Magnetic Flux

[Gazoo]

I am doing research into building a wind turbine, I like the VAWT design and i am planning on using axial generators. I see a popular design is the brake disk design.

I have a few general question on magnetic flux. Are the magnets stuck to the brake disk making contact with the disk or are they seperated by an insulating layer. Does it matter?

The way I understand the theory is 50 percent of the magnetic energy is expended at at each pole, so if you stick the magnets on to a steel plate, the magnetic flux is channelled to the other poles of the the magnets beside it therefor reducing the backside magnetic flux loss. I guess the front side of the magnet would then have 95% or something like that of the magnetic flux directed towards the coils, making the generator more efficent.

If I stuck 12 1/4" thick magnets on to a 7 1/4" sawblade would i have the same effect( I want to build a smaller version 10 to 100 watt windmill for a childs playhouse) would a sawblade be thick enough to acheive an efficent generator.

I have seen some designs that use laminates for the back side of the coils in a 1 disk brake design, what could I expect if I placed the magnets in to a precut hole in plywood/aluminum or some other material, and put laminates on the backside of the magnets.

Gazoo

[finnsawyer]

A couple of points.  There are two components to a "magnetic field".  One is called, surprise, the magnetic field and is usually given the symbol H.  It is due to the strength of the magnetic domains in the magnet or due to the strength of currents flowing in a wire (electromagnet).  It is not affected by materials.  The other component is called the magnetic flux density and is usually designated by B.  This is affected by materials and is what gives rise to induced voltages.  In sum, currents create the magnetic field, and flux or rather the time rate of change of flux creates the induced voltage.  In general B = uxH, where u is called the permeability of the material or medium.  The u for iron is about 1,000 times as strong as for air (free space).  It is also non linear, as the relation between H and B has an S shape.  That is, it can reach saturation.  In a magnetic circuit the magnet acts like a battery (called magnetomotive force as opposed to electromotive force).  This causes magnetic flux (analogous to current) to flow from the north pole of the magnet to the south pole through any materials that constitute a closed loop around the magnet.  This loop could be air only, in which case the flux would be low.  If the loop is entirely iron, as in a power transformer, the flux will be high.  The ability of the material to retard the amount of the magnetic flux is called reluctance (analogous to resistance).  Air has a much higher reluctance than iron.  In the case of a properly designed alternator the coils in the stator act much like air.  The result is that these air gaps effectively determine the amount of flux due to their high reluctance.  Ideally 100% of this flux will flow through all parts of the magnetic circuit.  In reality some flux (leakage flux) will flow directly form one magnet to its neighboring magnets of the opposite polarity.

By the way, the magnetic energy is not expended at either pole, or anywhere, for that matter.  It exists as long as the magnet exists, which could be for decades or more.  Nor do the magnets give up any of their energy when used in an alternator.        

[Gazoo]

WOW it is going to take me awhile to digest all this, but i will try, I got more research to do, what you said in a paragraph I am going to have to break it down a bit at a time to understand it in a more simpler understanding and fit the above back together, hehehe. I see some nice easy designs here and i will build one or two but I want to tinker with coils and material so I want to have a good understanding of what i am doing.

[finnsawyer]

I should have said that the curve of B versus H for iron is S shaped.  That is, the value of B can reach a saturation value.  There is more.  In an alternator or a transformer some or all of the iron parts are run up and down this curve as the flux changes direction.  Unfortunately B does not exactly follow the curve as it comes down.  The result is that the actual parametric curve for B versus H in operation makes an S-shaped closed loop.  This loop, called hysteresis, results in energy loss and heating in the iron.  The hysteresis occurs because it takes time for the magnetic domains to change direction.  This time delay can also degrade performance at very high speeds, and is why power transformers and the grid operate at or near sixty cycles.  It's not likely to be a problem for the speeds at which alternators for wind power operate.  All of this can easily be demonstrated in a laboratory setting, and is something that students of Electrical Engineering are required to do.