VFD Motor Troubleshooting Guide : 20 Common Problems and Solutions

 

In every industrial facility, Variable Frequency Drives (VFDs) are critical for motor control — managing torque, speed, and efficiency across pumps, fans, conveyors, and compressors.
Yet when problems occur, engineers often find themselves stuck between electrical, mechanical, and control possibilities.
This comprehensive guide answers the 20 most common VFD Motor Troubleshoot questions, based on real issues faced in plants and workshops.

1. Why does the motor not start even when the VFD shows “RUN”?

This is one of the most frequent questions in VFD troubleshooting.
Possible reasons include:

• Run command mismatch: The drive is in Local mode while command is sent from Remote (PLC).

• Output disabled: The drive might be in a fault or inhibit state.

• Wiring issue: Output terminals (U, V, W) disconnected or contactor open.

• Motor parameters not set: Incorrect motor frequency or voltage limits may prevent energizing.

Solution:
Check control mode (HMI → Operation mode), inspect digital inputs, and confirm output voltage at U/V/W when running.

2. Why does the drive run, but the motor doesn’t rotate?

When output voltage exists but the motor remains still:

• Motor winding open or loose terminal.

• Mechanical jam in coupling or gearbox.

• VFD output phase missing due to a damaged IGBT.

Tip: Disconnect the motor and check VFD output using a multimeter in voltage mode. If voltage is unbalanced, the fault is internal to the drive.

3. Why does the motor start and then immediately trip?

Common causes:

• Incorrect acceleration time (too fast → overcurrent trip).

• Motor overload or stuck rotor.

• Faulty current feedback circuit.

• Drive’s current limit parameter too low.

Fix:
Increase accel time (e.g., from 1s to 5s), verify mechanical load, and inspect drive’s current limit settings.

4. How to identify if the fault comes from the VFD or the motor?

Disconnect the motor and run the drive in “no-load” mode (without connecting U/V/W).
If the fault still appears (e.g., overcurrent), it’s an internal VFD problem.
If the drive runs fine without load, connect another motor or insulation-test the original one.
A megger test below 1 MΩ indicates winding deterioration.

5. What causes frequent overcurrent trips?

Overcurrent faults indicate the drive is delivering current beyond rated limits.

Causes:

• Shorted motor windings.

• Rapid acceleration or high torque demand.

• Incorrect motor FLA set in parameters.

• DC bus voltage instability.

Solution:

1. Check motor current with clamp meter.

2. Increase accel time.

3. Verify drive’s rated current vs motor FLA.

4. Inspect motor insulation and mechanical load.

6. Why does the drive trip on “Overvoltage” during deceleration?

When a motor decelerates, it regenerates energy back to the DC bus.
If the drive cannot dissipate it, DC bus voltage rises above limit.

Causes:

• Decel time too short.

• Braking resistor not installed or failed.

• Weak DC link capacitors.

Solution:

• Increase decel time (e.g., from 2s → 8s).

• Check braking resistor circuit.

• Measure DC bus voltage; add dynamic braking if needed.

7. What causes “Undervoltage” faults even when supply seems normal?

It’s often due to:

• Loose input terminals.

• Weak power supply or long cable drop.

• Faulty input diode bridge.

• Momentary brownout during starting.

Fix:

• Tighten terminals and verify input voltage with a meter.

• Compare measured voltage with drive’s rated level (±10%).

• Add line reactor if voltage dips under load.

8. Why does the VFD trip randomly without consistent cause?

Intermittent tripping may relate to:

• Poor grounding or EMI interference.

• Vibration causing loose connectors.

• Moisture or contamination on control boards.

Action:
Inspect cable shield grounding, clean with dry air, and log fault history — it often shows repeating fault codes that reveal the pattern.

9. Why is there vibration and noise after installing a new VFD?

New VFDs generate PWM waveforms that can excite motor mechanical resonance.

Other causes:

• Imbalanced rotor or fan.

• Incorrect carrier frequency.

• Unshielded cable radiating noise to nearby systems.

Solution:

• Adjust carrier frequency parameter (e.g., from 2kHz → 8kHz).

• Check alignment of couplings.

• Always use shielded motor cables with grounded terminations.

