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

We take so much for granted. The alarm clock buzzes and we make our way to the bathroom and turn on the light. We reach over and turn the water on and it’s there. We flip the switch on the electric razor. We really do take it all for granted. The en­ergy that flows through the wires to the switch that you flip comes from a device called an alter­nator. Yes, somewhere close to where you live is a power plant where that energy that makes the world go around is created.

Alternators come in all kinds of configura­tions. Your car or truck even has one. That’s right. An alternator is needed to charge the battery and provide electric power for your vehicle. If you have a recreational vehicle (my wife’s idea of really roughing it) you probably use a generator to power that air conditioner that would be tough to live without these days. This device is handy for watching TV, too.

Simply stated, an alternator is a converter: It converts mechanical energy into alternating elec­trical current. It’s a generator: It generates (creates) alternating electrical current out of me­chanical energy. How’s it doing that? It’s magic! Nah, not at all. I’ll show you. By the way, starting here, I’m going to refer to the alternator as a generator. It has one less letter in it and, besides, that’s what most folks call it anyway. 

Back to basics 
First, we need to know how a generator works. For demonstration purposes you can create an al­ternating current (AC) generator very simply. All you need is a simple coil of wire, a voltmeter, and a permanent magnet. Look at Figure 1. (Please forgive my elementary art attempts.) If you build a set-up something like this you can actually create AC power! By wiggling the magnet up and down the coil the cur­rent changes direction in the coil and you can see the voltage go plus and minus. Primitive as it may be, it is a genera­tor. So what do we need to do to create an AC generator? All you need is a coil of wire and a moving magnetic field. 

Well, it’s a little more complicated than this, but basically that’s it. The only problem is we re­ally can’t do anything with this except to move a meter needle. Just remember that this step is to demonstrate how it works. Now for the real thing. 

Our example in Figure 1 can be dubbed a two-pole AC generator. The two poles are moving the magnet one direction, then the other. If you were to insert a shaft through the center of the magnet and turn it instead of wiggle it you would have created a two-pole rotor. 

By turning the rotor close to the coil of wire the result is the same. We have created AC current. It’s just a whole lot more practical to rotate the poles than it is to wiggle them. 

Okay, so we have figured out that we need three things to generate AC current: 

1. A coil of wire. Let’s get a little more specific here. But let’s not get into a whole multi-chapter thesis on theory, metallurgy, and electrical engineering. The coils of wire that we need to generate AC are wound into an assem­bly somewhat like the electric motor stator. 
2. A rotating field. Let’s just call this the rotor. As noted before, the rotor holds the poles that turn inside the stator. 
3. A prime mover. It’s what turns the rotor. Lots of things do this. Here in West Texas the power plants use huge coal burning or gas fired boiler-powered steam turbines to turn the shaft of huge two pole AC generators. Maybe where you’re from they use massive slow turning turbines powered by nothing more than falling water to turn a rotor with many poles…like at the Hoover Dam.

Or you may live in a smaller community that generates power with a few big gas or oil burning engines at the municipal power plant. What I’m getting at here is that the prime mover can be anything that can turn the rotor shaft. 

Generating AC power 
Now that we know the fundamental parts of the AC generator, let’s see if I can stretch your mind. To generate useable AC power, we require even more stuff! Previously I was using a permanent magnet as the poles for our rotor. There are practi­cal uses out there for permanent magnet rotors. The railroad uses generators set up this way to power vibrators because they can speed up and slow down the engines to speed up and slow down the motor of the vibrator. I haven’t covered this phenomenon yet. Let’s cover it now. 

Frequency 
Why does an AC induction motor run the speed that it runs? (Tick…tick…) I’m waiting for the an­swer (tick…tick) Come on! (Tick…tick…DING!). Okay…the answer is frequency. The motor follows the AC generator’s frequency. In the U.S., we oper­ate everything at 60 Hz (hertz), while our friends “down under” and in many other parts of the world operate at 50 Hz. Another term that you will run into is “cycles.” “Hertz” and “cycles per second” are exactly the same thing. Well, that is, sort of. 

