VFD Trouble Shooting

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                         Variable Frequency Drives
  Before I get started with this rambling I wish to explain here that this technical article is definitely
NOT intended for the Drive Techs out there among you. It is intended to educate that poor, confused
soul that might be timid when it comes to dealing with these high tech appliances…Just as I once
was. I hope that this helps you all.

   Is anyone sleepy? Then welcome to VFD's 101! I've always said that if anyone has a problem
sleeping just tune in the History Channel. Well, here's another sure cure. Let's learn about VFD's!

   In order to troubleshoot one of these things you kind of need to know for sure what one is. So let's
see if I can suitably define a Variable Frequency Drive. First off they call these things several different

1. VFD…My personal favorite. That's short for Variable Frequency Drive.
2. ASD…That's Adjustable Speed Drive. This is a pretty generic term. It covers anything from the old
   Vari-Speed (belts, pulleys, and a crank) to DC Drives to our subject, the AC Drive.
3. AC Drive…a VFD.
4. AFD…That's Adjustable Frequency Drive.
5. Inverter…People call these VFD's…Inverters. The inverter is just part of the conversion. What's an
   inverter anyhow? We'll discuss this later.
6. You SORRY @*#*%!!!…That's what we're here for!

    Definition of a Variable Frequency Drive: Technically…it's a device that converts a single or
three phase fixed voltage and frequency to a variable three phase voltage and frequency used to
operate an induction motor at any speed that suits a multitude of processes. Basically…it's just a
converter. Converts AC to DC and DC back to three phase AC. Whew! All that to turn a shaft. How
does it do that?..

                                               This is how!

  All VFD's have a bridge rectifier front end. That's tech talk for the input. You know…where you
connect the electricity. The VFD converts the AC voltage to DC voltage…first thing. Different
manufacturers do this differently.

   What's a rectifier? The dictionary says it's a device, such as a diode, that converts alternating
current to direct current. A diode is simply a device that will allow current flow in one direction only. A
bridge rectifier is a set of these diodes configured in a network in such a way as to provide full wave
rectification. See Figure 2 to see an example of one. "Full wave" just means that both the positive and
the negative portion of the AC sine wave are utilized. It makes a much smoother DC as compared to
half wave, which requires half the diodes.

   In the old days this "bridge" was a whopping big set of heat sinks with diodes that you could
replace if they failed. Today all of the manufacturers use "hybrid bridges". They contain the same
components only in a much smaller package, which aids in miniaturization and are produced in
packages that are easier to test and to install. If one of these fail you just replace the whole thing.
FYI…The dictionary also says that a rectifier is "a worker who blends or dilutes whiskey or other
alcoholic beverages." I always thought that "worker" was called a bartender. Go figger!

    Please note figure 1. Here you see the power structure for one of today's VFD's. What I am
calling the power structure is the portion of the drive that conducts the power that turns the motor
shaft. We're not worried about the wimpy little control stuff right now. Here you see the "bridge
rectifier" on the "front end" of the drive. When the AC power is applied the first thing that happens is a
conversion from AC to DC. The one thing that is not indicated here is the DC bus. That's what the DC
supply is called in a VFD. The Inverter section draws its power from the DC Bus. More about the
Inverter section later.

                                                                                             Figure 2
Figure 1

                                     Figure 1

   Note that there are some capacitors in parallel with the DC bus, otherwise known as bus
capacitors. These are monster capacitors and act as a battery/filter sort of thing. A capacitor acts
kinda like a boss, that is it resists any change. In this case the change that it resists is voltage. If
when you were young, dumb, and insane have ever played around with electrolytic capacitors and
charged one up with a line, and I absolutely do not recommend that you try this, you would have
probably noted the violent pop (along with blackened finger-tips) when the cap was charged. The
pop is caused by the very sudden current draw when the cap is charging. The same thing happens
when a cap is discharged suddenly. The caps that are used in VFD's are large enough to trip circuit
breakers and blow fuses when an attempt is made to charge them suddenly…So the caps must be
"soft charged".

