Need help/ suggestions regarding iron core AFPMG.

[Aamir]

Hi to all friends and respected senior members.

   I want to make an iron core dual rotor axial flux generator having 16 coils and 32 (16+16) neodymium disc magnets. This is my first time working on AFPMG therefore I need help and guidance. Keeping aside all issues (cogging, eddy current, etc), my main goal is to get maximum output from it that's why I chose iron core. Output will be converted to high voltage dc so frequency and waveform is not an issue as well. I have 20mm diameter solid iron cores (1.5 inch & 2.25 inch). Magnets are 20mm x 10mm/ 20mm x 20mm disc magnets. Output will be single phase and coils will be connected in series or series-parallel combination. Stator and rotors will be made of acrylic or aluminum. But before I proceed, I would like to ask a few questions.

1) Which magnets will be good for the job. 20mm x 10mm, 20mm x 20mm or 25mm x 25mm?
2) Should I use smaller core (1.5 inch) or bigger one (2.25 inch)?
3) What should be the maximum diameter of output coil?
4) Any advantage of bifilar coil over single wire coil?
5) Is 5mm air gap ok between magnets and coils?

Kindly point out other important factors as well which I may have left. Any kind of help will be highly appreciated. Thanks.

[electrondady1]

welcome to the forum . this is the right place to learn how to build alternators.

I build dual rotor axial flux alternators  but there is no iron core.
the alternators consists of two iron ( mild steel) disks with multiple magnets placed on the surface.
the disks need to be iron in order to contain and redirect the magnetic flux.
between the magnet disks  a stator is suspended . consisting of copper coils immersed in a plastic resin disk.

i am unsure what you mean by iron core unless you mean placing iron in the center of the copper coils.

[DamonHD]

Welcome!

[MagnetJuice]

Welcome to Fieldlines Aamir.

"I want to make an iron core dual rotor axial flux generator"

As far as I know, there is not much information about iron core stators for an axial flux alternator, not even in this website.

Questions:
1) Which magnets will be good for the job. 20mm x 10mm, 20mm x 20mm or 25mm x 25mm?
2) Should I use smaller core (1.5 inch) or bigger one (2.25 inch)?
3) What should be the maximum diameter of output coil?
4) Any advantage of bifilar coil over single wire coil?
5) Is 5mm air gap ok between magnets and coils?

Those questions are hard to answer without more information.

The round steel bars on your picture are not the best material to use for cores. The best material would be laminated steel sheets similar to the ones used for transformer cores.

There could be some advantages of an iron core stator for an axial flux alternator, but only if it is designed and built in a way that eddy currents and cogging are kept to a minimum. It would take a lot of experimentation to find that. That could be one of the reasons why not many people build iron core axials.

Ed

[Adriaan Kragten]

If you have iron in the coils, you will get very strong preference positions in the clogging torque for the given number of poles and stator coils and this will certainly give starting problems. It is possible to make an axial flux generator with iron in the coils but then you need another ratio in between the number of poles and the number of coils. This ratio can be 10 : 9. This ratio is also used for hover board motors for which one uses 30 poles and 27 coils.

Assume that you use 10 poles and 9 coils. So the armature pole angle is 360 / 10 = 36° and the coil angle is 360 / 9 = 40°. If you have two iron armature disks with the magnets at the inside and the stator with coils in between, you need 20 magnets. You can best take circular magnets and circular coils. The coils of the three phase U, V and W have to be laid in the following sequence: U1, U2, U3, V1, V2, V3, W1, W2 and W3. Coils of the same phase are connected in series.

If the north pole N1 is just opposite coil U2, the south south poles S5 and S1 are almost opposite to coils U1 and U3 but there is an angle of 4° difference. But this is no problem as it results in a small positive phase angle of 20° in between the voltages generated in coil U2 and U1 and a small negative phase angle of -20° in between the voltages generated in coil U2 and U3. This small phase angle makes that the resulting voltage is only a little lower than if all three voltages would be exactly in phase. As a south pole is about opposite to coil U1 and U3 when a north pole is opposite to coil U2, coils U1 and U3 have to be wound left hand if coil U2 is wound right hand! It is easy to prove that there is a phase angle of 120° in between the voltages generated in coils U2, V2 and W2, so the given winding is a 3-phase winding. You need a 3-phase rectifier with six diodes to rectify the AC current.

