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Interfacing to the tachogenerator

The engine has a tachogenerator connected to the gearbox to give an indication of the gas-generator rotational speed.

The tachogenerator itself is a large unit mounted at the top of the gearbox:


As it was looking rather tired, and as a chance to check the bearings were OK, I dissasembled and repainted it:

Rotor and thre-phase coil

Rotor and three-phase coil

Coil and top bearing

Now that the tachogenerator has a shiny new coat of paint it needs to be characterised so we can convert the AC (ish!) output to the number of RPM its shaft is rotating at.

Using a phototachometer (30 quid on eBay), an oscilloscope and a cordless drill to drive the tachogenerator I got the following results (pins 1 & 3):


Input (RPM)      Output (Hz)     Output (RPM)

198                   6.6                    396

394                 13.08                  784


So we can see that the tachogenerator output frequency is twice the frequency of the input to it. After a bit more testing I also found that the voltage is directly proportional to the RPM at 38mV/RPM. This is really of no consequence as I'm only interested in the zero-crossing points and will be clipping the waveform to lower levels.

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