"In reading the descriptions of alternator construction on OtherPower.com I've noticed that 3-phase seems to be preferred in larger devices."
I'd say it's preferable in all size generators unless single phase AC is our final product. If we're rectifying the output to DC, then 3 phase is the way to go. Size is not an issue.
"I'm having trouble visualizing what the output curve looks like from a 3-phase alternator. I'm guessing it produces lower peak voltage but at a higher AC frequency. "
No... the frequency and voltage can be whatever we wish in either case. 3 phase is delivered on 3 wires and at any time youll have single phase AC between any two wire. Each set of two wires carries single phase AC. So we sort of have 3 single phase outputs, except that they share conductors. So we have current flowing through all the conductors all the time. Single phase has at least a moment where no current is flowing in the line, during that time our whole machine, and the line between the machine and the batteries is doing nothing. With 3 phase, everything is working all the time so were making more efficient use of conductors (and magnets).
When there's 0 current flowing in a single phase machine, there's also 0 torque required to turn the alternator. At the peak of the sine wave (maximum current flowing) it takes maximum torque to turn the alternator. So it 'cogs' while producing power. You can feel that in your single phase machine if you short it out - itll feel stiff to turn, but very lumpy. This makes noise and tends to rattle things apart. 3 phase is very smooth because we're making power all the time.
" I'm trying to decide how to wire my second alternator. What would make you chose one method over the other?"
I would make it 3 phase!
Sorry for the rough picture there.. But you can see in the picture at the top left what single phase AC looks like. On the right top, is what you get if you rectify that to DC. I put a line at the top there and called it 12V (imagine were charging a 12 V battery). The DC is very lumpy, and current only flows when the voltage is over 12V. With single phase, any time the voltage is below that line - our alternator and all the wires are doing nothing. So at, or near cutin on a wind turbine - much of the time no current is flowing at all in a single phase machine. Even at full output there would be a good deal of time where no current is flowing.
The bottom left picture shows what 3 phase looks like and you can see that once we rectify it to DC we have very nice smooth (slightly lumpy) DC current.
Here is a good link that helps explain this too on Ed's page: http://www.windstuffnow.com/main/3_phase_basics.htm
I understand much better now.
Rob [ Parent ]
Your question of 3 phase vs single phase power production is quite interesting when you refer to the prairie turbine site. Prairie turbine is claiming to make 5500 watts of single phase power using a Nord 7.5hp 3 phase induction motor. They claim to use one phase for voltage regulation, and one phase to generate electricity.....Up until I read that part, I was quite interested in Prairie Turbine's goal of creating a low cost, anyone can build, simple turbine.
If you look at their site, you see the reason they use a three phase rather than single phase motor is because it was readily available surplus from an industrial crane.
However, as they can only use one of the 3 phases to cogenerate power at a single phase household, they will only be able to extract about 2.5 hp which equates to about 1.7kw rather than the 5.5kw they claim in their breezy 5.5 ad.
If someone can correct me if my conclusions above are wrong, I would appreciate it. Otherwise, Prairie Turbine should be looked at as another 'snake oil' sales site.
Gary[ Parent ]
I haven't bought their plans yet, but from what I can tell you are using 2 phases. If you stop and think about it, 220vac is using 2 phases(2 sets of windings). Don't know what they mean by the 3rd phase is just voltage regulation. I would guess it is something like making a phase converter, where you just energize the 3rd phase with caps.Goose[ Parent ]
Not knowing how you arrived at your 1.7kw figure I'll explain how we arrived at ours. The method we use is to actually clamp an amp meter to the supply lines to the motor and measure the current flowing in the circuit. Then we use the formula: P= IV to arrive at the power being produced. When Breezy 5.5 is max'd out (actually when our turbine begins to limit the torque applied to the motor shaft), we measure 23 amps flowing in the circuit. Using the formula above we simply multiply 23amps X 240volts = 5520 watts (maybe we should call it "Breezy 5.52").
