Axial flux alternators overheating

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

Hi alan,

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.



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Grid-tied 2F windmill self-construction project with Jonathan in Austria

Small wind turbine Do-It-Yourself Workshop incl. Installation

Windmill type:  2F 16p piggott (2m rotor, grid feeding)
Location:  FH Technikum / Energieforschungspark Lichtenegg
Participants:  15 people
Country:  Austria
Date:  Spring 2017

The machine is currently under test alongside some larger ones and doing well.

For an ebook with details of how to build the 2F click here.

read more (in German) here

Posted in construction, courses, ferrite magnets | 1 Comment

Build a Small Wind Turbine with Kostas Latoufis in Portugal August

One Week Course:

Build a Small Wind Turbine with Kostas Latoufis

August 21 to 27 2017, Montemor-o-Novo, Alentejo, Portugal

The wind turbines built in the workshops are based on the designs of Hugh Piggott, a renewable energy expert living in the off-grid community of Scoraig (Scotland), as described in his ‘Wind Turbine Recipe Book’. These wind turbines can be constructed with the use of common materials, simple manufacturing techniques and basic tools, providing a low cost small wind turbine that anyone can built. Hundreds of homemade small wind turbines have been constructed worldwide using these designs, and have proven to be robust, efficient, easy to maintain and quick to repair. Typical applications of locally manufactured small wind turbines are development projects such as electrification of village communities in the global south, educational projects in universities and schools and sustainability projects such as renewable energy electrification of ecovillages. ‘Nea Guinea’ has been organizing small wind turbine construction workshops since 2009 and has empowered many people with the knowledge of harnessing power from the wind.


Course Program

During this one week course a 600W small wind turbine with 2.4m rotor diameter will be constructed from scratch, using simple raw materials like wood for the blades, steel for the mounting frame and tail, copper wire for the coils and neodymium magnets cast in resin for the generator. The wind turbine will be the product of enthusiastic team work, as we will work in groups focusing on woodworking, metalworking and generator fabrication, along with short breaks to share what we have learned with the whole team. Woodworking will involve hand curving the blades of the rotor using drawknives and spokeshaves, planes and chisels. Metalworking will involve fabricating the steel frame to support the generator, blades and tail, with the use of welding, grinding and drilling techniques. Generator fabrication will involve winding coils, placing magnets, preparing plywood moulds and casting the stator coils and rotor magnet disks into resin. Basic woodworking and metal working skills will be taught during the course, and will enable all participants with enough hands-on experience to complete the wind turbine construction.

The course will be led in English by Kostas Latoufis. Kostas has been active in the off-grid renewable energy sector for 10 years. After completing his electrical and electronic engineering degree at the Imperial College of London (UK) in 2000, he traveled to Central America and South East Asia while also training as a silversmith. In 2004 he started working as a renewable energy researcher in the Smart Grids Research Unit (SmartRUE) of the National Technical University of Athens (NTUA). He started constructing Hugh Piggott small wind turbines in 2007 with the F.A.R.M.A. collective and continued with educational construction courses for students, in the Rural Electrification Research Group (RurERG) of the NTUA. In 2009, he co-founded the Nea Guinea not-for-profit organization, with which he has organized many small wind turbine construction courses, building windmills with rotor diameters from 1.2m to 4.2m, and many solar panel and pico-hydro construction courses. While working with Nea Guinea, he has carried out several off-grid renewable energy installations in permaculture farms in Greece and in rural development projects in Central America and East Africa. He is currently a member of the executive board of the Wind Empowerment association, a global network for the promotion of locally manufactured small wind turbines, and he is also pursuing a PhD in the NTUA on the same topic.


