George Stratton 
G.E. Jones Electric Co., Inc. 
Amarillo, Texas 
Technical Education Committee Member 

Before I get started with this rambling about variable frequency drives (VFDs), I wish to ex­plain 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 who might be timid when it comes to dealing with these high tech appliances…just as I once was. I hope that this helps you. 

Is anyone sleepy? Then welcome to VFDs 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 VFDs! 

Part 1 of 3

Start with definitions 
In order to troubleshoot one, you need to know some basic definitions. So let’s see if I can suit­ably define a Variable Frequency Drive. First of all, keep these terms in mind:. 
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 VFDs…inverters. 

The inverter is just part of the conversion. What’s an inverter anyhow? We’ll discuss this later. 

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. It 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. Each manufacturer does 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. “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! 

VFD power structure 
Please note Figure 1 on the previous page. Here you see the power structure for one of today’s VFDs. 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. With 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 DC bus is 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. 

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 you 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. 

Silicon controlled rectifiers 
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 inverter 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 Figure 2. Actually, transistors are just on/off switches. If you and two buddies had a setup like this, and were really quick and coordi­nated, you could make a motor shaft turn! 

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. 

Pulse width modulation 
Now…let’s 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-so­simple part. 

In Figure 3, the drawing shows the transistor in the inverter section of the drive where the switches were in Figure 2. These transistors are used in most of today’s drives and are called IGBTs or In­sulated Gate Bipolar Transistors. 

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 your buddies! This is called the Carrier Frequency.

Let me explain about IGBTs. 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. 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 is that it 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 sense; 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. 

Motors used on VFDs
Lets Talkabout 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 4. 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.

 

Now lets look at Figure 5. 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 (hertz) 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 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 6 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 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. 

VFDs offer flexibility 
You can do so many things with VFDs. If you operate one of the positive transistors you can cause the rotor of the motor to lock down which is 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 may 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. 

I want to thank Yaskawa Electric America, Inc. and Siemens Energy & Automation, Inc. for the use of their resources. They supplied some of the graphics used in this article. The manufacturers continue to educate us in order that we may serve customers better. The words are mine, though; don’t blame the manufacturers! 

About now you are probably saying to yourself “I thought this was a trouble shooting guide. When’s he gonna get to that?” Well not yet. 

In Part II next month, we will go over how these VFDs know what to do.

Part 2 of 3

A few years ago Variable Frequency Drives (VFDs) 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. Troubleshoot­ing, however, 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. 

The same holds true 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’re fixed. As far as all those other boards…better have some spare boards or other drives just like them so you can start swapping out boards. 

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. It’s a 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? 

Signals to run VFDs 
Now that I have attempted an explanation of all the scary stuff, let’s look at some of the signals it takes to run the VFD. The 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. Let’s look at Figure 7. 

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 manufacturers 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…things like that. There can be one or more of these relay outputs depending on the brand and model of the drive. 

Analog input signals 
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 go?) and the other one for feedback (how fast am I going?). The pro­cessor in the drive compares 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. 

The definition of PID: Proportional, Inte­gral, Derivative refers to the automatic means used to adjust a device that controls a process. Figures 8, 9, and 10 walk us through each of the steps involved. 

Figure 8 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. The only problem is that different processes do different things and react differently to the correction. 

Figure 9 shows us what takes place with 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 Figure 8…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.

In Figure 10 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. 

Troubleshooting tips 
Tip Number One. If you’re not on the Internet…get there! Those of you who have not yet seen the light cannot begin to realize the infor­mation available on the Web. Almost all of today’s drives are programmable. And everybody does it differently! 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 
• www4.ad.siemens.de/csinfo/ livelink.exe?func=cslib.csinfo&siteid=cs&lang=en&objid=4 000957&aktprim=0&objaction=csopen 
• www.ab.com/manuals/dr/index2.html#1336  ...Allen-Bradley 
• www.abb.com/global/seapr/seapr035.nsf viewunid/ b9f108fb0c8b4410c12568fd0047d 5d3!OpenDocument&v= 63136&e=us…ABB 
• 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/P­Home...Mitsubishi
• www.hitachi.com/products/industrial/acvarless500/index.html …Hitachi 
• www.ch.cutler-hammer.com/NASApp/cs/ ContentServer?pagename=C-H/Document Support/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
• www.sea.siemens.com/training/step2000/courses/index.asp ...Siemens

I’m sure there are others out there. 

Tip Number Two. Inside most of these opera­tors’ manuals you will likely find some troubleshooting 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. In Part 3 of this series, I will pro­vide a generic flowchart to help you troubleshoot AC drives. 

Tip 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. 

Tip Number Four. If all else fails, call the manu­facturer. Most drive manufacturers have a customer service phone number where you can access techni­cal assistance. 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. 

Tip 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 person up.

Part 3 of 3

Part three (provided below as a downloadable PDF) provides a series of flowcharts and diagrams that should be helpful in troubleshooting variable frequency drives (VFDs). But first, always exercise extreme caution when troubleshooting electronic equipment with doors open and guards removed! Voltages up to 800 VDC are present at any given time!

Always follow the manufacturer’s recommended safety procedures as outlined in the “Operator’s Manual.”

For additional information remember to refer to the Web sites listed in Part Two of this series. 

Categories: Drives/controls, Troubleshooting & failure analysis

Tags: Variable frequency drives

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