# Generator output frequency and rpm

1. May 9, 2015

Hi, as I was reading on generators I noticed that apart from the homopolar machine(faraday disc) all other generators are basically AC , or if they are DC then the very current generating loop or part involves changing and time varying currents.

They usually have poles on the rotor or stator depending on construction and that leads to a certain frequency once they rotate as the frequency is tied to how fast the poles move by the coils , but I wonder is there a generator that has an output frequency that is independant from it's rotational rpm?

the only thing that would come close to what I desbribed I found online in youtube and is explained by a guy in a video.

2. May 11, 2015

bump

3. May 11, 2015

### Staff: Mentor

Hi. I think the lack of answer means people don't think there is such a thing.

4. May 12, 2015

by " such a thing" you mean that there aren't any frequency from rpm independant generators ?
well that was what I was thinking too, except maybe for the generator shown in the video.

5. May 12, 2015

### sophiecentaur

Does it actually work? The video only shows an animated diagram.

6. May 12, 2015

### Averagesupernova

I would say it has several potential problems. The resonant circuit as part of the transformer cannot be expected to maintain the correct frequency under all load conditions. Interesting concept though. Something to build from.

7. May 13, 2015

well in theory and i believe both in reality this things works as its basically the same faraday disc just with a changing magnetic field instead of a static, which gives the ability to use step up transformer on the output so to improve the single disc or few wire turn generator's low voltage.

so by theory the rpm and the output frequency of this device can be made truly independant , since you can create a separate excitement circuit for the AC exciter field of the disc and the output from the disc would follow accordingly. The drawback probably being the need for slip rings or brushes.since a homopolar type macine has to have one part of the loop rotating.

So is this the only example of a frequency from rpm independant generator or are there some others which I'm not aware , please tell me ?

8. May 13, 2015

### sophiecentaur

Until I see one actually working, I would say that is the main "problem" with the idea. Surely it can't be too difficult to knock one together and make a video of it (if the idea works). If one wants to be taken seriously, an animation is a pretty lame way of presenting an idea. Shades of Tesla's constant output of ideas which never worked.

9. May 13, 2015

As I am thinking about this I am reminded of an Induction Disk KWHM? -- The disk moves at a variable speed with a fixed input (60 Hz) source? - Actually a related but different assembly to the Faraday disk. So instead of a external current loop - there is a 2nd magnetic element - their interplay generates (the proper) torque on the induction disk. Actually these have to be manually set up - otherwise the will creep with only the V or I sources applied - or if these V and I are 90 out of phase ( no power in the primary circuit) -- also note there is a such thing as a KWVM -- to measure reactive power - which registers only the V and I element that are out of phase. -- if the primary circuit has 1.0 PF this type of meter is NOT moving.

Note in this case you have 2 coils - V and I -- I have no idea what happens if you, for example, apply the line voltage with no current and the spin the disk ( apply mech energy) as with a generator" --

10. May 13, 2015

But when you approach an idea you don't think of the last names of who made it but you use your scientific and educational background to see if it works or if it doesn't.
Now I'm no Phd in electrical engineering but then again I don't see any reason why this shouldn't work, we know a faraday disc works, the only difference here is that the magnetic field is changing.

are you saying tha this device looks similar to that of a electric wattmetter in it's workings?
well to me it's perfectly clear its a generator because you have the rotating loop then you have a magnetic field coil that produces the eciting field for the rotating loop , it's all there.the supply current for the magnetic field coil can be made to be either separate or taken from the output directly , well separate would be bether in terms of frequency stability as the output will tend to sag under load.

Last edited: May 13, 2015
11. May 13, 2015

### Averagesupernova

I will post it again. Salvador, do you understand what the above means? I am not saying it will not generate an output. I am saying that the loading will change the frequency. Yes, a faraday disk works. Yes, a faraday disk would output AC if exited in the proper manner. The catch in this setup is that the output depends on the condition of the tank circuit and the tank circuit depends on the amount of power taken from the output. No matter how bad you want to believe something reality always wins.

