Hi, Im no engineer, just studying for pilots exams and Im looking for a good website or other source on Induction tachometers and the differences between magnetic, DC, AC or 3 phase AC indicators. Any ideas, I was under the impression that the Induction tacho operated on the principle of a Notched wheel in a magnetic field but I have seen it written that it is the rotation of asynchronus motor energised by an alternator. :confused:

Induction motors are also called assynchronous motors It's just a terminology matter.
Induction motors work by the principle of subjecting the armature(which could be a Squirrel cage or a notched wheel or whatever you call it ) to a rotating magnetic field (if it's 3 phases or 2 phases) or simply 1 pole of AC winding assisted by an auxiliary winding shunt (for cranking it initially which later on gets disconnected) supplied by a generator.
When the armature is subjected to the rotating magnetic field the bars of the squirrel cage induce voltage and high current due to the low impendance of these bar which are connected in a loop manner which sets up a secondary magnetic field that reacts with the primary magnetic field (the one supplied by the ac generator) and tries to line up with it and due to the momentum of the bars mass a rotation ensues, the faster the rotating magnetic field (which is a function of frequency) the faster the rotation you'll get at the armature and hence the speed indication that the tachometer is supposed to give.
If you apply the same principle but for a rotating coil supplied with DC rather than a rotating field set up by an AC generator you get the DC type.
Thats it in a short and shallow explanation.
If you need further information you can pm me.

Yeah, wot he said.

I need to get back to my books.:confused:

Thank you Flame Bringer for shedding some light on the subject, I think I may finally be getting to grips with it. :ok: