Permanent Magnet Generator - Fixed Load?

What does this mean "keeping it real"?

My explanation was a little basic, but then I was not interested in a generator theory lecture, but rather, the little ohms law problem (that I felt) the OP faced. I didn't think I was factually in error, because, well, I didn't (and still don't) know that there were any permanent magnet generators that did not need to modify their AC output to get a DC output. This would normally be done by commutation or full wave rectification of course.

Thanks for setting me straight....I see that some linear motors are permanent magnet based, as are some rather exciting linear generators. All of which are extremely rare. I am surprised you would have run into them in a rewind shop. Generators which have alternating current in their rotors can produce DC when the speed is correct, but they are not permanent magnet rotor'd generators. So, um...I'll stand by my comment. Even a wave actuated variable stroke pm generator puts out the opposite polarity when the magnet comes back down the pipe.

Thanks for the links. None were for rotating electrical equipment which put out DC, all were for rotating electrical equipment which modified their output from AC to DC. Perhaps this link or this link may be useful as well. There is no doubt a great difference between a generator, an alternator and an AC generator, but they all require a magnetic field to cross a coil of wire....the movement by its nature creates AC.

I had to remove one of your "Off Topics", you are fully on topic to my mind.

JRaef is correct in saying that virtually all treadmills (etc) that supply resistance by loading a generator are PMDC. The DC generators in cars used until the 1960's have always been considered DC devices. These devices, when used in generating mode, are named for their output, (what you get at the terminals on the outside of the generator) and that output is slightly pulsed DC, not AC.

Engine driven DC welders have used DC generators for many decades. DC generator is a term that has a very long use, and most generally means a generator that has a commutator and brushes (and may have permanent magnets or wound separately excited field.) Commutators are associated with DC motors and generators.

So given that our OP is new to the function of motors and generators, I suspect that calling all generators AC might confuse him. For many decades motors and generators with commutators have been called DC. It probably has to do with the fact that the commutator is an integral part of the machine, whereas rectifiers can be, and ofter are outside the machine. Thus car alternators are still called alternators, even though the rectification has been moved to inside the unit, and what shows up at the terminals is DC.

In permanent magnet motors, the distinction is pretty clear: these work equally well as generators, and when spun in one direction produce DC in one polarity and when spun in the other produce DC in the opposite direction. Many people would find it hard to call such devices AC, I think.

If the OP wants to mess around any more with generation systems, he would do well to learn some basic electricity (which he seems to be doing by asking questions here). I applaud his quest....it is a complex field, full of difficult concepts. The habit of electrical practitioners to play fast and loose with terminology creates a barrier to understanding, and often scares outsiders away. Perhaps that is the point! Electricians who call a relay a contactor, and engine a "prime mover", and a signal wire a cable merely erect barriers.

My answer was intended to guide his further studies. I don't think it confused the issue at all. Did you really think it confused him?

Did you really think it confused him?

We'd have to ask him. But I certainly think it is potentially confusing to say that all generators are AC generators, given that the term "DC generator" has been used by well-educated electrical engineers for many decades. He came here correctly understanding that there are AC and DC generators. There are AC motors and DC motors; there are AC generators and DC generators. In the permanent magnet types the OP was writing about, motors and generators are exactly the same machines, and every PM motor acts as a generator simultaneously, as it is acting as a motor. (Thus, the self limiting speed/voltage relationship.)

Many people apparently do not understand the differences between AC and DC motors and between AC and DC generators. But simply calling all generators AC does not explain or elucidate the differences, it obfuscates. I'd say it is just plain wrong, because highly regarded companies have whole divisions devoted to designing and manufacturing DC generators.

Here is a paragraph from "All About Circuits" edited to eliminate the use of the term DC generator. (See: http://www.allaboutcircuits.com/vol_2/chpt_1/1.html I think that you will see that it becomes non-sensical:

1. While DC AC generators work on the same general principle of electromagnetic induction, their construction is not as simple as their AC counterparts. With a DC AC generator, the coil of wire is mounted in the shaft where the magnet is on the AC alternator, and electrical connections are made to this spinning coil via stationary carbon "brushes" contacting copper strips on the rotating shaft. All this is necessary to switch the coil's changing output polarity to the external circuit so the external circuit sees a constant polarity: Figure below

Perhaps it is only a US EE thing, but here, DC generator has a meaning different than AC generator. Here, a generator is not simply a wire moving in a magnetic field, it is what we call an electric machine, and includes, in the case of a DC generator, a stationary field coil or permanent magnet field, and a rotor with windings connected to commutator bars. That machine is much different that an AC machine. JRaef's understanding (that DC generators are different than AC generators) is typical usage for US EE's.