For the given ratio 10 : 9, you will get 10 * 9 = 90 small preference positions per revolution, so a preference position every 4°. The advantage of using iron in the coils is that the air gap is much smaller than for a generator with no iron cores in the coils and therefore the generated voltage per turn will be a lot higher. The peak torque will also be a lot higher. However, if you use massive iron bar as core, you will get eddy currents in the iron and these eddy currents make that the cores will become warm and that you need an extra torque for these iron losses. Dissipation of the heat generated in the cores and in the coils is difficult as the cores and coils are enclosed by the armature disks. So the peak efficiency might be lower than for a stator with no iron in the coils. But it might work if you limit the maximum rotational speed.

If your generator isn't big enough for 10 poles and 9 coils, you can also use 20 poles and 18 coils but in this case the coil sequence becomes U1, U2, U3, V1, V2, V3, W1, W2, W3, U4, U5, U6, V4, V5, V6, W4, W5 and W6. In this case you have the option to connect a coil bundle of three coils in series to the other coil bundle of three coils of the same phase for a high voltage and a low current or to connect them in parallel for a low voltage and high current.

[electrondady1]

that seems very complex for a guys first alternator and from the description is sound like it will  cogg like a pig.
 Aamir , how will you power this alternator and what will you use the electricity for?
 

[SparWeb]

The others have asked pertinent questions, so until we know more about your use and what you expect, I will wait to suggest any more about the electrical stator.

In the meantime, can I suggest that instead of solid steel bars, as you show in your photo, that you will have more success if you use a layered core instead.  I am assuming that you intend to insert steel/iron into the center of the stator's wound coils. You will have less cogging and less eddy-current heating if these centers have layers instead of being solid.  The cogging will not be eliminated, just reduced slightly.  The heating of the core by eddy-currents will be your biggest problem.

You will also need to layer the cores in an inconvenient way.  Disks won't work.  The layers have to cross the diameter.

[Adriaan Kragten]

that seems very complex for a guys first alternator and from the description is sound like it will  cogg like a pig.
 Aamir , how will you power this alternator and what will you use the electricity for?

If you use 10 armature poles for nine coils, so for nine cores, there will be only one magnet just opposite to one core at a time. So the peak on the fluctuation of the clogging torque will be very small. The winding isn't very complex if you follow my description. However, if you make the design according to the original idea, so with 16 poles and 16 cores, all 16 magnets will be opposed to all 16 cores at the same time and this will give an enormous peak on the clogging torque. You also get a 1-phase winding with a very strong fluctuation of the DC current.

Very long ago I have checked a generator with such a winding made in, I thought, Vietnam. If I remember well, this generator had 20 poles and 18 coils. This generator was used for a water turbine and the rotational speed was limited by limitation of the water flow. One used circular ferroxdure magnets, circular coils and a massive iron bar as core. A tapered concentrator was mounted on each magnet to concentrate the magnetic field in the core. So a core had a much smaller diameter than a magnet. The generator had only one armature disk and the stator coils were mounted at a steel plate. So you have also eddy currents in this steel plate. This generator was manufactured in large quantities and rather cheap at that time. So this idea can certainly work. If you use neodymium magnets, you need no concentrator, so the cores can have the same diameter as the magnets.

It's true that it is better to use laminated iron for the cores but as the lamination must be in parallel to the direction of the magnetic flux, making of such cores is difficult. I think that clogging will not be less if you use laminated iron because laminated iron guides a magnetic flux almost as good as massive iron. You get a peak on the clogging torque if there is a position of the armature for which the magnetic flux flows easiest from the armature to the stator. But eddy currents will be much less for laminated iron.

[Adriaan Kragten]

There are three reasons why an unloaded PM-generator requires a torque to rotate the armature at low rotational speeds. These reasons are: friction, eddy currents and preference positions.

1) Friction
Friction is caused by the bearings and by the seal on the generator shaft. Ball bearings have very low friction even if they have rubber seals at both sides of the bearing. However, these rubber seals are not enough to prevent entrance of water if the bearing must have a long lifetime. So an extra spring loaded oil seal is required on the generator shaft. The space in between the front bearing and the oil seal has to be filled with grease. This grease reduces the friction of the seal and it also reduces the wear of the seal. The back bearing cover can be closed, so no seal is required at the back bearing. The friction moment of a spring loaded oil seal can be about a factor ten larger than the friction moment of the bearing itself so the oil seal is the main cause of the friction moment. The friction moment is about independent of the rotational speed, so it is about a horizontal line in the Q-n graph.

