home made alternator ?

[bloadie]

Hi, I'm Interested in making a home made alternator run of a stationary engine

I fell comfortable with Otherpower 10 Foot Turbine plans and would like to model the alternator in that way

http://otherpower.com/turbineplans.html

  If I wanted to optimize this alternator design for 400-600 RPM, I'm assuming I would need to widen the Air gap? Does anyone have an estimate as to how much the gap should be widened  or if that is correct?

Stationary Engine Power 6.HP @ 600 RPM

12. Volt output
    
9. Coils, 2 in hand #14 gage wire 36 turns
   
1. " x 2" x 1/2" N magnets on 12" inch diameter rotors.
   
   

Thanks,

Matt

[Flux]

For 12v you are going to struggle with wire size.

To run at that speed I suggest you use about 10 turns per coil and you will need an awful lot of strands in parallel.

You may be forced to split the winding into 3 star circuits with about 30 turns ( your proposed 36 may be ok). Use 3 separate 3 phase rectifiers in parallel.

You should not need to alter the air gap, but it would give you some additional control if you can't shift the engine speed enough.

You certainly don't want to widen the air gap and use the existing winding, there is no justification for working at very low efficiency with an engine driven generator, the heat will be far worse than the intermittent rating of wind power.

Flux

[DanB]

I've given some thought to a similar project for my 'listeroid'.  Trouble is that with a PMA, the greater the load the more the voltage will drop and it's tricky to regulate the thing.  If it's just for battery charging, I question if there's advantage in efficiency to build a direct drive machine like this, vs simply running a smaller more conventional generator at higher rpm with a belt.

But direct drive would be fun and it does have some advantages.

If I do it with that engine (6hp, 600 rpm) I'll build a much larger alternator.  In order to match the power of the engine it would need to stand up to continuous output of around 3 - 4 KW.  With 12 poles, you could have 60hz and if you design it right it could do a nice job of both battery charging and producing 120VAC for normal loads.  It might also be useful for light welding!

At any rate - I think you need to consider a larger alternator than the one we build for 10' wind turbines if you really want to put that engine to full use and be reasonably efficient at full output.

[DanB]

Also - if you are wanting to charge 12V batteries, perhaps you could do like Jerry does and rectify each coil individually - this would allow the use of thinner wire.  You might also be able to have the option of wiring the coils in series to get 120VAC.

We made something like this a couple years ago for a smaller 2hp 1200 rpm diesel and it's worked out really well.  That stator had 4 coils, each individually rectified for charging at 12V, and then two coils in series which gave us 120VAC.  The frequency was wrong (120hz) so lots of loads wouldnt like it - but lights, and most power tools don't care.

I've yet to make a page about that whole project but if you look through my posting from the summer before last you should find it.  Its scattered through several months of postings.

[Flux]

Whatever you do if you want to use the full 6hp you will have a dreadful time rectifying it at 12v and I think it will take half a telephone exchange battery to absorb that current for long.

Rectifying each coil as jerry does would let you use those common 35A rectifier bridges.

I don't see much point in using permanent magnets for such jobs, especially for 120v where you need voltage regulation.

If you want to add this thing to take part of the engine power for battery charging and  perhaps have the option of another alternator for 120v loads then it would make more sense.

The 10ft alternator wound for higher speed would easily do 2kW and that is about as much as I would want to rectify for a 12 v battery. Far more efficient than a single phase battery charger from a 120v single phase alternator or a bank of car alternators, but not a cheap option and one where you have to be fairly exact with the number of turns.

Flux

[bloadie]

Thank you very much for the info. I have my heart set on building one of these machines, but I am not sure I have the skill level to design one from "scratch." So, I think I will plan on building a battery charger to run in tandem with a "store bought" AC generator head which is run off a pulley. If I still kept to something close to the 10' plans and used a fractional pulley to bring the RPM's down -- what would be the optimal rpm be; meaning most power sustainable for long periods without overheating? From what I have read it sounds like 300 RPM would give about 700 watts. Not sure if that is right though. I wish I lived in a place where I could put up a windmill.