Detailed Program

Day 1

– Arrivals during the morning and introduction of participants

– Workshop activities (woodworking, metalworking, generator fabrication)

– Introduction to off-grid renewable energy systems (solar panels, solar pumps, wind turbines, hydro turbines)

Day 2

– Workshop activities (woodworking, metalworking, generator fabrication)

– Dinner and evening break

– Introduction to small wind turbine basics (wind energy, aerodynamics and blades)

Day 3

– Workshop activities (woodworking, metalworking, generator fabrication)

– Dinner and evening break

– Introduction to small wind turbine basics (electromagnetism and generators)

Day 4

– Workshop activities (woodworking, metalworking, generator fabrication)

– Dinner and evening break

– Introduction to small wind turbine basics (furling tail system and tower installation)

Day 5

– Workshop activities (woodworking, metalworking, generator fabrication)

– Dinner and evening break

– Introduction to small wind turbine basics (electrical system and battery connection)

Day 6

– Workshop activities (woodworking, metalworking, generator fabrication)

– Dinner and evening break

Day 7

– Workshop activities (woodworking, metalworking, generator fabrication)

– Departures during the evening



General Information:

When: 21 to 27 August 2017

 Where: Oficinas do Convento, Montemor-o-Novo, Alentejo, Portugal

Maximum participants: 17 persons


Until 15 July: Full course: 250€ – Full Course + Accommodation + Meals: 320€

After 15 July: Full course: 300€  – Full Course + Accommodation + Meals: 370€

Accommodation and Meals: Oficinas do Convento have space for 15 people accomodations in a common space. Theres also the possibility of camping. Breakfast, lunch and diner are are included.

 FB event:



 Nea Guinea:

 Cooperativa Integral Minga:

 Oficinas do Convento:

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Evento que se celebrará a la hora, en la fecha y ubicación siguientes:

Sábado, 24 de junio de 2017 a las 09:00

– hasta –

Domingo, 25 de junio de 2017 a las 18:00 (ART)

Maestro M. Lopez esquina Cruz Roja, AVEIT, Córdoba capital

Maestro M. Lopez esquina Cruz Roja


Ver mapa

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Arduino based PWM diversion control system

I am a bit fanatical about using surplus energy from wind and solar systems.  I just had an article published in Home Power magazine about this subject.

I like using Tristar controllers in diversion mode and then triggering more usable diversion (AC water heating) from that using my Tristar Follower idea.  That’s my latest trick anyway and I am proud of it.  It’s done using hard-wired analog electronics.

However it turns out that there are folk out there who like things a lot more complicated, and I must say it looks like quite a lot of fun too.   Will Eert wrote to me to tell me about his Arduino based PWM diversion control system that talks to his Midnite Classic controllers and uses surplus solar energy to heat his water tank based so as to limit maximum current into the battery, and limit its voltage to the prevailing charging setpoint.

You can learn a lot more about Will’s project on the Midnite discussion forum here.

Here is a basic summary of what it does:

  1. The control uses a series of PID controllers to divert under various conditions. This means it modulates diversion at a variable rate based on the amount of power available. The control normally turns on when there 10w available of excess power anywhere in the system.

  2. The control has a dynamic high amp limiter which uses the WBJr amps for an limit amp signal. This lets me “over array” the battery bank but not worry about putting too many amps into the the battery bank (when conditions permit) while still being able to utilize the full output capacity of the array at all times.

  3. The control diverts power based on high amp flow to the batteries. This lets the control divert in Bulk if the excess power is available. I adjust this set point 5 amps lower than the dynamic amp limiter. At present I only divert into one HWT element which is two KW in size. If the amp diversion is at maximum then the amps to the battery increase and the dynamic amp limiter limits array output.

  4. The control also diverts power based on voltage. It always maintains the batteries within .3V of the battery charge setpoints. The divert setpoints are set .3V lower than the Classic charge setpoints. When the divert setpoint is exceeded diversion starts automatically. It is possible to be amp and voltage diverting while dynamically amp limiting at the same time.

  5. The control reads the Classic setpoints and adjusts its setpoints to match. This means that once the control is setup all the owner needs to do is adjust their Classic as they desire. The diversion control will make all needed adjustments to its setpoints to ensure diversion continues to happen optimally.

