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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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