12. May 13, 2015

well yes I understand this part , as I said in my previous post the output would sag so the field coil current should be separated from the output.
When I said that this has to work I was thinking about the very idea not the details.

13. May 13, 2015

### sophiecentaur

It is far better to be shown something working or, in some cases, a reputable reference can be quoted. With neither of those, I am usually a bit skeptical (based on a limited background in that particular direction, I admit). I ask myself why it hasn't been tried before. It is highly unlikely that it hasn't been tried in the past century of electrical experiments so has anyone written up a successful experiment?
That could settle the matter.

14. May 13, 2015

well i'm no "guru" either, but from searching google , searching my own mind for what i have came across in my ife and seen or read about , I haven't heard that someone would have made or is producing a frequency independant generator that is similar to this.
Best I have read about are some faraday type generators in a vacuum used for energy storage but they are made to output DC, so nothing new there except the storage vessel and so.

by the way I have seen you sophie saying multiple times over the years on this forum that basically everything has been made already and if not then atleast thought about and usually had a flaw or drawback so we dont have it.
And I tend to agree on you about this, as even all our moder " chip devices" are basically just super scaled down circuits invented decades ago, yet still I kinda feel there is room for improvement , or maybe by putting together what we already have we can make someting with properties that we didnt have before.

In this case I think a generator that could have a independant frequency would come handy in many applications especially where the shaft rpm is low , you could use higher frequency to require less metal for the generator make it lighter and taking up less space?

15. May 13, 2015

### sophiecentaur

I am guilty as charged. But my skepticism is so often proved right. There have been many brilliant inventions over the years but they have nearly been arrived at when something about technology changes. There is nothing about this 'thought invention' that couldn't have been done a hundred years ago and all those monkeys and typewriters would surely have produced a winning combination by now, if it were feasible.
I am not 'down' on new inventions - when they actually work. (e.g. E-ink and quantum computing.) Thought experiments, based on anything but a deep and well founded knowledge often have serious flaws. Then there are the blatant scam-type inventions that promise something for nothing.
In this case, I am totally prepared to be gobsmacked when I see such a device in action. I have a bit of a problem with the basic operation principle. The transformer coupling makes me wonder about the impedance seen by the supply and how the power (electrical and mechanical) is likely to flow in the system.
You are right about it being a very handy form of variable frequency source. It's a form of amplifier with a mechanical power source - pretty desirable.

16. May 13, 2015

### Averagesupernova

This is also how I saw it.
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As for new inventions, often time something that appears totally new has been conceived of in the past at least in a limited form. However, there may have been one key technology that was missing so the whole idea had to hit the scrap pile and was forgotten about. Then years later that key technology is made feasible and the whole idea is conceived by someone who thinks it is something totally new.
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The issue with this generator is what usefulness does it have? Faraday disks are typically low voltage high current devices. Of course with a transformer we can step up the voltage since we are now talking about AC but all of the power coming out of this generator has to come from a pair of brushes. I wonder what would happen if we tuned the tank to 1 Mhz and used a crude AM detector on the output of the disk? Could we make a crude AM receiver that is powered ONLY by mechanical energy? Even if we required multiple disks? Like I said in an earlier post, something to build on. I seldom throw any idea completely away. I always try to imagine an idea could be good for something some day.

17. May 15, 2015

well I believe it's usefulness is in the fact that it has variable frequency.Many mechanical loads like those of steam turbines, water turbines , wind mills etc have different rotational speeds , different by many thousands of rpm.If you use a typical generator you have to change its rotor pole count and geometry to make ir even closely useful for that particular application.Also to match the grid 50 or 60 hz the generator has to either have fixed rpm atwhatever rpm it is designed or it has to have some electronics that hel fixing the otherwise variable output frequency from a variable input rpm, as in wind farms because wind changes its strength all the time.