2) Eddy currents
Eddy currents happen if the coils have an iron core. Eddy currents happen only if the magnetic flux in the iron is varying. The direction of the eddy currents is perpendicular to the direction of the magnetic field. Eddy currents can be reduced if the iron is laminated with an isolator in between the sheets and such that the direction of the lamination is the same as the direction of the magnetic field. The losses due to eddy currents result in an extra torque. This torque is zero at n = 0 rpm and increases about linear to the rotational speed. This torque is higher for delta than for star rectification because higher harmonic currents can flow in an unloaded winding for delta rectification. The higher torque for delta rectification is not really caused by eddy currents but because circulating currents in the winding cause a certain internal generator load. You will also get an increasing torque for a PM-generator with no iron in the coils if the winding is rectified in delta and if there is no external load. The steepness of the Q-n curve for eddy currents is much stronger for non laminated iron than for laminated iron.

3) Preference positions
Preference positions only happen if there is iron in the coils. Preference positions are independent of the rotational speed but are active if there is a position of the armature for which the magnetic flux flows easiest from the armature to the stator. You get a preference position if an armature pole is just opposite a stator core. If you have a 16-pole armature and a stator with 16 coils with iron cores in the coils, you will get 16 very strong preference positions per revolution because if one armature pole is opposite to one stator core, this will also be the case for all other fifteen poles and cores. The torque due to preference positions varies about sinusoidal. A complete sinus corresponds to a rotational angle of 22.5° for an armature with 16 poles. Assume that the angle of rotation measured from the preference position is called alpha. So the torque is zero for alpha = 0°, for alpha = 11.25° and for alpha = 22.5°. The torque has a maximum for about alpha = 5.625° and a minimum for about alpha = 16.875°. The zero point for alpha = 11.25° is instable which means that from this point the armature will turn backwards to alpha = 0° or turn forwards to alpha = 22.5°.

The final torque at a certain unloaded low rotational speed is the sum of all three different torques. So first the torque due to eddy currents has to be added by the torque due to friction. This is the average torque. However, the torque fluctuation due to preference positions is also active when the armature is rotating. So the momentary torque can fluctuate very strongly if the torque fluctuations due to preference positions are high. The peak torque due to preference positions can easily be a factor twenty or more higher than the friction torque if the number of armature poles is the same as the number of cores. This torque fluctuation is supplied by the flywheel effect of the rotor. So it isn't a real problem once the rotor is rotating at a sufficient high rotational speed. But if the rotor is not rotating, there is no flywheel effect and the starting wind speed is then mainly determined by the peak torque due to preference positions.

The fluctuating torque due to preference positions can make a PM-generator rather noisy at high rotational speeds. I have measured a 28-pole hub dynamo of a bicycle for 12 V battery charging. This dynamo has 28 rather strong preference positions per revolution and became rather noisy at high rotational speeds. You don't feel the preference positions if you spin the wheel but if you take the wheel out of the bike and if you turn the shaft, the preference positions appear to be rather strong.     

[MattM]

Correct me if I'm wrong, but every magnetic field creates an electrical field and vice versa.  So eddy currents are simply natural counter rotating fields that occur on the edges of main field.  So if you don't want eddy currents you either have to keep any electrical or magnetic field in strict dimensions or utilize the eddy flows.  Where they form is directly in relationship to the strength of the fields, which is why shaping the fields is probably easier than utilizing them.

[Aamir]

Thank you so much all of you. I am sorry I didn't mention that this alternator will be driven by a variable speed bldc motor and it's output (low amps high voltage dc) will be used to power a hybrid solar inverter. I want to make it for experimental purpose and if I succeed then I intend to make a bigger multi rotor alternator. I will currently use round solid iron cores for stator coils because they can be easily made in any size. I will definitely go for laminated iron core if this machine worked.
 
Rotors will be made of aluminium for light weight or iron if necessary. Holes will be made in rotors for perfect alignment of magnets with coils. Aluminium will be used to hold stator coils. One bearing in the stator and two bearings at the ends will be used. Shaft dia will be 10-15mm and rotors will be moveable to adjust air gap. Required power output from this alternator is 1000 watts or more.

