Generator coils, core or no core?

[Njguy]

If you have a permanent magnet generator would it be more efficient to have coils with iron cores or air cores? I am wondering because it seems that the flux from the permanent magnets is there already regardless of the core. Would there really be that much of an efficiency increase with ferrous cores? Also you wouldn't have to worry about the permanent magnets on the rotor being attracted to the iron cores and slowing it down further.

 

[tcmtech]

There is a reason that all power generators use laminated iron alloy cores and not air cores.

 

[unclejed613]

the cores actually increase the flux, because you can get a very close gap spacing. with no core, the field lines cut through very few of the conductors, since the strongest field lines loop around the magnet itself. the cores guide the field lines so that they effect all of the conductors, and with the closely spaced gap, the strongest field lines follow the core to the gap, look up any textbook that describes motor and generator theory, and you will likely find a pictorial representation of this, and it will make sense

 

[shortbus=]

Axial flux generators don't use iron cores.

 

[Njguy]

Axial flux generators don't use iron cores.

That is the exact reason why I created this thread. I wasn't sure why axial flux generators aren't made with cores.

 

[crutschow]

If you look a this article, you will see that the rotor consists of two parallel plates with permanent magnets on each plate. The gap between the two plates is very small such that the magnetic field is still very strong without any added magnetic material between the plates. The thin, flat windings are placed between the two plates.

A typical generator needs the added magnetic material since otherwise the air gap would be large, greatly reducing the magnetic field through the coils.

 

[johansen]

the reason some axial flux generators don't use cores is because the topology of the generator does not lend itself to a very dimensionally stable system.
so many just get rid of the core than deal with thousands of pounds of forces exerted on a laminated magnetic stack that is more like a roll of fish tape than a block of laminations that are fitted together and then inserted into a steel shell, which is often welded together.


essentially what a magnetic core does is it enables you to put 50 times as much copper into the generator as you would otherwise be able to

 

[johansen]

the article crutschow linked: https://www.warlock.com.au/10kwgenerator-abstract.htm

I'm going to be generous and give them 3KW at 515 rpm. at 50% efficiency.

32 magnets, each 2 inches by 1 inch by .5 inches.
that is 32 cubic inches of magnet.

with 1.5 cubic inches of magnet, I got 138 watts from this:
**broken link removed**
at only 8.5% voltage drop, and used not even one pound of copper.
which means there's 11 watts lost in the copper.
probably 50 watts lost in the iron, but i don't have an iron loss number yet.
so if i pick a reasonable number like 50 watts copper loss that comes out to a 300 watt machine, at 500 rpm, and 75% efficiency. 400 watts in, 300 out.

so divide 3 kw by 32 cubic inches of neodymium and you get 93 watts per cubic inch.
about the same as what i got.

now surprisingly we get about the same value, but mine is running at 8.5% copper losses, theirs at 50%.

but one relatively unknown property of electrical machines is they are supposed to scale at the 4/3rds power of the specific size.
axial machines also scale torque with radius cubed, as opposed to radial machines with the radius squared.

because air core machines are always copper limited,an axial machine that uses 32 cubic inches of magnet, and is on the order of 16 inch diameter, 15 poles, should generate something on the order of 38 horse power, at 500 rpm.. at 95% efficiency.

i think there's something wrong with that number but one benchmark you can use is the toyota prius pulls 80HP out of a 4 pole motor using 18 cubic inches of neodymium magnets at about 600 hz or 9000 rpm iirc.
a 16 pole motor would spin at 2250 rpm to generate that much power and you would have to double the radius of the machine, but you would use exactly the same amount of magnet, and copper to get the same mechanical power.

