Just for comparison sake I quickly designed a direct drive ferrite magnet generator for a 3.8 meter turbine with the following specs:
..... the big three phase would be able to produce about 47 amps before it maxed out the windings - I am delivering power on four wires, the three phase only uses three....
Chris
I have tried to reproduce this design and I find that I can use much thicker wire and also the flux/voltage is somewhat stronger due to the larger holes inside the coils, but I am not about to build one and prove it.
There is a flaw in the logic of this comparison because the current in the two single phase stators is working in pulses - one at a time effectively - which means that the resistance and the losses of this winding are effectively much higher, relative to the 3-phase example given. I think that the relatively high losses in the 2 single phase units are helping to produce a the optimum loading on the blades and make this machine very successful, but you could also build a 3-phase, direct drive one with the same losses.
The obvious virtue of direct drive is that we don't all have a machine shop to build a chain drive. So far as I understand it, bearing life is measured in revolutions, so a lower rpm helps with that and so does the ability to mount the rotors directly on the bearings rather than cantilevered out on the side of a gear case.
So much for theory. I gave up building ferrite machines in practice ten years ago, but I have noticed that they seem to last longer than neo ones. The biggest axial flux one we built was a ten footer with 12 2x2 magnets on 16 inch rotors.
![Generator with ferrite magnets 3704-0](https://www.fieldlines.com/index.php?PHPSESSID=f787d9fed12ce97b7038e817ab7bb662&action=dlattach;topic=145700.0;attach=3704;image)
That is still going strong on its original bearings about 12 years down the line. The stator has never burned out from overload yet. That one has a very long wire run that helps to push the blade speed up and keep them from stalling. It's horribly inefficient in strong winds, but the long wires take it up to a very windy spot, and wind energy is free!
![Generator with ferrite magnets 3705-1](https://www.fieldlines.com/index.php?PHPSESSID=f787d9fed12ce97b7038e817ab7bb662&action=dlattach;topic=145700.0;attach=3705;image)
Hugh