I once read in an article that diameter of an iron core stator coil can be 1.5 times greater than it's length because of concentrated magnetic field in the core. If it's true then in my opinion it will be easy to get high voltage output (300-500 volts) from 16 coils and 32 neodymium magnets. I hope 20mm x 20mm disc magnets with 20mm solid iron cores inserted in stator coils will do the job but as I said before, experts help and guidance is requested to proceed further on this project. Thanks.

[joestue]

Unless this generator is very slow turning, solid iron cores are not going to work.

Take surplus pallet banding strap, burn it in a fire lightly.. not for long. enough to build up an oxide layer and temper the steel. coil it up into rings, drill a hole in it and rivet the last turn together would be far better than solid iron core.

an aluminum disk with holes cut for the coils will not work either. you can have thin fingers of aluminum reach into the magnetic field path to hold the coils, but you can't have a thick chunk of it, nor can you have the aluminum make a complete loop through the magnetic field.

[Adriaan Kragten]

Correct me if I'm wrong, but every magnetic field creates an electrical field and vice versa.  So eddy currents are simply natural counter rotating fields that occur on the edges of main field.  So if you don't want eddy currents you either have to keep any electrical or magnetic field in strict dimensions or utilize the eddy flows.  Where they form is directly in relationship to the strength of the fields, which is why shaping the fields is probably easier than utilizing them.

Eddy currents are only created when the magnetic field varies and have a direction perpendicular to the main field. So you have no eddy currents if the armature isn't rotating. That is why the preference positions can't be caused by eddy currents. If the armature is rotating and if a certain eddy current is flowing, this current creates a magnetic field which is perpendicular to the eddy current and this secondary magnetic field is opposite to the original magnetic field. That is why eddy currents result in a certain torque. But as eddy currents are flowing in the iron of the core and not a the copper of the coil which is wound around the core, eddy currents have no contribution to the electrical power which is generated. Once a current is flowing in the copper coil, this current also gives a secondary magnetic field which is counter acting the original magnetic field and this is the reason why you need a torque if the generator produces electrical power. The torque because of the current flowing in the coils is much larger than the torque because of the eddy currents flowing in the cores.

Eddy currents are not only created in iron but in any material which guides an electric current. So if the coils are mounted in an aluminium disk, you will also get eddy currents in this disk.

[MattM]

Every magnetic field creates an electrical flow at a righthand perpindicular to the magnetic field, and vice versa.  You may not be able to measure the electrical field in say a vacuum, but it is there.  It's a rotating set of fields, always perpindicular to the other.  All 'eddy flows' likewise are locked in flipped alignment to the original.  So a stationary rotor still has them, they just are static.

[Adriaan Kragten]

Every magnetic field creates an electrical flow at a righthand perpindicular to the magnetic field, and vice versa.  You may not be able to measure the electrical field in say a vacuum, but it is there.  It's a rotating set of fields, always perpindicular to the other.  All 'eddy flows' likewise are locked in flipped alignment to the original.  So a stationary rotor still has them, they just are static.

I can't agree with this statement if I understand you correctly. A voltage is only created if the magnetic flux varies. If you keep a coil in a constant magnetic field, no voltage is created and so no current is flowing in the coil. If you make a stator with coils with only air in the core of the coil, there will be an electric field created in this core if the armature is rotating but as air has a very high electric resistance, no substantial eddy current will flow. Therefore you will have no increase of the unloaded torque at increasing rotational speed. This is just one of the reasons why people make generators without an iron core in the coil.

But if you have a coil with an iron core, the magnet of the armature will take position such that the magnetic flux flows easiest from the magnet to the core. This is the case if the magnet is just opposite to the core. If you turn the armature over a small angle, the overlap in between the magnet and the core becomes smaller and this makes that the flow of the magnetic flux is hindered. So the core is pulled back to the position of maximum overlap of magnet and core. This creates a moment in between armature and stator and the moment is larger as the overlap in between the magnet and the core is smaller. But the moment is there for a certain angle even if the rotational speed is zero. So this moment is not caused by eddy currents.

