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Gear Box for Wind Turbine?

05/18/2012 9:39 AM

Which is the best type gear box for small Wind turbine? How to calculate torque if we know the weight of wind turbine blades & hub? Suppose weight of our wind turbine blades is 6kg & length is 1.2m, rpm is 50, how much torque this blades is apply on the gearbox input?

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#1

Re: Gear box for Wind turbine?

05/18/2012 10:37 AM

The torque depends on how much power the wind inputs to the blades and at what speed. The weight of the blades and hub constitutes the overhung load on the gearbox shaft. The gearbox mfr should have data on what is acceptable.

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#2

Re: Gear box for Wind turbine?

05/18/2012 11:15 AM

Why use a gearbox at all? If you search the Internet, you can find all kinds of information on small wind turbine/generators that don't need a gearbox.

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#3
In reply to #2

Re: Gear box for Wind turbine?

05/18/2012 12:12 PM

Low speed permanent magnet alternator price is too high. We found permanent magnet DC motor of 2700rpm in very low prices, about 10$, therefore we attach gear box with it.

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#4

Re: Gear box for Wind turbine?

05/18/2012 12:27 PM

50 RPM is too slow for that small of turbine.

I have built ones in the 2 meter and under range that flew in the 2000 - 3000 RPM without issue on windy days.

If it was me I would have no issues with doing a direct drive with a 2700 RPM motor as the generator and a good set of blades.

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#5
In reply to #4

Re: Gear box for Wind turbine?

05/18/2012 1:53 PM

I want to install turbine on roof top of building & in urban areas there is not much wind speed to operate turbine on 2000rpm & operating wind turbines in high rpm is also dangerous in urban areas.

I design turbine blades for 100rpm & if I use 1:20 gear box it becomes 2000rpm & in high wind speed blades speed is 2500rpm. So, motor maximum rpm (2700 rpm) can not exceed in high wind speeds.

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#6
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Re: Gear box for Wind turbine?

05/18/2012 3:03 PM

At 100 RPM on that small of turbine you probably wont get enough starting torque to over come the friction of the 20:1 gearbox with anything but the highest of wind gusts.

I have experimented with homemade wind generator systems since I was in my early teens some 20+ years ago and I can assure you for a fact wind generators under 4 meters in diameter are near useless when using gearbox's for any stepping up.

Match your generator to the blades or the blades to the generator. Thats your two most practical cheapest and most effective choices.

Why cant you spin your generator over 2700 RPM?

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#7
In reply to #6

Re: Gear box for Wind turbine?

05/18/2012 4:45 PM

First of all, I made blades from light wood & in my opinion 2700rpm speed using wooden blades is difficult to achieve.

Second thing, I mount wind turbine in the urban area on the roof top, so spinning blades with 2700 rpm is dangerous to nearby population in high winds.

By the way, the diameter of my blade is around 2.4m & it should spin around 70 rpm with gear box.

One more thing, using wood what is the best root width? i.e. equal to the tip width or wider as most of wooden blades are constructed.

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#8
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Re: Gear box for Wind turbine?

05/18/2012 6:01 PM

Years ago a Buddy and I built a number of homemade wood blades out of nothing but good clean pine 2" x 6" lumber.

A single piece two blade set we made once was just short of 2 meters long and I flew that for about 5 years at speeds well over 3000 and occasionally up into super sonic tip speeds when the wind was real bad without problems.

It finally just weathered itself to death and lost all its aerodynamic shape. Since then I have tried, and wasted a lot of money, on a number of factory built fiberglass blades sets that have all left me disappointed in their quality, durability and overall performance for what they cost.

I have no answers for whats the best design but apparently a good clean 2"x 6" piece of construction lumber and a few hours of work with a good belt sander can make a surprisingly effective and cheap but durable set of blades!

The only advice I can give is thick blade roots are the key to safe high speed operation and durability.

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#13
In reply to #7

Re: Gear box for Wind turbine?

05/19/2012 9:55 AM

First of all, I made blades from light wood & in my opinion 2700rpm speed using wooden blades is difficult to achieve.

How fast do wooden airplane propellers turn? They also transmit a lot of horsepower. I'm pretty sure most of them are laminated, but you might want to check the design standards.

