Permanent magnet alternator 3 phase wind generator design

[myfaithnka]

Hello,

I am trying to make my first wind turbine. The wind speed is low in my area,so chose a VAWT.

Since VAWT is a slow speed one,I am adding 21 poles and 30 magnets. my calculation is that I get a 21x( 30/20 ) cycles in a full rotation,that is 315 cycles. This makes a rotation of 60 degrees by turbine gives me 50 cycles.

Is 21 poles and 30 magnets a good combination for a 3 phase PMA ?

my magnets are N35 round, 30mm dia with 4 mm thickness. can I produce a 40 Watts using them?

I am in need of a 12 V system since my batteries are 12 V(18 AH),I am not sure How can I choose the number of turns for a certain thickness ?

Many websites tells the coils should be wound with leaving a central hole in the size of magnet, is it necessary ? I am curious because this increases the generator size.

will making stator and rotor made of mild steel instead of wood increases output ?

Can somebody help me,please forgive my ignorance.

thank you.

 

[chuckey]

I feel that the number of poles(coils?) and magnets should be the same. My reason is that first of all its 3 phase so that will be 7 poles and 10 magnets per phase, so lets consider just one phase. Lets look at the 7 poles, their voltages should be in phase and adding. Suppose pole 1 is just on the peak of a N field from a magnet1, then as the angle between it and pole 2 and magnet2 cannot be the same so pole 2 is not on the peak of magnet2 N field and it gets worse as the angle increases, by the time you get to pole7 and magnet7, it might be a whole magnet out so its getting a S field.
I cannot understand the amateurmade generators, they normally have a large steel rotar (ex-disc brake from a car) with the magnets stuck/bolted on to one surface. Fixed close to that surface they have their coils, pointing towards the rotar. As the rotar rotates the magnetic field, cuts the coils and a voltage is induced in them. The problem is that the magnetic path is diabolical as there is no magnetic material within the coils. ALL professional installations use magnetic material EVERYWHERE except for the necessary air gap to keep the rotating part from rubbing on fixed parts. FWIW a DC generator for a bike (rotating magnets, fixed coils), is stuffed full of magnetic materials. This causes the action of it to be "notchy" as the magnets move from pole to pole. I have often wondered if this is the effect that amateurs are worried about, but it also means that you can use a much smaller winding in the coils as the magnetic field is much stronger.
FRank

 

[myfaithnka]

Hello frank,

I will increase the number of coils to 30, will this configuration be able to produce a 40 Watts ?

I feel that the number of poles(coils?) and magnets should be the same. My reason is that first of all its 3 phase so that will be 7 poles and 10 magnets per phase, so lets consider just one phase. Lets look at the 7 poles, their voltages should be in phase and adding. Suppose pole 1 is just on the peak of a N field from a magnet1, then as the angle between it and pole 2 and magnet2 cannot be the same so pole 2 is not on the peak of magnet2 N field and it gets worse as the angle increases, by the time you get to pole7 and magnet7, it might be a whole magnet out so its getting a S field.
I cannot understand the amateurmade generators, they normally have a large steel rotar (ex-disc brake from a car) with the magnets stuck/bolted on to one surface. Fixed close to that surface they have their coils, pointing towards the rotar. As the rotar rotates the magnetic field, cuts the coils and a voltage is induced in them. The problem is that the magnetic path is diabolical as there is no magnetic material within the coils. ALL professional installations use magnetic material EVERYWHERE except for the necessary air gap to keep the rotating part from rubbing on fixed parts. FWIW a DC generator for a bike (rotating magnets, fixed coils), is stuffed full of magnetic materials. This causes the action of it to be "notchy" as the magnets move from pole to pole. I have often wondered if this is the effect that amateurs are worried about, but it also means that you can use a much smaller winding in the coils as the magnetic field is much stronger.
FRank

 

[chuckey]

So many factors to take into account. I would not use a VAWT, they are are no good at generating electrical power but a nice piece of dynamic art. have you Googled " home made wind turbine"?
Frank

 

[myfaithnka]

hi frank,

I have a place to stay near beach,where there is wind right at ground level, I want to know more about why you suggest same no: magnets and coils...

thanks

 