If you make an armature with ten poles and nine cores, you have a fluctuating moment for every magnet. However, these fluctuating moments are not in phase to each other. If a north pole is opposite to a core, the south pole positioned at an angle of 180° with this north pole is just in between two cores. Therefore there is a shift in between the moment patterns of these two magnets such that the resulting moment is about zero. This is the case for all opposite magnets and the resulting moment of all ten magnets is therefore almost zero if you rotate the armature slowly.

[MattM]

But you're talking about an additional vector of field rotation, with an additional component of time being introduced.  But it only moves the field around points, it doesn't suddenly create it.  Nor does absence of rotation destroy it.  Every circuit moves in both direction from the positive and negative poles at the speed of light when there is motion.  As the fields rotate the polarity shifts.  But the motion doesn't create the electrical or magnetic fields, it simply gives them direction.  The fields are still there even when your fields are otherwise static. 

[joestue]

But you're talking about an additional vector of field rotation, with an additional component of time being introduced.  But it only moves the field around points, it doesn't suddenly create it.  Nor does absence of rotation destroy it.  Every circuit moves in both direction from the positive and negative poles at the speed of light when there is motion.  As the fields rotate the polarity shifts.  But the motion doesn't create the electrical or magnetic fields, it simply gives them direction.  The fields are still there even when your fields are otherwise static.

i honestly have no idea what you're saying.

no one uses solid iron cores in an alternating current motor or generator. you can sort of get away with solid iron cores in the dc field of an alternator but even 100 years ago they moved away from cast iron pole pieces on dc machines.

[Adriaan Kragten]

But you're talking about an additional vector of field rotation, with an additional component of time being introduced.  But it only moves the field around points, it doesn't suddenly create it.  Nor does absence of rotation destroy it.  Every circuit moves in both direction from the positive and negative poles at the speed of light when there is motion.  As the fields rotate the polarity shifts.  But the motion doesn't create the electrical or magnetic fields, it simply gives them direction.  The fields are still there even when your fields are otherwise static.

You might be right but I can't follow you. I have simply described how the torque fluctuates if you move the armature from the position when an armature pole is just opposite to one core.
Assume that you have ten armature poles and only one stator core. In this case there is a preference position when an armature pole is just opposite to a core. It isn't important if this is a north or a south pole. So for 36° rotation of the armature you will get an about sinusoidal fluctuation of the torque and so you have ten stable preference positions per revolution.
Assume you have ten armature poles and ten stator coils. The peak torque will now be a factor ten higher as the torque fluctuation of all armature poles are in phase to each other. You still have ten preference positions per revolution.
Assume you have ten armature poles and nine cores. Now the situation is completely different. If a north pole is opposite to a core, the opposite south pole is just in between two cores. If the torque fluctuation would be exactly sinusoidal, both torque fluctuations will cancel each other and no torque will remain. However, the torque fluctuation isn't exactly sinusoidal and therefore some small fluctuation remains. Now you get a preference position every time when a magnet is opposite to a core. This happens every 4° and that is why in practice you will get 360 / 4 = 90 small preference positions per revolution.
So a ratio 10 : 9 in between the number of magnets and the number of cores gives a fluctuation of the clogging torque which is very much lower than if the number of cores is the same as the number of armature poles.

Next assume that the number of cores is the same as the number of armature poles. For the strong torque fluctuation at a very low rotational speed, you will get about the same peak torque if you use cores out of massive bar or laminated cores. However, when the armature is rotating, you will get much higher eddy currents in massive cores than in laminated cores. So the average torque will rise much faster at increasing rotational speed for massive cores than for laminated cores.

[MattM]

When I hear 'only', it is meaning something exclusive.  When you said, "Eddy currents happen if the coils have an iron core. Eddy currents happen only if the magnetic flux in the iron is varying."  I was simply saying that isn't quite true and why I believe that.  I was reacting to your responses that used terms that have dual meanings, some that do not meet the context of your explanation.  When you use the word 'varies' it seems out of context.  Moving wire through varying levels of the same polarity doesn't create the voltage and impact the amperage.  Moving it through a shift in polarity does induce a voltage.  Rotating a magnet to shift polarity of the magnetic fields does directly affect the magnitude of electricity potential for amperage and voltage.  The 'flux' that matters most for generating electricity is where these lines of polarity intersect.  If someone uses iron cores it is possible to get benefit as long as it is shaped specifically to get flux where they want it. When someone says iron core it doesn't mean all iron cores are blobs of metal flakes imbedded in epoxy or even simple plugs of ferrous material.  Modern EV motors are being creative with flux to get torque and power in ways previous motors never could, by using iron cores in very distinct forms to shape their magnetic fields.  I guess its just some of your choice of wording that triggered my responses.

