alternators: what formulas can I use to help predict

[brandnewb]

So in my en-devour to make a 3 phase alternator that can produce 50+ VAC at one revolution every 8 seconds (7.5 RPM) I am a bit worried about what I have read here.

A senior member here warned me about the speed not being high enough or something when not increasing the RPM of the alternator.
I guess what was meant was the frequency of field reversals that needs to be higher.

I am failing to find a formula though that underlines this need.

All I found thus far is
e(voltage) = B(flux) x l(conductor length) x v(velocity)
https://www.youtube.com/watch?v=KUrMt6ic53o
As one can see there is no frequency component in that formula.

Which related formula should I be aware of that does include the frequency of field reversals?

[mbouwer]

7.5 RPM requires a very large generator diameter and many coils ( frequency is related to that ) and iron in the coils.
With this setup on page 3 of my topic with only 30 cm diameter and 60 rpm I had 50 volts between the phases.

[brandnewb]

Yes i know.

I have made a proof of concept that showed the potential of around 250 VAC at 60 RPM having 96 poles at 40CM diameter.

But 60 RPM is far too fast, dangerous, even, when the alternator is directly connected to the turbine. Hence the need for something more tame like 7.5 RPM.

So I take it you also do not know of any formula regarding this?

[MagnetJuice]

The frequency component in that formula is v(velocity)

The link below will take you to a discussion where Hugh Piggott (scoraigwind) explains his equation in detail.

His equation is derived from Faraday's Law of electromagnetic induction.

You should copy it and paste it to a document, so you can take your time and study it.

Then try working some examples until you get it. It is not that difficult to understand, but it requires concentration and practice.

If you don't have Hugh's book, you should go to his website and get it. The digital edition is only $5. Great investment.

https://www.fieldlines.com/index.php?topic=138646.0

https://scoraigwind.co.uk

Ed

[Adriaan Kragten]

You have to distinguish the open voltage and the loaded voltage. The open AC voltage is proportional to the strenght of the magnetic flux flowing through the wires of the coil, the number of turns per coil and the rotational speed. It is very difficult to predict the open voltage for a certain armature construction for a certain rpm. So once the armature construction is chosen, you simply lay one winding with a certain number of turns per coil and measure the AC voltage at a certain rpm. Then you multiply this voltage with the number of coils per phase and this gives you the open AC voltage per phase. If this voltage is too low, you have to increase the number of turns per coil. However, there is only a limited space for the amount of copper and so more turns per coil means a thinner wire and so a larger resistance. In my public report KD 340, I give the formulas with which the open AC voltage of one phase can be transformed into the open DC voltage after the 3-phase rectifier depending if the winding is rectified in star or in delta.

If the generator is loaded by a resistance, the open DC voltage is reduced to the loaded voltage. The reduction of the voltage is stronger as the load resistanc is lower. But the current increases as the load resistance is lower. You get the maximum current for short-circuit. The maximum current is reduced by the internal resistance of the coil. The are two kind of resistances, the ohmic resistance and the impedance. The ohmic resistance changes somewhat by a changing temperature. The impedance is a kind of AC resistance which depends on the frequency. The higher the frequency, the higher the impedance. The impedance makes that the current is limited at high frequencies.

[Mary B]

Yes i know.

I have made a proof of concept that showed the potential of around 250 VAC at 60 RPM having 96 poles at 40CM diameter.

But 60 RPM is far too fast, dangerous, even, when the alternator is directly connected to the turbine. Hence the need for something more tame like 7.5 RPM.

So I take it you also do not know of any formula regarding this?

why is 60rpm dangerous at the "alternator"? Gearing up slow speed is doable especially if you have plenty of torque available

[brandnewb]

@MagnetJuice,

I bought that book (Kindle) you are referring to on 17. November 2021 gave it a quick scan and did not find anything applicable to my scenario back then.
I just gave it another quick scan and found the page that explains the formula that is also being covered in the discussion you referred to. Very interesting indeed this 1.73 x the VAC when using a star configuration. I definitely missed that aspect first scan.
Other than that the book seems to assume a classical build (horizontal and also classical coil arrangement).

