Hard to start?

[GerryS]

What makes windmills hard to start as far as the alternator goes? How can you make an alternator start turning with less power input?

[vawtman]

#1= the builder

#2= a weaker builder.

 Dont know what type alt.Gerry

 

[GerryS]

I think my question is about axial alternators that use lamination strips behind the coils (like the wood AX and Volvo 140).  I've done some research and it seems that the laminations become magnetized and this is what causes the difficult starting.  How do you avoid this in single rotor machines? I've seenn people suggest annealing in a fire.  What is annealing?  And what does it do to the metal?

[ghurd]

The laminations won't become magnetized, much. Or my refrigerator would hold up paper clips by itself.

Annealing is cooking them in a fire until they are red hot, and then letting them cool.

It changes some of the properties of the steel.

Metal strapping is not the best choice.

Take apart a transformer for a better material. It won't need annealed.

Older microwaves have some nice big transformers.  File or grind the welds and most come apart fairly easy.

I think I have a picture I could upload if you want to see how I did the laminations.

Pretty much just a wooden circle, with each piece added one at a time so there was very little space between them.  Medium duty packing tape for the insulation between layers.

Never did finish that one because I ruined most of the other parts.

A bit of drag comes with using laminates.  I don't believe they cog unless the laminates are slotted for the coils.

G-

[GerryS]

I would indeed like to see how you did them!

[TomW]

Gerry;

I thought this was so obvious, I didn't comment earlier but here goes.

Well, surprise, surprise. Your magnets are attracted to the metal behind the coils. Lots of posts here on the pros and cons of that. Bottom line is the magnets love steel and want to stay close to it. Assumption being it only does the cogging when the magnet rotor is on. Otherwise it is mechanical and not my forte.

Pretty ancient prototypes you are trying to mimic there, why not come on into the 21st century?

Again, just one mans opinion.

Cheers.

TomW

[GerryS]

Hi Tom.  I'm coming into the 21st century slowly.  I only have 12 smallish magnets 3/4" diameter by 3/8" deep and 22AWG wire right now.  So I'm trying something really simple.  I am planning to build it 3-phase and maybe put a second rotating disc behind the stator and use a 4-ft diameter prop.  I am planning to build it three-phase as well.  So that's kind of 21st century.  The one hurdle I have is 3-phase vs one phase mathematics.  Everyone says three-phase is much more efficient. I believe them too.  But I haven't been able to see the mathematical reason.  I keep falling back to this concept:  I wound some test coils and if one coil produces 1 volt, then 12 in series will produce 12 volts.  But if I take 9 of those coils in 3-phase they will only produce 1Vx3 magnets per phase = 3V per phase.  3V per phasex1.73=5.19V output for the three-phase machine. That's 12 volts RMS for single phase verse 5.19V constant for three-phase.  I saw a great graph from flux that shows the advantage of three-phase experimentally.  I wish someone could show me mathematically where the advantage of three-phase is instead of everyone just saying "oh, three-phase is better!  It's the only way to go."

[ghurd]

It's not difficult.

Transformer pieces stay how they are bent.

Easier to work with than many other materials.

Pieces get twisted up during seperation, but pound flat with a hammer.

I think the whole donut came from one very old, very large, microwave transformer.

That's a speaker plate with crappy 'rare earth' magnets.

The plan was for overlapping coils.

[finnsawyer]

I thought we were over this.  If a coil passes over a single magnet you will get a complete cycle out, say a positive pulse followed by a negative pulse.  So, if each cycle gives one volt then twelve coils and twelve magnets gives your twelve volts.  The trouble is that for the three phase a coil doesn't pass over a magnet and then start over the next one.  It can straddle two magnets.  Tecker and I just had a little discussion about some consequences of that.  Anyway, when the coil straddles two magnets its output is the sum of the effects due to each magnet considered by itself.  This is called "The Principle of Superposition".  That is, in linear systems you can add effects that have the same physical attributes.  So, we get a voltage due to the coil moving off of one magnet added to the voltage of the coil moving onto the following magnet.  Since the magnet polarities are opposite, the voltages add and will give a waveform having a voltage greater than one.  So, you are understating the voltage from the three phase.  If you go back and study my alternator design again you will observe that when the coils of one phase are centered over north poles, the coils of the following phase are just starting over south poles.  The two phases added together will produce the same voltage waveform as the coils move to the second phase centered over the south poles, as the single phase does in the three phase alternator.  We can say this due to that Principle of Superposition.  Of course, the times of transition have to be the same.  In practice the rotor diameters will be different, also affecting the output voltage due to the different transiting times.