Actually “hertz” pertains to the electrical aspect of things where “cycles” can pertain to lots of other stuff including electrical. We won’t get into the other stuff. Where do hertz come from? Here go the fig­ures again. 

Look at Figure 2. Here is another, more practi­cal, drawing of an AC generator. Please note that we are rotating the AC windings and the field is stationary. This is not usually the case. However, there are generators out there that are set up like this. For demonstration purposes, this works better.

Figures 3 though 7 represent the different stages that the AC generator goes through in order to complete one full cycle of AC power.

Please note in Figure 7 that the two-pole nature of our beast must turn one complete revolution in order to accomplish this amazing feat.

Now look at Figure 8. Here we have a four-pole version of our simple little AC generator. Please note that one full revolution results in two full cycles. 
Is this revolutionary? Not really, but it sounds good. The advantages? There is only really one ad­vantage of a four-pole unit over a two-pole one. The slower that something runs, the longer the life of the parts tends to be. It just makes sense. 

You can use this reasoning as the poles increase. Also there is a disadvantage. To generate the same power, the four-pole unit must be physically larger. The same physics apply to the prime mover. Smaller must be faster and slower must be larger to achieve the same results. It’s just physics! 

Frequency is a very important component of the AC generator. Frequency is determined by the prime mover. Here in America the generator must provide 60 Hz AC current and 50 Hz in many other parts of the world. So much for hertz, at least for a little while. 

Voltage 
The next component that the AC generator must provide is voltage. What is voltage and where does it come from? The dictionary says it is electromotive force or potential difference that is usually expressed in volts. Not much help here for us dummies. What’s electromotive force…of, relating to, or producing electric current? Hmm. Okay, it’s electric current or a flow of electric charge. Now we’re getting some­where. Our little generator here generates a flow of electric charge. 

Looking at Figure 1 again we see that by wig­gling our magnet across the little coil of wire we create that flow of electric charge or voltage. This voltage can be increased or decreased by three things. These things will become important later. 

1. Increasing or decreasing the number of turns of our AC coil. 
2. Increasing or decreasing the speed of the field (or, wiggling the magnet faster or slower). 
3. Increasing or decreasing the magnetic force of the field. 

We now need to define some things. What is our desired product? It’s what we use every day: a standard product that will suit our application. We need a constant voltage at a constant frequency whether it’s here in the U.S. or anywhere. 

Beyond the basics
All that I have gone over so far are the very basics of AC power generation or the “where does it come from” part. Now for the “this is how it re­ally is” part.

In order to provide a useable AC power sup­ply, all the variables of the generator must be controlled. Those variables are frequency, volt­age and current (amps). By the way, variables are the things that change…either by accident or on purpose. Current has not been discussed much up until now but it is a very important variable. Current is the actual work, let’s call it load (that’s what us tekkies call it), we want done. We can have volts and hertz and every­thing is just swell but nothing is getting done until the load is applied. The load is the great en­emy of the generator.

Let’s look at frequency. As an example, here in the U.S. we require 60 Hz. How fast we move our field through the winding controls the fre­quency. As long as the speed remains constant we will maintain our 60 Hz, right? Okay, let’s turn on a load. All of a sudden current comes into the pic­ture. If we don’t change anything as far as the prime mover goes, the load will slow the rate that the field is passing though our winding and the frequency, as well as voltage, drops. Well, we can’t have that. Our appliances all require 60 Hz so we must maintain the rotor speed. We must control, or govern, the prime mover.

Governor -1. A person who governs, espe­cially: a. The chief executive of a state in the United States. b. An official appointed to govern a colony or territory. c. A member of a governing body. 2. The manager or administrative head of an organization, a business, or an institution. 3. A mili­tary commandant. 4. Chiefly British. Used as a 
form of polite address for a man. 5. A feedback device on a machine or an engine that is used to provide automatic con­trol, as of speed, pressure, or temperature. 

I always like to look at definitions. Some­times it’s pretty funny and sometimes they nail it. We can assume that the point of this defi­nition is that a governor is a controller. Yes, just as with politics, an AC generator without a gov­ernor would allow our system to become unstable. So a governor of some sort is required to control the speed of the prime mover, or the legislature in the case of politics. Only the prime mover’s governor actually works. 

