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 | 3 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 | 4 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

Still time to get a place on the Scoraig Workshop 15th April

I will be holding a wind turbine building workshop here at my home on Scoraig from 15th-22nd April 2017.   Kostas Latoufis will be helping me this year.  Most of the places are taken but there is just room for one more, maybe two if I get a cancellation.  Please get in touch soon if you are interested in details of this event.  Photos of last year’s workshop are here.

The happy group

Posted in construction, courses, Notices, Scoraig | Leave a comment

Have we got our priorities right?

I can’t understand a society where people are pleased and proud to own a fancy car that will never pay for itself, and pollutes the planet, but hang their heads in shame if their wind turbine does not pay for itself and make them money. Why not own a wind turbine and be proud of using renewable energy? Why does it have to also pay before it’s a socially acceptable thing to own??

Posted in UK small wind scene | 4 Comments

Fixing a 2F turbine on Scoraig

The other day I had to take this 2F windmill down because it had stopped working. Turning slowly as if on the brake.  It’s a few years old and I thought most likely the flexible wiring inside the tower had got twisted up a bit and somehow shorted out, although the wires did not look that bad a ground level.

I was pretty shocked when I saw it close up, because the plywood had somehow got shattered at the front.  Birch plywood is very strong, but I suppose I was pushing it a bit to use 9mm ply here.  It’s not the oldest 2F on Scoraig using that plywood but it definitely taught me something.  Always more to learn!

I replaced the 9mm with 12mm birch.  I figured out a quick way to lay out the screw holes in the plywood so they are neat and symmetrical, and avoid the bolt holes for the mounting studs.  I used a cardboard template that matches the shape of the contact area between blade and ply. (Click to see larger images.)

Punching through the dots on the cardboard gave me my hole centres for drilling.  Next I went looking for the shorted wiring.  But I found to my horror that the alternator was “braked” even when totally disconnected from the wiring, so the short was internal.  Maybe I needed to make a new stator?

First I set to work to find the location of the short.  I used a battery, a magnet and a photo that I miraculously found of the stator before it was cast (back in 2013).  

Passing current through the coils between each pair of wires gives rise to a pattern of attraction and repulsion of the magnet in the winding that reveals where the current is going.  (Usually this will be “push, pull, nothing, push, pull, nothing, push, pull, nothing.”)  One pair of wires gave rise to a very strong field in just one coil and the photo revealed where the short must be between the output wire off a neighbouring coil and a series connection from that coil to the next.

The fact that it was a fault on an incoming wire meant I could simply cut that wire and make a new incoming connection to the first coil in phase A where the wire comes close to the surface of the casting to enter the coil.  So there was not too much digging around to do in the resin (using a drill and a gouge).

That was pretty lucky, and a quick solder joint had it all sorted out.

Balanced the blades, greased the bearings, untwisted the tower wires with a cordless drill and got the machine back into action after a relatively painless repair considering how much worse it could have been!

Not a breath of wind of course.

Posted in construction, ferrite magnets, my own projects, Scoraig | 2 Comments

Wind turbine workshop in North Carolina, USA

Handy Village Institute, 5840 Jewell Road, Graham, NC 27253 are hosting a wind turbine building workshop with Dan Bartmann of Otherpower March 20 – 25, 2017.

Posted in construction, courses | 1 Comment