Troubleshooting

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

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 bridge rectifier rectifier front front end. That's That's tech talk for the the input. You know…where know…where you 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  device that that will allow allow current current flow in one direction direction only. only. A current to direct current. A diode is simply a device 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! 1

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

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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.

Figure 3

Figure 4

+

 A

B

C

Now…lets talk about PWM. Pulse Width Modulation is a term used to explain the process that controls the current that the drive 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 Bipolar Transistors. Figure 4 is an example of one. This particular  example is a hybrid 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 second or 15 kHz. Much faster than you and

 _  Figure 5

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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 preinverter 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

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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 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 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.

Figure7

460 Volts

230 Volts

0 30 Hertz 1/2 Speed

60 hertz Full Speed

You can do so many things VFDs. If you operate 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 speed up a process. Take care doing this as you 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 5

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? L1 L2 L3

T1 T2 T3

Power  Supply

S1 S2 S3 S4 S5 S6 SC Signal Common

Digital Inputs

Forward

Multi-function Input Terminals

FS +15VDC P O T

 Analog Inputs

FV 0-10V (20Kohms) FC Freq. Ref. Common FI Freq. Ref. 4-20 mA (250 ohms) 0-10 V available

Motor  Multi-function Output 0-10VDC

 AM  AC

2mA maximum  Analog Output MA Multi-function output terminal MB 250Vac/30VDC 1A MC M1 Multi-function output terminal M2 250Vac/30Vdc 1A Digital Output

Figure 9

Now that I have attempted an explanation of all the scary stuff lets look at some of the signals it takes to run the VFD. First thing worth mentioning is the digital signal. A complicated way to say an on/off  signal. They further complicate this by assigning numbers to the signals. Off  is a 0 and on is a 1. Lets look at figure 6. Here we see a typical control terminal arrangement. If you apply a switch across terminals SC and S1 there is now a means to give the drive a forward command. 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 drives. 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 

Set Point  Offset 

Proportional Control  Figure 10

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  programming. The analog signals require a 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 want me to Timego?) and the other one for feedback (how fast am I going?). The processor in the drive compares 6 the two inputs

and simply speeds up or slows down the drive to keep the process running where the reference tells it to. This is PID control.

Output 

The definition of PID: Proportional, Integral, 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.

Set  point 

Proportional Integral Control  Figure 11 Output 

Set point 

PID Control  Figure 12

Time

Fig 10 shows strictly proportional control. A 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. Only problem is that different processes do different stuff and react differently to the correction.

Fig 11 shows us what takes place with proportional control along with a little integral Time 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 anything. 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:

www.drives.com/products ...Yaskawa www.baldor.com/support/literature_manuals.asp …Baldor  http://www4.ad.siemens.de/csinfo/livelink.exe? func=cslib.csinfo&siteid=cs&lang=en&objid=4000957&aktprim=0&objaction=csopen ...Siemens www.ab.com/manuals/dr/index2.html#1336 ...Allen-Bradley www.abb.com/global/seapr/seapr035.nsf/viewunid/b9f108fb0c8b4410c12568fd0047d5d3! OpenDocument&v=63136&e=us …ABB 7

www.reliance.com/docs_onl/online_stdrv.htm#manual …Reliance www.namc.danfoss.com/techlit/index.html …Danfoss & Graham www.meau.com/eprise/main/Web_Site_Pages/Public/P-Home ...Mitsubishi www.hitachi.com/products/industrial/acvarless500/index.html …Hitachi www.ch.cutler-hammer.com/NASApp/cs/ContentServer?pagename=CH/DocumentSupport/DSMainPage …Cutler Hammer  www.squared.com/us/products/atv18.nsf/DocumentsByCategory?OpenView&count=999 …Square D www.geindustrial.com/cwc/library?famid=13 …General Electric www.actechdrives.com/Library.htm …AC Technologies Also…Siemens has a terrific training web site. I'm sure there are others out there. http://www.sea.siemens.com/training/step2000/courses/index.asp 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!