The power produced by the motor has much more to do with the turbine driving it then the size of the motor (easy guys, I'm still talking about asynchronous generation here). If you connect a smaller motor to Breezy's turbine, a 5 hp for example, you would be over driving the motor and probably burn it up shortly (regardless of what the motor data plate says). This is so much a fact with wind power that you can almost determine the power capabilities of a wind turbine by simply looking turbine it's self. When comparing Breezy 5.5 to other types of turbines consider that Breezy's turbine is turning (and pretty much held) at speed around 120 RPM and is a four blade turbine over 18` in diameter.
Gary, I see that you have referred to our web site as "should be looked at as another snake oil site". I must say that I find that remark quite offensive. And as for myself I would be quite sure of my facts before making such a comment especially if I were as ignorant of the principles of this type of generator as you would lead us to believe you are by your statements.
In contrast we have done our research and also relied on the research of others to present this turbine and the plans for it. But you don't have to believe me. In 1974 at the Technical Institute of Vaasa in Finland a study of how "squirrel cage" 3-phase motors excited and driven as generators behave unexpectedly under different wiring and load configurations. I believe some of the results of that study may still be available at: http://www.saunalahti.fi/elepal/moottori/gener_e.html. Also on a Danish website I have found an excellent description of the principles of asynchronous generation, you may want to take a look at this: http://www.windpower.org/en/tour/wtrb/async.htm .
Since I believe in the phrase "put up or shut up" I would like to extend an invitation to anyone wishing to see and measure the output of our turbine for themselves. We're located in central Kansas 20 miles south of Abilene. My only request is that you call first so that we can schedule a time. If you can't make it yourself and have a friend or relative nearby, we'd be happy to show them too. You may find our contact information and a phone number at: http://www.prairieturbines.com www.prairieturbines.com[ Parent ]
Since single phase is lumpy and bumpy output, on/off sort of. Would that act in part similar to a pulse charger?
If 3 phase is always sending power, smooth currant, always on, then that would NOT act as a pulse charger right?
Now question, if I take a 3 phase machine, rectify each line seperate (3 brigdes) and run each to it's own battery bank, then would I in effect have a 3 phase machine, smooth output, always on, but get the pulsing effect to each battery bank from each seperate phase?
If that's somewhat correct and might work, then when I do my wiring here soon, I still need the batteries in the truck for portable power, I could easily wire a bridge so I can connect to the main house bank or the bank in the truck. The ones in the truck of course get abused alot, I like to pulse charge those every so often.
All bridges would be connected at all times, thus 3 phase, just to different banks instead of all to one. If you think it will work that way I'll try it, if not I'll probably just wire in a switch to flip from one bank to the other as needed like I first planned.. nothing to lose
Spelin and tpying are my strong points, not electronics.[ Parent ]
You're right about some batterytypes liking to be charged with pulsed voltage; I've built a batterycharger once, and when it was finished, I heard that batteries should be charged with pulsed voltage; rectified AC, unfiltered (no electrolytic caps) should do the trick, if you can limit current & overvoltage.
As far as a 3phase machine providing 3 separate single-phase voltages; this can be done if you have 6 wires coming from your alternator; i.e. 2 wires per phase, and none of the 3 phases internally connected; i.e. 3 completely separate phases.
Then you can individually rectify them and end up with three single phases; however, if you don't load all phases about the same, your alternator will see a lumpy load (vibration, etc.).
Perhaps you could also use 3-phase star, where you use 4 wires: 3 phase wires, and the '0' (zero) from the center of your phases. That's the way the utilities provide electricity: they produce 3 phase, but each house normally only gets 1 phase (& other wire being the common 'zero' of the star). Unless you have a 3phase electric connection, obviously. Since most houses use about the same power, the 3phase system is equally loaded (what goes for your little genny about 'bumpy-ness' also goes for the big one from your power company).
Like I said, I'm not sure whether in a 3phase star-situation (with extra 'zero' wire) you could individually rectify each phase; when each phase is completely separately available (6wires) you can.
Hope this is a bit clear; my explanation won't win a prize for clarity, I'm afraid.
