How to Fix VFD Faults - Step-by-Step Troubleshooting Guide

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How to Fix VFD Faults: The Ultimate Troubleshooting Guide for Technicians

If you work in maintenance, you know the sound. That sudden silence when a fan, pump, or conveyor stops running, followed by the frantic call on the radio: "The drive tripped again!"

 

How to Fix VFD Faults - Step-by-Step Troubleshooting Guide

When you walk up to a Variable Frequency Drive (VFD) with a red fault light blinking, it can feel like you’re defusing a bomb. But here’s the secret: VFDs rarely die without a reason. They are excellent tattle-tales. They will tell you exactly what is wrong—if you know how to listen.

In this guide, I’m going to walk you through a universal 6-step process to fix almost any VFD fault, and then we’ll look at the "Big 3" faults that account for 80% of your downtime.

The "Golden Rule" of VFD Troubleshooting

Stop pressing the Reset button.

I know it’s tempting. You just want the machine to run. But repeatedly resetting a fault without finding the cause is the fastest way to turn a $50 problem into a $5,000 explosion. If the drive tripped, it did so to protect itself. Find out why before you ask it to try again.

The 6-Step Master Troubleshooting Workflow

Before you dive into complex parameters, follow this physical checklist.

1. Safety First (LOTO)

Never open a drive cabinet without Lockout/Tagout. Wait 5 minutes after killing power. VFDs have massive DC bus capacitors that store lethal voltage even after the screen goes dark. Always measure the DC bus terminals (PA/+ and PC/-) to ensure they are at 0V before touching anything.

2. The "Smell" Test

Open the cabinet. Does it smell like burnt popcorn or ozone?

  • Yes: You likely have a blown capacitor or fried IGBT. Stop immediately; the drive probably needs replacement.

  • No: Proceed to step 3.

3. Check the Fans

Is the cooling fan on the heat sink spinning? VFDs generate a lot of heat. If the fan is clogged with dust or dead, the drive will trip on "Overheat" (OH) constantly. Clean the vents!

4. Tighten the Screws

Vibration loosens wires over time. A loose input wire causes Undervoltage faults. A loose output wire causes Overcurrent faults. Grab a screwdriver and torque down every terminal on the power strip.

5. Separate the Motor

If the drive won't reset, disconnect the motor leads (U, V, W) from the drive.

  • Try to run the drive without the motor (in V/Hz mode).

  • If it runs: The drive is fine; your motor or cable is the problem.

  • If it faults: The drive is likely damaged internally.

6. Check the Display

Now, look at the specific code. Don't just say "it tripped." Did it say OV? UV? OC? This code is your map.

How to Fix the "Big 3" Common Faults

1. Overvoltage (OV / OU)

What it means: The DC bus voltage inside the drive got too high.
Why it happens: This usually happens when the motor is stopping. A heavy load (like a large fan) continues to spin, turning the motor into a generator. It pushes power back into the drive.
The Fix:

  • Increase Decel Time: If your Deceleration Time is 5 seconds, try 10 seconds. Give the load time to slow down naturally.

  • Check Input Power: Is a capacitor bank on the main grid switching on? Sudden spikes in plant voltage can cause this.

2. Undervoltage (UV / UU)

What it means: The drive isn't getting enough power to keep the brain alive.
Why it happens: Usually a power dip in the building, a loose fuse, or a bad contactor upstream.
The Fix:

  • Check Fuses: You might have blown just one of the three input fuses (single-phasing). The drive can sometimes sit idle on 2 phases but will trip the moment you try to run.

  • Check the Contactor: If you have a contactor feeding the drive, are the contacts pitted or worn?

3. Overheat (OH / OHT)

What it means: The drive is cooking itself.
Why it happens: It’s almost always airflow.
The Fix:

  • Clean the Heat Sink: You’d be amazed how much "fuzz" collects on the back of a drive.

  • Check Cabinet Fans: If the drive is in a box, are the box fans working? Is the filter clogged?

  • Lower the Carrier Frequency: There is a setting called "Carrier Frequency" (switching speed). Lowering this (e.g., from 8kHz to 4kHz) makes the drive run cooler, though the motor might whine a bit more.

When to Call the Pros

If you have disconnected the motor, checked the power, and the drive still shows a fault like "IGBT Short" or "Ground Fault" the moment you power it up, the internal components have failed. At this point, board-level repair or replacement is the only option.

Summary:
VFDs are logical.

  1. Overcurrent? Check for jams or short circuits.

  2. Overvoltage? Slow down the stopping time.

  3. Overheat? Clean the fans.

  4. Undervoltage? Check your incoming power.

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