Thanks Again.

Matt

[DanB]

Hi Flux -

"I don't see much point in using permanent magnets for such jobs, especially for 120v where you need voltage regulation."

The advantage I see (one reason I'll do this myself - the other reason... it'd be fun) is that it'll get rid of the belt.  On cold days the belt makes it a good bit harder to startup with a crank.  The belt also adds a good bit of cost and makes the machine larger.  I don't think voltage regulation is that big a deal so long as the resistance in the alternator is low enough to remain at a reasonable voltage under full load (the biggest load the engine can handle).  So long as we're happy with say... 125V at no load at 115V under full load then I think regulation is no problem.  It would have to be a big alternator though.  It's not easy (or cheap I expect) to find a 600 rpm direct drive solution for one of these machines - I think the cost of building an alternator from scratch would probably be reasonable.

"The 10ft alternator wound for higher speed would easily do 2kW and that is about as much as I would want to rectify for a 12 v battery. "

Yes, 6hp is a lot to clobber 12Volts with - the current would be significant.  Most battery banks aren't up to that.

"Far more efficient than a single phase battery charger from a 120v single phase alternator or a bank of car alternators, but not a cheap option and one where you have to be fairly exact with the number of turns."

It shouldn't be that hard to get the number of turns/coil just right, and if one started on the side of 'overbuilding' it would give plenty of room to adjust the airgap.  There is a bit of room to adjust he rpm of the engine too.

I think it'd be a great project...  I've been wanting to do this ever since I got my listeroid.  I'm really happy with how the little one we built worked out - it's been very reliable, it's compact, I think it's pretty efficient - and it was a fun project.

[Nando]

If you are in USA for 60 Hz.

The cheapest way would be a 1:3 multiplier coupled to a generator or a Induction motor as generator ( 4 Pole unit) you will not need a controller for voltage regulation. If you have 6 HP then think in not more than 4 HP out ( 3 KW)(some may disagree with the power) using a 7.5 HP motor converted to generator.

I do not recommend any of the Chinese generators available in the market, like those sold by Harbor Freight ( not good and they are 3600 RPM).

Nando

[Flux]

Don't use pulleys to reduce the speed to suit that alternator, the most significant advantage as Dan has said is in the elimination of belts. You would still be limited by the alternator output in its original form and that would realistically be about 500W at continuous rating with no wind cooling.

If you are happy with about 2kW out or a bit less, direct drive then If you settle on your magnets then I am sure Dan or myself can sort you out some suitable winding.

If the engine is not committed to anything else then you can alter speed to control output. If there is another alternator committed to 60Hz then your only control of the battery charging is by air gap adjustment or by ballast resistors.

Flux

[Flux]

Dan I hope this comes up in the right place.

My comments were intended for those who want a normal cheap method of producing ac power. I entirely agree with you that for someone who wants to do it themselves and are prepared to accept the high material cost then the permanent magnet scheme is perfectly viable.

I also agree about belts, vee belts are not too clever except at full load, timing belts are better but have other issues and the pulleys are costly.

Commercial alternators are generally no better than 80% efficient at that power level and the self excited capacitor things that now seem to dominate the market will have no better regulation than a decent pma. They are speed sensitive and as with the pma, the regulation will be significantly influenced by the engine governors.

Many loads are not particularly frequency or voltage sensitive these days with built in regulators so that is not the issue that it used to be.

At 600 rpm, with decent diameter discs, 4kw should be reasonably easy to obtain at 120v ac. May be worth incorporating a battery charger winding as you did on the other one or it may be simpler to use 3 discs with ac and battery charging stators.

It would not be rocket science to add a servo speed adjuster to the engine governor to maintain a very good compromise between voltage and frequency, with a slightly rising frequency with load in place of the normal engine drooping characteristic.

Nice project.

Flux