 

[johansen]

If you have a permanent magnet generator would it be more efficient to have coils with iron cores or air cores? I am wondering because it seems that the flux from the permanent magnets is there already regardless of the core. Would there really be that much of an efficiency increase with ferrous cores? Also you wouldn't have to worry about the permanent magnets on the rotor being attracted to the iron cores and slowing it down further.

iron cores have static iron losses that scale with the square of the rpm.

ironless motors do not have much eddy current to speak of unless we're talking 20K rpm and above, in which they also have air drag losses that scale with the square and the cube of the rpm as well..

the ironless generator does not have any airdrag, or iron drag losses at low rpm.
they do not have an "efficiency" value either.
the copper losses increase with the square of the power drawn from the machine, and that does not change with rpm.

you could make them infinitely big, and 99.9% efficient (bearing losses only) but they would cost 100 times as much as a properly sized generator. they would also weigh 10 times as much.

practically speaking axial iron less generators are sized to "not burn out" at the maximum loading.
because they are almost never run at those conditions, they are built to extract usable power at less than 20% electrical loading..
at which their "efficiency" is 99 to 80%. dropping to about 50% at burn out conditions.

 

[shortbus=]

That is the exact reason why I created this thread. I wasn't sure why axial flux generators aren't made with cores.

There are many, many pages in Google devoted to axial flux generator "thesis". Lots of good information out there just by adding the word thesis to a search.

Both axial motors and generators don't use a core iron because both magnetic poles are together at the coil. North on one side, south on the other, of the coil. In a radial motor or generator, the iron core is needed to get both poles, to complete the magnetic path.

 

[Njguy]

I understand this. So it goes back to my original question. Would adding an iron core be of any benefit, or is the flux already at its most dense since the path is being focused straight though the coil? Or would the magnetic domains of the iron being lined up in a core have a multiplying effect?

 

[MrAl]

Hi,

I tend to think that adding a separate iron core to each coil would mean better magnetic coupling and so the generator would produce more power at slower speeds. However, there's a big disadvantage to this because the better the coupling the more magnetic drag and that means if it is used in a windmill type application the windmill may not be able to start up in light winds and depending on the forces between the magnets and the core this could mean it might not even be able to start up in moderate winds.

 

[johansen]

I understand this. So it goes back to my original question. Would adding an iron core be of any benefit, or is the flux already at its most dense since the path is being focused straight though the coil? Or would the magnetic domains of the iron being lined up in a core have a multiplying effect?

Its not about concentrating the flux, its about forcing the magnetic field to flow around more copper, thus reducing copper losses.
my statement than an iron core allows you to use 50 times as much copper was a bit of hyperbole, but you get the idea.
neodymium magnets cost on the order of 50-100$ per pound, copper is 6-9$.

for neodymium air core machines, the optimum amount of copper is on the order of the same thickness of magnet.
iron core machines are practically limited to about ten times as much copper than magnet.

air core machines are practical when dealing with low temperature superconducting coils, on the order of 5 to 10 tesla field strengths, here, iron is of no use because it saturates at 1.95T for the best iron.

 

[dr pepper]

Saturation being a key point when designing alternators or dynamo's, the amount of iron and the air gap to an extent depict when saturation begins(as well of course as the magnets flux), this can effect both the max output and the max rpm of your machine.
Iron cores also ought to be laminated to reduce eddy's.
Sometimes I wonder if some axial flux wind gens are designed that way for constructional simplicity, or even if the designer doesnt understand how to incorporate an iron core.
'cogging' as mr al inffered also increases with iron cores, its a trade off you need some drag to be able to produce energy, some alternators have parallelogram shape laminated iron poles, the idea being to introduce the amount of iron gradually as the magnets rotate, reducing the onset of 'cogging', but not the total amount, still however lowering start up torque.

 

[johansen]

one way to make a partly iron core axial flux machine is to use a toroidal transformer core, in which all the 15+ coils are located in segments, wound toroidally as normal, with the exception that the magnets are located on rotor disks that fly over the windings. The magnetic flux lines are cutting through the individual phase coils, but they meet in the middle and flow through the core to make a return trip across the coil again.
This offers a number of improvements, the first being that you have a steel core for the copper to be epoxied to, and the surface area of the copper coils can be effectively doubled, (as with the typical disk coils, they have to be completely buried in epoxy. also, the steel core conducts the flux effortlessly, and with no cogging.

but the iron core itself still must be securely bolted and held in the center of the magnet disks.

 

[dr pepper]

I have a large 60's clock movement that works similar to that, must be effecient to run off batteries for months, obviously its a motor not a generator.