Converting 2 pole induction motor to generator, no. of poles?

[Flux]

This is getting complicated.

The figure for open circuit flux on the face of a magnet is no use here. basically all neo magnets can produce something like 1.2T into a closed circuit, depending on grade,  but that is near enough.

The flux you get in a machine depends on the relative length of the magnet and the air gap. With an  air Gap about magnet length ( typical axial ) you will have a gap flux of 0.5T. if your air gap is short compared with the rest of the magnetic circuit you will get higher, right up to 1.2T when the air gap is zero.

For a motor conversion with decent thickness magnets and a small air gap you will get something under 1T. This will only change a small amount with different grades of magnet. Typical neo N40/N42 will be fine and probably something over 3/8 thick to deal with a practical air gap. N50 will do a little better and cost a lot more.Those you are looking at should do well enough just forget the gauss on the face and lifting force, both figures are meaningless except as a direct comparison of similar size magnets of different type.

What you calculate from the emf divided by the circuit resistance is short circuit current and that is no use to you. As I said you need the voltage driving the current through that circuit resistance, the battery internal resistance is so near zero that you can forget it.

Flux

[SparWeb]

...F674 - 10mm dia x 10mm thick N42 Neodymium Magnets (Pack Of 10) (A8/Y) 
...A pack of 10 magnets

Far too few.  You will need dozens and dozens of those.
Try to imagine that when you take the rotor out of the motor, it will be (maybe) 10cm long and 10cm in diameter.  That's over 200 square centimeters of area that needs to be literally covered with those magnets, even after being machined down to size.  Sure there is a little gap between each because they fit on a curved surface and so on, but there is still lots of area to cover.  The magnets will cost you as much as that motor, if not more.

[damob]

'Basically open circuit volts less battery volts divided by effective resistance'
I have measured the resistance in each phase to be about 4.4 ohms, If i take my open circuit volts to be in or around 50 volts at 300rpm and go with say a 48 volt batterypack this gives me (50-48)/4.4 = 0.4545 amps.
Is there something i am doing very wrong that is giving me such low amps?
Also i understand that i need lots of magnets, they just sell them in packs of 10.
Thanks

[ghurd]

The general idea is there.

Increase the speed 25%.
375RPM will be 62.5V open circuit.
(62.5 - 48) / 4.4 = 3.3A

Increase the speed 50%.
450RPM will be 75V open circuit.
(75 - 48) / 4.4 = 6.1A

Part of the problem is VAWT and low RPM.

You are over thinking it. 

The 4.4 ohms is not what it will be when it is running anyway.

SparWeb said "IMHO using calculations to show that your motor, when converted, will do what you expect is a stretch."
Flux said "Predicting the output is way more complicated than you are imagining."

None of the numbers you have at this time have enough certain value to do any detailed math.

Cram it full of neos and hope for the best.
G-

[damob]

Thanks for the info!
So i just cant get my head full around this:
Power out at 300rpm, 50-48/4.4 = 0.45, multiplied by 50 volts gives 22.7 watts?
At 375 rpm 3.3 multiplied by 62.5 v gives 206.25 watt
I just cant understand how it is only 22.7 watts at 300 rpm, surely it would give out more?
Also i confused horizontal with vertiacl!! it will be used on a HAWT!
I need to have some calculations to show my supervisor before i start machining, that is why i am trying to work out some average figures, also my colleague rckons it will be difficult to get over 300 rpm

[Flux]

"Is there something i am doing very wrong that is giving me such low amps?"

Yes. You Will need an emf of well over 50v to charge a 48v battery. Your 50v at 300rpm open circuit would better suit a 24v battery.

Sadly this set up is never going to be efficient with such a high winding resistance, You can expect to loose about half your power in heat in the winding, I seriously doubt that you could get over 60% efficiency from such a small machine producing 300w at such low speed.

If you reduce the speed by 5 then for similar efficiency you need to cut the resistance by 5. If you squeeze power out at very low speed then you have to overload things or make it very much bigger.

Weight and cost goes up seriously as you reduce speed and try to keep the same output.

If you could find a 300rpm 2 hp induction motor you would find it was enormous in comparison with your 4 pole motor, but you won't find one, it wouldn't make sense when you could use a 4 pole motor and belt drive to do the job at 1/10 the cost and size.

The other sad point is that you can't do this in reverse and use a belt drive from your VAWT to drive the motor at 1500 rpm because you would loose all your power in iron and belt losses. You could possibly speed increase an axial air gap machine with no iron loss but you seem destined to go down the motor conversion route so you have to live with all the snags it throws at you.

Flux

[damob]

Thanks flux!
If i charge 24 volt batteries i get about 5.9 amps, multiplying by 5o gives about 300w, with 60% efficiency can i only expect 180 watts of power out?
Thanks

[Flux]

So you have changed your mind and it is going to be a HAWT then at least you have a lot more chance to get your 300W.

I seriously think your speed is miles out for a small 300W HAWT. You could get your 300W from a 5ft HAWT and that should comfortably run up to something over 800rpm. Cut in could be not much over your 300rpm that you have been worrying about.

Once you get the speed up then your chances of doing something increases dramatically.

I anve no idea of the intended blade design so it's not reasonable to comment on speeds but I think you are way off the mark.

Flux

[Flux]

"Thanks flux!
If i charge 24 volt batteries i get about 5.9 amps, multiplying by 5o gives about 300w, with 60% efficiency can i only expect 180 watts of power out?
Thanks"

What you get out depends only on two things. Firstly on what you put in in terms of mechanical power and secondly on the point where you burn it out.

If you want 300W into the battery at 60% efficiency there is no problem as long as your turbine rotor can provide the 500W or so input power. That motor won't burn out supplying 300W to a 24v battery.

[ghurd]

Do you have any idea how big the blades will be to do what you are talking about?
Big.
If they won't let you do machining without numbers, they certainly will not let you put up something that size.

[damob]

Thanks, i dont know off hand what size the blades are, i will look into it