Diagnosing VFD Problems

When a variable-frequency drive, or VFD, malfunctions, costly downtime often ensues. It is important to take the correct measures to determine the cause and resolve the problem expediently.

Check the controller display. Fault codes can indicate high current issues (usually blown fuses and overload trips), high and low voltages on the input and output sides, high temperatures, internal faults, and other diagnostics.

Check your connections. Loose connections are frequently the culprit when a VFD goes down. Visually inspect connections, then check for voltage drop across the connection if still powered up, or for resistance if you are powered down. Be sure to isolate the connection for an accurate reading. You can also tell if a connection is loose if its temperature is hotter than the connecting wires.

Check drive and motor temperatures. A frequent cause of VFD problems is overheating due to insufficient or degraded motor insulation, causing shorts. Oftentimes these shorts are too small to blow a fuse, or too intermittent to trip an overload, but enough to shut down a controller.

New installations. VFD problems on new installations are often due to improper application, drive selection, setup, or installation of the motor or drive. Other issues can be traced to problems in the sequence of operation.

Breaker trips. Where applicable, most drives should use a soft start function to reduce inrush current. Verify that breakers are sized and/or adjusted properly

If the problem still cannot be traced, you should also check: excess distance from the motor; current to the motor; voltage notching; inductive noise in signal, control, or power wiring; cable routing; damaged signal, control, or power wiring; power and control wire intermixed; and current through the controller during sudden load changes or during speed ramps.

About MSC - The Go To Guys
We are a service sub-specialist in the technical intricacies of HVAC, process cooling, and building automation systems in commercial, industrial, and institutional environments. MSC is recognized as the leading industry expert in solving elusive problems and attaining peak performance from clients’ mechanical equipment and overall systems.

Located in northern New Jersey, MSC is the only specialized contractor of our kind in the Northeast and Mid-Atlantic regions. We are MSCA STAR-qualified, NEBB-certified, fully-licensed in electrical, plumbing and HVAC, and our engineers and technicians are the most knowledgeable in our field.

Mechanical Service Corp.
41 South Jefferson Road
Whippany, NJ 07981
732-884-5000
http://www.mscnj.com