   This is where different manufacturers do this differently. In figure 1's case the caps are charged
through a "precharge" resistor. You will note that there is a contact supplied by the main or
"precharge" contactor. This contact will close, removing the circuit through the resistor, when the DC
bus voltage is near the fully charged state allowing the DC current to flow through the contacts as the
drive is operating. The precharge resistor is sized to charge the caps for a very short duration. If the
contacts fail to close, and the drive is allowed to operate, the resistor won't in very short order. Makes
a really good heater for a little while.

   Other manufacturers will use a hybrid bridge rectifier that utilizes SCRs otherwise known as silicon
controlled rectifiers or thyristors to charge the caps. These are electronically controlled devices very
similar to diodes except that they can be turned on anywhere in the positive half of the AC cycle.
They turn off when the voltage reaches zero. So…the voltage required to charge the caps can be
ramped up electronically with these SCRs. This can provide some other benefits, such as controlling
the DC bus voltage level.

     The next thing to note in figure 1 is the DC bus fuse. This manufacturer chooses to use this. Most
others that I know of don't. This fuse will not keep this drive from failing! That's not what its function
is. Its function is kinda like a suicide mission. It fails in order to keep the other output and control
components in the VFD alive. Never change the fuse and apply the power before checking the
output transistors! If you ever see that the DC bus fuse is open there is a 99% chance (no such

thing as perfect) that there is a failed output transistor and perhaps some associated electronics. The
good news is that there are some good parts left because of the fuse's ultimate sacrifice.

    The inverter section of the power structure of the VFD contains the output transistors. The
definition of invert is 1. To turn inside out or upside down. 2. To reverse the position, order, or
condition of. That's all that happens here. Looking at figure 1 again you will note that there is a
transistor above each of the output lines to the motor and one below each one. There are three sets
in order to make three phases. Every positive half cycle the positive transistor conducts and every
negative half cycle the negative transistor conducts. Then it all happens again, and again, and so on
and so on…its just that simple! The not-so-simple part is how the transistors know when to conduct.

                                                             For a really simple look at what I tried to
                                                         explain look at it like in figure 3. Actually,
                                                         transistors are just on/off switches. If you and
                                                         two buddies had a setup like this, and were
                                                         real quick and coordinated, you guys could
                                                         make a motor shaft turn!

                                                             In steps 1 and 2 switches A+ and B- are
                                                         closed. The voltage from A to B is positive. In
                                                         step 3 B- is opened and B+ is closed along
                                                         with A+ making A to B zero volts. Then in
                                                         steps 4 and 5 A+ is open and A- is closed
                                                         sending a negative voltage to the motor. And
                                                         so on and so on…until your buddies or your
                                                         fingers get tired of switching those switches or
                                                         the motor has completed its process.

                                                            Now…lets talk about PWM. Pulse Width
                                                         Modulation is a term used to explain the
                                                         process that controls the current that the drive
                            Figure 3                     produces and the motor uses to perform its
                                                         process. This is the not-so-simple part.

                                                            In figure 5 the drawing shows the
                                                         transistors in the inverter section of the drive
                                                         where the switches were in figure 3. These
                                                         transistors are used in most of today's drives
                                                         and are called IGBTs or Insulated Gate
                            Figure 4                     Bipolar Transistors. Figure 4 is an example of
                                                         one. This particular example is a hybrid
                   A    B   C                            module that contains two transistors. In fact
             +                                           the two transistors are the two required to
                                                         make up one of the three phases in the
                                                         inverter section of the drive. There would be
                                                         three of these modules required to produce
                                                         the three phases. Note A, B, and C in figure 5.
                                                           These IGBTs are capable of being switched
                                                         on and off at up to and beyond 15,000 times a
                            Figure 5
                                                         second or 15 kHz. Much faster than you and
your buddies! This is called the Carrier Frequency.

