Medium Voltage Variable Frequency Drives are fairly reliable pieces of equipment, and in an ideal world, they would never fail. In an ideal world, I would be able to dunk a basketball but unfortunately, we do not live in an ideal world. At EMA, we make a living from failed VFDs, so it is bound to happen at some point to your drives as well. There are multiple factors, internal and external, that lead to Medium Voltage Drive failures and to list all of them would be impossible (and very boring). However, in our over 30 years of experience, we do see some trends of common items that fail on VFDs. All of these items should be checked and inspected during normal, regular preventative maintenance.
Voltage-source drives all use some type of capacitor to make up the “DC Bus”, which essentially is the main power supply for the drive and what the inverter section pulls power from to create the output to the motor. Almost every low-voltage drive utilizes electrolytic capacitors, which have a typical useable life of 7-10 years. Many medium voltage drives utilize electrolytic capacitors as well, but film and oil-filled capacitors are also widely used in MV drive topology mainly due to their longer useable life. Electrolytic capacitors are high failure items in VFDs in general and at EMA we spend a lot of time replacing capacitors and repairing damage caused by them. For a more in-depth description of how they work, see our blog here. I like to equate capacitors to the timing belt on your car; you can run your car past the recommended 100,000-mile timing belt replacement, but every mile you do, the risk of catastrophic damage increases. There are tons of drives out there still running with degraded electrolytic capacitors, but the risk of catastrophic damage will continue to increase year after year. If your medium voltage drive is past the 10-year mark, it would be wise to consider a capacitor replacement to avoid costly repairs and downtime.
Of all the issues we’ve seen over the years, loose connections are the #1 culprit. These connections can be internal or external to the drive, but cause problems nonetheless. As power connections become loose, arcing takes place, which builds up oxide, produces heat, and will eventually break the connection altogether; you can see in this video that this happens even on household circuits. The extreme heat resulting from loose connections will cause all kinds of unwanted issues, particularly in high current applications. Low power connections, such as those on controls, will not arc but it can cause a litany of problems as well. If a control connection is loose, the VFD’s controller will literally be receiving “mixed signals”. If you’ve ever troubleshot a loose control connection, you know how frustrating this can be, because it is often intermittent. Tightening all connections is a MUST during any preventative maintenance.
This seems like an obvious one, but fans are often overlooked as a critical component to a medium voltage drive. As with any electronic component, heat is a major killer. The switching components within a drive produce heat that needs to be dissipated. If the cooling fans on the VFD are not functioning, that heat has nowhere to go and can prematurely degrade components and cause unwanted overheat failures. Typically, the cooling fans on medium voltage drives are 230 or 480V, 3-phase, squirrel-cage fans, which allow you to test them without the medium voltage being applied. Checking and testing cooling fans and the environment around the drive is another critical part of any preventative maintenance program.
When I say “power supplies” I am referring to the low voltage power supplies that every drive will have to power the internal ICs, chips, and components within the VFD’s circuitry. Manufacturers use 12V, 15V, 24V, and many other combinations of low voltage power supplies within the drive. These power supplies are typically easy to change out and a spare should be kept to avoid unwanted downtime. These power supplies use electrolytic capacitors for filtration (as do most circuit boards) which also have a usable life of around 7-10 years. If those capacitors lose their ability to hold charge, the high ripple can cause the power supply to be inconsistent and mess up modern controllers on VFDs. A failing power supply can manifest itself as intermittent issues, which is no fun at all. Power supplies should be checked for tolerance and ripple during normal preventative maintenance.
In a medium voltage VFD there are going to be several types of transformers (control power transformers and potential transformers) but I’m referring to the large, multi-pulse transformers that are integrated into most medium voltage drives. These transformers are used for harmonic mitigation as well as to step down the voltage to what is used by the drive’s power cells. Because they are often upstream of the main “guts” of the drive, they are often overlooked during normal maintenance. Among other things, transformers can fail due to high heat, arcing due to vibration, or defective insulation. We’ve seen several extreme cases where the transformer literally caught fire! MV transformers are unfortunately expensive, heavy, and mechanically difficult to replace. If your transformer hums louder than it used to or the colorization of the windings looks wrong, that is a sign of degradation and it needs to be checked out further by a transformer expert.
Does this seem overwhelming to you? Don’t worry, EMA is here to help you if you need it. We do preventative maintenance and field service on Medium Voltage Variable Frequency Drives every single day and there is no problem we haven’t seen. If you need help with your MV drives, call us, email us, or utilize the chat function on this site. No One ANYWHERE Is Better At Drives Than We Are!