@Mary,
I am trying a direct drive. Meaning that the drag type VAWT turbine is directly coupled to the alternator thus the turbine RPM equals the alternator RPM.
As my builds are still of amateurish quality I would feel uncomfortable having them spin rather fast in an effort to safe local wildlife ( i.e. my family ).

I might introduce a gear of sorts to have the alternator spin 2 or more times faster than the turbine but it's just another point of failure I'd like to see if I can do without first.

[Adriaan Kragten]

@MagnetJuice,

I bought that book (Kindle) you are referring to on 17. November 2021 gave it a quick scan and did not find anything applicable to my scenario back then.
I just gave it another quick scan and found the page that explains the formula that is also being covered in the discussion you referred to. Very interesting indeed this 1.73 x the VAC when using a star configuration. I definitely missed that aspect first scan.
Other than that the book seems to assume a classical build (horizontal and also classical coil arrangement).

@Mary,
I am trying a direct drive. Meaning that the drag type VAWT turbine is directly coupled to the alternator thus the turbine RPM equals the alternator RPM.
As my builds are still of amateurish quality I would feel uncomfortable having them spin rather fast in an effort to safe local wildlife ( i.e. my family ).

I might introduce a gear of sorts to have the alternator spin 2 or more times faster than the turbine but it's just another point of failure I'd like to see if I can do without first.

The factor 1.73 (square root of three) is wrong! The correct formula for star rectification for the transformation of the AC voltage of one phase into the DC voltage is given by formuala 13 of my public report KD 340 if the voltage drop over the rectifier diodes is neglected and by formula 14 if this voltage drop is taken into account. The effective AC voltage Ueff in these formulas is the AC voltage in between the star point and one of the phases. The effective voltage is the voltage which you measure with a volt meter. The AC voltage in between two phases is a factor square root of three higher than Ueff if the winding is connected in star. So if you measure the AC voltage in between two phases, the term "square root of three" must be cancelled from the formulas. Especially for a low battery voltage, the voltage drop over the rectifier diodes can't be neglected. So if you want to understand how rectfication of a three phase winding works, you should study report KD 340.

[mbouwer]

If you stick with this design, you will have enough space to mount a large diameter direct drive.

[brandnewb]

@mbouwer
My apologies, first time I saw you using my images I thought there were alterations made to it. So I got nervous and asked for removal. Now I see there are no alterations but still I miss the relevance of a lift type turbine image in the context of a drag type context. One thing we are are is on the same page of having enough space.

@Adriaan,
Thank you brother for once again pitching in. But I lack the intellect to wade through your papers.
Would you be willing to digest for us a formula and present it here?

[joestue]

one square meter at 1 T field strength at 1 Hz would produce 4.44 volts per turn assuming a sine wave.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

[mbouwer]

@brandnewb,

What you are doing: working towards a slow-running direct drive generator fascinates me.

[Adriaan Kragten]

@mbouwer
My apologies, first time I saw you using my images I thought there were alterations made to it. So I got nervous and asked for removal. Now I see there are no alterations but still I miss the relevance of a lift type turbine image in the context of a drag type context. One thing we are are is on the same page of having enough space.

@Adriaan,
Thank you brother for once again pitching in. But I lack the intellect to wade through your papers.
Would you be willing to digest for us a formula and present it here?

A problem with the formula is that it contains square root signs which I type as a Greek letter in my reports and that it contains indices which I can't make on this forum. So I give the formula here on this forum without square root signs and without indices. Formula 14 out of KD 340 for star rectification is then given by:

Udc eff = 0.955 * square root of 3 * square root of 2 * U eff minus 1.4      (V)

Udc eff is the effective DC voltage. U eff is the effective AC voltage in between the star point and one of the phases. The term 0.955 is to compensate for the small fluctuation of the DC voltage. The term 1.4 is the voltage drop over two silicon diodes of the 3-phase rectifier. So if Germanium diodes or Schotky diodes are used in the rectifier, the voltage drop can be smaller.

