Anemometer - Jan 27, 2007

We needed a precision anemometer for determining wind speed for an upcoming multi-million dollar project. *
Our research on anemometers (which began when we started building this one) proved that they were expensive.
We decided that money was indeed an object so we spared all expenses and just  made one.

In the air:

Mounted on a piece of thin wall 1.5" pipe and supported by an old PA speaker tripod that had been lying about.
(Naturally, the day was windless and I didn't feel like adding 'Motion Blur' in Photoshop)


The Rotor:

The cups were made from 20 oz. drink bottles cut with the Hot Wire Plastic Cutter.
These precision components proved to be the most costly part of the project ($3.89).
The brass tubes were heated with a soldering iron until they melted slots in the bottle caps.
The tubes were then cut to length, then trimmed with a Dremel to balance the rotor. 


The Generator:

Made from a hard disk spindle motor. The clamp ring holds the brass tubes in place.
The tubes were flattened about 0.25" from the ends, jammed under the clamp
120 degrees apart and the clamp screws tightened mercilessly.
Extreme precision is needed here so we eyeballed their positions above (kind of) the motor's mounting holes ;^)


A hard disk motor is a three-phase stepper motor usually with four leads or contacts. One lead/contact is "common".
If a stepper motor is rotated by an external force, it generates three sine (almost) waves from three of its four leads or contacts
referenced to common with frequency AND voltage determined by rotational speed.
Getting rotational information from one only requires only two wires; "common" and any one of the three phases.  

Finding common is easily done with an Ohmmeter.
To find common, start with the Ohmmeter on any two leads. Note the resistance. 
Measure all leads to all other leads. "Common" will have the same resistance to all others. 

The one we used measured 1.5 Ohms from common to any phase and 3 Ohms from any phase to any other phase.
Use sub-miniature shielded wire (we had many scraps of Mogami X-Series audio cable) and connect the shield to
common and ONE of the conductors to any phase. The other end of this cable goes to the connector of your choice.
 We used XLR type connectors so that standard mike (that's right; MIKE, not MIC **) cables could be used.

It's inportant to have this connexion scheme so as not to place strain on the tiny motor connections. 

The output voltage AND frequency varies with windspeed. Use whichever is convenient for you.

 Typical HDD motor wiring


 In the time you've wasted reading this, you could have built one! Get Busy!

 * OK, it's a multi cent project!
** I'm an old fart and use the old spelling so fsfk off!