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Medium Voltage VFD Troubleshooting Tips and Tricks

Troubleshooting is an art form that few people truly possess. At EMA we’ve seen all kinds of troubleshooters, and it rarely takes long to essentially come to this conclusion: you either have it or you don’t. There is a famous troubleshooting call by Charles Steinmetz, the legendary German electrical genius, on a generator at Henry Ford’s plant. Ford’s internal engineers spent days troubleshooting a generator and finally called in Steinmetz to help. Steinmetz made a chalk mark on the side of the generator and told Ford’s engineers to remove the plate where that mark was and replace 16 windings from the field coil. After it worked perfectly, Steinmetz sent a $10,000 invoice to Henry Ford for the service call. Ford, flabbergasted at the price, asked for an itemized invoice and Steinmetz complied: “Making chalk mark on generator = $1; Knowing where to make mark = $9,999”. Steinmetz had it!

Any kind of troubleshooting, not just on medium voltage VFDs, requires a certain skills and many of them are not “technical”. Some of the smartest guys in the world can be horrible troubleshooters because they lack basic, necessary skills. When troubleshooting VFDs in particular, there are several “Tricks of the Trade” that we’ve learned over the years.

Keep it Simple, Stupid

This isn’t just good advice from Michael Scott (if you’re a fan of The Office, you get the reference); it applies to troubleshooting as well. Our brains tend to look at a problem and think the most elaborate and complicated issue possible as the culprit, but oftentimes, the problem is something very simple. We often reference a study by IEEE that showed loose connections contributed to almost 1/3 of all electrical failures! If you have a drive failing, it could easily be something like a loose connection or a wire in the wrong place or even the drive is in local mode. Try the most obvious things first, then grapple with the complicated.

The Drive Might Not be the Problem

Modern drives are quick to recognize issues and often shut down to protect themselves, this doesn’t necessarily mean the drive is bad. Overcurrent is a very common fault where the drive tells the user “Hey, I’m being asked to provide way more current than I’m rated for”. Ground fault is another good example; if the drive is tripping on Ground Fault, you may have a ground fault (imagine that). When we used to do commercial HVAC service calls, we saw countless instances where a customer would see the drive failed on ground fault, switch the drive into bypass mode, and then be very surprised when the bypass contactor welded shut and the breaker tripped! If your drive is tripping, maybe it is “trying” to tell you something. Another common complaint we hear on service calls is “The drive won’t run”. These types of calls almost always turn out to be something external, like an E-Stop pushed, a bad PLC Card, or some sequencing issue. Rarely does a drive just not run, and it is not intuitive enough to just know when you want it to, it has to be commanded to do so.

Understand the Mechanical and the Electrical

Understanding the mechanical load, not just the electrical is critically important in troubleshooting medium voltage VFDs. A drive/motor combination is only rated to provide a certain amount of current and if the application is asking for more current than they’re rated for, it is likely a mechanical problem. Torque in simple terms is a twisting force. Torque is a mechanical term, and you can generally equate that with current which is an electrical term. The more twisting force the motor shaft requires, the higher the current draw. If too much cooled plastic has gummed up in your extruder screw, the drive/motor is not likely going to electrically overcome that mechanical problem. If the heat load has dramatically increased on your fan or the blades are out of alignment, the drive/motor is not likely to electrically overcome that mechanical problem. Regeneration is another very common mechanical issue that causes drives to fail. If some outside source is mechanically forcing a motor to spin, that motor acts as a generator that can regenerate back to the VFD and cause overvoltage trips. There are numerous other examples that I haven’t discussed but the main point is: mechanical problems cause drive failures as much as electrical problems. A good troubleshooter understands the entire picture of the machine, load, motor, and drive.

Visual Troubleshooting

Leaky capacitors on a medium voltage power cell

Expensive test equipment and tools are certainly helpful (and often critical) to have, but almost everyone inherently possesses an extremely valuable troubleshooting tool: your eyes. For someone with experience on drives, your eyes can often spot something abnormal quicker than test equipment can. Discolored transformers or resistors, leaking capacitors, and evidence of arcing are some of the common things we see. One of the reasons we suggest a comprehensive preventative maintenance program on your medium voltage VFDs is because it gives you the opportunity to visually inspect the VFD and recognize potential problems BEFORE they become major issues. Use and trust your eyes.

Stay Organized

EMA Service Engineer reviews parameters on a Siemens MV Drive

This seems obvious but organization is extremely important when troubleshooting VFDs. At EMA, we’ve made a lot of money over the years coming behind a customer who failed to stay organized. You’re already dealing with one problem, don’t create another for yourself by being disorganized. Here are some bullet points to avoid excessive time redoing what you’ve undone:

  • Label wires
    • If you remove wires (control or power), label them. Landing wires back in the incorrect place will leave you with more problems than you started.
  • Document parameter changes.
    • It is unlikely you can cause a drive to blow up by changing parameters, but you can certainly cause it to stop working altogether. BEFORE you start troubleshooting, make sure the original parameters are documented via software, a keypad copy, or pen and paper; with this, you can always get back to where you originally were.
  • Take pictures.
    • Smart phones have made this easier over the years. When I take a drive apart, I take tons of pictures so there is never any doubt of how to put something back together. Very few people can remember where things were before they were taken apart; you’re likely not one of them.
  • Write down what you’ve done.
    • In situations where VFD troubleshooting takes place over several days, it is important to document, in writing, what you have done so far. When troubleshooting a very difficult problem, you’re tired, overwhelmed, and frustrated, having documentation of what you’ve done so far will help you avoid needlessly repeating the same actions. Plus, when you sit down and look at all the steps you’ve taken, it often points you where to next.

Avoid Arrogance

Good troubleshooters are confident but not arrogant. Confidence is a positive mindset, born of experience, that you can figure out the problem. Arrogant people think they know everything and look down on others who might suggest something different. Arrogance will make you a BAD troubleshooter, there is always more to learn and others who might know something you don’t. One of the most valuable guys in a plant is the operator of the machine; I’ve spend hours troubleshooting problems only to have the operator tell me “well, I usually turn this switch” and everything magically came to life. Confidence is great, but arrogantly ignoring other’s contributions is dumb.

Know When to Call for Help

The last tip is an obvious one: Let experts help you when you’re stuck. Thankfully, I know a guy (or multiple guys)! Are you having trouble with a Medium Voltage Variable Frequency Drive that you can’t solve? Let us help you. We do troubleshooting calls all over the world on drives and we can help you as well. Email us, call us, or use the chat feature on our website.

Let us prove to you that No One, ANYWHERE, Is Better At Drives Than We Are!


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