One other thing that you need to consider is alternator resistance.  Twelve coils in series will have twice the resistance of six coils in series.  So, the twelve coil series will provide less current at a given voltage and therefore less power.  This may not be a problem if you are trying for less current and higher voltage at a given RPM.  Anyway, you need to look at all aspects when comparing these things.    

[GerryS]

Fin, in the pictures below both the 3-phase and 1-phase are straddling two magnets.  Please look at my math above and tell me just which calculation is wrong?

If my math wasn't wrong, then how do I change the picture below to make the three-phase more productive than the one phase using the same coils and 12 magnets?  If it is already more productive, then please show me the math (Fin you seem to be really great at math, so I know you can!).  It's not that I don't believe 3-phase is more productive.  I just want to see someone show me mathematically a comparison what happens when you have 12 coils that each produce 1 volt.  Mathematically compare the voltage of that situation from three-phase to single-phase.  From my picture, the voltage PER COIL won't change whether it's 3-phase or one-phase.  Fin, can you please provide me with my comparison so I can see the part I'm missing.  

[finnsawyer]

What are the shape of the voltages pulses?  And what are the time relationships (phase differences)?  Maybe you can determine them for round magnets and coils.  Knowing that it would be easy to add the pulses in the right way to get the voltages.  Rotor diameters and resistance will also figure in.  The use of an oscilloscope would be very useful in this regard.

[paNewbie]

i'm just prototyping HAWTs (no generator) and its proving difficult to get them to spin.

buttery smooth skateboard bearings, sharp-as-a-prison-shank PVC blades... and it takes a gulldarn high wind advisory to get them to move at all.

this is not an easy hobby in the least.

[alancorey]

I'm not sure this will answer any of your questions, but this is a theoretically perfect trace of 3 sine waves (with 120 degree spacing) done in Excel (yuck).  This is what clean 3-phase power would look like on a scope with at least 3 input channels if the alternator were giving a perfect waveform for each phase.

Notice how at any given instant in time (any position on the horizontal axis) there's some power produced.  Power is evenly distributed among 3 wires if you consider each color to be connected to a wire.  Aside from distributing power on 3 wires, not all of the work of making electricity is being done at one instant.  Single phase can cause vibration problems but because 3 phase is spread out over time there's less vibration.  With 5 or 7 phase there's even less.

Single phase is like rowing a boat with 1 oar.

  Alan

[TomW]

Gerry;

How do the pvc blades work on a nail or bolt [no load].

Lots of folks use them but my experience was they are touchy on shape and position relative to the wind. Plus they spin like crazy unloaded but suck driving a load.

Just some observations from my earlier attempts using them. Mostly documented on this site someplace.

My humble advice would to build proven, current designs for alternators and realize that PVC blades work for some but overall do not have a stellar reputation as a drivere to make power.

Good Luck.

Cheers.

TomW

Cheers.

TomW

[GerryS]

I'm not certain of the pulse shape, it is not a perfect sine wave in either case, but it is a similar shape in each case (two adjacent magnets pass over the coils in both cases in the same positions) so mathematically it shouldn't be a difference. I think the total integral under the pulse is the same in both cases.  The only difference in the pulse shape I can come up with is from the rotor diameters.  In the single-phase the diamter is bigger.  So at a given RPM the magnets are passing the coils faster for the three-phase than for the single-phase resulting in a higher peak in the single-phase and a wider wave in the three-phase. I'm curious if the equivalent resistance of the three-phase is less than for the single-phase.  Could it have to do with power loss?  Isn't power loss equal to IR^2?  Do 6 coils creating voltage in single-phase have a lower resistance than 6 coils creating voltage in three-phase because of the possibility of current flowing through the unused third phase (at those times when the magnets are in the coil holes of the third phase as in the picture below - even though the blue coils aren't producing, current from the red coils can flow through them thus dropping the resistance for the return of red current from the 3-phase junction by 1/2, or a drop in resistance of 33% for the whole system. )  If what I say is true about the lower resistance, then (assuming identical coils...either 12 for single-phase or 9 for 3-phase)the single phase case has the ability to produce higher voltage while the three-phase has the ability to produce more power.

