Author Topic: solid or laminated magnet rotor?  (Read 10681 times)

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collidog

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solid or laminated magnet rotor?
« on: November 23, 2015, 12:49:19 PM »
solid disc magnet rotor or laminated thin sheets as in transformers? I have tried building up rotor thickness gradually and measuring results. Initial results indicate  similarities with varying magnet/ coil gap, i.e. rotor speed increases to provide a fixed final power into a resistive load. Is this the expected result?

hiker

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Re: solid or laminated magnet rotor?
« Reply #1 on: November 23, 2015, 02:01:42 PM »
solid steel mag rotors....no need for laminated on rotors...laminated ...is used  for the coils..on motors..and gens that are built like motors..
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collidog

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Re: solid or laminated magnet rotor?
« Reply #2 on: November 24, 2015, 04:46:54 AM »
  Thanks for that. Let me reword the question slightly.
  Are there any DISADVANTAGES to using laminated rotors? (axial flux type, of course). I do not have the facilities to machine solid rotors, so some years ago when I began practical experiments, I used tinplate that I could cut with tinsnips then laminate the discs to required thickness, using various methods.  By gradually building up the thickness, this gave me a "real-world" awareness of how adjusting rotor thickness to magnet strength affected power output. I also wanted to minimise rotor weight and thus lower starting  inertia. If solid rotors have been proven in practice to be more efficient electrically rather than merely convenient for the car disc brake aficionados, then for my own convenience I shall have to live with it. Perhaps practical comparisons have not been made?

electrondady1

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Re: solid or laminated magnet rotor?
« Reply #3 on: November 24, 2015, 09:24:02 AM »
i found 10" table saw blades to work OK as rotors for ferrite mags.
if your using neo mags, you will need multiple layers of blades to contain the magnetic flux.
you can test with something like a paper clip on the back side to see is it wants to stick.
if it does you need more steel.


collidog

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Re: solid or laminated magnet rotor?
« Reply #4 on: November 24, 2015, 11:01:32 AM »
  Thanks for responses so far but let me rephrase the question again.
 If transformers use laminated cores to reduce eddy currents, as do electric motors, then unless there are no eddy currents in axial flux magnet rotors (expert opinion, please) surely laminated rotors would be beneficial, especially if the laminations were insulated from each other?

P.S. I do know about saw blades and paperclips. I have spent many interesting hours hunting through the forums

hiker

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Re: solid or laminated magnet rotor?
« Reply #5 on: November 24, 2015, 11:41:21 AM »
Theirs no eddy currents in the rotors...
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SparWeb

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Re: solid or laminated magnet rotor?
« Reply #6 on: November 24, 2015, 04:32:00 PM »
Eddy currents only happens if the flux through the material changes.  The flux in the STATOR changes a lot, so eddy currents would flow and generate heat if there is any thickness of metal there.  But in the ROTOR, where the magnets are attached, the flux is pretty constant, and eddy currents aren't a problem.  If it's practical to use several thicknesses of material to build your rotor, then go ahead.  There will be no benefit, but no downside either, provided you have a thick enough rotor to carry the flux from magnet to magnet.  Since you say you've been experiementing with paperclips sticking/not sticking to the back of the rotor, it sounds like you're OK.
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collidog

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Re: solid or laminated magnet rotor?
« Reply #7 on: November 25, 2015, 08:02:44 AM »
Thank you Hiker and Sparweb. Asked and answered.
Yes, I do find it convenient to make up laminated rotors. I do not have the facilities to machine sold rotors. I can build up thickness to exactly match magnets to keep rotor weight to a minimum, and use anular ring (doughnut?) configurations. I use non-metallic face plates with holes drilled to suit circular magnets which are press-fitted, and remain securely in place by magnetic attraction only. I can prise them out and refit if necessary. I was wary at first that centrifugal force might make the mags fly off, but over the years this has not happened, even when spinning up 8" rotors to 4000rpm.
Overall, I find it a versatile and convenient way to experiment with the "if in doubt, try it and see" method.

mbouwer

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Re: solid or laminated magnet rotor?
« Reply #8 on: July 24, 2018, 05:16:52 PM »
Shaping laminated magnet rotors is a nice way to get a light rotor I think.
Any dimension you want.


joestue

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Re: solid or laminated magnet rotor?
« Reply #9 on: July 24, 2018, 07:12:43 PM »
to find the minimum thickness for the magnetic backplate you could just use something thin like a saw blade, and then stack another one, or a third. if its weakly attracted to the back of the plate then i'd presume its not needed.

