Fault Diagnostics Lab

Learn to identify, diagnose, and resolve common VFD and PLC faults in the field.

Training reference only. Always follow OEM manual, lockout/tagout procedures, and site safety rules before troubleshooting live equipment.
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Preview: 3 of 10 faults
VFD OC High Severity

Over Current

VFD ka output current limit se zyada ho gaya — motor ya wiring mein problem.

Output current exceeded the VFD's rated capacity. Usually a motor or wiring issue.

Common Causes
  • Short circuit in motor cable
  • Motor winding insulation failure
  • Acceleration time too short for load
  • Undersized VFD for the motor
  • Mechanical jam/seizure in driven equipment
Diagnostic Steps
  1. Check motor cable insulation with megger (disconnect VFD first)
  2. Check motor winding resistance (phase-to-phase balance)
  3. Increase acceleration time
  4. Check if load is mechanically stuck
  5. Verify VFD current rating vs motor FLC
Quick Fix: Increase acceleration time. Check for mechanical jam. Megger test motor and cable.
Common Mistake: Megger-testing through a connected VFD — destroys the VFD.
⚠ Disconnect VFD output before megger testing. Never megger through a VFD — it will be destroyed.
Beginner Tip: If it trips during start only: acceleration time too short. If trips randomly: check motor and cable insulation.
VFD OV High Severity

Over Voltage

VFD ka DC bus voltage bahut zyada ho gaya — motor energy wapas bhej raha hai.

DC bus voltage exceeded safe limit. Motor is regenerating energy back into the VFD faster than it can absorb.

Common Causes
  • Deceleration time too short for high-inertia load
  • Supply voltage too high
  • No braking resistor on regenerative load
  • Sudden load drop (overhauling load)
Diagnostic Steps
  1. Increase deceleration time
  2. Check supply voltage (should be within VFD input range)
  3. Add braking resistor for high-inertia loads
  4. Enable stall prevention / overvoltage suppression feature
Quick Fix: Increase deceleration time. Enable OV suppression in VFD parameters.
Common Mistake: Making deceleration shorter instead of longer for high-inertia loads.
⚠ Do not ignore repeated OV trips. If supply voltage is genuinely high, it can damage VFD capacitors over time.
Beginner Tip: Most common cause: deceleration too fast on a heavy fan or flywheel. Double the deceleration time first.
VFD UV Medium Severity

Under Voltage

Input power supply voltage bahut kam ho gaya — VFD safe operate nahi kar sakta.

Input power supply voltage dropped below VFD's minimum operating threshold.

Common Causes
  • Power supply sag/dip (grid issue)
  • Loose input power cable connection
  • Undersized supply cable (voltage drop)
  • Multiple large motors starting simultaneously
  • Input fuse blown (one phase lost)
Diagnostic Steps
  1. Check input voltage at VFD terminals (all 3 phases)
  2. Check for loose connections at input terminals
  3. Check input fuse/MCB status
  4. Monitor supply quality with power analyzer
  5. Check cable sizing for voltage drop
Quick Fix: Check input power connections. Verify all 3 phases are present and within range.
Common Mistake: Ignoring single-phase loss — stresses DC bus capacitors.
⚠ Single-phase loss on a 3-phase VFD causes UV trip and can stress DC bus capacitors. Fix promptly.
Beginner Tip: First check: are all 3 input fuses/MCBs OK? Then check voltage at VFD input terminals with multimeter.

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