Your voltages indicated are undefined and the circuit itself is not well determined.

for one include the Fet intrinsic parasitic diode and you will see the differences and limitations.

The V1 voltage will be inserted into the circuit when its voltage is within V2 less 0.6 volts, also the 2N3906 Base Emitter junction is in reverse mode allowing the breakdown ( Zener voltage of it ) flow current causing transistors behavior to be affected.

Then if the V2 voltage is greater by 0.8 volts than the V1 voltage, the Fet intrinsic diode to be turned ON ( reverse mode operation)- which may give you problems if V2 does increase radically.

If you want and really need to measure the voltage at any time, then another circuit is needed, like to be installed up the tower and brought down either with wires or wireless device to send the data.

Proper data taken is BEST to define the generator Volt/RPM and RPM measurements will give you the generated voltage even under any load and if you have the generator internal resistance, then you may be able to calculate the stator watts dissipation at any instant.

One needs to think in practical terms the parameters and the design of a circuit to see if it is applicable for one needs.