Reversal of Field Connections

While it is running, or while it is stationary?

When it is in stationary?

Is this homework? Are you a service tech? Were you sick when they covered this in class?

Alternators & Three-PhaseCircuits

If you'll reverse the connections and then plug the motor/alternator back, it will start running in opposite direction.

So when u reverse the connections, it means u reverse the phase sequence as well. Yes rite.

RAMConsult, what about a brushless exciter?

I've always thought that it wouldn't make any difference since the DC, from the VR, is inducing a small field that is inducing 3 phase AC into the rotating exciter and it's converted, with 6 diodes, into DC for the main armature field. The 3 phase rectifier will always be positive on the Cathode and negative on the Anode side of the diodes going to the Armature field coils. BUT most generator exciter coil wires, to the AVR, are marked + and - so maybe it does make a difference. What do you think?

It makes no difference. In the brushless excitation that you're describing the output of the AVR goes to the field of the pilot exciter (which is stationary). The rotating part contains three windings whose output is rectified by the six diodes, the DC output of which goes directly into the main field winding of the alternator. Switching the leads of the field at the pilot exciter end has no effect since the output is still rectified by the six diodes.

There are a couple of reasons why the output of the AVR is marked +/-. First it's always good practice to to distinguish between DC and AC circuits in a mixed signal environment, +/- tells us it's DC, a letter tells us it's the high side of an AC circuit, G/N tells us the low/return side.

Secondly it can make a really big difference depending upon the exact type excitation system that you're looking at. In particular, a boost/buck system has two sources of DC, one from the manual control which provides a fixed/base level current, the other is the control/variable output from the AVR. These two are mixed/summed in one of two ways, either by the interaction of two separate field windings on the pilot exciter so the summing takes place in the magnetic field, or by adding the two voltages together in a single field. The result is the same, the AVR output is constantly adding to (boosting) or subtracting from (bucking) the manual/base excitation level. Reverse any of these leads and you have a positive feedback instead of a negative system.

Wrong! You have now stated that the thing is stationary. Nothing will happen, because the armature is not moving. When it does turn, assuming it is turning in the original direction, the phases will be reversed.

This is the same in both hemispheres, here on Earth.

Have we ever established which connections ravi142857 meant?

RAMconsult is right on the DC side the flip result in 180deg phase reversal of the DC magnetic field. Inconsequential when stopped. Interesting big time, if you attempt while running.

If you flip any two of the 3 phase legs, the rotational direction of the three phase is reversed. Some motors can take it, some equipment don't.

A pretty interesting idea, or basic homework.

Take your pick.

The simple answer is: a portal opens in space while at the same time the worm hole closes due to the decreased magnetic field and the flux goes haywire. (Not copper wire). It then prevents any free flow of our knowledge, ships and other, between the planets. Then we have no way of getting home. Reversing the connections; It causes the generated electric to be sucked back into the alternator, rather than pushing the electric along the wires. And that is, How it will be. Please dont flux it up!

All three phases will be reversed in polarity but the rotation will be the same.

Field connections?

Keeping in mind that what YOU mean by "Alternator" and what I mean by "alternator" may be two different things. I personally have never heard of a three phase alternator. If you mean "three phase generator", and IF the output is from the field, then of course, if you reverse the leads, you change the phase rotation.

I mean...how else WOULD you reverse the phase rotation?

(alternator...puts out rectified DC)

(field connections...normally stator fields but not always.)

(A B C is the reverse of A C B, and of course B A C. switch two leads, you reverse the phase rotation.)

I saw this back when I was a young tech on airplanes more years ago than I care to admit. A senior tech crossed two of the leads on the output of one of the two AC generators, causing all sorts of problems. Nothing ran backwards as far as I could tell, lots of things didn't care, but lots of other things didn't work. I found the crossed leads, and expected to be celebrated...visions of being carried across the hanger on the shoulders of my fellow techies danced in my heads, maybe an early promotion! At least a pat on the back as a "good guy" from the Warrant Officers. Then I discovered that getting the senior tech into trouble was not the path to career advancement. Subsequent events have proven this discovery to be correct...he retired as a Chief Warrant Officer, I retired as a Master Corporal. What does that tell you?

So Ravi, use a phase rotation tester, fix it, smile, get it working, and don't tell anybody how you did it.

OP was not talking about an alternator in the automotive/airplane sense.

By common convention a generator is a device that "generates" electricity without regard to whether it's output is AC or DC. An alternator is a device whose output "alternates" (changes polarity) on a cyclical basis (Alternating Current). All alternators are generators, but not all generators are alternators.

The usage of the term has been distorted by the automotive industry, what is commonly referred to as an alternator in fact is a three phase AC generator/alternator whose output is rectified (that's what the six rectifiers are for, two per phase) before it is ever used in the vehicle's charging system, which of course is DC.

In most modern generators the field winding is found on the rotor and the output is taken from the windings on the stator, although there are special purpose devices where the opposite is true, but most people rarely encounter them.

