Low wind gearing

[Bosman]

Again it sounds so obvious that there must be a snag, but why not gear PMGs to give better (longer periods more often) low wind output. I know you will have some gear/belt losses, but has anybody tried it?

[electronbaby]

Depending on the actual measured wind speed, there is not much energy available in low wind speeds. Any energy there happens to be would be mostly eaten up by friction loss in the gearing. This is not an efficient approach unless you can increase your swept area substantially. After that is done, you then need to make sure you can handle the HUGE exponential power increase you will have in the higher wind speeds. Wind energy usually is a balancing act between physical practicality, and financial feasibility. Also, dont forget serviceability (maintenance). There is a reason why there are a lot of direct drive units out there.

[Bosman]

Thanks Roy :-)

[Todd a]

You are fighting 2 issues.... RPM and torque.  If you gear the output of the turbine up in speed for the generator you deal with very low torque and with much load you will slow the wind turbine down.  If you gear down the RPM you will gain the torque and even under load you should be able to push the generator without any problem, but the lower RPM is going to massively drop the output of the generator.

You need to balance the RPM and torque from a wind turbine to the correct power generator and in the torque and RPM range for that generator.  These means if you have a low RPM generator like a DIY PM generator and you are running it on a large enough HAWT with a high tip ratio like 7 or more, you are looking at plenty of RPM, so you only need the higher wind speeds to increase torque under heavy load.  If you are using a lower speed VAWT or a generator that needs higher RPMs then you will need a belt and pulley set-up to increase the RPM (dropping the torque).  So say you have a savonios VAWT (drag-type) with a good surface area then you will have low RPM and high torque.  You would be lucky to hit 200RPMs at most times, but say you are using a DC PM motor for a generator and it need 1500RPM, then you would need a tiny pulley on the generator and a huge pulley on the turbine shaft.  A 2" generator pulley and a 15" turbine pulley will give you a 7.5:1 ratio.  This would run the generator at 1500RPM when the turbine is running at 200RPM.  You need to take into account the room to fit that 15" pulley and the offset of the generator.

[Basil]

All they say may be true. But It will work if you get it right. There is more maintenance in it. I did a 1:80 ratio ( almost 2:1 ) It worked out very well. I used a 38 volt PM Motor and it doubled the wattage. Wind that would just start my straight shaft blades would not start it. BUT When the straight one was turning and not up to 12 volts yet It would finely start going and make enough and more power to start charging. Right blade size for it is the trick. I would not go over 2:1 unless you have some big blades. Mine were 48in by 9 1/2 in out to 3 ft then went down to 4in at the end. Can't remember the angle. They looked like big fan blades.

[Todd a]

What I am saying is that since he was talking about gearing it to help in low wind... Low wind is low RPM and low torque already.  You can gear it down to increase torque, but the RPM would be way to low.... or you could gear it up to increase RPM, but the generator would stop the blades as soon as you added a load.

Variable pitch blades can fix a lot of issues.  They are normally there to help in start-up.  They are also there to keep the blades from spinning too fast.  They do not help it make much electricity at low RPMs, but it would help keep it spinning and when the wind does pick up it would start generating power much faster.  This is more important for larger HA wind turbine.  Derrius VAWT also can use variable pitch for start-up as this style of wind turbine takes a lot of wind to self start.  They either need variable pitch or a motor to spin it up.  At a certain point the wind will take over and it will accelerate up to speed on its own.

[Flux]

If you have an unsuitable generator then yes you can use gearing to make it work.

I thought the question was about using gearing in general.If you are building an alternator then gearing will reduce your output compared with having the alternator right in the first place. If you want lower cut in speed then wind it with more turns.

If you have an existing alternator that is too fast then you can gear it but the result will never be as good as a more suitable alternator driven directly.

Gearing hurts you most in low winds so if it is the low wind area you are trying to improve then it's not a good idea for best results.

The only sensible reason to use gearing is to reduce alternator cost and this may become an issue with large machines. It is possibly cost effective for machines above 12ft compared with using more magnets and copper, but in the long term nearly all forms of gearing reduce reliability and now that we have fairly cheap neo magnets I wouldn't consider any form of gearing below 20ft.

Before the days of neo there was a far better case for it and above 8ft it worked out cheaper, but in those days very low wind performance just wasn't viable so a bit of gearing loss was often offset with less iron loss.

Flux