--[Q0.0]-----------------------(self-locking)-----|
|--[Q0.0]--[ton t37, pt=500ms]-----------|  // Switching delay

// Network 2: Inversion of Control
|--[I0.1]--[I0.2]--[Q0.0NC]--[T38]--(Q0.1)--|  
|--[Q0.1]-----------------------(self-locking)-----|


​Interlocking Triple Guarantee::


    

  1. Programme interlock: Q0.0 and Q0.1 normally closed contacts in series
    2. 

  2. Hardware interlocking: KM1/KM2 auxiliary contact mechanical interlocking
    3. 

  3. Time interlock: T37/T38 delayed switching (anti-arc)
    4. 



Step 4: Simulation and debugging in four steps


    

  1. ​Force table test: Force X0=1 in PLC software and observe whether Y0 is lit or not
    2. 

  2. ​timing verification: Simulate X0→X1 fast switching and confirm that T37 delay is effective
    3. 

  3. ​fault injection: Disconnect X3 (thermal protection) and check that all outputs are reset
    4. 

  4. ​waveform analysis: Capture the Y0/Y1 signals with an oscilloscope to ensure there are no overlapping pulses.
    5. 



Step 5: Live wiring life and death


    

  • ​power line: ≥2.5mm² single stranded copper wire (e.g. YC 3×2.5+1×1.5)
    • 

  • ​control lines: Shielded twisted pair cable (e.g. RVVP 2 x 0.75) single-ended grounded
    • 





​lesson learnt through blood and tears: A workshop's Y0 malfunction due to inverter interference without shielding and grounding






Four, five high-frequency fault quick checklist






fault phenomenon
detection point
cure






No response to forward rotation
Measurement of KM1 coil voltage
Replacing burnt relay output modules




Smoke when reversed
Check main contact phase sequence
Switching L1/L3 inlet of KM2




Tripping during switchover
Test contactor action time
Increase T37 delay to 800ms




Self-starting after shutdown
Verify the stop button type
Normally open button to normally closed wiring




Frequent overload alarm
Calibration of thermal relay setting values
Reset at motor nameplate current x 1.1










Ten years ago I omitted the hardware interlocks on a mine hoist project and the result was a sticky contactor that caused the cage to fall -Always remember: the safety circuit must exist independently of the PLC!. In every ladder diagram today, that interlocking contact between Q0.0 and Q0.1 is a life preserver drawn with lessons learned.

Tags.

Related news