Governor depends on prime mover 
Where is this governor and exactly what does it do? It depends on the prime mover. If it’s a gas en­gine, it controls the throttle just as you would an automobile as you are cruising down the road.

If the speed limit is 70 mph (112 kph), your right foot maintains the speed. So, I guess that, in this case, your right foot is the governor! If the prime mover is a diesel engine, the governor meters the amount of fuel being fed to the fuel injectors. If the prime mover is a steam turbine, the governor controls a valve (or valves) that meters the amount of steam pressure that hits the vanes of the turbine rotor. I guess that you can call a governor a cruise control because that’s virtually what it is. 

Now let’s look at voltage. At the beginning of this installment, I pointed out three things that can affect the voltage of our AC generator. That mate­rial now becomes very important. As with frequency, when a load is applied to an AC gen­erator the voltage will drop, even with a governor to control the frequency. So, we must be able to control the voltage level as well as govern the speed. We can’t increase or decrease the turns in our winding because the turns are a fixed con­stant. It would be a little tricky throwing a few more turns in there on the fly. We can’t increase or decrease the speed of the rotating field because we require a constant frequency. The only thing left is to increase or decrease the force of the ro­tating field. We can’t do that using a permanent magnet for a field. So, what do we do? We must use electromagnets in order to increase and de­crease the magnetic force of the field. Thus, we are able to maintain the output voltage, as the governor maintains the frequency, of our AC gen­erator. 

Another important definition is that of the electromagnet. It is a magnet consisting essen­tially of a coil of insulated wire wrapped around a soft iron core that is magnetized only when elec­tric current flows through the wire. 

Our rotor consists of from two to many mul­tiples of two electromagnets. These electro­magnets are the poles of the rotating field that I have been talking about. If you were really paying attention when you were looking at Figures 1-8, (Part One, in the September issue) you would have noted that there is a North pole and adjacent to it is a South pole. This is very necessary in or­der to generate AC current. Where does the DC required to power our rotating field come from and how does it get there? The answer is the ex­citer. 

Excite -1. To stir to activity. 2. To call forth (a reaction or emotion, for example); elicit. 3. To arouse strong feeling in. 4. Physiology. To pro­duce increased activity or response in (an organ, a tissue, or a part); stimulate. 5. Physics. a. To in­crease the energy of. b. To raise (an atom, for example) to a higher energy level. 

Power must be adjustable 
From this definition we can gather that to ex­cite is to stimulate. Well, we’re going to stimulate our AC generator by generating some DC current and applying it to our rotating field. The power that the exciter generates must be adjustable and be large enough to supply sufficient DC current to the rotating field not only while the generator is operating at normal loads but should have enough reserve capacity to deal with changing load condi­tions such as electric motors and such. How do they do that without twisting up the wires that feed DC current to the rotating field, anyway? Following are a few ways. 

The remote mounted DC exciter—Usu­ally belt driven from the same shaft that turns the rotating field. The DC that is generated is con­nected to the rotating field through a set of collector rings.

This is probably the oldest way of generating our excitation current. The prob­lem with the DC generator is maintenance. One, it’s belt driven. Belt tension and wear issues plague us to death, especially in bad environ­ments like sand and gravel quarries and in the oil field where dirt is a-plenty. Two, brushes, com­mutators, and collector rings tend to wear very quickly in these type environments. Also, brushes tend to stick in their holders when the generator is inactive for long periods of time es­pecially in dusty environments. Another type of remote mounted exciter would be the motor/gen­erator (M/G) set. This is simply a DC generator that is powered by an electric motor. About the only place you would see this type of exciter is in a power plant. See Figure 9. 

The shaft mounted DC generator—The exciter is mounted on the same shaft, and turns the same speed, as the rotating fields. The DC ar­mature rotates inside a stationary field. The exciter output is connected to the rotating field, as with the remote mounted unit, through collec­tor rings. (It is essentially the same as the remote generator as far as brushes and stuff.) There is one plus, though. The belts and sheaves have been eliminated. Another possible plus is that slower operating speeds of this DC exciter would probably extend its parts life somewhat, just as in Figure 9. By the way, folks call collector rings “slip rings,” too. That’s okay. 