[Mary B]

As your magnetic field rotates around the stator it would cause polarity shifts in your iron cores creating eddy currents. Only time your field is non-moving is if you have no wind.

[Adriaan Kragten]

As your magnetic field rotates around the stator it would cause polarity shifts in your iron cores creating eddy currents. Only time your field is non-moving is if you have no wind.

There can be a lot of wind and still the rotor can not rotate. This happens when the starting torque of the rotor at that wind speed is smaller than the peak torque of the generator at stand still position. This is not caused by eddy currents. I have measured the torque required to rotate a copper disk moving in between two permanent magnets and this torque is zero at n = 0 rpm and increases linear with the rotational speed. A linear Q-n curve gives a parabolic P-n curve. All the input mechanical power is transferred into heat. So the copper disk becomes hotter as the rotational speed is higher. The torque is caused by eddy currents. So to get eddy currents, you need change of the magnetic field in the copper disk and this requires rotation.

Assume that you have a PM-generator with a high peak torque and you load this generator to almost the peak torque and keep it there non rotating. If there would be eddy currents during this state, these eddy currents would produce heat. So you get heat without any mechanical input power which means that you have created a perpetual mobile. This is another indication that the peak torque is not caused by eddy currents.

So I think that lamination of the core is useless if it is meant to reduce the peak on the clogging torque at stand still position. It would be good if someone would build two identical generators, but one with massive iron cores and one with laminated iron cores of the same diameter and thickness and measure the peak torque. The  peak torque for laminated iron cores might be a little lower but that isn't caused by the lamination but simply because less iron can be put in a certain volume if the core is laminated. If both generators are compared rotating with the same rotational speed, then the one with massive cores will certainly have a higher average torque level.

[electrondady1]

not sure how good  of alternator it would be but it might make a pretty good induction heater

[MattM]

An intetesting phenomenon with horseshoe magnets is their uniform field between the parallel sections.  Outside those parallel sections it is not uniform.  It makes me think iron flat stock in horseshoe shapes could extend fields out from magnets, especially if each leg is influenced by separate opposing polarities.  Could one wrap the horseshoe, wound around each leg in opposite spirals, to maximize coil size?  Not to inctease actual flux strength but volume under influence of the flux.  No eddy currents to worry about.

[Adriaan Kragten]

I have measured several PM-generators made from asynchronous motors on a very accurate test rig. These generators have magnets which are mounted in grooves which are inclined such that there is exactly one stator slot overlap in between the left and the right part of the armature. Therefore the overlapping area in between an armature pole and the iron of the stator poles is constant for every position of the armature. These generators therefore have no fluctuation of the clogging torque. The stator is provided with laminated iron and I have measured the unloaded torque for star and delta rectification. The measurements for a generator frame size 90 are given in public report KD 78. The starting behaviour of this generator in combination with the VIRYA-3B3 rotor is investigated in figure 8 of public report KD 484. It is good to study this figure to understand the starting behaviour.

The unloaded Q-n curve of the generator for star and for delta rectification start both at a torque of about 0.4 Nm which is very low as the peak torque of the generator is about 30 Nm for star rectification and about 38 Nm for delta rectification. The starting torque of 0.4 Nm is only caused by the friction of the rubber sealed ball bearings and the extra spring loaded oil seal on the generator shaft. If the unloaded Q-n curve of the generator for star rectification is followed, it can be seen that the torque at n = 200 rpm is about 0.6 Nm. So the rise of the torque due to eddy currents in the stator is only 0.2 Nm. If the unloaded Q-n curve of the generator for delta rectification is followed, it can be seen that the torque is about 1.2 Nm at n = 200 rpm. Only 0.2 Nm is caused by eddy currents and 0.6 Nm is caused by circulating higher harmonic currents in the winding for delta rectification.

In figure 8, I also give the Q-n curve of the rotor for a wind speed of 2.7 m/s. This wind speed is the starting wind speed as for this wind speed the rotor can produce a starting torque of 0.4 Nm. This curve is rising faster than the Q-n curve of the generator for star rectification which means that the rotor will really start if the wind speed is 2.7 m/s. The torque difference is used to accelerate the rotor and this torque difference becomes larger if the rotational speed is higher.

