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Troubleshooting Variable Speed AC Motor Drives
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by Eddie Mayfield - Electronic Maintenance Associates, Inc.
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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.
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The 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.
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First, 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.
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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.
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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 changed.
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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).
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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 t
o develop any expertise in repairing them.
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Effective 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?
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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!
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Stochastic 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 article.
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So how can you quickly troubleshoot a dead VFD?
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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.
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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.
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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.
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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 manufacturer). |
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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
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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. |
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Later PWM drives use IGBTs in the output section,
and they are much less likely to fail the driver sections.
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Nervous 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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provided courtesy of
Electronic Maintenance Associates.
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