  Let me explain this IGBT stuff. By the way…IGBT can be pronounced igbit. If you have ever been
close to a motor that is being operated by a PWM drive I’m sure that you have noticed the tone,
buzz, or whatever you want to call it. What causes this to happen? When the transistor is turned on
the voltage isn’t ramped up like the Power Company’s power is. It just turns on. BANG…the full
voltage of the DC bus is applied to the motor windings. Then just as quickly the transistor is turned
off. Well it’s kinda like hitting a flagpole with a baseball bat. You hear a ring. A few years ago most
PWM drives were operated at a carrier frequency of 3.5 kHz and lower, well within our audible
hearing range. And it can be pretty noisy. But back then that was about all that our old transistor
technology would allow. Then, as I said before, the IGBT came along and these transistors could be
switched at a much higher frequency. At 15 kHz the tone is barely perceptible, unless you’re a dog or
some other critter. The advantage…allows quiet operation for fans and pumps in areas where people
work or play. Those high pitched whines can be pretty distracting. The disadvantages are that the
high carrier frequency causes the drive and motor to run considerably hotter. It makes since…going
from being hit 3,500 times to 15,000 times a second would tend to heat stuff up. Another
disadvantage is the turn to turn insulation in the motor is further stressed by both the higher
temperatures involved and the abrupt voltage changes.

  Lets talk about motors that are used on VFDs. Will a VFD operate an existing three phase motor
even if it isn’t inverter duty? Yup...sure will! Any three phase motor can be operated with a VFD.
Could I experience problems with this motor if I run it with a VFD? Yup…sure could! I always warn
the customer that he could damage the motor windings with the added stresses that are caused by
VFDs. Well then…should I replace my motor with an inverter duty motor? I said that you could
damage the motor windings, not that you would. If that motor is performing a non-essential function,
in the case of many HVAC applications, my opinion is that if there is a budgetary problem with
replacement of the motor go ahead and run it with the VFD. Personally I have experienced very few
problems retrofitting existing motors with VFDs. But I didn’t say that I haven’t experienced any
problems. What I have noticed throughout my experience with AC drives is that the smaller pre-
inverter duty motors, up to 10 hp, tend to throw in the towel more quickly than the larger motors. I
have experienced good results recommending the application of a reactor on the output of the drive
to help reduce the effect of the abrupt voltage changes on these little motors.

                                                        NOW lets discuss the PWM output that makes
                                                      these drives so useful. Look at figure 6. At the
                                                      beginning of the sine wave the transistor is
                                                      turned on and off very quickly. The next pulse will
                                                      be a little longer as will the next pulse after that
                                                      and so on and so on until the transistor is on
                                                      almost continuously. This is the high portion of
                                                      the wave where the current is at its highest.
                                                      What goes up must then come down. The
                                                      voltage pulses begin to get shorter until it is time
                                                      for the negative transistor to do its thing. It does
                                                      the same thing only negative. We then have a
                                                      complete sine wave.

                       Figure 6
                                                        Now lets look at figure7. How does the drive
                                                      increase and decrease the voltage and the
                                                      frequency? Notice that the DC bus voltage is fixed
                                                      at around 650 VDC. This voltage is representative

                                                   of a 460 volt three phase input. A picture is worth a
                                                   thousand words. The top waveform represents a
                                                   lower voltage and frequency and the lower one a
                                                   higher level. The voltage is modulated in order to
                                                   provide a variable voltage and frequency.

                                                     The low voltage and frequency is accomplished
                                                   by switching the transistors on for short periods of
                                                   time while the higher volts/hertz (frequency) levels
                                                   the transistors are switched on for longer periods
                                                   of time.

                                                     Let’s define Volts/hertz ratio: The voltage and
                                                   frequency level at which a motor can maintain full
                                                   torque at any given speed. Figure 8 is a graph
                                                   representing the volts/hertz curve. The line angling
                     Figure7                       up from 0 and intersecting at 460 volts and 60
                                                   hertz is the curve that I am indicating. Not much of
                                                   a curve…looks pretty straight, huh. Actually it’s
 Volts                                             pretty much the way that drives are set up out of
                                                   the box except for a little voltage boost at the
                                                   lowest hertz level. This drawing is just an example
                                                   of where the volts/hertz level is at any speed along
                                                   the curve. This is what the VFD is all about.