If the air gap flux is .7T then for 1.4T through the backplate you only need steel about 1/4th as thick as the width of the magnet. this does not include paths that reach radially beyond the magnets, or if the magnets are not at the edge of the plate then the outer perimeter of the plate is also a path for the flux.


so at 8000 rpm for a 16 pole motor yes eddy currents in the magnets and the magnet backplate will start to become a problem, but eddy currents in the copper coils will probably swamp the effect. there are papers on this topic with worked out examples you can download to get an idea.. eddy currents follow the cross section, so 16 awg wire will have half the loss of a 13 awg.

if you have an air core machine then the eddy currents in the magnets come from the harmonics drawn by the rectifier, for a regular machine they are intrinsically produced by the cogging torque and the harmonics.


basically the harmonic content pushed into the motor or drawn from it increases and decreases the value of the magnetic flux in the air gap. since these are harmonics they don't travel with the magnets but some travel backwards or forwards at a multiple of the frequency, causing voltages at those frequencies to appear.. and they cause current to flow wherever it can.
« Last Edit: July 24, 2018, 07:27:15 PM by joestue »
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Ungrounded Lightning Rod

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Re: solid or laminated magnet rotor?
« Reply #10 on: August 23, 2018, 04:24:40 AM »
If you want to use laminated magnet backplates to fight losses from rectification harmonics varying the flux through the magnet, you need to align them with the motion of the field line loops so they go "sliding down (into) the razor blade" rather than crossing from lam to lam or (worse) entering the flat side, then dragging a giant bullseye of eddy around as the entering/exiting field loop moves sideways.

A disk made by wrapping a looooooong strip around the axle (best is concentric N-gons, where N is the number of magnets and the n-gons' corners are under the magnets' centers so the sides are straight lines from magnet to magnet) would do the right thing, as the varying lines from the back of one magnet to the back of another entered and left the backing plate.  A stack of plates would be about as bad as a solid disk.
« Last Edit: August 23, 2018, 07:50:46 AM by Ungrounded Lightning Rod »

Adriaan Kragten

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Re: solid or laminated magnet rotor?
« Reply #11 on: September 07, 2021, 04:12:42 AM »
The advantage of using iron in the stator coils is that the magnetic flux flows much easier through iron than trough air. So you will get a much stronger magnetic field in the coils and therefore a much higher voltage for a certain number of turns per coil and a certain rotational speed. However, the fluctuation of the magnetic field in the stator iron causes eddy currents and these eddy currents heat up the stator and increase the required torque which results in decrease of the efficiency. The eddy currents flow in a direction perpendicular to the direction of the magnetic field. The eddy currents are normally reduced by taking laminated iron sheets for the stator. These sheets are electrically isolated from each other and so the eddy currents in different sheets don't strengthen each other. PM-generators made from asynchronous motors have laminated steel in the stator as the standard motor housing is used. But no laminated steel is needed for the armature as the magnetic flux in the armature is almost not changing (see my public report KD 341 for this type of generator).

In my public report KD 679, I describe a simple 8-pole axial flux PM-generator for the VIRYA-1 rotor using a bicycle hub as generator housing. This generator has a 3-phase winding with six coils. There is no iron in the coils so the generator has no preference positions. But there is an iron sheet behind the coils to give some strengthening of the magnetic flux in the coils. This iron sheet isn't laminated, so rather large eddy currents will be generated in this sheet. I have measured the rise of the temperature in this sheet and this is acceptable for the maximum rotational speed which happens in the windmill. Measurements are given and discussed in chapter 9 and 10 of KD 679. Detailed drawings are given in appendix 1 at the end of this report. The output of this generator is much higher than for the Nexus hub dynamo which is used for the VIRYA-0.98 and the VIRYA-1.04 (see Pel-V curves of this windmills at the menu VIRYA-folders at my website: www.kdwindturbines.nl).


Warpspeed

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Re: solid or laminated magnet rotor?
« Reply #12 on: March 31, 2022, 05:52:52 PM »
A solid steel rotor works fine in an alternator as already stated.
Rotors in car alternators are a solid casting.
That works well with either dc excitation or a permanent magnet rotor.

The only disadvantage might be "cogging" where the rotor and stator tend to attract very strongly when the rotor poles line up with the stator poles. That can create some initial resistance to turning in a wind machine in very light wind conditions.
One of the advantages of axial flux alternators is that air cored coils are completely free of the cogging effect.