The Q-n curve of the rotor for V = 2.7 m/s coincides about with the Q-n curve of the generator for delta rectification for rotational speeds in between 0 and 25 rpm. So the rotor won't really start at V = 2.7 m/s for delta rectification. In figure 8, I also give the Q-n curve of the rotor for V = 3 m/s and this curve is lying higher than the Q-n curve of the generator for delta rectification. So the rotor will certainly start at V = 3 m/s for delta rectification.

The starting torque for V = 3 m/s is about 0.5 Nm. The optimum torque for V = 3 m/s is about 3.55 Nm at about n = 125 rpm. So the ratio in between the starting torque and the optimum torque is 0.5 / 3.55 = 0.14. This is rather low but a good value for a rotor with a design tip speed ratio of 6.5. So figure 8 shows that torque loss due to eddy currents is not a problem for a PM-generator made from an asynchronous motor. But these motors have stator sheets with a thickness of only about 0.5 mm and with electric isolation in between the sheets such that eddy currents can't flow from one sheet to another. So eddy currents can flow only in one sheet and are therefore rather small. The friction of the bearings and the seal in combination with the starting torque of the rotor for a certain wind speed determines the starting wind speed.

For wind turbines of Hugh Piggott, one uses the shaft and bearing housing of a car wheel. This shaft is provided with tapered roller bearings and a rather large seal on the shaft. So a PM-generator with this kind of bearings might have a much higher starting torque than a generator with ball bearings made from an asynchronous motor. I think that this is the main reason why one has chosen for an axial flux generator with no iron in the coils as this makes that there is no extra peak torque due to clogging and no extra unloaded torque caused by losses due to eddy currents.

Figure 8 out of report KD 484 has been added as an attachment
 Figure 8 KD 484.pdf (10.09 kB - downloaded 33 times.)

[clockmanFRA]

For wind turbines of Hugh Piggott, one uses the shaft and bearing housing of a car wheel. This shaft is provided with tapered roller bearings and a rather large seal on the shaft. So a PM-generator with this kind of bearings might have a much higher starting torque than a generator with ball bearings made from an asynchronous motor. I think that this is the main reason why one has chosen for an axial flux generator with no iron in the coils as this makes that there is no extra peak torque due to clogging and no extra unloaded torque caused by losses due to eddy currents.


(Attachment Link)

It really depends on the bearing hub used.

On all three of my 3.7m diameter Hugh Piggott design Wind Turbines, and after some in depth research on motor industry wheel hub designs, in the end i found the Peugeot Boxer van, Citroen relay van and the Fiat Ducatto van all have the same casting and design system.

These van bearing hubs are 5 stud, a good diameter for the hubs and a good diameter for attaching the magnet discs.  However they now use the single long bearing arrangement that does not allow for any slack take up like a taper roller bearings does.

But a bit of machining to the substantial casting soon rectifies that and all my wind turbines have no oil seal taper roller bearings. However i do make and machine a dust/oil cap to keep the grease/oil from contamination from the elements of the weather.

At first i found that removing the tension spring around the bearing oil seal rubber helped, but after a few years the rain eventually got in.

Also i have found in light winds that the wind turbine with Fiberglass blades, and 3 times as heavy as my cedar wood blades, generates about 20% more each year than the others. Its as if the heavier blades keep the momentum and the turbine running. While the lighter blades need time to get back up to generating speed. Start up wind is the same on all 3 of my wind turbines.

[Adriaan Kragten]

Also i have found in light winds that the wind turbine with Fiberglass blades, and 3 times as heavy as my cedar wood blades, generates about 20% more each year than the others. Its as if the heavier blades keep the momentum and the turbine running. While the lighter blades need time to get back up to generating speed. Start up wind is the same on all 3 of my wind turbines.

(Attachment Link)

This is strange, I would have expected just the opposite. If a rotor has a large moment of inertia, it works more as a flywheel. So following changes of the wind speed takes a longer time than for a rotor with a low moment of inertia. This means that a heavy rotor will turn longer at a tip speed ratio which is lower or higher than the optimum tip speed ratio and the average Cp will therefore be lower.

