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Troubleshooting VFDs

Apr 1, 2000 12:00 PM, By Paul Frank, PRIMEDIA Workplace Learning

Find more articles on: Variable Frequency Drives

When a motor drive goes down, how do you resolve the problem quickly?

When a variable frequency drive (VFD) goes down, you're under pressure to get it back online fast. Don't let this pressure make you take even longer to resolve the problem. Instead, remember the following basic checkpoints about troubleshooting VFDs.

Check the controller display. Most VFD controllers include an interface to set up the drive for operation, and display information about its operation. Although the information varies, most controllers tell you about high current, high and low voltages on the input and output sides, high temperatures, internal faults, and even some advanced power diagnostics.

Check the connections. If the fault codes can't help you track down the problem, check the connections. Surprisingly, loose connections are among the most common causes of faulty operation in VFD applications. Just eyeballing a connection is sometimes enough to know it's loose. But, you can also check for a voltage drop across a connection if you're still powered up; or resistance through a connection if you're powered down. Don't forget to isolate the connection to ensure a reliable reading.

Check temperatures. Checking the temperature of connections with a temperature probe or IR-thermometer is one way to tell if they're loose. They should never be hotter than the connecting wires. You can check temperatures in the drive and at the motor. For example, if a controller's heat sink overheats because of infrequent cleaning, it can shut down the drive. Or, if the motor insulation is unsuitable for VFDs, it'll gradually degrade until it develops a short. Such shorts are often too small to blow a fuse, and too intermittent to trip an overload. However, they're enough to shut down a controller. An IR thermometer can show what is going on. Also, use your nose: If a motor smells hot, it is.

Sure, you can do more (see sidebar, below). But usually, just checking the basics will be enough to uncover the problem and get the system running again. And that can give you the time you need to schedule a permanent fix.

Sidebar: A Closer Check

In new motor installations, problems with drive performance are often due to improper application, selection, setup, or installation. Sometimes what seems like drive problems are due to process control logic -- not the drive at all. In the case of frequent breaker trips, you may need to examine protection coordination to ensure your breakers are the right size from the drive all the way back to the service -- and possibly other branch and feeder circuits. Before you do that, however, see if your drive can reduce inrush current with a "soft start" function. You also want to check: current to the motor (ammeter); voltage notching (oscilloscope); inductive noise in signal, control, or power wiring (oscilloscope); cable routing (visual inspection); damaged signal, control, or power wiring (insulation resistance, TDR); and current through the controller during sudden load changes or speed ramps (controller display).

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