Second thing, I mount wind turbine in the urban area on the roof top, so spinning blades with 2700 rpm is dangerous to nearby population in high winds.

Why? There is not much population on top of buildings. Could you have a roof-level fence around the turbine to keep the few people away?

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#15
In reply to #13

Re: Gear box for Wind turbine?

05/19/2012 12:02 PM

Why? There is not much population on top of buildings. Could you have a roof-level fence around the turbine to keep the few people away?

because in storms, or in high wind speeds, there is danger of wood breaking & if blades fall from some height above the building, it causes damage to the near by population.

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#12
In reply to #6

Re: Gear box for Wind turbine?

05/19/2012 9:49 AM

At 100 RPM on that small of turbine you probably wont get enough starting torque to over come the friction of the 20:1 gearbox with anything but the highest of wind gusts.

Note this! If the turbine won't start, the electricity generation will be zero. A small educational turbine at the local historical society takes several MPH of wind to start. I don't know if it is gearbox or direct drive; nor diameter, but guess 8'.

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#9

Re: Gear Box for Wind turbine?

05/19/2012 4:56 AM

Who told you that a decent (efficient, reliable and silent) gearing stage for that ratio range would be cheaper than a lower rpm generator of same power? The only possibility to have some gain is to use a 3:1 to 5:1 helical gear set (like a readily made spare for a rear wheel car final drive), and an accordingly smaller and faster gen. But it would still require a lot of time and effort to make it work, so it all depends on your time cost. S.M.

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#10
In reply to #9

Re: Gear Box for Wind turbine?

05/19/2012 8:19 AM

Low rpm used PM alternator of 300W is around (50-60)$ where as we find used 2700rpm bike motor in 10$ & used gear box of 1:20 in 10$.

I don't understand why 1:20 gear box is not running with 1.2m blades & 2700rpm motor in wind speed of 5m/s or higher.

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#14
In reply to #10

Re: Gear Box for Wind turbine?

05/19/2012 10:32 AM

Well it looks like you are getting exactly what you paid for. Cheap mismatched parts that don't work together.

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#11

Re: Gear Box for Wind Turbine?

05/19/2012 9:43 AM

The weight of the blades, housing, gearbox, etc will NOT contribute to the torque input to the gearbox. The weight will contribute to the support design though.

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#16

Re: Gear Box for Wind Turbine?

05/19/2012 12:19 PM

So what is the purpose of this wind generator and expected output?

At its present size and speed its not likely going generate enough electricity to be of use for much of anything.

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#17
In reply to #16

Re: Gear Box for Wind Turbine?

05/19/2012 12:32 PM

Expected Average output at 5m/s (assuming Cp=0.3) = 1/2*1.2*0.3*3.142*1.2*1.2*5*5*5= 100W

Expected Average output at 6m/s (assuming Cp=0.3) = 175W

Expected Average output at 7m/s (assuming Cp=0.3) = 280W

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#18
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Re: Gear Box for Wind Turbine?

05/19/2012 3:17 PM

Assuming those are very optimistic numbers.

What type of gearbox are you trying to use?

Inline helical gear? inline planetary? right angel worm?

Even if you assumed 100% gearbox efficiency and zero torque loss here is what you are up against.

Your motor is most likely a PM brush type and give you said its a bike motor it probably has around .3 NM of initial drag to get it moving. Now multiply that by 20 and your blades have to produce around 6 NM of torque to get it moving. Add in the gearbox taking at least half that much again itself and your blades now have to produce around 9 NM of torque at zero RPM to stat the system moving.

Basically direct drive reduces your theoretical required starting torque by about 27 times with that motor. Use a bigger brushless PM alternator designed for lower speed operation and you are still likely to see a starting torque reduction of at least 10 times what your present concept would likely need.

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#19
In reply to #18

Re: Gear Box for Wind Turbine?

05/20/2012 6:38 AM

Thanks for the reply.

I know higher rating motor is the best solution, but there is some problem that we want lowest cost solution of producing electricity that can light up one or two energy savers of 23W, 12V.& the PM motor, I use is available easily & its maximum power rating is 300W & rated torqe 0.3N-m.

I have calculations that I share with you.