The static exciter—Here the term “static” means no moving parts. This can be a solid state electronic assembly that is remote mounted (not shaft mounted). These units require power that is generated by the alternator as shown in Figure 10. The static exciter also serves double duty as a voltage regulator. I’ll get into that later. The DC current that it generates is fed to the ro­tating field through a set of collector rings and brushes; that’s another annoying maintenance is­sue. However, we do eliminate the DC exciter’s commutator and its brushes. Static exciters can be used to replace a remote mounted or a shaft mounted DC generator after all of the smoke contained therein has been used up (generator has failed). Figure 10 is a representative drawing of a static exciter. 

The shaft mounted brushless DC gen­erator—This is the most modern of these devices. What happens here is the rotor of the brushless exciter is a three-phase AC winding that turns inside a stationary DC field that is excited with current provided by a static voltage regulator (again, more about this later). The AC voltage that is generated by the above mentioned three-phase AC winding is rectified by a diode bridge rotating along with the exciter rotor/rotating field assem­bly and wired directly to the rotating field. There are no belts, sheaves, brushes, commutators, or collector rings to deal with. This works! 

Now for the voltage regulator. First of all, what does a regulator do? 
Regulator -1. One that regulates, as: a. The mechanism in a watch by which its speed is gov­erned. b. A highly accurate clock used as a standard for timing other clocks. c. A device used to maintain uniform speed in a machine; a governor. d. A device used to control the flow of gases, liquids, or electric current. 2. One, such as the member of a governmental regulatory agency, that ensures compliance with laws, regu­lations, and established rules. 

We just can’t seem to get away from that gov­ernment thing, can we? But when we get right down to it, the voltage regulator is just another form of a governor. It governs, or regulates, our AC voltage at the level that we desire. 

Okay, we have all of the parts in place in or­der to generate our AC current. As I mentioned before, our requirements for good AC power are that our frequency and our voltage must be stable.

After all, it would be a bit difficult to read this article if the lights were gyrating and flash­ing. In Figure 9, the DC generator type exciter, I drew in a rheostat to control the level of DC that is required by our generator to maintain the voltage. This is perfectly okay if the load is fairly constant and little adjustment of the volt­age is required. 

Use your imagination 
The best way that I know to explain how a voltage regulator works is to ask you to imagine that there is a very tiny little “fellow” inside that regulator box. We provide a voltage to the regula­tor and we tell the little guy to watch that voltage on a tiny little voltmeter on a tiny little panel and make sure that it stays where we say to keep it. The little guy has his tiny little hand on a tiny little rheostat on the tiny little panel. When a load is applied to the generator the voltage will drop. That’s a physical law; just accept it! The little fel­low sees the voltmeter drop below what we told him so he cranks hard on the tiny little rheostat trying to maintain the voltage. When the load is taken away the little guy backs off his tiny little rheostat keeping the voltage where he was told to. 

What does the little guy do? He is comparing the output voltage with a reference (set point) that we select and reacts accordingly. That’s what the voltage regulator does. We supply a voltage to the regulator for the purpose of sens­ing and also to provide power for the output to the exciter field. We supply a potentiometer for a reference device in order to select the level of voltage that is to be maintained. So, the regulator compares the output voltage to the reference (set point) and reacts accordingly. That’s how it works, electronic or otherwise. 

The voltage regulator performs exactly the same function as that little fellow that’s watching the voltmeter and making adjustments. If the AC volts go down, it increases the voltage to the ex­citer field. That, in turn, increases the voltage to the rotating field, which maintains our AC voltage at the desired level. If the AC volts go up, it’s vice versa. The regulator is just a lot better than he is at this because reaction time is much quicker. 

Besides that, who would want to sit and watch a voltmeter all day? Wow, that’s just too much trouble. Let’s do this automatically. 

