Wireless cycle computer as an RPM monitor
Along with the usual amperage and voltage measurements usually associated with wind generators to monitor it's performance, another useful measurement you can make is the RPM of your turbines blades. You can buy a wireless cycle computer from a bike shop for about £25, then you’ve basically got a cheap RPM meter, amongst other things.
You can fix the magnet to your propeller, (watch the balancing) and the sensor to the generators mounting bracket so that they pass each other closely without touching.
Unfortunately the wireless facility on the cycle computer only has a maximum range of 70cm, if this is unworkable, a wired version can be used, along with the twisted cables!
The sensor sends pulses to the bike computer wirelessly, which then displays many useful measurements including:
· Current Speed
· Maximum Speed
· DUAL Average Speed
· Total Distance (Odometer)
Useful for servicing intervals?
· Trip Distance
· Auto Start/Stop
· Manual Start/Stop
· DUAL Trip Time
· Back Light for night viewing
Visitor Adam V. kindly helped me out with the actual calculations:
"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).
In order to help with balance you could put a magnet on two opposite points, or even on every blade. In which case simply divide the figure given by the number of magnets used (basically the counter will see each pass as a whole revolution when in fact it is one half, quarter or whatever). The nearer the magnets are to the hub the more stable the arrangement will be (the linear velocity of the magnet increase with radius, so the forces required to pull it round in a circle increase and cause more wobble). Remember that to measure revs the radius is not a factor
The better quality cateye cycle computers offer excellent water resistance as well. Cateye site here