Maximum potential output?

[makenzie71]

At the risk of recieving answers which suggest you guys believe I am as smart as you are...is there a calculation for a stator’s maximum potential?

What I’m hoping to find is a metric i can use with all the turbines i bring in and inspect/review.  Right now all I’m doing is providing a comparison between claimed iutput and real world performance.   The problem with this is that it only really tells us whether or not the seller lied. What I would like to be able to do is find a way to calculate what the maximum potential of a stator would be assuming the rotor and blades were themselves perfect. This would not only give me a better idea of how well the turbine is actually performing, But would also give me a better standard to judge it against the other turbines that I’ve reviewed. We already know that almost all the turbines out there the sellers claims are bogus or at the very least unrealistic, so when I confirm that it’s never surprising.  I think knowing what the maximum potential is would be better information.

[MagnetJuice]

Over the years, I have gathered some data based of experiments that I have done. In addition, I have collected data from dozens of alternators that others have built and posted the electrical power output generated by those systems.

My studies led me to conclude that one cubic inch of neodymium magnet with N42 strength, has the potential to produce 42 watts.

I am strictly speaking of axial flux alternators, but I am sure that there is a correlation with the radial alternators that those wind turbines that you are testing have.

Just because you can get 42 watts from a cubic inch of magnet, doesn't mean that you always will. It would depend on the overall construction of the alternator. Magnet gap, wire size and other factors like quality construction will dictate your final output.

Another thing is that the grade of the magnets, normally are not given in the specifications of those alternators. They could be anywhere between N35 and N52. More likely, it will be N40 or N42.

I think that the best way to test those turbines would be to have a setup where you turn them with a powerful variable drill. Then, you will need to have a setup that can read VOLTS, AMPS and RPM accurately. By using ohms law, you will know how much power is being produced at a given RPM.

For the load, you should use a piece of resistance wire. 30 Ft of 15 Gauge Nichrome 60 wire will give you about 6 ohms. That should be enough to run tests of 12 to 48 volts at up to 1 kW.

I buy my wire here:

https://jacobs-online.biz/nichrome_wire.htm

They are located in Washington State.

Ed

[makenzie71]

My plan with magnet material was to go about it with a scale and a 1" mild steel cube.  I've been lead to believe that maximum potential yield for N42 magnets is 75 watts per cubic inch...which is still a lot of guesswork but does give me a baseliine.  I measure the breaking strength of a 1" N42 cube and compare it to the breaking strength of the magnets in my rotors to get an idea of how strong those magnets are.  Saw the breaking force required for an N42 cube is 50lbs, but the magnet in my turbine is 22lbs...well, that's only 44% of the magnetic strength...so potential yield is 33 watts per cubic inch.  Again, as an example.  Not nearly as scientific as some fellows here, but it's at least some variety of baseline.

[Adriaan Kragten]

To my opinion there is no fixed maximum power for a certain PM-generator but every PM-generator has a certain maximum torque level. This maximum torque level can be measured on a test rig if you make short-circuit. The maximum torque level for short-circuit in delta is larger then for short-circuit in star. If you measure the generator for a certain voltage, you get the same Q-n curve as the short-circuit curve but it is only shifted to the right. The higher the voltage, the more it is shifted (see measurements given in KD 78). For every voltage you will get a certain maximum electrical power out of the generator if you stay below the rotational speed which belongs to the maximum torque level. The higher the voltage and so the higher the rotational speed, the higher the maximum power. So basically you can get any power out of a generator if it runs fast enough.

However, a problem is that the generator has a certain efficiency and so heat will be generated in the generator and therefore the generator must be cooled at high powers. So finally insufficient cooling will limit the maximum power which can be generated. PM-generators used in direct drive wind turbines run that slow that normally no extra cooling is required. But if a small PM-generator would turn at 10.000 rpm and if it would be cooled enough, you can get a very high power out of it.

If a certain PM-generator type with a certain size has been measured for a big range of loads (like I did for the 4-pole generator as described in KD 78), the characteristics for a bigger housing can be estimated because for the same flux density in the air gap, the torque level for a radial flux PM-generator increases proportional to the increase of the armature volume. This can be understood as follows.

A certain flux density in the air gap means that a certain tangential stress is possible which can be compared to a shearing stress. The shearing force F is proportional to the product of the shearing stress times the cylindrical area of the armature situated within the stator stamping. The moment is the product of the force time the radius of the cylinder. So finally the torque is proportional to the armature volume.

For an axial flux generator one has to take the total magnet area multiplied by the radius in between the heart of the generator shaft and the heart of the magnets. This finally also has the dimension m^3. But as the flux density of axial flux generators with no iron in the coils is much lower than for a radial flux generator made from an asynchronous motor, a much larger volume is needed for the same maximum torque level.

[oneirondreamer]

Using magnet volume to calculate max output will only produce a useful number when combined with rotor speed, or Volts per RPM.   It may be that you could develop a magnet volume/voltsper rpm metric.   I think that's also in agreement with what Adriaan's saying, that magnet volume to torque is the number you are looking for.