Voltage regulators have been around for quite some time. Let’s look at some of the older units: 

1. Vibrating Contact – This used a half wave rectifier that supplied current to a solenoid coil that caused a set of contacts to vibrate together and apart. The time that the contact was touching was when the field got its power. When the contact opened it removed the power from the field. If the AC voltage dropped, the contacts would touch for longer periods of time. When the AC voltage increased, the opposite would take place. This worked pretty well except for some maintenance issues. The current, being DC, caused one contact to wear faster than the other contact. A polarity switch had to be reversed occasionally for the contacts to wear more evenly.
2. Carbon Pile – Just as the name implies, it is a pile of carbon disks that were squeezed together for less resistance and loosened for more resistance. This required a servo of sorts and springs along with a considerable amount of room. It worked well, though! 
3. Rocking Contact – This was a neat gadget. It was a servo of sorts that operated a curved device, similar to a rocker of a rocking chair, that in turn operated several contacts that were hooked up to a gang of resistors. The more contacts that were closed, the less resistance and therefore the more excitation. The less
4. contacts that were closed, vice versa. 
5. The Saturable Reactor – The exciter field is powered by rectified voltage (usually 120 volts) that is supplied by the generator. Two of the load leads are wound through the saturable reactor that in turn creates a secondary voltage that is placed across the AC input of the same rectifier. The more load, the higher the voltage to the field rectifier. If everything is set up correctly the voltage is regulated. I don’t know what anyone else calls it but I call it summing. The sum of the voltage from the reactor plus the nominal excitation voltage provided by the generator itself equals a voltage regulator. It works pretty well! 

Except for the saturable reactor, we see very few of the above mentioned regulators. What we do see a lot of, these days, is the electronic volt­age regulator. In Figures 10 and 11 you can see some examples of the application of the electronic voltage regulator. 

Okay, let’s review the stuff that makes the AC generator tick. 

1. The prime mover’s job is to turn at a constant speed requiring… 
2. A governor that controls the RPM of… 
3. The rotor shaft assembly that turns the exciter, except in the case of a static exciter, both of which supply DC current to… 
4. The rotating field that provides the moving magnetic poles that pass through the coils of wire (the AC winding) causing AC current to flow… 
5. Resulting in voltage that must be constant so a voltage regulator must be used. 

Troublshooting AC generators
Okay, after reading the first two parts of this series, you should know how an AC generator functions. If you don’t, then I give up! Let’s get into troubleshooting. First things first. As we have covered, there are several things that take place in order to generate AC current. Some very dangerous conditions exist. There are rotating things like fans and belts on engines that can remove your ex­tremities in a heartbeat, not to mention very hot parts. Also, on the other end of the gen­erator, the protective covers may need to be removed in order to check things like voltage and frequency. 

Here’s a flowchart that might help: 

There is a test setup that I recommend for troubleshooting AC generators. You need a 120-volt, 10 amp variac (variable transformer) and a diode bridge wired to the output. If the generator checks good (not grounded with the wye disconnected from ground) simply remove the exciter field wires from the regulator. Attach them to the test setup and start the prime mover. Now slowly turn up the variac increasing the DC voltage to the exciter while monitoring the AC amperage draw of the variac. If everything goes well you should be able to see the AC voltage rise to a normal level with a fairly small amount of excitation. 

The prime mover is the backbone of the AC generator. As stated before, the prime mover must maintain the frequency as well as carry the load. So, the testing of the prime mover is very straightforward. Actually simply observing the reaction of the prime mover to a change in the load is all that is needed. If the prime mover slows without recovering when a load is applied, then the governor is not reacting properly. A frequency meter can be handy. Remember, the hertz, cycles, or frequency is a direct result of the speed of the prime mover. 

The main AC winding is pretty much the same as single or three-phase motor windings. Just test these the same way that you would a motor winding. Depending on the brand of the generator there may or may not be some special windings installed along with the main winding that provide power that particular voltage regu­lator converts into DC voltage for the exciter field. These are generally seen when the excita­tion voltage is very low (less than 10 VDC). These windings need to be noted before rewind­ing the generator stator. Another thing to note here is that there are a lot of star/wye connected generators out there. It is common practice to ground the star/wye. When you test the stator for a ground be sure to disconnect the star/wye from the ground lug or you might think that the stator is bad and waste a lot of time. 

