Variable frequency drives, or VFDs, are the MVPs of many operations because they deliver the ability to control motor speed and torque with flexibility and precision. VFDs are used in everything from appliances and HVAC systems to massive pumping systems for water and wastewater operations. Their benefits include energy savings, the ability to optimize processes, and data collection.
Although VFDs do sometimes fail, that’s rarer than you might think. Knowing how to properly diagnose what’s actually happening can help you extend the life of your system and keep your drives running smoothly for the long term. The reality is that most VFD faults are symptoms of upstream or downstream problems — not failed drives. VFDs protect themselves when something goes wrong.
Let’s examine what typically causes VFD faults, why diagnosis can be trickier than expected, and how to correctly identify the source of the trouble.
A VFD fault is not the drive failing. A fault or alarm simply means that an abnormal condition has been detected, whereas a failure is when there has been an internal hardware breakdown, much less common.
To ensure performance, VFDs constantly monitor current and voltage output, which provides insight into many system conditions. If something isn’t right, the VFD will stop or display a fault to prevent damage.
Typical causes of VFD faults may include:
Understanding what the drive is telling you is key to solving the problem. The underlying issue is often external to the VFD and possibly not an electrical issue at all.
For example, as torque requirements rise, current output must increase to compensate for the additional load. Because the drive is monitoring the current, the rise in current can cause the drive to generate an overcurrent fault and shut down.
This could be caused by several issues, including bearing failure over time, closed valves increasing back pressure, or material caught in the impeller blades. None of these issues are internal to the VFD or even the electrical system; however, the VFD is protecting itself (and the system) by telling you, “Hey, it’s taking too much work to run this motor.”
Because VFDs are always part of a larger system, you need to think systemically when analyzing the problem. Every VFD is connected to other components and equipment that include:
Most often, faults are a sign that the VFD is reacting to the system. The VFD itself is rarely the cause. Understanding your unique system is key to the diagnostics process.
The causes of faults tend to fall into a few common categories.
All too often, when there is a VFD fault, the VFD is blamed. Instead, focus on finding the root cause, which is likely to be an upstream or downstream issue.
The following scenarios are examples of common VFD troubleshooting issues and are not intended to replace application-specific diagnostics or appropriate electrical safety procedures. Troubleshooting steps will vary based on the equipment, application, and fault conditions. Consult your MCE rep for help identifying the root cause and determining the appropriate corrective action.
Because most VFD faults are symptoms of larger system issues, effective troubleshooting starts by stepping back and looking at the full picture. A systematic approach can help you identify the true root cause faster, avoid unnecessary replacements, and reduce downtime.
Start by asking a few basic questions about the system:
Seemingly small changes in the system can trigger VFD faults if the repercussions are not thought through.
Some of the most useful diagnostic clues come from simple observations. Use your senses to identify potential problems:
These observations can narrow down the cause of the issue whether it is electrical, mechanical, or environmental.
The VFD itself can be one of your best diagnostic tools. Review the drive’s fault codes and history log to identify trends and recurring conditions. Pay close attention to:
Fault timestamps can help connect drive alarms to specific operating conditions or process events happening elsewhere in the system. Is it occurring every time another system in the plant starts up? Maybe it occurs when the well reaches a certain level? Or maybe it is tied to weather conditions or certain pump speeds. These observations can indicate deeper system-wide issues.
Once you’ve gathered initial clues, start working outward from the drive and inspect the larger system. Check:
Remember that overcurrent, overheating, and communication faults are often caused by issues like clogged pumps, damaged bearings, poor grounding, or electrical noise — not the drive itself.
Although true VFD failures are less common, they do happen. After ruling out system-related causes, look for signs of an actual internal drive failure, such as:
Replacement is the last step, not the first. In many cases, the VFD is doing exactly what it was designed to do: protecting the system and alerting you to a larger problem.
Shock-Proof Your Investigation
Before we go any further, it’s important to stress the importance of safety procedures. Always wear PPE (personal protective equipment) when working on VFDs. Make sure that you follow lockout/tagout procedures to ensure power is fully disconnected.
Even after incoming power is removed, capacitors inside a VFD can retain a dangerous electrical charge. Follow the drive manufacturer’s specified discharge time before beginning work. You might want to consider this a good opportunity for a coffee break.
Do not rely solely on an indicator light turning off. After the required waiting period, verify that hazardous voltage is no longer present using an appropriate voltage tester before touching internal components.
Many VFD faults can be prevented long before they lead to downtime. Because drives operate as part of a larger system, reliability depends not only on the drive itself, but also on the surrounding environment, wiring, programming, and maintenance practices.
Taking a proactive approach can extend equipment life, improve system stability, and reduce unnecessary troubleshooting and replacements.
Heat is the biggest threat to VFDs. Excessive temperatures, poor airflow, and contamination can shorten the lifespan of internal components and increase the likelihood of faults. To protect your drives:
Improper grounding and poor wiring practices can create false faults, communication problems, and unstable operation. Because VFDs use high-speed switching components, they can generate electrical noise that interferes with nearby equipment if systems are not installed correctly.
Best practices include:
Proper grounding and cable management can prevent many intermittent or difficult-to-diagnose issues.
Incorrect drive settings can create performance problems that appear to be equipment failures. Reviewing programming during startup and troubleshooting can help eliminate unnecessary complications.
Pay close attention to:
Most quality VFDs now have automatic motor tuning. Make sure to run this with any motor change as this tunes the VFD performance to the individual characteristics of the motor and pump.
Routine preventive maintenance can help VFDs operate reliably for 20 years or more. Regular inspections also make it easier to identify developing problems before they lead to shutdowns. Recommended maintenance practices include:
Consistent maintenance improves uptime while reducing the likelihood of recurring faults.
Monitoring normal operating conditions can identify changes before they become failures. VFDs provide valuable system data that can be used to spot developing trends and diagnose issues early.
Helpful monitoring practices include:
Almost any parameter or datapoint in the drive can be sent over communications. Document key performance indicators of the VFD in the SCADA system such as current, frequency, input/output status, and phase conditions.
The more baseline data you have available, the easier it becomes to recognize abnormal behavior and pinpoint root causes quickly.
VFDs are designed to protect both themselves and the systems they control. When a fault occurs, it’s often a sign that something else needs attention whether that is a mechanical issue, wiring problem, environmental condition, or control-system error.
Approaching troubleshooting with a system-wide mindset can help teams diagnose problems more efficiently, avoid unnecessary drive replacements, and reduce costly downtime. By combining careful observation, preventative maintenance, and a structured diagnostic process, operations can improve reliability while getting the full long-term value from their VFDs.
Need help troubleshooting persistent VFD faults? Whether you’re dealing with recurring faults, power quality concerns, motor issues, or preventative maintenance planning, our team is ready to help. Contact MCE to discuss your system and find the right solution for your operation.