Variable Speed Axial Flux Alternator

[Jim508]

I'm designing a small grid tie wind turbine that is based off a variable speed axial flux alternator. My aim is for 30kw turbine with a shaft speed of about 300-400 rpm. Does anyone know the limitations or problems associated with a large axial flux alternator? What type of inverter is ideal for this application?

Best Regards,

Jim

 

[potteryshard]

There are several homebuilt axial-flux alternator construction documentaries on the web. While you're looking for something much more substantial, the homebuild dairies give a pretty good understanding of the problems and constraints.

 

[Sparweb]

3 limitations typically crop up:

Heat,
Heat,
and heat.

The stator is sandwiched between the two magnet rotors, where it receives some but not a lot of cooling airflow. The stator must hold with wire coils firmly enough to resist the reaction torque and vibration, yet not be made of metals that can develop eddy currents or divert the magnetic field lines. Any material that does develop eddy currents gains yet more heat. Usually one is left with high-tech polymers and the limiting temps can be below 200C. The resistance in the wire develops heat with the I-squared-R law, so to develop 30kW you need heavy wire gauges unless you are extracting power at very high voltage. Even at 600v, the current is upward of 50 amps. That's no small wire, unless you use a lot of paralleled phases.

Lastly, the electronic architecture of a wind turbine is not a secret. Sorry-no specific answer it's not my strong suit, but I do find discussions of inverter design, from time to time. It's been studied and different schemes have been tried by Sandia over the years, not to mention various commercial products that do this. If your turbine does not regulate its speed aerodynamically, then as you say the load will vary in frequency, forcing you to convert the wild AC through multiple steps before it becomes steady 60Hz. Each conversion of the power involves some loss, again, as Heat.

At the 30 kW level, you should consider the option of using induction motor/generators with aerodynamic speed controls. It sounds a bit 1980's, but it does work, and the "heavy-metal" is very resilient. Start-up and shut-down controls are mandatory, but that's easier than making a custom-built inverter.

Steven Fahey, CET