Use voltage drop method 
The best way to test the main field is to use the voltage drop method. This test works very well to determine whether or not there is smoke left in it. Simply apply 115 VAC to the two rotor leads. Voltage measurements are taken across each coil. If one or more of the coils are bad you will see lower voltage reading than the voltage readings of the good coils. The measurements should be +/- 10%. You can also use DC voltage. I’ve been known to use a constant current DC welder. I like it because the open circuit voltage is high enough to give us good resolution. Also because of the constant current nature of the beast, the voltage just droops if the load is too great. The measurements with the DC power supply should be in the +/- 5% area. You can even use an automotive battery in the field. 

The DC exciter is virtually the same as a DC motor or generator. The exciter field, whether it’s a brushless exciter or otherwise, can be tested exactly the same way that the main rotor is tested.

The EASA Technical Manual gives you some good guidelines on troubleshooting DC equipment in the field. See Section 4: DC Machines. Also, the Rec­ommended Practices section in the Technical Manual come in handy. Further, as mentioned pre­viously, the DC generator type exciter is somewhat of a maintenance problem with the brushes and the commutator. There are solid state exciters available that don’t take a rocket scientist to install. 

In order to supply DC to the main rotor, the brushless exciter requires a rotating rectifier because the exciter armature is actually a three-phase wind­ing. This rectifier is no more than a three-phase diode bridge that rotates along with the main field and the exciter armature, all integral parts of the ro­tor assembly. The rotating rectifier is really simple (Figure 12). The rectifier is made up of a positive plate and a negative plate having 3 diodes each. One plate uses 3 positive anode diodes; the other plate has 3 positive cathode diodes. I have seen cases where the user has missed the polarity difference when a bad diode was replaced. The thing just won’t put out! This is something that can usually be checked quickly, but overlooked often. I am includ­ing a chart, Table 1, to help you test the rectifier. 

Testing the exciter armature, like the main AC stator, is usually easy. They pretty much fail like three-phase motors—with one exception—that be­ing overspeed. 
Here’s something interesting to note about the brushless exciter. In almost every case the exciter will have more poles than the main field. For ex­ample, a 4-pole (1800 rpm) generator may have 6, 8, or even more poles. Driven at 1800 rpm, a 6-pole exciter will generate 90 Hz, an 8-pole 120 Hz, and so on. Manufacturers do this to get more power from a small package. Also, the higher frequency has a smoothing effect on the output. If someone rewinds the exciter, and assumes that the poles would match the main generator poles, the output voltage will be very low or worse.

The electronic voltage regulator is a very bulb on the DC output should glow brightly. If complex device. But, in most cases it is fairly not, the regulator is bad and will require repair or the AC in, DC out, and terminals for a remote voltage-adjusting rheostat. Some have separate ter­minals for the power supply of the regulator and DC out to the field and a separate AC input just for sensing (multiple sensing voltage taps available) along with the remote rheostat terminals. Some have single phase sensing and some have three phase. Fig­ure 13 is an example of how I test regulators. Be sure to check with the particular manufacturer. I haven’t found one yet that doesn’t have some sort of a testing procedure that they would share. 

Note suppression device 
Please note the suppression device that is pointed out in Figure 12. If it’s there, always check this device for a circuit. It should always check open. If this device is shorted the genset won’t gen—at all. The suppression device is there to protect the diodes from high voltage spikes replacement. If bright, begin apply­ing voltage to the sensing terminals with the variac. As you increase the voltage, at some point the bulb should begin to dim as you ap­proach the desired voltage level. At this point turn the rheostat up and down. The bulb should brighten and dim accordingly. If the bulb contin­ues to glow brightly, chances are that the electronics have malfunc­tioned and the regulator must be repaired or replaced. 

If there are no sensing voltage terminals present just do the above except for the variac part. Just make sure that you use the voltage re­quired by the regulator. As the DC output bulb glows brightly, adjust the voltage adjusting rheo­stat up and down. The DC output bulb should glow and dim accordingly. If it doesn’t, just pitch it and replace it. 

I haven’t mentioned anything about testing the other types of voltage regulators because most of them are obsolete and should be replaced with the newer, electronic versions. 

Categories: Alternators/generators

Tags: Generators

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