10. Why does the motor rotate in the wrong direction?

A simple yet common issue:

• Two output phases swapped (U-V-W).

• Wrong polarity in direction input command.

Fix: Swap any two motor output leads or correct the digital input logic.

11. What causes “Phase Loss” or “Phase Imbalance” alarms?

Causes:

• Blown input fuse or loose cable.

• Asymmetrical voltage supply.

• Unbalanced load on the output side.

Check:
Measure each input phase with a meter; imbalance >3% can cause tripping.
Use phase monitoring relays for protection.

12. Why doesn’t the VFD respond to the PLC run command?

Check the control interface:

• Wrong source selected (local/remote).

• PLC output not energizing digital input.

• Control signal wiring error or 24V supply missing.

Tip:
Monitor drive digital input status via keypad to confirm PLC signals are reaching the VFD.

13. Why is there a delay between PLC command and motor start?

Possible reasons:

• Soft start delay (parameter Pxxx delay start).

• Interlock waiting for feedback signal.

• Ramp-up time intentionally programmed.

Action:
Review sequence logic; if unneeded, reduce delay time or disable external start confirmation.

14. How to fix “Fieldbus Communication Fault” or “Modbus Timeout”?

Causes:

• Loose communication cable or poor termination.

• Incorrect baud rate or parity setting.

• Network noise or grounding loop.

Fix:
Check all nodes’ settings, replace suspect cable, and ensure shield grounding at one end only.

15. Why does the motor speed fluctuate under steady load?

Causes:

• Faulty feedback (encoder slip or noise).

• Loose speed reference wiring.

• Poor PID tuning in closed-loop control.

Solution:

• Check feedback signal waveform on oscilloscope.

• Secure wiring and ensure proper shielding.

• Retune PID or switch to open-loop for testing.

16. What parameters must always match the motor nameplate?

• Rated voltage

• Rated frequency

• Rated current (FLA)

• Rated speed (RPM)

• Power (kW or HP)

• Motor type (induction, synchronous)

If these values differ, the drive’s internal model miscalculates torque → leading to trips or poor performance.

17. How can incorrect acceleration/deceleration settings cause faults?

Too fast acceleration increases current draw → Overcurrent.
Too fast deceleration increases DC bus voltage → Overvoltage.

Good practice:
Set acceleration/deceleration between 5–15 seconds, depending on load inertia.
Use dynamic braking or S-curve profiles for smooth transitions.

18. How to check if IGBTs or output section are damaged?

If drive powers ON but gives no output:

• Measure output line-to-line voltage while commanding RUN.

• If one phase shows zero consistently → suspect IGBT failure.

• Also check for “ground leakage” using insulation tester.

Warning: Only trained engineers should perform this test — VFD DC bus retains high voltage even after shutdown.

19. What’s the best way to perform preventive maintenance for VFD and motor?

Regular motor maintenance and inspection dramatically reduce downtime.

Task	Frequency	Description
Visual inspection	Monthly	Dust, moisture, and terminal tightness
Filter/fan cleaning	3–6 months	Prevent overheating
Check capacitor health	Yearly	Replace if ESR rises
Megger motor windings	Yearly	Should exceed 1 MΩ
Parameter backup	Yearly	Save settings for quick recovery

20. What are the most common hardware components that fail inside a VFD?

1. Cooling fans – wear out due to dust and heat.

2. DC link capacitors – lose capacitance after 4–5 years.

3. IGBT modules – damaged from overcurrent or heat.

4. Control boards – affected by humidity or surges.

5. Input rectifier – burns due to shorted DC bus.

Tip: Always check ventilation, maintain clean panels, and monitor drive temperature using built-in diagnostics.

Conclusion

Effective VFD Motor Troubleshoot depends on understanding both electrical and control aspects of the system.
By methodically addressing common problems — from startup failures and overcurrent trips to communication errors and parameter mismatches — engineers can cut downtime, protect assets, and extend system lifespan.

Regular inspection, parameter review, and preventive motor maintenance remain the best defense against costly failures.
In today’s industrial plants, knowing how to interpret fault codes and apply structured troubleshooting is not optional — it’s what separates downtime from continuous production.