is there such a thing as "too fast"? :-\

[michael.i]

Ok, simple questions like this normally have horrifically long and many answers, so let er rip.

For a dual axis PM Alternator, is there a nominal speed that one should shoot for? I mean aside from centrifugal force being an issue with magnets flying off and whatnot, is there an electrical reason that a rotor can spin too fast? can the pulses be too short? or alternatively too long if too slow?

I am just curious of course. I mean say you were doing a hydro setup with a transmission, is there a speed that you want to gear for?

I have more questions, but I will stop here in case you guys or gals answer them with the above.

Thanks in advance, Michael

[rossw]

is there an electrical reason that a rotor can spin too fast?

The coils are... well, "coils". They have an inductance. As frequency increases, inductive reactance increases. As inductive reactance increases, the effective internal resistance of the source increases, meaning the total current it can deliver reduces, so you get less power out of it.

If your machine has steel or iron cores, the magnetising and demagnetising effort required is approximately constant per event - more events per second means more wasted energy, more heat. So you get less power out.

If you use conventional diodes to rectify your AC for use (ie, if it's not being used as raw AC for example, to run a heater), diodes have a finite recovery time. The higher the frequency, the less efficient they are, so you get less power out.

If you have have lengthy cables from your machine to where the power is used, you have greater inductive (and capacitive) losses in the cables at higher frequencies, so you get less power out.

Short answer? Yes, you can "go too fast"

[artv]

Hi Michael,....I like this question...
Rossw,....."As frequency increases,inductive reactance increases".....
In this case frequency means rpm right? If you increase the # of poles this also increases frequency?
So can you have too many poles?
Obviously more poles means bigger rotor, there is a limit, but max poles would mean lower rpm??

artv

[Flux]

Within the normal limits you will meet,you probably won't go too fast. For a hydro with step up transmission I doubt that you will get to speeds where there are electrical issues.

Within reason the faster you drive an alternator the smaller it is for a given oputput. Friction and windage and transmission loss have to be taken into account so I would not be looking at speeds in excess of 6000 rpm and typically more in the 3000rpm region. Bearing life and other things have to be considered.

Low speeds tend to be more reliable but the aiternator size increases. For small machines I would say go for 3000 rpm for larger size machines you may be looking at slower.

Beyond 20 000 rpm you start having to think about other issues but that sort of speed is outside anything sensible for a hydro set up.

Flux

[Flux]

Hi Michael,....I like this question...
Rossw,....."As frequency increases,inductive reactance increases".....
In this case frequency means rpm right? If you increase the # of poles this also increases frequency?
So can you have too many poles?
Obviously more poles means bigger rotor, there is a limit, but max poles would mean lower rpm??

artv

If you have a fixed frequency limit ( say 60hz) then you have a fixed number of poles for your speed options. If you are converting to dc or have a load that doesn't worry about frequency then you have a lot of choice.

Within normal limits you can have large variations without much ill effect. Normally it makes sense for large low speed machines to have many poles and a large diameter. For high speed working you tend to use long thin rotors with smaller numbers of poles but larger area. This is mainly dictated by mechanical constraints.

For the normal air gap alternators running at safe mechanical speeds you benefit from larger numbers of the same size magnets on larger diameters but you can use the magnets in groups to form smaller numbers of poles with more area. The trade off is complicated but usually mechanical limits rather than electrical ones will decide the choice. When frequencies start approaching 400 Hz you need to be careful electrically but that is not going to be an issue with wind power, it could be with hydro.

Flux