[GerryS]

Ha ha ha ha....oh man...First off, excellent comparison...a boat with one oar.  Good laugh there!  

Power is evenly distributed among 3 wires if you consider each color to be connected to a wire.

I think this might lie at the heart of my question.  I made a graph earlier too.  Same as your's basically, 3-phase with each phase 120 degrees apart. The Total Rectified Output plot is a sum of the absolute values of the sine waves from each individual phase (not including rectifier losses).  As you pointed out, it's basically a constant output (at 1.73xindividual phase voltage).  The 12 coils connected in series is the single-phase representation.  These are voltage graphs though, not power.  It's obvious that the integral under the 3-phase graph is less than the 1-phase.  But I bet the graph of power-vs-RPM has a larger integral for the 3-phase machine.  But why?  I am starting to think it's because we can send all the current of 6 coils through 2 wires (not 3 because 3 are never flowing the same direction) in the 3-phase whereas all the current of the proportional 6 single-phase coils are going through just one wire.  

[GerryS]

I think something was screwed up for a minute on this thread.  I responded to a message that was supposedly from Fin but now turns out to be from AlanCorey.  You responded to a message from me, that was actually from PA Newbie.  Oh well...server mix up~  

[GerryS]

What are transformer pieces made of?  

[Flux]

Yes Gerry, things often end up in strange places here and sometimes cause confusion.

I will have one more go at trying to explain the issue that is bothering you.

In your particular example with the coils producing 1 volt you are quite right that the single phase case has a far higher voltage and for a given speed and would be producing output long before the 3 phase case.

Your example is totally fictitious, you have arbitrarily chosen the same coils for the single phase and 3 phase case.

I tried to explain previously that the number of turns per phase is the factor that decides voltage. For arrangements with different numbers of coils per phase the turns in a coil will be different. You can make 6 coils of a 12/9 3 phase produce the voltage of one phase of a 12/18 or 12/36 three phase winding by altering the number of turns per coil.

Forget 3 phase for a moment, concentrate on the single phase case. The obvious one is the case where you have 12 coils in series and with your example you would have 12v.

It is perfectly possible and in fact very common to produce that winding with only 6 coils. If you still want 12v you will need to wind each coil with twice as many turns. you can do this because you have extra space that you can use where the missing coils would have been. Ignoring minor differences the 2 windings are exactly the same.

Now going back to the 3 phase case, you can wind the coils with more turns to still have the same ac line or phase volts so you can achieve the same cut in voltage at a given speed as the single phase case. The turns per coil will be different for single phase, 3 phase star and for 3 phase delta. Also the wire size will need to be different in the 3 cases.

Volts alone produce produce no power, you must have a cut in speed above battery volts and the current that flows will be dependent on the voltage difference and the effective circuit resistance. Power is the product of battery volts and the dc current flowing.

The fact that you have had to use different sizes of wire to fill the same space with the desired number of turns adds a confusing factor that is rather more difficult to grasp and you need to know the way in which each type of winding delivers current via the rectifier.

Just bothering yourself about integrals under curves is not going to solve your problem, that integral only affects voltage.

I hope this helps, it is my last attempt to explain.

Flux

[finnsawyer]

I think you are missing the point that I was trying to make.  If a single coil in space passes over a single magnet starting from far away, we would see a voltage pulse rise gradually from some distance away reach a maximum as the coil approached the magnet, go through a zero and reach another larger and opposite maximum as the coil moved onto the magnet, go through another zero as the coil becomes centered over the magnet, go through another again reversed maximum, another zero crossing as the coil exits the magnet, and finally another maximum with a gradual drop off as the coil moves away from the magnet.  The two lower maxima come about because the magnetic field outside the magnet is directed opposite to the magnetic field over the magnet, as is the flux.  But it is also weaker.  It should be obvious that this voltage waveform is not a sine wave, nor should we expect that the part over the magnets is a perfect sine wave either.  The waveform may rise slowly to the maximum and then fall quickly to zero.  In fact, you may be assured that with different shape magnets you will get different shaped waveforms.  In the case of circular rotors and stators we don't get those lower voltage pulses.  You might as a learning tool consider a hypothetical waveform that rises linearly for 3/4 of the time it takes to reach the center of the magnet and then drops rapidly for the remaining 1/4 of the time.  Continuing on, the waveform continues dropping for 1/4 the time it takes to move off the magnet and then rises to zero for the remaining 3/4 time.  Well, with the three phase alternator or with my design you have at the same time a pulse with a coil moving off of a magnet and a pulse with a coil moving onto a magnet adding to give the actual voltage.  You can do this graphically by inverting the negative half of the cycle and overlaying it on the positive half.  You will note that the maxima do not coincide.  If they did you would get twice the voltage (or two volts in your case), but in this example it will be less.  A pure sine wave would double the voltage, but is not likely in practice.