According to the Dictionary link I recently posted, your post is only partially correct....I also understood the same as the Dictionary....

An Alternator is not a generator, ever. Even one that has built in rectification, see any (correct) car sales pamphlet or technical description.....

But many do not understand that, especially here on CR4.

I looked here:-

What_does_the_generator_do_in_a_car

and found this:-

The generator makes power for the car so the battery doesn't drain flat in a few miles/kilometers. Everything a car does takes power, and batteries alone just can't cut it on long trips. However, generators have not been used in today's American automobiles for nearly 50 years. The alternator took its place in most cases in 1963. It does the exact same job.

Andy, by your definition Wikipedia is wrong, as is the entire library of EE textbooks.

The reason automotive "alternators" are called that has to do with history not definitions. When the automotive industry started to switch from DC generators to AC alternators their first step was to use a three phase AC alternator whose output went directly to a six diode, three phase, full wave rectifier that was mounted alongside of the alternator. As time went by the rectifier module was built in to the alternator housing so that there was just a big red positive output terminal called "BAT".

Car pamphlets are written for sales results, not engineering precision. Anyone who worked on a generator equipped car remembers that it was twice as big and put out half the current of an alternator, something the cost accountants in Detroit were quick to grasp once semiconductor rectifiers were miniaturized and ruggedized for automotive use. Here's a conversion with some pictures of the difference between them. There also many articles on how to remove/bypass the rectifier bridge and use the three phase portion to generate three phase AC power or as a three phase synchronous/induction motor.

Your link is excellent, but it supports exactly what I said, Alternators produce AC. The very first sentence is:-

An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.

By the way, I did not say any weblink was wrong, if you read my post carefully, but defining an alternator as a generator that produces AC is a bit long winded.

If you look at this weblink:-

Alternator

It shows that three different renowned dictionaries define an Alternator as:-

An electric generator that produces alternating current.

But how else would you describe it if the word alternator is wrong as you appear to say...?

The word Alternator for me describes the machine exactly, in one word only, you appear to want more....

Or correct your post to reflect exactly what you are trying to say, but as I read it, you probably misread my previous post and we are both saying the same thing.....

My statement would be that an alternator produces AC, even if (as in the case of cars) it is nowadays rectified in the same case, its still an alternator......knowing only how a DC generator works would not help someone trying to repair/understand an alternator, now would it?.....

Game, set and match to me I feel.....but please feel free to change my thinking if you can!

Automotive alternators produce DC

People using alternators to generate DC

General usage of "alternator" in day to day use to describe a DC output.

More general usage

charging systems

So you see Andy, most of the world seems to think an alternator creates DC in a car for charging batteries. I think if you asked a dozen people at random, if they even knew that an alternator created electricity, they would assume you were talking about an automotive alternator. And I don't think any of them care how its made, they only care about the output. If it stops charging the battery, you just change out the alternator. Why do you think it is not important to be clear on this. Do you suppose that just because it is long winded to call it an AC generator (or whatever) that is somehow wrong to say so? (game)

You don't seem to me to be the kind of guy to walk down the street saying "they are all wrong, only I am right because I have the dictionary right here". Well, I have 5 links above to different dictionaries... (set)

I have found that electrical equipment is almost scary in the number of different names they have for nearly the same thing. A contactor, for instance, is a relay, which of course is a sort of switch. We have a situation where we are all right because we all have an authority on our side.

game....set.... match (I like this game)

(well, you asked me to try to change your mind...grin! Hard to do considering that in actuality you really ARE right...but then...so am I.) (and yes, oh you pedantic anonoymous poster you....this has NOTHING to do with the OP's question. So what!)

I at 21 was a Petty Officer on a RN county class destroyer, in charge of what was then 20 odd million pounds of electrical supply and distribution (probably close to 200 Million pounds nowadays at a guess!).

I was trained fully and correctly by the RN, as it was my job, we were trained to use the correct terminology at all times, not to MISuse the names.

Using the correct terminology at all times in the RN was a requirement and simply reduces dramatically the possibility of misunderstandings, which is what we have here - misunderstandings and wrong terminology.

But if that's what floats your boat carry on with any name you want for anything, but you will not get me doing it, ever. I find it to be sloppy, but that is only my opinion, each to his own!!!

You are probably right that many people make this simple mistake, still not my problem......I do believe it is a "modern" though!

So you feel that calling an alternator an AC generator is using wrong terminology. Sloppy.

Interesting.

The automotive industry is wrong then. Shame on them.

I guess I don't use the RN dictionary.... Who does? Well, the RN, I suppose, and the Wikipedia people for sure. Certainly not the automotive industry. Was the SAE responsible for this?

aaaaaannnnnnd.... volley.....

How do you use those really cool emoticons! Oh, here we go...

Nah...we will have to agree to disagree. You can call that thingy under your hood any thing you want. I think Tommy is a good name. I honestly don't know how to get AC out of it though.... Or how to reverse field connections (ANY) field connections.