  6. The control diverts at 10HZ. This is tunable however this frequency keeps the inverter very happy.

  7. If the HWT tank thermostat opens the PIDs turn off and the Classic limits array output. Fully automatic.

Thanks for documenting all of this, Will!  I love it.

Posted in construction, Diversion load control | 4 Comments

More free designs at

Adriaan Kragten writes:

Dear Hugh

Recently I have developed two new small VIRYA wind turbines with axial flux generators, the VIRYA-1.25AF and the VIRYA-1.81. The design reports and the drawings can be copied for free from my website:  A folder in which all seven free VIRYA designs are described, can be found at the menu “VIRYA-folders”. The main specifications of all seven free VIRYA designs are given at page 3 of the folder “Extended specifications VIRYA windmills”.

The calculations and the drawings of the VIRYA.1.25AF are given in report KD 626. The 8-pole generator of this wind turbine makes use of the front wheel hub of a mountain bike. The rotor is mounted to the front flange and the armature sheet is mounted to the back flange. An assembly drawing of the rotor and the generator is given in figure 1 of KD 626. Detailed drawings of the rotor and the stator sheet of the generator are given in the end of KD 626. The remaining drawings of the generator are given in report KD 608 of the VIRYA-1. The remaining drawings of the head are given in the manual of the VIRYA-1.36. The VIRYA-1.25AF is a rather simple design but has an acceptable Pel-V curve for 12 V battery charging.

The calculations of the VIRYA-1.81 are given in report KD 631. An assembly drawing of the rotor and the generator is given in figure 2 of KD 631. Detailed drawings of the rotor and the generator are given in a separate manual. The free manuals are given at the bottom of the list with KD-reports. The 8-pole VIRYA-1.81 generator has only one steel armature sheet with circular magnets glued to the back side. It has a synthetic stator sheet and synthetic coil cores and therefore no eddy currents are generated. As this construction results in a rather long way for the magnetic field flowing through air, rather strong and thick magnets have to be used. But as the magnets can be ordered at a rather cheap Polish company, the magnet costs are still acceptable in relation to the other costs which have to be made to build the VIRYA-1.81.

Yours sincerely

Adriaan Kragten

I cannot say whether these are good designs or not, but they are free, and Adriaan has done some proof-of-concept testing to find the best turns/coil and support the conclusions.  If you build a turbine according to any of these designs then I am sure he would love to hear about it.  He says “I am 70 years old now and I still like it to design things but I no longer feel the need to build what I have designed.”

I (Hugh) am not very comfortable with using a steel disk backing the stator, because of the losses in that disk and the thrust load on the bearings, but Adriaan is confident that these are both quite acceptable.

Adriaan’s work is very strong on detail, and theoretical analysis, which can be very helpful especially to students of small wind turbine design.  For example this diagram showing the matching of the alternator (Pmech) to the blades best tip speed ratio is especially helpful.

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Scoraig 2017 wind turbine workshop photos

The self-timer group photo session.

On this year’s course we built a 2F machine but with 16 magnets instead of the usual 12.  I have a cheap source of these smaller magnets.  The 2F is a “2 metre diameter Ferrite magnet wind turbine” which is documented in my 2F wind turbine construction manual.

The rpm is a little higher than the 12-pole version.  We played with adding an extra layer of magnets to one rotor, and got about 15% higher flux density (hence lower rpm) but I don’t think I will build any serious alternators stacked like that.

Most of the blade carving was done with drawknives but I did not get any pics of that.  It’s an easier procedure than the one in my Recipe Book and seems to give good results for this size of turbine.

I am very grateful to Kostas Latoufis for coming to help, and for putting up with me so patiently.  Also grateful to the five participants for their hard work and good company.


Posted in construction, courses, ferrite magnets, Scoraig | 4 Comments

What’s a kiloWatt and a kiloWatt-hour?

I see a lot of people talk about energy and power using terms like “kW per day”.  Let’s be clear with our language.  kW is a measure of power. Power is actually the rate of energy use.  Energy is power multiplied by time in hours.  So if you use a kW all day long then you will use 24 units of energy per day.  That’s a lot.  Only in North America does a typical house use that much.