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!!

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 input terminals?

No

Check power  switch.

 Yes

Is run Indicated?

No

Fault Code Indicated?

 Yes

No

Is stop indicated?

 Yes

Check operator  manual for definition and correct the fault.

Remove stop command.

No  Yes

Is run commanded?

No

Initiate run command.

 Yes

Is control voltage present at control terminals?

No

 Yes

Is reference signal present at analog input?

No

Check Wiring. Check control volts. Replace power supply

Turn up the speed pot or set speed with keypad. Check external wiring and repair.

NEXT PAGE!

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From Page 1

Is the proper  voltage present at output terminals?

Check interface cable connections internal to drive. 2. Faulty drive.

No

 Yes

Is voltage present at motor terminals?

No

Check wiring to motor.

 Yes

Is the voltage balanced at the motor terminals?

No

Check interface cable connections internal to drive. 2. Faulty drive.

 Yes Motor has run out of smoke.

10

Overvoltage Fault Indication…

Does the drive trip during deceleration?

Increase decel time if process allows.

 YES

NO

Nuisance trips due to electrical noise.

 YES

Install snubber circuits to all coils of every contactor and every relay close by…also may need to add line filter on drive input.

Still trips from OV.  YES

NO

Bad control or  base driver  PC board.

Install proper  dynamic braking 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 process 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.

11

Overcurrent Fault Indication

Is output current at 200% or more?

Increase accel time. Utilize current limiting function if available. Reduce load. Load jammed.

 Yes

Must Be!

Does over current trip occur instantly?

 Yes

Check motor for  shorted condition.  Yes

Replace motor.

 Yes

*Check power  transistors.

 Yes

Replace transistors if  feasible.

Checks Ok

Nuisance trips due to electrical noise?

 Yes

Install snubber circuits to all coils of every contactor  and every relay close by… also may need to add line filter on drive input.

No

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 therein.

12

Undervoltage Fault Indication

Check input voltage.

Low

Check wiring and connections to the drive.

Ok

Is DC bus voltage within acceptable range?

No

 Yes

Check power  structure connections internal to the drive.

Undervoltage trip when motor  begins to run?

 Yes  Yes

Check for DC bus voltage drop.

Nuisance trips due to electrical noise?

No

 Yes

Install snubber circuits to 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.

13

No Display.

No

Voltage present at AC input terminals?

 Apply Power.

OK

 Yes

DC bus voltage present?

No

1. Check for open precharge resistor if  applicable. 2. Precharge circuit inoperable.

 Yes

Is control voltage present at digital control terminals?

No

Is control voltage present at analog control terminals?

No

Replace power  supply board.

 Yes

Is control voltage present at analog control terminals?

No

Power supply or  control board bad.

 Yes

Check interface cables internal to drive.

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Diode Module Test + ON L1 L2 L3  _   _   _  L1 L2 L3 + + +

- ON + + + L1 L2 L3  _   _   _  L1 L2 L3

NORMAL

ABNORMAL

READING

READING

READS LIKE  A DIODE 5-10 OHMS RX1

INFINITE

INFINITE

READS LIKE  A DIODE 5-10 OHMS RX1

CAP TEST - RX1

+

 _ 

CHARGE THEN BLEED DOWN

INFINITE OR SHORTED

+ L1 L2 L3

-

15

Transistor Module Test * + ON

- ON

+ + + T1 T2 T3

T1 T2 T3  _   _   _ 

T1 T2 T3  _   _   _ 

+ + + T1 T2 T3

NORMAL

ABNORMAL

READING

READING

INFINITE

READS LIKE  A DIODE 5-10 OHMS RX1

C1A

+ G1A

SHORTED

INFINITE OR SHORTED

C1B

G1B

C1C

G1C E1C C2C E1B C2B

T2

E1A C2A

G2A

-

T1

G2B

E2A

T3

G2C

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 therein. 16