230                                                  You can do so many things VFDs. If you operate
Volts                                              one of the positive transistors you can cause the
                                                   rotor of the motor to lock down…DC injection
                                                   braking. Comes in pretty handy when you need to
                                                   stop a wind-milling fan or pump before you start it.
                                                   You can raise the voltage some when you start the
                                                   motor…torque boost. You can increase the
                                                   frequency above the 60 cycles if you need to
                  30 Hertz       60 hertz          speed up a process. Take care doing this as you
                 1/2 Speed      Full Speed         my sling something apart on the rotor. You can
                                                   perform a speed search when the power fails and
                                                   the motor is coasting down. The drive applies a
                                                   small amount of voltage to the motor throughout
 the frequency range. When the current drops the drive knows where the speed is and resumes
 operation and brings the motor back up to speed. If the appliance that the motor is powering shakes
 at certain frequencies you can program the drive to skip those frequencies. And many other things.

  About now you are probably asking yourself “I thought this was a trouble shooting guide. When’s he
 gonna get to that?” Well not yet. Now we gotta talk a little about what tells these VFDs what to do.

   A few years ago these drives were pretty darned complicated. They had input boards, output
 boards, the control board, the power supply board, the input logic board, output relay board, the
 analog input board, the analog output board, the base drive board, the firing board…puleeeeze
 gimme a break. Trouble shooting these things isn’t quite as bad as it looks. The inputs on these
 things operate with a DC control voltage of some value…mostly 24 VDC. Just check from the signal
 common terminal to any of the input terminals for that voltage. Same thing for the analog input and
    output boards. If the voltage wasn’t there you would check the interface cables (the wiring from board
    to board). If it’s none of those things change the power supply board and maybe you were fixed. As
    far as all those other boards…better have some spare boards or other drives just like them so you
    can start swapping boards out.

      The drives have gotten a lot smarter today. Most of them include the power supply board as a part
    of the base driver board (provides the signals to the IGBTs). The digital and analog inputs, outputs,
    and relay outputs are found on the control board. They still have option boards but they simply plug
    into the control board and use the same power supply that the control board uses. Much better way
    to do things. The proliferation of surface mounting technology in the circuit board manufacturing
    industry has really helped us here. Where would we be without the Space Program?

                                                                                       Now that I have attempted an
                         T1                          L1                                explanation of all the scary stuff
     Supply              T2                          L2            Motor
                                                                                       lets look at some of the signals it
               Forward S1
                                                                                       takes to run the VFD. First thing
                                         Inputs                   Multi-function
                                                                  Output 0-10VDC
                                                                                       worth mentioning is the digital
                       S3                                         2mA maximum          signal. A complicated way to say
                       S4                                                       Analog an on/off signal. They further
                               Multi-function       MA                          Output
                       S5      Input Terminals      MB Multi-function output terminal  complicate this by assigning
                       S6                           MC 250Vac/30VDC 1A                 numbers to the signals. Off is a 0
      Signal Common SC
                                                    M1 Multi-function output terminal
                                                                                       and on is a 1. Lets look at figure 6.
                          FS +15VDC
                          FV 0-10V (20Kohms)
                                                    M2 250Vac/30Vdc 1A                 Here we see a typical control

                          FC Freq. Ref. Common                 Digital                 terminal arrangement. If you apply
                          FI Freq. Ref.                        Output                  a switch across terminals SC and
                             4-20 mA (250 ohms)
        Analog               0-10 V available
                                                                                       S1 there is now a means to give
        Inputs                                                                         the drive a forward command.
                                   Figure 9
                                                                                       When the switch is open the drive
    sees a 0 signal. What you and I see is 24 VDC, which the drive supplies, across the switch. When
    the switch is closed the drive then sees a 1 signal. We see a 0 VDC signal. Gosh…I guess that
    means that the switch is closed. The drive runs forward. The same digital thing goes for all those
    other switches. The difference is that all of these terminals except for S1 and SC are programmable
    for several different functions that require an on/off (digital) signal. S1, in this case, is always going to
    be a run command. There will be contacts assigned for a run command and a signal common with all

