Overlapped coil stator construction

[JeffD]

In 2009 I changed the stator in my small wind turbine.  There was nothing physically wrong with the stator that was in use that had been built back in 2007.  Here is a picture of the original stator.


It was a 6 coil 3 phase star configured stator.  Each coil had 80 turns of 22 AWG wire and had a DC resistance of 0.45 ohms.  DC resistance across two phases was 1.8 ohms.  AC peak voltage to RPM ratio was 0.014.

  In 2008 I had modified the stator to be used with a boost converter to improve load matching between prop power and alt power.  The boost converter operates from 2 m/s to about 6 m/s and then the main rectifiers take over once the alt voltage is above battery voltage. 

 In 2009 I wanted to try a different design to reduce the amount of copper in the stator and reduce resistance by at least half.  I could have just used 2 22awg in hand to reduce coil resistance by half, I had the room.  But I decided to try the overlapped coil design that had been discussed many years earlier on this board.  I was curious to see how well it would work.  After about 3 months of trial and error I was able to determine how many coils and turns I needed for each phase and how to lay them out.

  The magnet rotor seperation went from 0.5" to 0.275".  Turns per phase went from 160 to 100.  Coils per phase went from 2 to 8 so each coil in the new stator would have 12.5 turns. Calculated resistance of a phase based on turns and coil size was about 0.45 ohms.  AC peak voltage / RPM ratio was decreased from 0.014 to 0.012 since the resistance had dropped in the new stator and power output would be higher (wanted to keep TSR up to at least 4 at 12 m/s, design TSR is 5 for the blades).

Attached is a picture of the new stator now in use.

Construction details to follow in the next post.

[JeffD]

Attached is a picture of the new stator but with colouring of the coils so its easier to see phases and individual coils that belong to each phase.  The red and pink coils belong to the first phase and are in series with each other.  The pink coils are wound opposite in direction to the the red coils.  The blue coils make up the second phase and the green coils make up the third phase.  The number of coils in a phase equals the number of magnet poles which was 8.

I think the coil layout will be clearer once the construction photos are up.

[JeffD]

This is how I built the stator.  Each phase is made of 8 coils of 12.5 turns each wired in series.  I wound each coil on the stator form using the coil form that was temporarily attached to the stator form.  One continuous piece of wire (22 AWG) was used to wind all 8 coils.  The first coil was wound anti-clockwise then the second coil to the right was wound clockwise then the third coil anti-clockwise and so on.  After winding a coil I used sewing thread to tie the coil to the stator mold form.  The coil form was then removed and moved to the right to start the next coil winding in the opposite direction.  Total time to make one phase was about 20 minutes.  There are only two wire ends to deal with for each phase: the star point and the output to the control box.

[JeffD]

The second phase is shifted over and started on top of the completed first phase.

[JeffD]

And now for the third and final phase.  There are now 6 wire ends to deal with.  3 ends get tied together and the other 3 go off to the controller.

Once all three phases were completed it was time to set up the mold.

[JeffD]

The mold is nothing fancy, just two 5/8" pieces of plywood.  I used plasticine (kids have lots of it) to form the mold seals.  East System Epoxy was used to bind the coils.

[JeffD]

And then the fun part.  Removing the stator from the mold.  I used Cyan blue Crayon to coat the mold forms with wax.  Worked realy well.  The amine blush on the epoxy picked up the wax but rinsing in warm water with a little soap removed most of the blue colour from the stator.

[JeffD]

Finally out of the mold and ready for mounting.

The end result was that the stator has performed as calculated.  It has been in operation since early December 2009 every day 24 hours a day.  It has been through a lot of heavy wind through the winter and no physical signs of deteriation, yet.

  With such a thin stator (3mm in magnet track area) I've been a little concerned with the stator cracking in high winds.  I should have put some 2 ounce/sqyd glass cloth on either side but never thought about it until I removed the finished stator from the mold.  Can still do it but will let it go until end of June to see if it holds up.

  Some numbers from the data logger that monitors the turbine:   At around 12 m/s, the little turbine is pumping out about 3.2 amps into the battery bank (13.5 volts), 43 watts.   The turbine blade diameter is 20.5 inches and at 12 m/s its spinning at about 1800 rpm.  Furling happens between 12 and 14 m/s.

[taylorp035]

At around 12 m/s, the little turbine is pumping out about 3.2 amps into the battery bank (13.5 volts), 43 watts.   The turbine blade diameter is 20.5 inches and at 12 m/s its spinning at about 1800 rpm.  Furling happens between 12 and 14 m/s.

That impressive from a 20.5" diameter.  I made a 2 bladed CNC 30" diameter blade with a small 10 amp dc motor.  My record was also 43 watts, but it was spinning at 3000 rpm.  If you figure a tsr of 8, it comes out to about 33 mph winds or about 14-15 m/s.  For my rpm, I had made a rpm vs. power table on a piece of paper, so I knew how fast it was spinning.  Unfortunately, my cut in rpm was 2200...
Nice write up!