1. I design blades at TSR=7 i.e. for every 1m/s wind speed, blades move 7 times faster.

If wind speed is Vw=5m/s, blade speed becomes Vb=35m/s & in radians per second = (35/1.2)= 29 rad/sec, & rpm's = 278

so torque becomes T= P/Vb, 100W/29.1rad/sec = 3N-m

3N-m torque is produce if blades rpm is 278 & if we add gear box of 1:20, power in the wind remains same whether using gearing or not using gearing i.e. 100W & if the speed decreases it means torque increases & suppose if blade run at 50rpm (6.248rad/sec) instead of calculated 278rpm, it means torque become 100/6.248 = 17 N-m ?

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#20
In reply to #19

Re: Gear Box for Wind Turbine?

05/20/2012 8:03 AM

Try building it and see what happens. Homemade wind generators with unknown blade designs generally don't do anything close to what theoretical scribblings on paper suggest.

Trust me I have tried building enough of them over the years to know that gearing on small units just leads to poor results in anything but the highest of wind speeds. And then at that point once they do get spinning they tend to be difficult to control and overdrive the generators.

Also the starting torque of a wind turbine with a fixed blade pitch can be about 1/10th or less of the torque it produces when up to its proper running speed. At speed the blades can catch about 60% of the energy in the swept area. At stall they only catch a percentage of the area of their leading face surface which is only a few degrees off from being 90 degrees to the wind itself.

So how do you plan to slow this thing down when the wind gets above the 300 watt output limit of your generator? I have built a few units with rotor diameters similar to yours with direct drive generators and at high wind speeds I have had hard times keeping them under control with 3000 watt loads holding them down.

As far as lowest cost goes you get what you pay for which is usually non functioning or very limited functioning junk and wasted money.

build it and prove me wrong.

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#21
In reply to #20

Re: Gear Box for Wind Turbine?

05/20/2012 11:54 AM

I use automatic or manual furling method to slow wind turbine down.

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#22

Re: Gear Box for Wind Turbine?

05/20/2012 2:26 PM

I'm pretty sure that your alternator is WAY bigger than your propeller can drive.

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#23
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Re: Gear Box for Wind Turbine?

05/20/2012 2:52 PM

I'm pretty sure that your alternator is WAY bigger than your propeller can drive.

Hi tornado, can you plz explain in detail?

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#24
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Re: Gear Box for Wind Turbine?

05/20/2012 4:27 PM

Not without knowing its actual ratings. And even then a quantitative explanation still might not suffice any better than the previous qualitative one.

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#25

Re: Gear Box for Wind Turbine?

05/21/2012 11:13 AM

Wind power depend on the speed of the wind, not on weight of turbine blades. 1.2 mtr radius blade wind turbine can not be governed to rotate at 50 RPM only. Average tip velocity of turbines blades is in range of 250 mtr/sec., if this is true, your turbine RPM will be in range of 2000.

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#26
In reply to #25

Re: Gear Box for Wind Turbine?

05/21/2012 11:30 AM

Hi mukesh,

I design blades for tip speed ratio '7' where Cp value is maximum & most of the turbines blades design on '7' TSR, it means that for 5m/s wind speed, blades rotate 7*5=35m/s & in rpm '168'

& if I design wood for 250m/s, it means TSR in 5m/s speed is (250/5)= 50? I can't see blade design which operates on TSR=50

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#27
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Re: Gear Box for Wind Turbine?

05/21/2012 11:42 AM

Thats because a good set of high speed blades can have a TSR of 10+ and the wind can blow far faster than 7 M/S as well.

Where I live 5 - 7 M/S wind speeds are on the low end when and its common to see sustained winds in the 15+ M/S range with peaks well over 30 M/S which is why its necessary to build the whole unit robustly enough to survive at those speeds!

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#28
In reply to #27

Re: Gear Box for Wind Turbine?

05/21/2012 1:03 PM

In the urban areas (5-6) meter/sec is the average wind speed & most small turbines give rated output power at 12m/s, which is in the kW range & 30m/s is much heavy wind that blows once in a year in our area.

my question is that can we design wood blades at TSR =50? so that my turbine will run at 2000rpm, but I can't see blade shape design at TSR = 50, but most of the books suggest that TSR=7 is the best value. & at high TSR's value of Cp decreases so that the output power

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