      The digital output signals are provided by relays mounted on and interfaced to the control board.
    Some manufactures choose to use transistor outputs that can be used to operate interposing relays.
    These outputs can be programmed to operate when you need a specific indication of something. It
    can indicate that the drive is running, at speed, a fault, you know…stuff like that. There can be one or
    more of these relay outputs depending on the brand and model of the drive.
                                 Output                                     The analog inputs are the inputs that tell the
                                                                          drive how fast to run and how to react to a
                                                                          changing process. A lot of today’s drives can
                                                                          perform process control from within their
     Set Point
                                                                          programming. The analog signals require a
                                                          Offset          control signal of some sort with 0 -10 VDC or 4
                                                                          – 20 mA being typical. Also there are almost
                                                                          always two analog inputs. One for actual
                                                                          speed control or reference (how fast do you
                              Proportional Control                 Time
                                                                          want me to go?) and the other one for
                                   Figure 10                              feedback (how fast am I going?). The
                                                                          processor in the drive compares the two inputs 6
                                                               and simply speeds up or slows down the
                            Output                             drive to keep the process running where the
                                                               reference tells it to. This is PID control.

                                                               The definition of PID: Proportional, Integral,
   Set point
                                                               Derivative refers to the automatic means
                                                               used to adjust a device that controls a
                                                               process. Figures 10, 11, and 12 walk us
                                                               through each of the steps involved.
                    Proportional Integral Control
                                                               Fig 10 shows strictly proportional control. A
                             Figure 11
                                                               control signal based on the difference
                                                               between a real condition (feedback) and the
                                                               desired condition (setpoint) is produced. The
                                                               difference is the “error”. The VFD speeds up
                                                               or slows down to compensate for the error.
   Set point
                                                               Only problem is that different processes do
                                                               different stuff and react differently to the
                           PID Control                       Fig 11 shows us what takes place with
                           Figure 12                       proportional control along with a little integral
                                                           tweaked in. It’s a math thing. What it amounts
to is the drive looks at the offset (see fig 10…the difference between the real and desired condition)
over a period of time and then calculating a correction. It helps to stabilize the process so that the
offset is as low as possible.

  Fig 12 we see that the signal settles down more quickly with some derivative tweaked in. About the
only time that you would want to mess with this setting is if there is a severe stability problem.
Derivative anticipates the error and puts some braking action in the signal. About all that I can really
tell you about setting up an application that requires PID is that every application is different.
Sometimes there's a lot of adjusting going on and then sometimes you don't even have to touch

                                          Finally…Trouble Shooting Tips

  Tips number one…If you’re not on the Internet…get there! Those of you that have not seen the
light yet cannot begin to realize the information available to you on the web. Almost all of today’s
drives are programmable. And everybody does it different! In order to program them you will need to
have an operator’s manual. Most manufacturers have these readily available on their web site…
FREE! A lot of them will charge you if they have to ship the hard copy. I have taken the liberty of
listing some of the manufacturer web sites that I know of, or can find, where you can get this
information. I can’t guarantee that these sites will be there when you look because they do change
web sites from time to time: ...Yaskawa …Baldor
aktprim=0&objaction=csopen ...Siemens ...Allen-Bradley!OpenDocu
ment&v=63136&e=us …ABB
                                                                                                                7 …Reliance …Danfoss & Graham ...Mitsubishi …Hitachi
H/DocumentSupport/DSMainPage …Cutler Hammer …Square D …General Electric …AC Technologies

 Also…Siemens has a terrific training web site. I'm sure there are others out there.

  Tips number two…Inside most of these operators’ manuals you will likely find some trouble
shooting flow charts. It starts out with something like “Motor won’t run”. In other words…the flow chart
will take you though a comprehensive series of steps until the problem is solved, or not. I have
supplied as generic a flowchart as I can provide to help you trouble shoot AC drives. However it is
better to use the manufacturers chart if it is available to you. Some of your customers even keep
these manuals!