I do know that if the idiot light burns out you can't excite the poor thing, and it turns but puts out nothing. Sort of like me vollying these posts.....

You want to slow down your reading of my posts, you misread them again, I was not quoting RN dictionaries as you appear to imagine , I was quoting technical dictionaries (with a weblink) from the web, accessible for all here.......

Maybe if you took the time to read and fully understand carefully, we would not have any need to be writing quite so much. (Though it is fun!!)

If you cannot find it yourself, look here:-

http://www.thefreedictionary.com/alternator

It was easy linked in post #25 for reading......

By the way, if the RN had a dictionary of such technical terms (not in my day,but it is in the technical manuals), it would fully agree with those other dictionaries I quoted, so on that point, you would have been correct!!! But as they don't, you aren't!

Thats okay Andy, I'll still call that item under my hood an alternator, even though it puts out DC.
I didn't invent the thing, so I have no claim on its name. Not like its MY kid....

We all agree that the terminology the original OP used was fine though. And that he was referring to a rotating device which produced alternating current. The rest is terminology, and great fun, in a sort of flaming interwebbish sort of way.

I fear we have bored the rest of the team though. They have likely unsubscribed from this thread in droves. Or would it be throngs?

And here we go again...

If they don't stick around, they miss all the fun!!!

I am a stickler for terminology, when I know it, as it helps in preventing misunderstandings, we see enough of those here....but I know that many here tend to be very lax, their choice.

Have a great day.....

Hi Yusef,

I think you misunderstood the question in several ways........

Alternators are by the nature of the name "alternating" current producers.

Generator as a name needs some sort of qualification, but is generally accepted as a DC producer.

Dynamos are generally DC as well (see the dictionary I mentioned before), but if a push bike version, can also produce AC, but is really wrongly named if not a DC output......

Changing the polarity of the rotor of such an alternator does not change anything worth mentioning by the way.......as I previously posted. Purely a theoretical situation, not a practical one...

I have always called an AC generator an AC generator, and a DC generator a DC generator. Was this a problem? This is standard practice on aircraft.

I dunno, he said he wanted to switch around the "field windings". Does this mean the rotor field or the stator field? Depending on the configuration, a brushless AC generator would need transformer action to get power to the rotor. Diodes in the rotor will convert that into the DC which all generators use. (thats one way...grin!) The non-brushless (bush type?) type uses a commutator to do the same job.
Either way, we are agreed that switching rotor leads around will have no effect.

That he was talking about phase rotation at all implies (to me) that he is looking at the output of an AC generator, and for some reason calling it an alternator. I don't have a problem with that. How do you measure phase rotation on an automotive alternator? So it must be an AC generator.

I stand by my answer....English is not the OP's first language, and I like to be very clear about terms I use.

The OP was very unclear in his question, that I have to agree with you completely.

If you look at post #5 from RAMConsult, it is clearly stated which connections are to be rearranged:-

OP is talking about the DC connection to an alternator field, not the AC "field connections" that are made to the stator in the terminal box.

"...Generator as a name needs some sort of qualification, but is generally accepted as a DC producer..."

I guess these lecture notes from an MIT course on AC machines are wrong, "...The synchronous machine is used, essentially interchangeably, as a motor and as a generator..., and Siemens is wrong as well, "...causes the motor to act as an alternator(A.C. generator)...", along with GE making the same mistake.

Semantics is a difficult enough subject amongst peers, and it gets even tougher when it starts crossing regional/cultural/language/educational/product boundaries. Just stick with this and you won't be wrong, "All alternators are generators, but not all generators are alternators". Say it five times fast and then write it down with all the possible combinations of AC/DC/electric power substituted appropriately, and you'll agree that it's syllogistically correct.

Its a shame for you that technical dictionaries do not follow these (unquoted, so who knows where and when said what) errors you name, because if they did, you would have then been right!!!

In the future, please demonstrate your theories by at least quoting researchable links, not just words off the top of your head with no way to prove them right or wrong. I did place the links for 3 dictionaries quoted online, if you had bothered to look!!!

If you don't I personally simply don't believe what I read......many of us are like that!!!

Actually, this is only a theoretical question and answer as yes, when the connections are reversed, the rotor poles change polarity, but when running, how will you notice that in a practical way?

Nobody will be checking the value of voltage induced in the alternator field windings, taken with regard to the position of the rotor.....but in actual fact, it will be different by 180 degrees!

So from a real practical point of view, no change will be noticeable at all.

Only reversing two field alternator connections or reversing the prime mover will change field rotation.

For those here who do not understand what the true meaning of the word alternator is, please refer to this page:-

http://www.thefreedictionary.com/alternator

It defines it as being a generator of "alternating" current, correctly as in the question above.

In the same dictionary, the meaning of the word generator, when that generator generates AC is further defined as an alternator (on a different page of course!).

I trust that this clears up any misconceptions that some people may have between generators, dynamos and alternators.....

What we have here, is failure to communicate! (From Cool Hand Luke)