While it is true that the power supply in a typically PC has AC flowing in some of its parts, it can correctly be called a DC power supply, because, like a generator, it is named for its output.

I do not find the Wikipedia description of prime mover to be incorrect. ("In engineering, a prime mover is an engine that converts fuel to useful work.^[1] In locomotives, the prime mover is thus the source of power for its propulsion.^[2] The term is generally used when discussing any locomotive powered by an internal combustion engine. The term is also applied to engine-generator sets, where the engine is termed the prime mover, as distinct from the generator.") I have certainly called an engine a prime mover (in both locomotives and in hydraulic machines in which the engine is the prime mover for the hydraulic system). Why do you believe that an engine cannot be called a prime mover? What barrier is erected by this usage?

All permanent magnet generators put out AC. The AC has to be modified to create DC.

If it is done inside the machine...they call it a DC generator. If it is done outside the machine they call it an alternator.

Your own wiki links say this.

Why do you dispute this? The only exceptions are linear generators, which are actually nothing more than linear motors being placed into a braking mode. Exciting technology for maglev vehicles, but beyond the scope of this forum thread.

I asked the OP if this was confusing him. He did not answer me. I suspect that English is not his first language, in which case we have to tread VERY carefully with regards to terminology. I also suspect that he has moved on, hopefully with a better understanding of how to apply this ancient technology. I truly and sincerly hope it was not because I confused him.

You seem to seizing on a part of my explanation which was not even the main point of the post. And spending a large number of minutes in explanations which are mostly beside the point, but admittedly hard to find fault with. What was my point about the term "prime mover? Well, did you not read my post before replying? In that off topic post, I objected to the plethora of terms which meant the same thing, which result in obfuscation. Cables versus wires...contactors versus relays, that sort of thing. I am trying to remember the last time I bought a car and asked the salesman "so dude, what is the wattage in that prime mover?" Um....nope... come to think of it, I never have.. and I think the salesman would have been right to have the blank look on his face that our OP has now!

If I have confused the OP, then I truly regret it. You..I cannot confuse, because you clearly know it all.

What was my point about the term "prime mover? Well, did you not read my post before replying? In that off topic post, I objected to the plethora of terms which meant the same thing, which result in obfuscation.

Obviously I read your post, because your post is the first in which the term "prime mover" is mentioned in this thread. Your first post does not make the point that you are trying to make now, and can easily be interpreted to make the opposite point (which is how I interpreted it): namely that more precise language, not less precise, should be used. For electricians, a relay and contactor are not the same thing, and if you use the terms interchangeably, you are using the terms in an incorrect, confusing way. Prime mover does not mean "engine", but a prime mover can be an engine -- you should not use the terms interchangeably.

All permanent magnet generators put out AC.

That is not merely confusing; it is just plain wrong. Why on earth do you think the term "DC generator" is widely and commonly used? DC generator does not "put out" AC. Said in an equivalent way, A DC generator does not have an AC output. It is the output, (not the internal circulation) that defines a generator. Even the internal circulation is not AC in any significant sense, because alternating current suggests the there is a complete circuit, and there is not a complete circuit for the coils or bars not connected to the load (via the commutator and brushes) at any instant.

An automotive "alternator" is still called an alternator (even when the electronics that rectify and regulate are built in) because as a machine it produces AC, which is subsequently rectified to DC. The term enables a meaningful distinction to be made between that type of machine and a the "generators" (commutator equipped, stationary field) that were used until the 1960's. They have different output curves, different mechanisms, different support circuit requirements.

You..I cannot confuse, because you clearly know it all.

Correct.

Clearly we are on different planets.

I will simply say...

What-ever

And withdraw from this thread.

Good afternoon sir.