  Tips number three…Fault Codes: Most of these drives will tell you what is wrong with them! Some
with a simple LED blinking out different codes, to three letter codes like “uU1”, and then there are
those that can even spell it out for you in plain English (or most any other language) with from one to
multiple line alphanumeric displays. Some of these drives will store anywhere from the present and
the previous fault to an indeterminate number of faults. Some even time and date stamp them! This
can help if you have one of those “if it ain’t broke, I can’t fix it” otherwise known as “intermittent” type
failures. By checking the previous fault codes you may be able to determine the problem. Chances
are that you will need the operator’s manual to decipher these fault codes because, even with the
elaborate alphanumeric displays, the fault can be stated in some wording that you have never heard
of. So…review Tip # 1.

   Tips number four…If all else fails call the manufacturer. Most drive manufacturers have a
customer service phone number that you can access technical assistance from. Speaking from
experience…don’t spend a whole lot of time trying to fix these things. The manufacturer pays these
folks to help you and your customer fix their drives. What may take you hours or days to figure out
may take one of these highly trained people a few minutes to determine with a little clear and
common sense communication from you. I hope that this article can provide you with a little of that.

  Tips number five…In order to keep from messing up some of the volts, amps, and ohm readings I
use an analog Volt/Ohm meter (that's one with a needle for all you young folks out there). Especially
when checking the output voltage. A digital meter will show the voltage really high. I know that there
are digital meters out there that will indicate correctly but with all that auto scaling and stuff it can
mess a guy up.

 Speaking of manufacturers I want to thank Yaskawa and Siemens for the use of some of their
resources. They supplied some of the pictures and graphics. The manufacturers continue to educate
us in order that we may serve their and our customers better…Sincere Thanks. The words are all
mine…Don't blame them for that!

!!                                                                               !!
         Always exercise extreme caution when troubleshootingelectronic
       equipment with doors open and guards removed! Voltages up to 800
         VDC are present at any given time! Always follow manufacturer's
      recommended safety procedures as outlined in the Operator's Manual!

                              Motor will not Rotate

                               AC voltage present at     No        Check
                               input terminals?                    power

                         No                            Yes
                                     Fault Code               Check operator
                                     Indicated?               manual for definition
                                                              and correct the fault.

                         No                            Yes
         Is run                       Is stop                    Remove
       Indicated?                   indicated?                    stop


                                      Is run            No      Initiate run
                                   commanded?                   command.


                                Is control voltage              1. Check Wiring.
                                present at control     No       2. Check control
                                    terminals?                     volts.
                                                                3. Replace
                                          Yes                      power supply

                                    Is reference                1. Turn up the speed
                                 signal present at                 pot or set speed
                                   analog input?                   with keypad.
                                                                2. Check external
                                                                   wiring and repair.


   From Page 1

                                  1. Check interface cable
    Is the proper
                        No           connections internal to
 voltage present at
 output terminals?
                                  2. Faulty drive.


Is voltage present at        No           Check wiring
motor terminals?                           to motor.


                                  1. Check interface cable
  Is the voltage        No           connections internal to
 balanced at the
 motor terminals?
                                  2. Faulty drive.


   Motor has run
   out of smoke.

                                       Overvoltage Fault

              Does the drive           YES                                         Increase decel time
               trip during                                                          if process allows.


                                               Install snubber circuits
              Nuisance trips       YES            to all coils of every                   Still trips
             due to electrical                   contactor and every                      from OV.
                  noise.                        relay close by…also
                                                may need to add line                            YES
                                                 filter on drive input.

              Bad control or                                                           Install proper
               base driver                                                            dynamic braking
               PC board.                                                                equipment.

NOTE: The main cause of over voltage trips on AC drives is inertia. Inertia is the force that resists
acceleration when the motor starts Only in this case were decelerating that same force...same thing
only different. The amount of inertia depends on the weight of the rotating elements. What happens is
that inertia overpowers the motor when the drive decelerates the motor causing it to become an
induction generator. This in turn causes the DC bus voltage to increase to a damaging level. Typically
modern drives are designed to trip at about 800 VDC (460 volt system, 400 VDC for 230 volts). This
happens often on high inertia loads such as large fans and other heavy rotating loads. Even small
amounts of inertia can give problems if decelerated too quickly. If the proc ess allows, the problem can
be solved by simply extending the decel time. If the process must be stopped quickly, dynamic
braking can be utilized. Dynamic breaking utilizes an internal or external braking transistor module
and resistors to regulate the DC bus voltage at a safe level. When the DC bus voltage aproaches the
trip level, the braking transistor module conducts redirecting the excess voltage to the resistors. In the
case of really BIG inertia loads such as elevator and hoist applications there are regenerative drives
available from most manufactures. A regenerative drive is capable of returning the excess energy
back into the power supply lines.

