Single stator and rotor 3 phase alternator

[oramafanis]

Hi there!
I am building a 3 phase generator with on stator and one rotor.
I will go either to 12-9 or 16-12.
What I want to ask is about the iron I will use.
I will put an iron plate behind the magnets but I don't know what to do with the stator.
Do I have to put a back plate to it also?
There are 3 possibilities.
1st I will not use plate or cores.
2nd I will use only iron cores without backplate.
3rd I will use both iron cores and backplate.

Which way I should go for better performance?

Till now I made some measurements with one coil with core and without and it doesn't seem to make any big difference.
In my test I don't use iron plates for both the stator and rotor though.

[SparWeb]

The iron disk for the magnets is the key ingredient.
No don't put iron/steel in the stator.  Let the field lines pass straight through the coil.
What you can do instead is put another disk on the side opposite the magnet disk.
As long as you mount it to the same shaft the magnet rotor turns on, and they turn together, it works well. 
If you put magnets on the second disk, then you end up with a dual-rotor axial flux gen, which is the ideal.
If you can't buy that many magnets, then you still want 2 disks on opposite sides of the stator coils (one with one without magnets).

[Adriaan Kragten]

I have designed, built and measured an 8-pole, 3-phase axial flux generator with only one armature disk with magnets on it. The design of this generator and the results of the measurements are given in report KD 679. The first idea was to use a synthetic sheet on which the six coils are mounted. However, this has as disadvantage that now there is magnetic loop from the north to the south pole which has about a half circular shape and which is rather long. This results in a rather low flux density in the air gap and therefore in a rather low voltage per turn of a coil. Finallly I decided to use a galvanised iron sheet behind the stator coils. Now the flux density is higher resulting in a higher voltage. However, a stationary sheet behind the coils has as disadvantage that eddy currents are generated in this sheet. I have measured the temperature rise after long turning at the maximum rotational speed which occurs at the wind turbine and it was only about three degrees. So this isn't a problem. The energy loss in the stator sheet is also not a problem because the VIRYA-1 rotor for which this generator is meant, is strong enough at high rotational speeds and high wind speeds. The torque due to eddy corrents increases about proportional to the rotational speed and therefore the power loss increases about quadratic. So at moderate rotational speeds, the power loss due to eddy currents in the stationary stator sheet is only low.

A steel sheet also has as advantage that it is stronger and stiffer than a synthetic sheet of the same thickness and that it is easier available in developing countries.

[oramafanis]

The iron disk for the magnets is the key ingredient.
No don't put iron/steel in the stator.  Let the field lines pass straight through the coil.
What you can do instead is put another disk on the side opposite the magnet disk.
As long as you mount it to the same shaft the magnet rotor turns on, and they turn together, it works well. 
If you put magnets on the second disk, then you end up with a dual-rotor axial flux gen, which is the ideal.
If you can't buy that many magnets, then you still want 2 disks on opposite sides of the stator coils (one with one without magnets).

SO since i dont want to deal with a second plate attached to the rotor it is better not to attach a plate at all?
and what about the iron cores inside the magnets? do i have to use them?
Which way is better to go from the 3 i have mention in my post?

[oramafanis]

I have designed, built and measured an 8-pole, 3-phase axial flux generator with only one armature disk with magnets on it. The design of this generator and the results of the measurements are given in report KD 679. The first idea was to use a synthetic sheet on which the six coils are mounted. However, this has as disadvantage that now there is magnetic loop from the north to the south pole which has about a half circular shape and which is rather long. This results in a rather low flux density in the air gap and therefore in a rather low voltage per turn of a coil. Finallly I decided to use a galvanised iron sheet behind the stator coils. Now the flux density is higher resulting in a higher voltage. However, a stationary sheet behind the coils has as disadvantage that eddy currents are generated in this sheet. I have measured the temperature rise after long turning at the maximum rotational speed which occurs at the wind turbine and it was only about three degrees. So this isn't a problem. The energy loss in the stator sheet is also not a problem because the VIRYA-1 rotor for which this generator is meant, is strong enough at high rotational speeds and high wind speeds. The torque due to eddy corrents increases about proportional to the rotational speed and therefore the power loss increases about quadratic. So at moderate rotational speeds, the power loss due to eddy currents in the stationary stator sheet is only low.

A steel sheet also has as advantage that it is stronger and stiffer than a synthetic sheet of the same thickness and that it is easier available in developing countries.

so you use an iron backplate... but what about the iron cores? do you use them also? WHich is the prefered way from the 3 i mentioned in my first post?

[Adriaan Kragten]

I have designed, built and measured an 8-pole, 3-phase axial flux generator with only one armature disk with magnets on it. The design of this generator and the results of the measurements are given in report KD 679. The first idea was to use a synthetic sheet on which the six coils are mounted. However, this has as disadvantage that now there is magnetic loop from the north to the south pole which has about a half circular shape and which is rather long. This results in a rather low flux density in the air gap and therefore in a rather low voltage per turn of a coil. Finallly I decided to use a galvanised iron sheet behind the stator coils. Now the flux density is higher resulting in a higher voltage. However, a stationary sheet behind the coils has as disadvantage that eddy currents are generated in this sheet. I have measured the temperature rise after long turning at the maximum rotational speed which occurs at the wind turbine and it was only about three degrees. So this isn't a problem. The energy loss in the stator sheet is also not a problem because the VIRYA-1 rotor for which this generator is meant, is strong enough at high rotational speeds and high wind speeds. The torque due to eddy corrents increases about proportional to the rotational speed and therefore the power loss increases about quadratic. So at moderate rotational speeds, the power loss due to eddy currents in the stationary stator sheet is only low.

