[mattW]

Ok i have no idea whether this works or not but could you use the voltage across the motor to work out it's rpm? According to my old physics textbook as the rpm increases so does the back emf, so could you just whack a voltage gauge across the motor and use it like a tacho? (i am assuming it doesn't change with load). If you knew the voltage drop at max rpm then you could just change up then, maybe even work out a gear change light. Kiwi i think i remember you talking about playing with the tacho somewhere but i cant find the thread atm. Anyone know whether this would work?

 

[mattW]

But what i am saying is, when a voltage, say 120V is applied across a motor, The electrical energy that is converted to kinetic energy has to be accounted for (it isn't 'free'). When the moving electrons in the moving wire pass through the magnetic field, the field tries to slow the electrons down, which reduces the individual energy of the electrons but it also pushes the wire. So a motor with 120V accross it might have a back emf (electromagnetic force) of -50V meaning the total voltage difference across the motor is 70V. So you can skip the small generator step. But i only know this from a text book, you would have to test it. What is the voltage across the motor when it is stationary (i.e when the power has just been turned on) and at full rpm? I don't have a motor i can test it on atm so i'm hoping someone else can.