Peter, The Netherlands.[ Parent ]
One more question about this. Your drawing on how to wire 3-phase shows the inputs as been A, B, C and the outputs are X, Y, Z. That makes sinse to me but the 3-phase Basics doucment (http://www.windstuffnow.com/main/3_phase_basics.htm) says the outputs are A, C, E. Looking at the drawings in both documents I don't understand why the 3-phase Basics document says the outputs are A, C, E. Your drawing on the otherhand is very clear. Which is correct?
Thank you, Rob [ Parent ]
Since the coils are not overlapped nor magnetically linked, as they are in a car alternator, the difference between 1 and 3 phase's specific efficiency is minimal, if not more dependent upon the specific variations, not topology.
MPPT as far as i know needs some "intelligence", whether it be an analog circuit, or a 50 cent 8 bit micro controller.
If you are suggesting using silicon controlled rectifiers (SCRs) to control the output voltage, then the alternator will need to be rated significantly higher. Reason is, I^2R losses are increased during voltage reduction. At the cut in speed, the conduction angle will be the highest, As the wind increases, the conduction angle must decrease, to avoid a stall. What happens next depends on the alternator's resistance, and the blade's output power curve.[ Parent ]
I don't know that I'd use SCRs. I'd probably try an NPN junction power transistor with its emitter terminal in series with the positive terminal of the battery. The base voltage would be held down until that part of the pulse cycle was reached for conduction to occur. The transistor would either be cut-off where essentially no emitter current flows or hard on where the collector to emitter voltage is small, around 0.3 volts. With 10 amps through the transistor when on the power dissipated in the transistor would be 3 watts or less. Obviously, other circuitry is needed to track the turbine's power curve and determine the on time. That's where the challenge is, but such circuitry need not consume much power either.GeoM[ Parent ]
If you can find me anything other than a mosfet that will drop less than .8 volt, please, I'm all ears. (btw this voltage don't matter too much, since you got rid of the diode(s))
If I understand you correctly, then naturally commutated scr rectification is still more eff if it is three phase, at the turbine, on the tower. all it takes is 3 diodes, 3 scrs, 3 capacitors, and 3 variable resistors.
what you want is on page 13 of this pdf: http://www2.ing.puc.cl/power/paperspdf/dixon/21.pdf
and there is a bunch more here: http://www2.ing.puc.cl/power/publications/electronics.htm
I hope that is of some help. post a schematic please. [ Parent ]
http://www.fieldlines.com/story/2006/4/21/16237/9933
The arguments that I give show that for the same investment in magnets you can get considerably more power out. That is where "the rubber hits the road" as I see it.
The junction transistors that I have known and loved all had a saturation (hard on) value of Vce in the 0.3 volt range. This is true whether they were PNP or NPN.
To a certain extent an argument of SCRs versus Transistors is pointless. You still need to control the on or conducting time and relate that to the power output characteristics of the wind turbine so as not to drive it into stall at any point along the power curve. Both the single phase and 3 phase alternator output current have the basic frequency information built in due to the ripple. You can pick that off by bleeding some current through a 100 - 500 microfarad blocking capacitor to a suitable resistor. From there you can process the information any way you see fit. You might need to add a resistor in series with the battery, although with a transistor in the off state in series with the battery you might not need to to get the basic information.
By the way, if the SCRs would work as you describe, why are people even bothering with the bulky and expensive buck converters? GeoM[ Parent ]
The reason SCRs are not used is due to increased copper losses as the conduction angle decreases. If Scrs are used to reduce the voltage by 50% and 66%, but the average rectified current into the battery remains the same, then I^2R losses in the coil are going double, triple, respectivly.
Buck regulators maintain a full 120 degrees of conduction in a 3 phase rectifier. In addition to this, if a buck regulator is used on a single phase unfiltered rectifier, than the conduction angle can be increased further, up to say about 150/160 degrees, at a dc output of 1/4 of the ac peak voltage. (on a sinewave machine) PMA's oftem produce trapezoidal waveforms, and that won't be a significant difference. [ Parent ]