0.955 * square root of 3 * square root of 2 = 2.3393

If a complete 3-phase winding is laid and if you measure the AC voltage in between two of the three phases, this AC voltage is a factor square root of three higher than the voltage U eff in between the star point and one of the three phases. So if you use this voltage, the term "square root of three" is cancelled in the formula for star rectification.

If the winding is rectified in delta in stead of in star, the AC voltage is a factor square root of three lower and the AC current is a factor square root of three higher. So the formula of the DC voltage differs from the formula for star rectification such that the term "square root of three" is cancelled.

[Mary B]

ALT251 on your keyboards yields √

[Adriaan Kragten]

ALT251 on your keyboards yields √

I have tried Alt251 but nothing happens. May be this works on an American keyboard but not on an European one. When I type a report as a Word file, I use Alt153 and then I get a capital O with two dots above it. Next I have to change that letter into a Griek letter symbol to get the square root sign.

[Mary B]

Hmmm I would need to know what code you are using for your particular KB.. only dealt with 1 EU kb and that was a very long time ago!

[brandnewb]

thanks all for the continued input.

Please allow me an animated GIF of the turbine at hand.

Sure I am worried about the stability of the central column but those are just problems to be solved.

As for now I do not see any problem yet I can't solve

[brandnewb]

now if anyone could please explain in layman's terms how a rather large slow spinning turbine would not be able to produce any usable power to charge a 48VDC battery bank than I am all ears.

Let's assume I can build an alternator that can go up the required voltage at low RPM. Let's keep in mind the insane torque we got here.

If it really is a no go then I will consider going back to the lift type blades but it will take me a life time to build something study enough to not be dangerous.

[DamonHD]

Just for reference, from someone living in the London (UK) 'burbs, that device is larger than my entire back garden.  And we have walls!

What did work (a little) for me was this design:

https://www.earth.org.uk/wind-power-pilot-autumn-2007-MotorWind.html

But it eventually pulled itself out of the wall, and I replaced it on that wall with some second-hand PV.

Rgds

Damon

[brandnewb]

Damon,

I am not sure what it is you are bringing to the table. Is it good or bad news?

[DamonHD]

I'm not sure what I'm bringing to the table either!

But any urban design has to be safe, acknowledge that some people have *tiny* outdoor spaces for any device, and work with really low available messy wind power.  That MotorWind design is simple and did that reasonably well for me.

Maybe I'm mixing my threads and should back out now!  B^>

Rgds

Damon

[brandnewb]

Never back out brother

I just never knew threads should be about a specific design category like you stated.

Is that actually correct? that all designs discussed here on this site should adhere to the guidelines as laid out by Damon?

I am sure you meant to say that it would be great if designs would have those aspects yes?

[DamonHD]

Goodness no I am not laying down any (design) rules here.  Ignore me and carry on as works for you!

Rgds

Damon

[Mary B]

Damon's point is we all have different wind conditions to deal with. I have had 25mph sustain with 45mph wind gusts for 5 days... just starting to die down...

[brandnewb]

understood Mary,

Also Damon I never meant to discourage from commenting.

On the contrary.

However, given the time that has passed since, am I save to assume there is no law of nature/physics that dictates a slow spinning generator can not produce enough watts to charge a battery bank?

If not then not all is lost, then it just means I will spend more time on the separation of the turbine and the alternator. Having a gear ratio yet to be determined.

I am asking for advice;
* Plow through with the direct drive (assuming it can reach voltages high enough for a 48VDC battery bank (I know I can do it it's rather easy))
* Go for separation of turbine and alternator with a yet to be determined gear ratio, so that the RPM of the alternator can be increased by the gear ratio