Troubleshooting Variable Speed AC Motor Drives
Mayfield - Electronic Maintenance Associates, Inc.
| Solid state
electronic AC motor drives are becoming increasingly more
commonplace within industrial and commercial facilities.
They control air handlers, chillers, pumps, conveyors,
machine tools, mixers, draw lines, and numerous other
applications once considered either strictly DC
applications, or constant speed.
increased usage of variable-frequency drives (VFDs) presents
the maintenance and service community with both the
opportunity and challenge of learning to maintain,
troubleshoot, and operate this equipment.
modern VFDs are reliable, in fact, they are extremely
reliable. Keep this fact in mind if a continued or
repetitive failure is observed on a particular application.
It usually isn't the VFD, regardless of the number of
fingers pointing toward it. Unfortunately, this hasn't
always been the case.
| Back in the
late 1970's when I first began doing field service on AC
drives, the typical service call involved pulling the
charred remains of drive components out of a cabinet, and
listening to the plant personnel remembering exactly what
they were doing when the VFD exploded. Just in case you're
wondering, explode is not an exaggeration. I've seen
cabinet doors blown off the their hinges, foil traces
hanging from the circuit boards, and shrapnel all over the
motor control room.
| I arrived at
one site in Texas to repair a 200 HP VFD, where in
maintenance supervisor, prior to my arrival, had picked the
unit up with his fork lift, and thrown it into the dumpster.
He threatened to kill the next OEM that installed one in his
plant. Reliability, top say the least, wasn't what earlier
versions of VFDs were noted for, Fortunately, that has
| A block
diagram of a typical Pulse With Modulated (PWM) VFD is shown
below, PWM drives are the most prevalent type of AC drives
being sold today. You'll note that the AC line is converted
to DC (within the converter Section of the drive) and
the synthesized back into a variable frequency, variable
voltage output. The theory behind this is, motor speed is
varied by changing the frequency, and motor torque is
maintained by keeping the volts to frequency ratio constant
(for most applications).
| How do you
troubleshoot it? Not to disappoint the electronic techs out
there, but you are going to do very little electronic
circuit board repair on a modern, microprocessor based motor
drive. Fortunately, most failures are not on the circuit
boards, but rather within the power sections, and that isn't
really difficult to troubleshoot. Companies like ours can
repair these boards, but the typical plant maintenance
person simply isn't going to see enough of the failures to
develop any expertise in repairing them.
troubleshooting, whether on a VFD or an old automobile
requires a methodical approach. I have generally observed
two techniques, one is the classic divide and conquer,
and the other which I'll term stochastic. The classic
method is taught by most technical schools, and is
particularly effective when knowledge of the equipment is
only rudimentary. The classic method is best illustrated in
figure two. Assume a signal is present at A, but is not at
B. What are the most number of steps a good
troubleshooter would use to isolate the box that isn't
passing the signal?
| The answer
is three! A classic troubleshooter would divide the circuit
in half, by checking between boxes 4 and 5. If the signal
were absent, then the next check would be between blocks 2
and 3. If the signal were present here, then a final check
between blocks 3 and 4 would isolate the problem to either
block 3 or block 4. Remember, divide and conquer!
troubleshooting takes advantage or more in-depth knowledge
and experience, and attempts shortcuts. The term
stochastic refers to an educated guess, based upon
random tests and observations. Basically, the more familiar
someone is with any particular piece of equipment, the
better they can deduce the problem from random tests,
without a time consuming disassembly in order to make
measurements, as would be required with the classic method
of troubleshooting. This is why I often stress to young
drive technicians the necessity of understanding drive
theory. That theory, however, is beyond scope of this
| So how can
you quickly troubleshoot a dead VFD?
| First be
careful. The capacitors within the power section can
maintain a lethal charge even after the power is removed.
Don't put your hands into the power section before
determining that the capacitor voltage has been discharged.
| With the
power off, check the power sections. Here's how. Place your
digital VOM in the Diode check mode. Find the positive DC
Bus (Sometimes this brought out to terminal, sometimes it
isn't). Place your negative lead on the + Bus, and then
check in turn with your positive VOM lead each incoming
phase. You should read a diode drop from each phase. If it
reads open, then the charge resistor (see figure 1) is open.
This is a common failure.
| Next place
the positive VOM lead on the - Bus, and place the negative
VOM lead on each incoming phase again. You should read a
diode drop, not a short or an open. Place both VOM leads on
the Bus, one on the + and the other on the -. You should
read the capacitors charging; you should not read a short.
| To check the
inverter section, place the positive VOM lead on the - Bus,
and reach output phase by placing the negative VOM
lead on each one. You should not read a short, and in fact
should read a diode (there are diodes connected across each
output transistor). Check the remainder of the inverter
section by placing the negative VOM lead on the + Bus, and
checking each output phase again with the positive VOM lead.
You should not read a short, but rather a diode drop again.
If you read open from either of these checks, then the Bus
fuse is open (see figure 1) ( The charge resistor and the
Bus fuses may be in the + or - Bus, depending upon the
| If no
problems are present within the power section, and the unit
will still not go, you have it either connected improperly,
programmed wrong (the most common problem we find),
or you have a bad circuit board.
| If you do
have a shorted transistor ..... here's a word of caution. If
you have an older generation PWM drive which uses Darlington
transistors in the Inverter section, be careful if you find
one shorted. When they short the almost always fail
the base driver circuit as well. This means that if you just
charge the transistor again as soon as you attempt to start.
| Later PWM
drives use IGBTs in the output section, and they are much
less likely to fail the driver sections.
about all this? Consider that the cost of VFDs has dropped
dramatically within the past few years, along with an
increase in features and reliability (just like computers).
So, buy a spare and sent the broken one to us! We make a
living repairing them for you!
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