magnet/coils and some rotor idea

[Adriaan Kragten]

OK, lets go back to the original poster. He bought 50 very small and very thin rectangular magnets. To my opinion this is a bad choice if you want to make a high pole number PM-generator for a squirrel cage rotor. I think you can better reduce the pole number up to eight and use 6 coils for a 3-phase winding. As he can't buy thick circular magnets, may be he can glue some of the small magnets together. Assume that you use only one armature sheet with magnets on the back side like I do for my axial flux generators. So in this case you need eight magnets. Assume you use 8 small magnets for one large combined magnet. So you need totally 48 magnets. You can't end up with a circular magnet but a square combined magnet is the next most optimum shape if you use circular coils. You have only two options to realise this, two layers with four magnets per layer or four layers with two magnets per layer. One should chose the option which results in a rectangular magnet which is closest to a square magnet as possible. A problem with gluing magnets together is that the magnets are repelling each other at the small side of the magnets, so in this direction the magnets have to be clamped together firmly.

To find the optimum winding for a certain DC loaded voltage at a certain rpm is a rather complicated procedure. I have explained this procedure in all of my reports about axial flux generators. You start with a test winding and measure the generator with this winding for different voltages. There is one voltage for which the matching with the chosen wind turbine rotor is optimal (you need to know the optimum cubic line of the windturbine and I don't know how to find this curve for a squirrel cage rotor). Next you modify the winding such that the wanted Pmech-n curve is gained for the wanted battery voltage. A problem with measuring of a generator is that it isn't enough to only measure the supplied electrical power. You should also measure the mechanical power and this requires measurement of the torque and the rpm which requires a rather sophisticated test rig. So if you can't measure the generator correctly, try and error is the only last option you have.

The idea of using eight poles and nine coils is useless if neodymium magnets are used and if stators are used with no iron in the coils. However, if only ceramic magnets are available, one sometimes uses iron cores in the coils to increase the magnetic flux. But if you use eight magnets and six coils, you get very strong preference positions every 15 degrees, so 24 preference positions per revolution. This will give starting problems for a rotor with a low starting torque coefficient and the generator may also become noisy at higher rotational speeds. But if you use eight magnets and nine coils with iron cores in the centre, you will get 72 preference positions and the torque fluctuations for 72 preference positions are much less than for 24 preference positions.

[electrondady1]

ok, if it was my project, i would put 2 of the 50 round magnets on the refrigerator.
then i would go to a saw sharpening shop and purchase 2 used skill saw blades
about 6-7" in dia.
making sure they were flat.
 i would put 24 magnets on each of the saw blades spaced 9mm apart.
creating a circle 137.5mm in dia .( approximately 5.5 " in dia.)

set up one magnet rotor on a shaft  so it can be rotated at a known speed from 0-100rpm

now your in a position to begin to experiment .
you can turn coils of various dia. and thicknesses finding out how many turns are required and what the thickness of the conductor should be .
you can decide at this point if you need a single phase or 3 phase stator
i f your making hydrogen you only need two volts
if your charging battery's you need to produce a charging voltage slightly greater than the battery voltage.
it may be your alternator won't produce what you want at direct drive speeds of your windmill.
  in that  case  you can build a pulley system to overdrive the alternator to a useful rpm

good luck
 

 
 


 

[Adriaan Kragten]

The idea of using 8 poles and 9 coils with an iron core in the centre to increase the magnetic flux might also be usefull for the original poster. For a horizontal axis wind turbine the fluctuation of the torque will probably be too high but a squirrel cage rotor has a rather high starting torque coefficient and it might still start at an acceptable low wind speed. I made a mistake in my previous post about this subject. If you have 50 magnets and if you want to make 8 combined magnets from that quantity, you need six magnets per combined magnet (and not eight) so you can make two layers with each three magnets or three layers with each two magnets.

The same principle of using one armature pole more or less than the number of stator coils can also be used for a 16-pole generator and a stator with 15 coils (so five coils of the same phase with alternating winding direction together). In this case you will have 16 * 15 = 240 preference positions per revolution and there will be only a little torque variation for that many preference positions. The positive effect of a much stronger magnet field flowing through the coils if the coils have an iron core is probably larger than the negative effect of the phase shift in the voltages generated in the left and the right side of the rather narrow coils. If the original poster is interested to try this option, he can send me a private e-mail and I will make a sketch of this option for him if he tells me the exact magnet dimensions. A problem with 16 poles and 50 thin magnets is that you can only make a combined magnet with three layers of one magnet or one layer with three magnets. The last option will certainly be too thin.