But it can also be that the heavy rotor has been made better to the optimum geometry or that the airfoil surface is smoother and that therefore this rotor has a higher maximum Cp. A high moment of inertia also means that starting of the rotor from stand still position takes a longer time if the wind speed goes up. On the other hand, once the rotor is rotating, it takes a longer time to stop if the wind speed goes down. I think that these two effects balance each other concerning the total output.

[joestue]

An interesting phenomenon with horseshoe magnets is their uniform field between the parallel sections.  Outside those parallel sections it is not uniform.  It makes me think iron flat stock in horseshoe shapes could extend fields out from magnets, especially if each leg is influenced by separate opposing polarities.  Could one wrap the horseshoe, wound around each leg in opposite spirals, to maximize coil size?  Not to inctease actual flux strength but volume under influence of the flux.  No eddy currents to worry about.

Again not sure what you're saying, but the whole point of using iron cores in alternators and motors, is so that you can get a whole lot more copper wrapped around the magnetic flux lines of the magnet. it only takes 1/1000th the magnetic "force" to generate a 1T field inside a piece of steel with minimal air gap, as it does to generate it in open air. we call this permeability.

The magnet itself is open air with a permeability of 1. . alnico magnets have a permeability of 10-14 iirc which is why they can be used for some interesting applications, and that's why they can be demagnetized easily.

if you were to stamp out horse shoe shaped iron plates, thin, like say .010" thick, group 90 of them together to make a 1" square cross section.. then yes you can make a good quality concentrated pole machine if you were to wind 12 of those horseshoe magnets, place 10, 1" diameter round magnets on a disk, and then spin the disk between the 12 horseshoe assemblies.

the machine would not be particularly efficient with regard to the use of the metal, but it would work

a better method is this design.
https://www.hoganas.com/globalassets/download-media/sharepoint/brochures-and-datasheets---all-documents/somaloy_axial-flux-solutions_1701hog.pdf

or this design
http://ancientpoint.com/imgs/a/j/h/i/s/antique_electric_bipolar_motor_toy_open_frame_cast_iron_paint_1_lgw.jpg

[Adriaan Kragten]

There is a big difference if a PM-machine is used as motor or as generator for a wind turbine. If the machine is used as a motor, it is no problem if the motor has a certain peak on the clogging torque. You only need a certain current to start the motor. But if the machine is used as generator, you will get starting problems if the generator is used in combination with a fast running windmill rotor with a low starting torque coefficient. This is the reason why many PM-motors can't be used as windmill generator.

I have investigated if the front wheel PM-motor of an electric bike could be used as windmill generator but this is rather difficult as the motor which I used had a rather strong peak on the clogging torque. But use of the motor in the bicycle gave no problem at all.

[MattM]

Joestue-

I'm not following why you would want thin plates if the horseshoe shape has shaped uniform lines of force by default.  It stands to reason thick flat stock would be better.  Less airgap along the parallel bars, the less losses.  Like you said, it permeates the iron 1000X better.  Stands to reason that your coil would have uniform flux across every strand.  The purpose is to reduce air gaps in flux by utilizing a third dimension in construction.  The question is, woul dit be an improvement?  If it worked the same but was easier to stay cool, then it could still have a use.

[joestue]

think of the perimeter of your solid iron bar as a "shorted turn" this time of iron, not copper.

take a look at the photo at the bottom of the page.
https://www.seventransistorlabs.com/tmoranwms/Elec_IndHeat9.html

the reason the perimeter of the plate gets hot first is because that's where the current is flowing. the magnetic field inside the coil is uniform, but also oscillating at 20Khz.

[MattM]

Luckily we aren't aiming for 80A at 300V and 20Khz, like your example.

I think we'd be safely in the lower left corner in any of his plots.

[joestue]

let us know how it works for you.

[MattM]

So I tried proof of concept.  The horseshoe shape does not magnetize.  I tried stacks of magnets and for some reason the field zeroes out.  I did see something interesting using a single bar with opposing polarities attached on each side of the bar.  Curious enough it was unintentional and only worked at a specific distance and faded being too close or too far from each other.  If not for this accident I'd of never stumbled upon it.  Best part it was with ferrous magnets.  I lack enough neodymium to try them for the same effect but I'm thinking it should work, too.  Just have to wait for those extra magnets to 'prove it.'

I started another thread to track that phenomenon.