# Mechanical Engineering/Figuring out RPM's

Question
If I build a wind powered generator, is there an equation I can use to determine the rpm's of the generator assembly in relation to the rpm's of the fan itself?  In other words, if I were to use an alternator for the generator, how do I determine the alternator has hit the necessary 2200 rpm minimum by the rmp's on the fan blades.

Louis - it is all dependant on where you are measuring the fan blades.  The outside of the blade (or the tip) is going to be moving slower than the inside of the blade (or the shoulder).
What is comes down to is as follows:
TSR = Tip Speed Ratio
TSR = Tip Speed of the Blade / Wind Speed

From MMPA.ORG
It is easy to find the distance travelled by the blade tip, but finding how long it
takes to go that distance can be tricky. There are several ways to find revolutions
per minute (RPM), but there is not a single simple way!
• Manually—try to count how many times the blades revolve in a certain
time period. It helps to mark or color one blade differently so you can be
sure of when the turbine makes a full revolution.
• Use a real tachometer—These are great, but can be pricy. Try the “Hangar
9 Tachometer” or the “Extech Pocket Tachometer”
• If you’re crafty, you can use a bicycle computer. Attach the magnet to the
rotor and the sensor to a non-moving part of the turbine. You can find detailed instructions here:
http://www.reuk.co.uk/Use-a-Cycle-Computer-to-Measure-Turbine-RPM.htm
You need to know how many seconds it takes the rotor to spin around one time. If
you found RPM, you will need to convert this number. There are 60 seconds in one
minute, so just divide 60 by your RPM value. That will tell you how many seconds it
takes to make one revolution.

Mechanical Engineering

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