A steel sheet also has as advantage that it is stronger and stiffer than a synthetic sheet of the same thickness and that it is easier available in developing countries.

so you use an iron backplate... but what about the iron cores? do you use them also? WHich is the prefered way from the 3 i mentioned in my first post?

No, I used no iron in the cores because this would result in strong preference positions and therefore in a high starting wind speed. If you have iron in the coil cores, there will be 24 strong preference positions per revolution if you have an armature with eight poles and a stator with six coils. For every preference position, two magnets are just opposed to two cores and the magnetic flux flows easiest for this position. I think that the disadvantage of strong preference positions is more critical than the advantage of a larger magnetic flux if the coil coils are made of iron.

In chapter 10 of KD 679 I have analysed the causes of the losses. It appears that the copper losses in the winding are the main cause of the rather low efficiency. So everything has to be done to increase the flux density in the air gap as this increases the voltage per turn of a coil. Increase of the voltage, allows less turns per coil and a thicker wire and so in a lower coil resistance. The advantage of lesser copper losses is larger than the disadvantage due to eddy currents in a stationary iron stator sheet.

[MattM]

I think some long held information from the forum got lost in this thread.

Eddy currents form across unbroken connects, like sheet or billet stock.  The reason iron filings by themselves is bad is because one flake is in direct contact with others around it, creating electrical paths.  The whole point of imbedding it in epoxy was to isolate contact paths.  Magnetic fields induce electrical currents, and vice versa.  Isolation of the material is absolutely critical, to minimize current generation through the material.  Simply pouring epoxy over powder would not isolate it, you have to stir it in thoroughly.  You are not changing the direction of flux using the powder, but rather lowering resistance of the path.  That flux potential is dependent on the strength and direction of the field through the medium.  This is akin to using better conductors to move electrical fields.  You may ask why metal to hold the magnets.  Because the magnets are static position to the metal holding them, there is no field generated.  Someone suggested a second set of magnets, to increase flux strength.  Again, thats true as long as the magnets all move in unison.  Move the magnetic disks at different speeds (or direction) and you create eddy currents opposing each other.  You already know, when current forms in your coils, you have magnetic fields generated.  Why are those fields not creating resistance?  Actually they do, which is why you never match a coil count evenly with magnet count.  The use of neodymium magnets makes this exercise with iron filings and epoxy an exercise in diminishing returns.  The most important lesson from this forum is that increasing magnet count is a better use of resources than increases in magnetic strength.

There is a simple test if your filings are not isolated.  Heat.

[Jackir]

For better performance in your 3-phase generator, I'd recommend using both iron cores and a backplate. It can improve efficiency, even though you didn't notice a big difference with just the cores.

[oramafanis]

thanks for the info. i have holes that can be mount m5 bolts so in the final stage i will be able to use iron or stainless stell bolts as cores...

[Mary B]

For better performance in your 3-phase generator, I'd recommend using both iron cores and a backplate. It can improve efficiency, even though you didn't notice a big difference with just the cores.

Introduce cogging... not a good idea unless you have high winds a LOT. Starting torque will be higher.

[oramafanis]

Heap.
Maybe I will not use.
Thought cogging in 3 phase will be very little.

[Mary B]

anytime you add iron in a magnet path it will attract...

[Adriaan Kragten]

It is possible to make a generator with a low peak on the cogging torque if the coil cores are filled with iron but not if the rato in between the number of armature poles and the number of stator coils is 4 : 3 (for a 3-phase, 1-layer winding). For instance, some hover board motors have 30 armature poles and 27 coils and so the ratio is 10 : 9. This results in 3 * 10 * 9 = 270 preference positions per revolution and that many preference positions will result in only a low peak on the cogging torque. But this requires a totally different orientation of the coils. Now three coils of one phase are lying against each other and the coils are alternately wound left hand and right hand.

There are other options if the housing of an asynchronous motor is used. I have designed several PM-generators using such a housing for which a magnetic pole is buit up from several mechanic poles and for which there is only a small difference in between the number of mechanical armature poles and mechanical stator poles (see report KD 718). The advantage of this construction is that the standard winding can be used.

[kitestrings]

For better performance in your 3-phase generator, I'd recommend using both iron cores and a backplate. It can improve efficiency, even though you didn't notice a big difference with just the cores.

I like the idea of the back plate, but I'm with Mary on the iron cores.  I wouldn't recommend this - the advantage of using open coils in an axial is there is little to no resistance at start-up.

[bigrockcandymountain]

I'm not 100% sure, but i think if the back plate is not attached to the shaft, but stationary with the stator, that it will cause iron loss and get hot from eddy currents. 

My gut says that if it doesn't spin, it is better to not have it at all. 

[MattM]

Backplates should move with the magnets as bigrockcandymountain says, or you create induction heating.

[Adriaan Kragten]

Backplates should move with the magnets as bigrockcandymountain says, or you create induction heating.

If a non rotating back plate will give problems with a too high rise of the temperature because of eddy currents, depends on the distance of the plate to the rotating magnets, on the maximum rotational speed of the generator and on the cooling capacity of the sheet. I have built and measured an 8-pole generator with a stationary iron back plate and I have measured a rise of the temperature after long rotation at the maximum speed which happens in the wind turbine of only about 3 °C (see for generator dimensions and measurements report KD 679). The generator efficiency is mainly determined by the copper losses and the losses due to eddy currents at moderate rotational speeds are rather low. So for the right circumstances, a stationary iron back plate is certainly possible. But it is important that the wind turbine has a proper safety system which limits the maximum rotational speed otherwise the sheet and so the coils may become too hot.

I used no iron in the coil cores because this would result in strong preference positions. The iron of the stator sheet gives no preference positions even if the sheet isn't circular. There is only a certain force with which the armature is pulled in the directcion of the stator sheet and this axial force gives a certain axial bearing load which gives a little extra friction torque.