                                          Fault Indication

                       Is output current at       Yes           1. Increase accel time.
                         200% or more?                          2. Utilize current limiting
                                                                   function if available.
                                                                3. Reduce load.
                                                                4. Load jammed.
                                 Must Be!

                     Does over current trip               Yes             Check motor for
                       occur instantly?                                  shorted condition.
                                 Yes                                            motor.

                         *Check power                Yes                       Replace
                          transistors.                                       transistors if

                                 Checks Ok

                       Nuisance trips due           Yes                Install snubber circuits to
                       to electrical noise?                           all coils of every contactor
                                                                          and every relay close
                                                                      by…also may need to add
                                                                        line filter on drive input.

                       Bad control or base
                        driver PC board.

*NOTE: When testing IGBTs be sure to check the gate (or base) terminal to the collector and emitter.
The reading should be infinite on RX1 scale. If there is any resistance replace the base driver board.
A very low DC voltage is required to fire the transistor. If the base shorts to the gate, the full DC bus
voltage is applied to the fragile gate control section of the base driver board removing all of the smoke

                        Fault Indication

Check input voltage.          Low            Check wiring and
                                             connections to the

     Is DC bus                                      Check power
   voltage within             No               structure connections
    acceptable                                  internal to the drive.

                           Yes                    Undervoltage trip
                                                    when motor
                                                   begins to run?


                                                   Check for DC
                                                    bus voltage

 Nuisance trips due          Yes            Install snubber circuits to
 to electrical noise?                      all coils of every contactor
                                               and every relay close
                                           by…also may need to add
            No                               line filter on drive input.

Bad control or base
 driver PC board.

                          No Display.

  Voltage present                 No                Apply
    at AC input                                     Power.

                                                   1. Check for open
   DC bus voltage                 No               precharge resistor if
     present?                                      applicable.
                                                   2. Precharge circuit

  Is control voltage     No   Is control voltage    No        Replace power
   present at digital         present at analog               supply board.
  control terminals?          control terminals?


 Is control voltage               No                Power supply or
 present at analog                                 control board bad.
 control terminals?


Check interface cables
  internal to drive.

              Diode Module Test
                     NORMAL         ABNORMAL
     + ON   - ON    READING         READING
     L1      +
     L2      +     READS LIKE
     L3      +      A DIODE          INFINITE
     _       L1         -
     _       L2    5-10 OHMS
     _       L3       RX1

     L1      _
     L2      _                      READS LIKE
     L3      _       INFINITE        A DIODE
     +       L1                          -
     +       L2                     5-10 OHMS
     +       L3                        RX1
                   CAP TEST - RX1
      +       _                     INFINITE OR
                   CHARGE THEN
                   BLEED DOWN




                              Transistor Module Test *
                                               NORMAL         ABNORMAL
                       + ON    - ON            READING         READING
                        +           T1
                        +           T2
                        +           T3         INFINITE        SHORTED
                        T1          _
                        T2          _
                        T3          _

                        T1          +
                        T2          +      READS LIKE
                        T3          +       A DIODE            INFINITE OR
                        _           T1          -               SHORTED
                        _           T2     5-10 OHMS
                        _           T3        RX1

                                     C1A             C1B           C1C

                              G1A              G1B           G1C
                                                                         E1C   T3
                                                       E1B                     T2
                                         E1A                                   T1

                              G2A              G2B           G2C

                       -             E2A             E2B           E2C

*NOTE: When testing IGBTs be sure to check the gate (or base) terminal to the collector and emitter.
The reading should be infinite on RX1 scale. If there is any resistance replace the base driver board.
A very low DC voltage is required to fire the transistor. If the base shorts to the gate the full DC bus
voltage is applied to the fragile gate control section of the base driver board removing the smoke

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