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Permanent magnet levitation toys - drag

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AndrewM

Fri Oct 10 2014, 11:04PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

In the classic "levitating desk toy" from a decade ago like this:

2744 Magneticlevitatorclassic 1

Apart from the axial bearing and windage, what is the source of drag and how would you estimate it? Eddy currents in the magnets? IS there another source of drag??

Thomas W

Fri Oct 10 2014, 11:18PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Well, most of the drag with this thing comes from the simple air resistance. if you wanted to measure the rate of loss of energy, i would get it up to a speed, measure the speed, then measure the speed as it slows down, plot it on a graph :)

BigBad

Sat Oct 11 2014, 01:24AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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If they're neodymium magnets then yes, there will be significant drag from eddy currents.

One biggish problem is that neos are conductive; as it spins, the magnets will see varying magnetic field strengths and that will inevitably cause eddy currents.

You can simulate this effect with FEMM if you simulate the magnetic field moving with phased electromagnets.

You can get rid of the eddy current effects with non conductive magnets; but they won't be quite as strong.

Alternatively, vertically hanging magnets with annular magnets would give you no eddy currents, because the fields will be DC or as nearly as you can manage to make them. But you'll have to be careful to make them laterally stable.

You could potentially get rid of air resistance by sticking it in a bell jar and pumping it down.

edit: oh yeah, nearly forgot, there will be a sort of 'rolling friction'; the magnets will always move in their mountings slightly no matter how rigidly you try to mount them-they will always distort a little bit unless it's vertical. However, with rigid materials this may be quite small.

Uspring

Sat Oct 11 2014, 08:41AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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You'll have eddy currents in the rotor caused by the magnets in the base, if the rotor contains conductive material. Depends on the amount and distribuition of metal in the rotor.

AndrewM

Sun Oct 12 2014, 04:04AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I'm interested only in the drag caused by the magnets themselves, not air drag or any other parasites.

So if I used a ferrite magnet (nonconductive) would there be zero magnetic/eddy current drag?

And what about the surrounding structure - if the magnet is uniform/cylindrical and its rotating around its magnetic axis, would there be any time varying field in the metal around it?

Sulaiman

Sun Oct 12 2014, 07:47AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In this toy I believe there will be (theoretically) no external time varying magnetic field so no eddy current loss due to external objects
but if the rotating magnets are conductive they will have internal eddy currents/loss.

BigBad

Sun Oct 12 2014, 06:06PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

wrote ...

I'm interested only in the drag caused by the magnets themselves, not air drag or any other parasites.

So if I used a ferrite magnet (nonconductive) would there be zero magnetic/eddy current drag?

It will be very low. There may be some hysteresis losses, but they're going to be very small provided you keep the magnets in the same quadrant.

wrote ...

And what about the surrounding structure - if the magnet is uniform/cylindrical and its rotating around its magnetic axis, would there be any time varying field in the metal around it?

You don't want any conduction in the rotor. The surroundings should see DC magnetic field and so no eddy currents.

AndrewM

Mon Oct 13 2014, 04:54AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

BigBad wrote ...

It will be very low. There may be some hysteresis losses, but they're going to be very small provided you keep the magnets in the same quadrant.

Talk more about hysteresis?

This is actually a work question - I'm working on an ultra-low drag bearing test rig and am trying to trade bearing types. I need ultra low bearing losses so I have to understand the "very small" things. Permanent magnet bearings (ala

) seem promising, but are very difficult for me to estimate.

Ideally I'd have a way to estimate the bearing drag relationship with load and RPM.

Andy

Mon Oct 13 2014, 05:08AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Hi AndrewM
At that large scale you will have physics against you, that you will have back EMF, and current BH field made, no matter what, but if you use say a induction motor ,but in reverse that cancels the losses you could brink it close to 99.99%, the induction motor pretty much has opposite polarity to the filed at placement that the filed is at present, you should be left with fiction, and some none charted loss. The induction motor would use the power that it stops throw losses, but the induction motor would be more effeiunect than randomness in these case.

Side thing maybe added another layer say the filed switch the current, you might slowly approach 100%, find a problem then patch then add to the patch.

Hope it helps

Uspring

Mon Oct 13 2014, 08:56AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

This is actually a work question - I'm working on an ultra-low drag bearing test rig and am trying to trade bearing types. I need ultra low bearing losses so I have to understand the "very small" things. Permanent magnet bearings (ala ) seem promising, but are very difficult for me to estimate.

Rotating a magnet, which creates an axially rotationally symmetric field around its axis doesn't change the field and causes no eddy currents in conductors around it. As far as I can see, magnetic bearings attempt to do just this.
You also have to think of the reverse effect, i.e. the stator rotating and the rotor fixed. If the stator also has a cylindrical symmetric field, it won't cause eddy currents in the rotor. This also seems to hold for magentic bearings.

In the picture of your original post, the stator does not seem to be able to create a symmetric field like this, so there might be eddy currents in the rotor. I don't know how well symmetricity conditions are met in real bearings. It all depends on that.

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