[rich4rdiez]

it may be your alternator won't produce what you want at direct drive speeds of your windmill.
 in that  case  you can build a pulley system to overdrive the alternator to a useful rpm

good luck

a pulley system between windmills (more than one) to obtain more torque or something like that?

[rich4rdiez]

But optimalisation also has to do with the power you get for the investment in magnets, copper and other generator components and maximalisation of the voltage per turn is one way to optimise the design.

thats exactly what I want, thanks for all the info.

[rich4rdiez]

So if you can't measure the generator correctly, try and error is the only last option you have.

I like that though

[Bruce S]

it may be your alternator won't produce what you want at direct drive speeds of your windmill.
 in that  case  you can build a pulley system to overdrive the alternator to a useful rpm

good luck

a pulley system between windmills (more than one) to obtain more torque or something like that?

I think he means a pulley between the rotating mill and one going to the generator part. Kind of like a bicycle's. Where the pulley for your mill will have a larger pulley and the generator part would have one smaller. Since your mill is going the route of a higher torque type, you could effectively have the pulley on the mill twice the diameter has the one for the generator side.
This will have the generator side going 2 revolutions for every one of the mill's.

GoVertical did a great job with pics of his builds.
http://www.fieldlines.com/index.php/topic,143852.486.html
2/3rds of the way down on the link above , you'll see his pulley setups (in Green) this should help you too.

[rich4rdiez]

it may be your alternator won't produce what you want at direct drive speeds of your windmill.
 in that  case  you can build a pulley system to overdrive the alternator to a useful rpm

good luck

a pulley system between windmills (more than one) to obtain more torque or something like that?

I think he means a pulley between the rotating mill and one going to the generator part. Kind of like a bicycle's. Where the pulley for your mill will have a larger pulley and the generator part would have one smaller. Since your mill is going the route of a higher torque type, you could effectively have the pulley on the mill twice the diameter has the one for the generator side.
This will have the generator side going 2 revolutions for every one of the mill's.

GoVertical did a great job with pics of his builds.
http://www.fieldlines.com/index.php/topic,143852.486.html
2/3rds of the way down on the link above , you'll see his pulley setups (in Green) this should help you too.

thank you for clearing that.

[JW]

Just curious about the pulley system, there's going to be friction losses by that configuration and its not viable. Given the lower amperes would it be better to use a old style auto coil instead. Single phase would be ideal. So wind some high impedance coils with some really thin wire and feed that into the secondary of the coil. Have the primary circuit rectified as output, if you will get up to ten amps(possible) rectifying the primary of the auto coil output probably I don't know..

 what the TSR would be.

There would be less eddy losses in the coils because of the number of conductor windings of smaller wire diameter would be increased per coil   

 

[electrondady1]

that's interesting.

[JW]

I remember the phrase "wire in hand" what this meant is that instead of using a thicker conductor to wind the coil. 3 wires of thinner wire connected in parallel and connected at each end, and winding the coil, this decreased eddy currents and made a better coil.

So it was like 2 or 3 wires in hand to wind a coil.

There once was a stator wound by DanB for 12volts using thick rectangular wire used to make the coils with the intention of lower ohms/impedance (to get more amps output) the conductor was solid and large, but it overheated due to eddy currents. And was not useable.       

[joestue]

the diameter of wire is arbitrary.

if eddy currents are significant, you can prove it. wind the same coil with 4 times as many turns of wire of half the cross sectional area and the eddy current losses will be about one 1/64th the original value.

if you really need to prove to yourself the math is right:

wind the coil with 8 wires in parallel, each of 1/8th the cross sectional area, and you shouldn't even be able to measure the eddy current unless you put the entire assembly in a vacuum chamber to get rid of windage losses.

[Adriaan Kragten]

I don't understand the story about eddy currents in a coil. Is that meant in the copper winding of a coil? I always thought that you have only eddy currents in iron components of the stator if there is a changing magnetic field in the iron. For a normal asynchronous motor, the stator is made of laminated iron sheets and there is a thin isolation in between the sheets to prevent that an electric current can flow in a direction perpendicular to the sheets. The eddy currents in this kind of stators are therefore limited strongly. In some of my small axial flux generators I use a steel stator sheet but as this sheet isn't laminated, the eddy currents at high rotational speeds can become rather large. But I use no iron in the centre of the coils and therefore these generators don't have a strongly fluctuating clogging torque.

[electrondady1]

i have turned two and three conductor coils to reduce resistance

i recall Dan building that square wire stator for his diesel powered alternator.
it burned out the coils.
 i also remember reading the current travels along the outer skin of a conductor.
 rather than the inner molecules

 ed lenz once posted an induction heater he built using a  drill powered mag rotor and a sheet of copper placed were the stator should go .