[chuckey]

its so on each phase, each coil gets the same field at the same time. If the magnets are at a different angle on every coil, some coils will get an increasing voltage while others a decreasing voltage, so when you add all the voltages up they will not be the maximum possible. For an example, suppose you look at the instant coil1 is getting +2V (maximum output), if all the coils were getting the same voltage the output would be 7 X 2 = 14V. Now if the magnet for coil2 is displaced , coil2 might get 1.8V, again, coil3, 1.6V, then coil4, 1.4V, coil5 1.2V, coil6 1.1V and finally coil7 1.0V. These voltages do not add up to 14V. when the magnets have rotated a bit more so coil2 is on 2V, all the other would have an increased output, BUT coil1 will be decreasing.
VAWTs, On a TV channel (UK CH4), they did a program on an ECO house called "The Ark" near Sudbury, this had a VAWT installed. This machine never worked properly and despite its costs of £40,000, never generated more then a 1KW of electricity. Which is about the same you could generate from a home built turbine costing a few hundred pounds. Look at battery charging turbines for yachts, they are all multi bladed turbines which generate a hundred watts or so. This is what you should be aiming for.
Frank

 

[red_alert]

The guys at fieldlines dot com have all the answers for you.
Anyway, for a three phase PMG, you should use 3:4 coils-magnets ratio. For example, for 9 coils (3 coils per phase) you need 12 magnets (actually, 24 magnets, for both rotor plates). And yes, you SHOULD use iron plates for rotors (they help to close and concentrate the magnetic fieldlines).

 

[red_alert]

its so on each phase, each coil gets the same field at the same time. If the magnets are at a different angle on every coil, some coils will get an increasing voltage while others a decreasing voltage, so when you add all the voltages up they will not be the maximum possible. For an example, suppose you look at the instant coil1 is getting +2V (maximum output), if all the coils were getting the same voltage the output would be 7 X 2 = 14V. Now if the magnet for coil2 is displaced , coil2 might get 1.8V, again, coil3, 1.6V, then coil4, 1.4V, coil5 1.2V, coil6 1.1V and finally coil7 1.0V. These voltages do not add up to 14V. when the magnets have rotated a bit more so coil2 is on 2V, all the other would have an increased output, BUT coil1 will be decreasing.

Actually, that's how three-phase (multiphase) should work. While one phase reaches the maximum output voltage, the voltage on second one it's decreasing and on the third one it's increasing. This way, you should have a much smoother output (after rectification) and it helps reducing vibrations.

VAWTs, On a TV channel (UK CH4), they did a program on an ECO house called "The Ark" near Sudbury, this had a VAWT installed. This machine never worked properly and despite its costs of £40,000, never generated more then a 1KW of electricity. Which is about the same you could generate from a home built turbine costing a few hundred pounds. Look at battery charging turbines for yachts, they are all multi bladed turbines which generate a hundred watts or so. This is what you should be aiming for.

The only problem with VAWTs are their somehow smaller efficiency regarding their widespread "sisters" (HAWT). Thus the sweept area of a VAWT should be bigger than a similar rated HAWT.
But surely you could (home)build a 1-2KW VAWT for 1000£ or something. Actually, I'm in a process of building such an "art" turbine using Ed Lenz's variant.

For OP: I'm using 2x12 poles (magnets) and 9 coils for a 2KW VAWT - but the magnets are much bigger (60 x 30 x 15 mm).

 

[FvM]

Actually, that's how three-phase (multiphase) should work. While one phase reaches the maximum output voltage, the voltage on second one it's decreasing and on the third one it's increasing. This way, you should have a much smoother output (after rectification) and it helps reducing vibrations.

It may be just ignorance, but I never heard of a synchrone generator with a similar design, generally speaking with different number of rotor and stator poles. It's neither in my electrical engineering text books nor do I remember a citation.

I think chuckey has explained the problem graphically.

 

[red_alert]

This is how the coils/magnets are disposed (overlapped):



.. and that's the waveforms:



For a more dynamic presentation I suggest this video (link).

In the picture above, the yellow phase has reached the maximum output voltage while the black one has reached half the output voltage and it's increasing and the green one has reached half the voltage too but it's decreasing (magnets rotating clockwise).