Magnetic wheels

[psc]

Hi,

 

I need an advice with magnetic wheels. I'm very new to this discipline and a I don't know where to ask else.
So, I have four magnetic wheels on moving platform. This platform will move in vertical direction. The surface will be metal and that's reason why I have magnetic wheels. When I want to move with the platform up I have to get over a gravity force and magnetic force that produce my wheels.
I don't know how to count these magnetic force. It shouldn't have to be precisely calculation, because I will add some overdesign. It's enought to calculate it approximately.
Could you please tell me how to do it or where to start (some webpages, books, etc.)?

If I didn't it explain clearly or if you have any question I would be very glad to answer and solve the problem.
Thanks in advance.

[Enthalpy]

Hi Michal, willkommen!

 

You have to decide if you magnetic levitation attracts or pushes. Pulling is done with normal materials (iron, copper...) at room temperature but needs an active feedback (sensors +loop +power electronics) to stabilize the distance. Pushing is naturally stable but uses superconductors (I know no exception, even at the size of a train) hence expensive materials and low temperatures. I regret the bad news...

 

Could you tell more about the wheels? Are these magnetic bearings, for rotating axles and normal wheels? Or just levitating supports - these don't rotate usually, because magnetic levitation makes translation easy.

 

One demo was a skateboard (over a linear and short way, but already fun) in a French university. Available over youTube.

[psc]

Hi Enthalpy,

 

I'm sorry but I didn't express myself clearly. The wheels will attract the platform to the metal surface. There is nothing like a levitation and no space between wheels and surface. This force react againts the move in both direction up and down like a rolling resistance.
Maybe if I increase the friction coeficient, maybe that is the solution. Am I right?

[Enthalpy]

Fun!

 

It is one solution. I hope no life depends on it, because it might not be perfectly reliable; especially, magnetic attraction decreases quickly over the distance.

 

The metal surface must be ferromagnetic, for instance steel. You will also have looses through eddy currents that will brake the vehicle's movement - unless the metal surface is a very special design.

 

You could split each wheel in two parallel steel disks, close to an other, with a disk magnet between both steel disks. The wall closes the magnetic path - but much flux leaks through the air all around the wheels.

 

One other design decouples the wheels from the magnets. Put the magnets under the car's body, use normal wheels, rather stiff and without susension, so the magnets are always close to the wall. Rubber wheels bring better grip and the magnets leak less.

 

The magnet under the car body could perhaps be an electromagnet: less dangerous, an easier to clean from iron dust. Though, electromagnets are more sensitive to distance, and are not always possible.

 

For the design of an attractive permanent magnet, you can get inspired by ones that hold knifes at kitchen walls, or by magnets that hold steel parts at grinding machines or milling machines. Better: buy one already built.

[psc]

I already know how to do it. Please do not post messages about how to build such a platform or how to place the magnets to the wheels etc.
I asked for the magnetic force that reacts against the movement. This force is acting like a rolling resistance and I don´t know how to count it.

I can promise you that no life will depends on it.

[Enthalpy]

Braking forces:

(1) Hysteresis in iron. Force essentially independent of the speed.

(2) Eddy currents in iron, magnets... Force proportional to the speed but saturates or could even decrease.

(3) Usual rolling resistance.

 

If the wheels of the wall are not smooth, or the wheels eccentric, they may well be the main source of resistance.

 

Hysteresis: you would have needed the magnetic data of the wall's material, which is not usual with normal construction steel. Either take steel for magnetic uses, expensive and mechanically poor, then the coercive force is known (or the energy loss per cycle and per kg) and gives a magnetic energy versus dispacement hence a force, or use normal steel and try to find magnetic properties for it or a similar one, with no guarantee.

 

At reasonable speed like 0.5m/s, eddy currents will create a modest resistance. I suggest to experiment, since predicting that is not very easy nor accurate (it would require a reasonable precision over the field distribution, uneasy with hand computation). In experiments on a horizontal track, swapping between magnets and equivalent weight separates the different forces.

[Enthalpy]

In case you want to reduce hysteresis losses hence braking force, and can choose the wall material, then Armco sells very pure iron that is cheap and has a tiny coercive field. It's a horror to machine, though.

[psc]

Thank you for your answer. It helps me.

[studiot]

The force between two ferromagnetic surfaces is given by (B²A) / 2U₀

 

It can be calculated by considering the virtual work of a small virtual displacement and equating to the energy stored in the magnetic field E = 0.5 HB

[psc]

Thank you, could you please post the source?

[studiot]

The calculation is a standard one for eelctromagnetic lifting cranes in scrapyards, where the force is of course vertical, but the principle is the same. It is treated in electrical technology textbooks such as the stadard by Hughes.

 

What more do you need to know?

 

It would be useful to know some more details of the magnetic circuit and its generation.