I got this question by email and I thought it worth answering it publicly.
Hugh, It seems home made axial flux pma’s are prone to overheating at fairly low speeds compared to conventional gas powered generators that run at 1,800 rpms or more. If so what’s the cause and how can they made to prevent overheating? Thanks Alan, Idaho, US
All generators will produce waste heat, and this can cause overheating if overloaded.
Low speed generators have a handicap which means their power to weight ratio is much lower so they are usually low power or very heavy. Higher rpm allows for a lighter and/or more efficient design but we need a low speed generator for direct-drive operation by large diameter blades.
Axial flux alternators are a good choice for small wind turbines because they are easy to build and have very low parasitic losses (magnetic drag and suchlike), meaning they are very efficient at part-load, but they do become inefficient on full load, and will ultimately overheat on overload.
When using any alternator it is important to know its limits and to design the turbine so that it will not be overloaded. Unfortunately the book published by Otherpower does suggest that you can push a very small alternator up to 1000 W output at low rpm and this has lead to a lot of burned stators in the USA. Burned stators are more or less unknown to me using my recipes because I make sure that the furling system works to protect the alternator from overload.
Building a big enough alternator is the best way to avoid overheating. And protecting it against overload. Using MPPT controllers and inverters can increase the power output available in strong winds by raising the operating voltage but this is costly. Often the power obtained in stronger winds is of less value since the batteries are already charged at this point.
I hope this helps! I am putting this on my blog as it is a good question to ask.