Variable Frequency Drives (VFDs), also known as inverters, AC drives, or motor controllers, are widely used in industrial, lift, compressor, HVAC, and manufacturing applications across the UK. When a VFD fails, it can cause costly downtime and operational disruptions.
This guide provides a structured troubleshooting process to help maintenance teams quickly identify and resolve common VFD issues while ensuring compliance with UK safety standards.
Safety First – Essential Precautions
Before working on a VFD, always follow these key safety protocols:
- Isolate the power supply and follow Lockout/Tagout (LOTO) procedures to prevent accidental energisation.
- Wait at least five minutes after disconnecting power to allow capacitors to discharge.
- Confirm zero voltage with a multimeter before touching any connections.
- Only trained professionals should work on live electrical equipment.
No Display or Power Issues
Some VFD failures do not generate error codes but still prevent the system from operating correctly. These are often power supply, control, or connection issues.
If the VFD does not power on:
- Check the incoming power supply using a multimeter to ensure it matches the drive’s rated input.
- For three-phase systems, verify that all three phases are present and balanced.
- Disconnect all non-essential connections (motor, control signals, I/O) and try powering up the drive again.
- If the issue persists, the VFD may have an internal failure.
Possible cause: Blown fuses, power supply failure, or damaged internal components.
Warning: Do not replace blown fuses without first checking for short circuits, as this could cause catastrophic failure.
Common VFD Error Codes and Their Fixes
Modern VFDs are designed with built-in diagnostics that display error codes when issues arise. While this guide covers the most frequent faults, it is crucial to refer to the manufacturer’s manual for specific details on your model. Keeping your manual to check these codes is essential:
- Different VFD brands and models use unique fault codes and troubleshooting steps.
- Parameter settings and wiring configurations vary across manufacturers.
- Many manuals include flowcharts for diagnosing complex faults.
- If your manual is missing, it can often be found on the manufacturer’s website or through an online search.
Phase Loss (Input or Output)
Cause: The VFD detects a missing phase on either the input or output side.
Fix:
- Check for blown fuses or tripped breakers in the supply lines.
- Inspect wiring connections for loose or disconnected wires.
- Ensure the motor is correctly wired and receiving all three phases.
Overcurrent Fault
Cause: The VFD detects excessive current in the output.
Fix:
- Reduce mechanical load (e.g., check for jammed conveyors or seized motors).
- Inspect motor windings and wiring for shorts or damage.
- If a new installation, increase the acceleration time to prevent sudden current surges.
DC Bus Overvoltage
Cause: Excessive voltage buildup in the DC bus.
Fix:
- Inspect the braking resistor and braking chopper circuit for proper operation.
- If tripping occurs during deceleration, increase the deceleration time.
- Check for excessive regenerative energy from the motor.
DC Bus Undervoltage
Cause: The DC bus voltage drops below operational thresholds.
Fix:
- Verify the power supply is stable and within the required range.
- Inspect input wiring and transformer taps.
- Check for loose terminals or faulty contactors.
Drive Overtemperature Fault
Cause: The VFD heatsink is overheating.
Fix:
- Clean heatsinks and cooling fans to remove dust and debris.
- Ensure the VFD’s enclosure has adequate ventilation.
- Verify that fans are operational (some only run under specific conditions).
Heatsink Thermistor Fault
Cause: The VFD’s temperature sensor is faulty or not reading correctly.
Fix:
- Inspect the thermistor connection and wiring.
- If faulty, replace the thermistor.
- Check if the heatsink is overheating due to ventilation issues.
Motor Overtemperature Fault
Cause: The VFD detects excessive heat in the connected motor.
Fix:
- Check the motor’s actual temperature.
- Ensure the motor’s thermal sensor is correctly wired to the VFD.
- Verify that cooling systems (fans, ventilation) are operational.
Communication Fault
Cause: Loss of communication between the VFD and an external control system (e.g., PLC, HMI).
Fix:
- Inspect communication cables and connections.
- Check baud rate and protocol settings.
- Ensure grounding issues are not causing noise interference.
Internal Memory Error
Cause: Corrupt firmware or incorrect parameter settings.
Fix:
- Perform a factory reset and reprogram the VFD.
- If the issue persists, update the firmware or replace the control board.
Ground Fault
Cause: The VFD detects a short circuit to earth.
Fix:
- Inspect motor windings for insulation damage by measuring resistance between motor phases.
- Use a Megger tester to check for ground leakage, Megger test between motor phases and Earth .
- Ensure the VFD is correctly earthed.
Undertorque Fault
Cause: The motor is producing insufficient torque.
Fix:
- Check for a broken belt, damaged coupling, or gearbox failure.
- Verify motor settings and torque limits.
- Inspect load conditions and mechanical resistance.
Short Circuit Fault
Cause: The VFD detects a short circuit in the motor or output wiring.
Fix:
- Disconnect the motor and check resistance for shorts.
- Inspect motor cables and insulation.
- Test with a different motor if available.
Precharge Failure
Cause: The VFD fails to charge the DC bus capacitors during startup.
Fix:
- Inspect the precharge circuit, including contactors and resistors.
- Verify power supply consistency.
- If unresolved, replace faulty capacitors or power components.
Final Tips for Effective VFD Troubleshooting
- Keep records of past faults and resolutions to speed up future diagnostics.
- Always refer to the manufacturer’s manual for error code explanations.
- Perform regular maintenance to prevent failures before they happen.
- Ensure staff are properly trained in handling VFDs and electrical safety.
Need Expert Help with a Faulty VFD?
At Inverter Drive Repair Ltd, we specialise in VFD repairs, refurbishments, and replacements across the UK. Our experienced engineers provide a fast and reliable no-fix-no-fee service. Give us a call today for a free inspection and quote.
📞 020 8150 1060 | 📧 info@inverterdriverepair.com