So here you can input any rpm you like or any the device can whitstand and by adjusting the AC field frequecy you get output voltage control, and since large mechanical loads like wind mill blades or turbine shafts cannot change their rpm fast due to inertia, the frequency adjusting would cope well in maintaining the output at desirded fixed level, ofcourse then again we face two other problems, first variable frequency if fed into and AC grid would need something that converts it to the fixed 50/60hz and that would be either a mechanical set or power semiconductors.the other problem with this is brushes ofcourse.

so there is a question I have tried elsewhere but with no luck, could a rotary transformer be used instead of brushes , since the output here is AC, my own guess is not because since it still operates in homopolar mode a rotary transformer doesnt provide the needed " one part rotates while the other one is stationary of the same loop" as in homopolar machines, since current is produced by lorentz drag.But since we have AC field that menas induction also comes into the picture , well I don't know , please tell me what do you think,

you folks have bee quite an interesting reading material so far :)

18. May 15, 2015

### sophiecentaur

We should try not to get too steamed up about this idea. As far as we know, it only exists in someone's head and has not been implemented. We should wait until someone comes up with some reliable evidence about it.

19. May 16, 2015

### Averagesupernova

I have no doubt that a periodically reversing magnetic field would result in a periodically reversal of polarity on the output of a faraday disk. Several questions arise after this. How fast? How high of a frequency can we use and still get an output? Could it be used as a signal amplifier as I implied in an earlier post? Probably pretty noisy due to the brushes unless liquid metal is used and I am not even sure about that. I don't ever see a taking power from a homopolar disk without brushes. You could arrange two disks side by side in a gear configuration and take power from the axle of each disk with the advantage here being a slower surface speed at each brush. There is still a potential for loss at the point that the two disks contact each other. The axles could have liquid metal brushes without as much splatter as a liquid metal brush would cause around the perimeter. Best thing to do is start building and experiment. Wish I had more time to tinker.

20. May 16, 2015

### sophiecentaur

If you want to show that this device can produce an output with varying frequency then, fine, But can / has you / anyone?
Given that it will produce an output. If the efficiency of power transfer is not at least 50% then there is hardly any advantage in using this system over using an alternator to power a high efficiency electronic amplifier.

21. May 16, 2015

### jim hardy

22. May 17, 2015

oh nice to see some more discussion,

jim, isn't the double fed wound rotor generator just a fancy way of calling an induction generator that has windings on it's rotor which being brushless gets its curent from an ac field coil which then is rectified in the rotor to supply DC for whatever the pole count on that rotor ?

23. May 17, 2015

### jim hardy

Close. That'd be a synchronous machine.

It's simply a wound rotor machine. They can be brushed or brushless.

If you short circuit the rotor windings it's an induction machine.
If you apply DC to the rotor windings it's a synchronous machine.
If you apply AC to the rotor windings , "double fed", well, now its field flux doesn't rotate at same rate as rotor is turning.. So you can shift the speed-torque curve off synchronous speed. That's handy for windmills.

Been decades now since i studied them..
Reason i mentioned them is i think eddy currents in the rotor will make that homopolar machine impractically inefficient.. Laminated rotor in wound rotor machine would be more tolerant i do believe.

Try a search on "homopolar machine efficiency". From some reading i did last night , In theory they could approach 100% except there's a tradeoff between speed, intensify of magnetic field, and eddy currents resulting from those two parameters. When field strength X speed becomes large enough to dwarf friction, which helps efficiency, eddy current loss becomes appreciable which hurts it.
I only read one article - see what you come up with?

food for thought ?

old jim

24. May 19, 2015

well , but if you would design the homopolar machine with litz wire type rotor or very thin laminated copper sheets pressed into a disc or something like that then it would solve your high frequency eddy current problem just as just have laminated soft steel in ordinary transformers and other parts to minimize the losses, otherwise they would be huge on any machine type.

But I would love to hear about the rotary transformer idea I posted a few posts back considering the brush problem for the homopolar devices , could you take on that jim ?

25. May 24, 2015

### Jeff Rosenbury

I had the understanding that induction motors would generate power if driven faster than their synchronous speed?

Since the synchronous speed is highly variable depending on the winding array and the output ties to the grid frequency, wouldn't this be a variable frequency both in and out? Simply vary the output (grid) frequency and select a slow synchronous speed. (Remember the windings can be set with multiple taps to give a wide range of motor frequency options.)