Troubleshooting a Failed VFD

Variable Frequency Drives, or VFDs, enable precise speed control of AC induction motors in a wide range of industries. They’re often found in critical processes that form the cornerstone of a company’s operations, and when they fail, it can cost a great deal in lost productivity, downtime, and parts replacement costs. It’s important to have a good understanding of the common reasons why VFDs fail so that the cause of the problem can be quickly diagnosed and corrected, and productivity restored as soon as possible.

Diagnostics

When diagnosing a VFD failure, the first step is to determine if the VFD is at fault or whether the problem originated in a different part of the system. VFDs feature a range of safety functions designed to trip when something goes wrong, to prevent damage to itself and the motor it’s driving. A good VFD will also offer extensive, easy-to-read diagnostics and error information to guide operators to the source of the problem. Analyzing this information is the first step toward determining the cause of a problem.

Over and Under Voltage

A common cause of VFD failure is over- or under-voltage, which prevents the unit from operating normally. If the voltage trip occurs during start-up, the problem is likely to be caused by poor input power. Bad connections and long cable runs throughout the power network can cause voltage drops resulting in insufficient voltage at the VFD power input. Cable lengths of 30 feet or more are likely to cause a significant voltage drop. To diagnose bad connections, look for connections that are hotter than the wires around them. They can also be found by testing the resistance across a connection when the power is offline or the voltage drop when power is applied.

Fluctuating input voltage places stress on the VFD’s DC bus capacitor and can quickly wear it out. This can be caused by poorly governed generator power or common-mode transient voltages from other devices in the network (such as power supplies and other VFDs). A line reactor connected to the VFD input can protect it from voltage surges and spikes. An isolation transformer can enable it to be fully isolated from other components in the system.

On the output side of the VFD, a common cause of an over-voltage fault is regenerative feedback created by the load spinning faster than the motor, especially during fast deceleration or the sudden release of a clutch, which rapidly increases the voltage on the VFDs DC bus and trips the safety cut-off. To solve this problem, implement a longer deceleration ramp or use dynamic braking to keep the load under control.

Over Current

Over-current faults often appear indirectly as overheating, especially when the current excess is relatively small. They’re often caused by too-fast acceleration at start-up, which demands torque and current above what the VFD is rated to provide. Over-current failure can also be caused by excessive loads on the motor and loads that fluctuate quickly, drawing surges of current that create heat that doesn’t have time to dissipate. Small amounts of excess current may not be enough to trip safety functions but sufficient to cause significant heat stress and premature deterioration of the VFDs internal components.

To solve this problem, increase the acceleration time of the motor, and ensure that the VFD is adequately cooled and ventilated, especially in hot environments. Choose a VFD that is de-rated sufficiently for the operation – it is not uncommon for a drive to be de-rated by 50% of the current requirements to ensure hassle-free operation. Additionally, when connecting a 3-phase VFD to single-phase power, ensure that the input of the VFD is rated to withstand the full current load and increased voltage ripple.

Operating Environment

Many VFDs operate in industrial environments rife with airborne particles, oil, humidity, and vibration. Unless a VFD is properly selected and maintained for the application, it’s at risk for short service life and premature failure.

In environments with high humidity, such as wastewater treatment plants, a VFD is at risk for circuit board corrosion from moisture sucked through the cooling vents. This can be compounded by infrequent use, enabling moisture to build up inside the unit without time for heat and ventilation to dry it out. To avoid this, ensure that both the operating environment and storage spaces for VFDs are clean and dry, and consider using a dehumidifier to dry out the air further. For washdown environments, a watertight enclosure such as a NEMA 4 or 4X will be necessary.

A common cause of VFD failure in heavy industrial environments is the clogging of cooling vents by airborne particles, especially when combined with moisture or oil particles. Metallic particles can also cause short circuits on the VFDs circuit boards. Ensure that the VFDs are always clean and well-maintained and that fans are operating smoothly without vibration and noise. If the environment is exceedingly dusty, a dust-tight NEMA 5 enclosure may be necessary to ensure the optimum service life of the unit.

Summary

By understanding the main reasons why VFDs fail, operators can quickly determine the cause of a failure or select the right VFD for the job, which helps prevent problems from occurring in the first place. eMotors offers a range of high-quality, tough and reliable VFDs for a variety of different operating environments and applications, with easy-to-read diagnostics and error information that will guide you toward correcting problems and restoring productivity to your operations with minimal downtime.