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puddingbag
Starting Member
United Kingdom
8 Posts |
 Posted - 27/06/2009 : 17:10:41
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In the project section of this site you talk about the 'cycle computer as an RPM monitor'
Which is really interesting but I cant get it to work
The size of my wind turbine is 24 inches wide (ie the same size as a bike wheel) but i cant work out the number i need to put into my cycle computer??
to get mph for a 24" wheel i type in 1916
does any one know what I need to type in so it shows RPM instead
many thanks
Tim
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gotwind
Forum Admin
United Kingdom
1564 Posts |
Posted - 27/06/2009 : 19:36:49
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Hi Tim.
For others here, I believe you are refering to this page I wrote a few years ago now.
http://www.gotwind.org/diy/cycle_computer.htm
To be honest I can't remember the actual methodology, however a guy named Adam offered an explaination for Km/h
"What you actually need is to input a ‘fake’ value for the diameter"
PI x d [cm] x RPM x 60 / 1,000,000 = speed [km/h – hence the million to get from cm to km]
Back calculating tells us that putting d = 5,305 would give a readout of the exact RPM (but displayed as if it were km/h). Many computers won’t display more than two digits plus the decimal, so it might make more sense to use 530.5 cm and the readout would then show eg 35.6 for 356 RPM
Similarly if the computer won’t accept a wheel this large we could aim for Hz (revs per second) instead of RPM and use 5305/60 = 88.4 cm. Hertz might actually be useful in working out the expected power out of a dynamo – I don’t know if they quote figures based on Hz or RPM usually.
So the two most workable numbers would be 530.5 (for RPM/10) or 88.4 (for Hz).
If anyone else knows a better explaination for mph please enlighten us, my brain hurts 
Ben. |
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ghurd
Average Member
USA
546 Posts |
Posted - 28/06/2009 : 15:05:27
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Adam's explanation makes my brain hurt too. And it is incorrect.
We do not care how big it is. We only care how many times per minute it goes around.
100 RPM is 100 RPM, regardless if it is 24" or 24 meters.
The magic number is the "wheel diameter" which when rotating 10 times per minute rolls along at 1kph.
This number never changes, and is always 1666.666mm.
Set the speedometer to kilometers per hour.
Set the wheel size to 166.7cm or 1667mm.
452 RPM will read 45.2 kph.
If the kph (RPM) reading is obviously incorrect and seems to jump around, it is probably the magnet is not close enough to the sensor. They are picky. A larger stronger magnet helps.
G- |
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ghurd
Average Member
USA
546 Posts |
Posted - 28/06/2009 : 19:05:54
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OK- I see what Adam did.
Every bike computer I ever used, from Garmin to the $1 ebay unit, uses circumference, not diameter.
Still just set it for 166.7cm or 1667mm.
Sorry about that.
G- |
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gotwind
Forum Admin
United Kingdom
1564 Posts |
Posted - 29/06/2009 : 21:33:55
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Thanks Ghurd.
166.7cm makes sense to me now.
Ben. |
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puddingbag
Starting Member
United Kingdom
8 Posts |
Posted - 01/07/2009 : 20:42:03
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Thank you ben and Ghurd for your help
I understand now! very cool and much cheaper way of knowing how many rpm the wind produces
many thanks
Tim
quote:
Originally posted by gotwind
Thanks Ghurd.
166.7cm makes sense to me now.
Ben.
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Ed_N
Starting Member
3 Posts |
Posted - 03/07/2009 : 21:08:03
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Hi,
New here, coudn´t resist the temptation to reply to this post. You could also measure rpm directly from the wind turbine generator output with a frequency to voltage converter. If your wind turbine ia a low voltage one, it will be easy to do.
If it is a higher voltage one (mains) then you´ll need some isolation electronics like LEM VT sensors |
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ghurd
Average Member
USA
546 Posts |
Posted - 05/07/2009 : 14:08:27
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Ed,
Generators make DC in this context.
Alternators make Hertz. The typical reader does not have a junk box with the parts necessary to simply add a LM2917 and measure RPM.
The $1 eBay bike computers work quite well.
G-
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how to get RPM on monitor??