As far as resistance is concerned, for three phase where six diodes are used to rectify the current you multiply the single coil resistance by six for the 9:12 configuration.  The other three coils are cut off by backward biased diodes and very little current will flow in them (modern diodes are very good).  The single phase with 12 coils will will have twice the resistance since all 12 coils are in series.      

[ghurd]

Layers of special iron, held together with paint and a few welds.

[GerryS]

I will try to locate an old transformer for the purpose...although, at Tom's, Flux's and everyone's suggestions I may try to come into the 21st centrury and put a second rotating disc behind the coils (without magnets).

[GerryS]

Flux, Thanks for all the help.  I do understand everything you have written here and in the other posts.  It has taken me from no 3-phase knowledge to a decent  conceptual knowledge.  


Flux wrote: "In your particular example with the coils producing 1 volt you are quite right that the single phase case has a far higher voltage for a given speed and would be producing output long before the 3 phase case."




I see why you said that this was a ficticious example.  An experienced person  winding three-phase would not use the same size wire or # of windings per coil that they would use for single-phase, so as you said the comparison isn't quite fair.  The part that kept confusing me was that I gave my example and people kept implying I was wrong about the voltage being higher in MY EXAMPLE when their real thought was probably that it wasn't the right example or a fair comparison (which you did say from the start).  So I couldn't figure out what was wrong with my understanding in MY EXAMPLE. But as you said, I was "quite right" about my example. My point is, that as long as people were telling me that I was wrong about MY EXAMPLE, that meant that my understanding of the concepts were wrong. But as soon as you made me realize that I was correct in my concepts in MY EXAMPLE, it made your points of increasing windings and using smaller wire make sense!  Now I see from your statement, that for MY EXAMPLE and MY EXAMPLE ALONE that the voltage would be higher for single-phase, but that nobody would ever wind a machine like that. I can see now, from you, that the winding would be different and THAT'S what should be compared.  This all takes me back to that graph you posted.  I deduced the single coil output from the three-phase output and then assumed that you could use 12 of those single coils.  But you couldn't wind the stator with 12 of the 3-phase coils in a single layer, there wouldn't be room.  I'm on this!!!  Thanks Flux.  Third time is the charm I guess.  

For my current machine I am already using AWG 22 wire and easily producing 1 AMP spinning by hand.  I saw that the conduction limit on AWG 22 is about 1 AMP (my wire isn't high temp insulation or anything...it's the AWG22 from Otherpower.com).  So I don't think I can go smaller.  I don't want to fry the stator.  So I need to put more windings on of the same AWG probably.  I'll make the holes a bit narrower I guess.  I will cover 80% (instead of 50%) of the stator by making the holes 60% narrower.      

[ghurd]

Great call.

All the other crap makes the cost of neos and wire insignificant, in most locals.

You seem determined. Watch the trash for giant old microwaves with wind up timers. ;-)

[Seaspray0]

Nice graph showing the 3 phases!  And yes, 3 phase is more efficient for two reasons:  1.  Where it takes two wires to carry power for a singe phase, you can carry 3 times that power by adding one more wire and using 3 phase.  This is why power companies love it.  2.  When rectifying 3 phase, you only conduct through the diode when the portion of the AC voltage exceeds the DC voltage.  That's only twice a cycle for single phase, but 6 times a cycle for 3 phase (which provides a smoother power flow, less ripple effect, and less of a surge current through the diodes).

Also, 3 phase motors/generators are smaller and cheaper to build for the same power, do not require starting capacitors, and provide smoother torque on the shaft.  It's a shame that houses are not equiped with 3 phase as a standard.  The power savings alone on the air conditioner would be substantial.