Power transmission

[mechjackt]

I'm planning on building the 700 watt alternator according to the plans in the just published Homebrew book (GREAT BOOK!!!) and spinning it with a Banki turbine (although the problem I have would be the same with a wind turbine) .  I've got plenty of water (~4cfs @ 20+ ft head)so getting 700 watts is within reason.  My batteries are 450 feet from the stream though.  Yeah, I could move them closer but then I'd have to run the AC from the inverter to the house that same 450 feet which would require lots of expensive copper.

My idea is to build a 12V 3 phase alternator, rectify to 12VDC, dump that into a 12VDC to 120VAC 1200 watt interter that I already have, run the 120VAC the 450 ft to the battery building over 10 gauge (probably overkill) copper and into a 120VAC to 12VDC charger.  Not terribly efficient, but I don't see any really more efficient alternative either.  I'm figuring that after all the gyrations I'll see about 400 watts into the batteries with a 700 watt alternator.   Sound reasonable?

Any alternative ideas?   (and no, I can't reroute the creek!)  

Thanks.

Jack

[scottsAI]

Mechjackt,

Wind the alternator for 120vAC output. Transformer at the battery.

Have fun,

Scott.

[Norm]

If you have a 700 watt alternator you already have

AC current....smaller and more wire would make it

higher voltage....then a step down transformer and voltage regulator and rectifier at the batteries??

[Ungrounded Lightning Rod]

Just a transformer and rectifier at the batteries.

Regardless of whether there's a transformer in the circuit the batteries provide a hard voltage load that limits the RPM of the turbine and turns extra torque into higher charging current (and a moderate RPM increase to raise the voltage until it pushes the current),

[Ungrounded Lightning Rod]

Also:  It will be important to take the frequency of the alternator into account when selecting transformers and voltages:

The magnetization of the transformer core is proportional to the current in the coil.  And the current in the coil is proportional to the INTEGRAL of the voltage applied across it.  So lower frequencies at a given voltage mean higher magnetization.

Unfortunately, the core has a limit to how much it can be magnetized.  Magnetizing it to that limit is called "saturation".  At that point further changes to the current don't appreciably affect the magnetic field through the core.  It's like the core isn't there and you have an air-core coil with very much smaller inductance.  The current rises abruptly.

Transformers are designed to operate close to saturation.  So running at a lower frequency (with the rated voltage) will saturate them and cause problems.

Fortunately, permanent-magnet alternators change voltage and frequency in sync.  If you get to a particular percentage of saturation at one frequency, then (neglecting resistance) you'll get to the same percentage of saturation at ALL frequencies.  (Resistance makes the voltage decay at low frequencies.  But if you figure with open-circuit voltages you'll set an upper limit and your transformer selection will be safe.)

So the way you pick your voltage and transformer is to use a transformer that is rated for the alternator's open-circuit voltage if it were spun to produce the transformer's rated frequency.

Now you turn that inside-out:  Figure out your under-load RPM of the turbine.  (Presuming for example you're in the US and can easily get 60 Hz transformers) you can build your alternator to produce 60 Hz and 120 V at that RPM and use a 120V primary transformer.  Or you could build one to produce 120 Hz and 240 V and ALSO use the 120 V transformer.  Or you could get a 240 volt transformer and build your alternator to produce 480 volts at 120 Hz, and so on.

A thing to keep in mind is that some types of turbine have an UNLOADED RPM that is twice the loaded RPM.  (Pelton is one of those.)  This will redouble your voltage if your load is disconnected from the secondary side of the transformers.  And if you sized your transformers to take into account the voltage drop from wiring resistance the voltage drop will be greatly reduced with the load disconnected from the secondary, so the primary voltage will rise further.  This could exceed the insulation strength of the transformers and cause insulation breakdown.  So I wouldn't design for more than, say, about 600 V on the input to a 120 V rated transformer assuming doubled RPM and no transmission line drop.  (Maybe Flux, or others with more experience with transformers, could give you a better estimate of how far you can push the voltage.)

[ghurd]

I am a bit concerned about "rectify to 12VDC, dump that into a 12VDC to 120VAC 1200 watt inverter".

It will need a battery on the inverters input.  And the battery will need a controller.

If the power is not being used and the house batteries get 'full', then the 120V charger sends less power,

the inverter feeding the charger uses less power, the turbine runs with less load, and faster,

and the voltage climbs and smokes the inverter.

I think I would go with the transformer at the battery after all the alternator frequency issues are sorted out.

Might find it is cheaper to use big aluminum wire.

Playing with the powerstream calculator, looks like winding for 24V, 40A, #2/0 AL wire, leaves 18V at the other end with a 23% voltage drop.

Sounds better than the 50% loss.

Not sure of all the implications.  Just a thought about it from another angle.

G-

[hydrosun]

Why do a 12 volt system? Why not generate 48 volts dc and then a 48 volt inverter at the house?

Chris

[nick1234]

 you said (I've got plenty of water (~4cfs @ 20+ ft head)) that will give at least 5000 watt 5 K of power as for line losses i run with 6% lose with a 700 ft 12g run

i would use 1 phase 240 ac or higher generator with governor and multi tap input transformer at house to 110/220 output

just my 2 cents

nick

[zeusmorg]

 This is how i would do it..

 First off, I wouldn't build an axial flux for a hydro situation I'd use a 3 phase motor as a generator and then run the a/c output to my battery bank where I'd rectify it to dc for the bank.. If you used a 480 3 phase motor your wire sizes would be quite small! and you would eliminate the double conversion losses..

 Since as you say you have "plenty of hydro" using the fields in the 3 phase would rob some power, but so what?

 It just makes sense to me to run A/c vs dc over long distances,and since the output of an axial flux, or a 3 phase system is already a/c why rectify it at the source rather than at point of use?

 I've advocated running wild a/c down windmills for years now, and still don't understand why people rectify it up on the mill. If a diode pops, you have to lower the tower to fix it. And yes there's 3 wires vs two,, but overall it'll be cheaper on the wire.