If you are talking about energy use then you should be talking “kiloWatt hours” or kWh because this is the standard unit of energy that you pay for in your utility bills (if unlike me you actually have utility bills).  Here are typical meters – old style and new.

To make this a bit more real, here is a chart of typical daily electrical energy use for homes around the world.  Prices vary from about 8 US cents to 40 US cents per unit.  Here in the UK we pay 20 cents or 16 pence per kWh unit.  The average British home uses about 13kWh per day costing about £2.

Renewable energy sources

Now suppose you have a 1 kW wind turbine.  Does this mean you will get 24kWh of energy per day?  Only if the wind blows non-stop full power.  Most small wind turbines only deliver about 15% of their rated output as an average figure.  (Wind farms get twice as much because they are sited on hilltops.)  What about 1kW of solar panels?  It depends on where you are, but in the UK we get an average of 4 hours of sun per day so again the energy output is only 17% of what it would be if the sun shone all of the 24 hours.

To match a typical UK home’s demand you’d need a 3.5kW wind turbine or 3kW solar array.  Or a bit of each.  Or why not both, and use more than average electricity by charging up an electric car and heating your house during gales?  In my household we use about 50% more electricity than the typical on-grid house and we love having ample renewable energy.  It’s not about the money.  It’s just so much better than burning stuff to make energy.  Burning fuel to make electricity is something we need to put an end to.  Or it will be the end of us.


Maybe the most interesting thing to learn from looking at kW and kWh is that hydro turbines can outstrip both wind and solar by simply keeping at it full time.  Whereas you need 3kW or more of solar or wind power capacity to power a typical UK house, you only need 530 watts of hydro power (on a fulltime basis).  530 watts doesn’t sound impressive but it’s all you need, and the fact that it’s ongoing is also a big advantage because your power consumption is also ongoing.  Having a big chunk of solar electricity around lunchtime, or receiving copious wind energy when the wind happens to blow is a lot less useful than having it coming in round the clock when you actually need it for your fridge and your lights and your other needs.

The average home uses less than one kW of power on average.  Multiply this by 24hours to get daily kWh of energy and then by 365 to get annual energy consumption in kWh.  Don’t talk about kw per day.  The correct unit for energy is kiloWatthours.


Posted in hydro, UK small wind scene | 2 Comments

Secondhand Proven 2.5kW 48V turbine

I have been asked to mention a 2.5kW Proven wind turbine (nowadays “KingspanWind”) that is available factory reconditioned.  I installed this turbine in the Highlands in summer of 2004, charging a 48V battery as part of a hybrid power system for a private client.  It caused no bother and was given no further attention, until it burned out last year.  I have not been shown any pictures of the damage but I understand this failure was attributed to a lightning strike, and was covered by insurance so the client got a new turbine under his insurance.  The original turbine has been factory reconditioned but is now for sale to cover the cost of this reconditioning.  It comes with blades but no tower.  It’s a solidly built machine that is good for at least another 12 years service, or more if the lightning will stay away.  Get in touch and I will send details of how to contact the seller as usual.

Tech specs 2.5kW

Posted in stuff on offer | 6 Comments

More secondhand stuff

People often let me know when they have secondhand stuff that might be of interest, and I am very happy to mention it here.  Often it finds a good home.  I got two emails today.

Neil on Barra has three windyboy 3800 inverters that he can’t use.  Also a 10kW Westwind load controller.  If you are interested then post a comment or email me and I will put you in touch.

Simon in the Highlands has a secondhand hydro turbine that can do 10kW at 330 l/sec on 8.3m head.  An Evans Engineering R&D model.   It’s a bit rusty in places but serviceable.  “It is all dismantled and ready to go to a good home (or the scrap-yard).  Would suit somewhere on a big river with a clean intake.”

Again I can put interested parties in touch.

Posted in stuff on offer | 2 Comments