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Common Faults On Medium Voltage Drives And How To Troubleshoot Them

An EMA engineer removes a power cell from a Siemens Gen 4 VFD.

Troubleshooting Variable Frequency Drives is what we’ve done at EMA for almost 35 years. So, we’ve seen and learned a thing or two along the way.

We’ve written posts before about troubleshooting tips and tricks. However, the purpose of this blog is to give you an idea of some of the more common faults that you’ll experience while working with medium voltage variable frequency drives and a few things to check when those faults occur.

Troubleshooting medium voltage drives is often easier than low voltage drives because MV drives are designed to be modular. Depending on the topology of the drive, the fault codes are often tied to individual “cells”, “power arms”, or “modules” that are designed to be quickly changed, allowing for less downtime. The fault codes often tell you which module to change! Even with specific fault call out, there is often a misunderstanding of what causes certain fault and why.

ATTENTION: This is by no means a comprehensive list, there are literally thousands of possible faults and alarms utilized by various VFD manufacturers; this blog simply lists a few of the more common ones.

Overcurrent (OC, OCA, CFA, etc.)

Overcurrent is caused when the drive is asked to provide 175% to 250% more current than it is rated for. This can be caused by mechanical means or internal damage within the drive, causing it to “think” it is being asked to provide more current than its rating. We recently had an example where a misfiring power cell was the cause of an overcurrent fault. Understanding torque and how it works mechanically is important for understanding overcurrent as well. If something on the load size has changed, is seized or otherwise impeding the motor shaft it can increase the torque needed, which increases the current, which is going to shut the drive down. The drive shuts down on overcurrent to protect itself and it typically happens instantaneously.

Troubleshooting Tips:

As with most output-related faults, the first step (if possible) is to isolate the motor and/or load and see if the problem persists. If you have an overcurrent and then isolate the motor and the overcurrent is gone, there is a good chance the issue is in the load/motor. On the flip side, if an overcurrent fault exists without a motor present, the problem is likely within the drive itself.

Overload (OL, etc.)

Overload differs from overcurrent because overload is the drive protecting the motor from drawing current over its FLA rating. During a startup, it is imperative to program the correct motor data within the drive so it can properly protect the motor from over amping. There are many applications where the drive is rated for much higher current than the motor, so by setting the proper motor data, you’re asking the drive to protect the motor from over amping even if the drive can provide that current. As the name suggests, this tends to be a “load” problem, and changes in the mechanical load being driven often contributes to this fault. Similar to what we mentioned with overcurrent, if you think in terms of torque, it will help you better troubleshoot this fault. If the motor is now asking for more current than it is rated for, there is often something mechanical that is causing the motor to require more torque to accomplish what it is trying to accomplish, thus the current rise.

Troubleshooting Tips:

Overload is also an output-related fault so isolating the load itself is the easiest troubleshooting step. It is advantageous to run the motor uncoupled and observe what happens. If the current stays high, you likely have a motor problem but if the current drops to the no-load current of the motor, there is likely a load problem causing this. Examples of load problems: belts improperly tensioned, impellers clogged, and misaligned sheaves.

Overvoltage (OV, etc.)

As the name suggests, overvoltage is the drive indicating that the DC bus has risen to a level it is not rated for and it trips out to protect itself from catastrophic failure. Failing DC Bus capacitors could cause overvoltage issues on drives as the capacitors lose their ability to store charge and the ripple increases. Electrolytic capacitor failure is common among certain medium voltage VFDs because capacitors have a useable life of around 7-10 years.

Regeneration is also a very common cause of overvoltage. If something is mechanically forcing the motor to spin faster, the motor acts as a generator and regenerates back to the drive, raising the level of charge on the DC bus. This is very common problem in applications that require high braking torque and often methods like dynamic braking or regenerative braking are used to avoid overvoltage. If your line voltage is too high, this can also cause an overvoltage. The charge on the DC bus will depend on the AC coming in, so the higher the line voltage, the higher the charge. In some instances, it may be advantageous to re-tap the line side transformer in order to avoid nuisance OV trips.

Troubleshooting Tips:

If you have a way to monitor your drive (we do), watching the DC bus level during different parts of your process is key to determining if regen is causing the fault. Checking your capacitors for age and visual signs of leaking will also give you an indication if they are the culprit.

Overheat (OH, OT, etc.)

dust dirt and debris can cause drive failures
EMA field service engineer cleans the transformer section of a drive to avoid heating issues

An overheat fault tends be related to a lack of air flow across and around the drive. Switching components, especially in high horsepower applications, generate massive amounts of heat and without a way to dissipate it, the drive’s internal temperature sensors will shut the drive down to prevent major damage. These faults are usually fairly easy to troubleshoot, a cooling fan is likely bad OR something is restricting air flow like a clogged heat sink or a filter. Most drives are designed to operate at an ambient temperature of 40 degrees Celsius, so if the environment around it is hotter than that, the cooling fans will not be able to do their jobs.

Troubleshooting Tips:

Checking cooling fans and filters is the most obvious and easiest thing to try first. If the drive trips immediately and will not reset, even after long periods of “cool down” time, then there is likely an internal temperature sensor bad.

Blown Power Fuses (FU, F, etc.)

Many medium voltage drives (but not all) will indicate if the main power fuses are blown. IF YOU HAVE A BLOWN POWER FUSE, DO NOT SIMPLY REPLACE IT. This seems obvious to some, but we’ve had literally hundreds of cases where someone had a blown fuse and simply replaced it, only to blow it again (and perhaps see some major sparks). Even semiconductor type fuses, which react very fast, are designed to prevent fire, explosion, and injury, not the VFD. If a fuse is blown, you almost certainly have a shorted semiconductor like a diode or a transistor.

“Dang, this fuse is shorted!”

Troubleshooting Tips:

If you have a blown fuse, static checking the drive is a must in order to determine which components are shorted. The fuse should be replaced and power reapplied only after the shorted component has been replaced.

Major Troubleshooting Tip:

The best troubleshooting tip I can offer you is: LET EMA TROUBLESHOOT IT FOR YOU! Medium Voltage VFD troubleshooting is what we do, and we’ve been doing it for over 30 years! If you’re having any issues with your medium voltage variable frequency drives, give us a call, email us, or utilize the chat feature on this website.

No One ANYWHERE Is Better At Drives Than We Are!

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