Monitoring the rotational speed of the wind turbine blades can help with power performance measurements.
This post shows various techniques to measure the rotational speed of a 'wild AC' output small wind turbine. In this system, the wild AC is rectified into DC and either delivered to a battery or to a grid-connected inverter.
The wild AC output of the turbine can be used to give us the rotational speed.
The AC waveform can be converted into a digital signal by a circuit such as this, from Noam Dotan of Comet Me:
This circuit includes opto-isolation, which means the data logging system is protected from any over-voltage.
The fuse protects from any short circuit and only needs to be a low value, 50-100mA.
The zener diode limits to voltage across the LED of the opto-isolator.
The output will be a digital on/off signal for every time the magnet passes the coil. As mentioned, the number of magnets can be used to calculate the actual RPM of the rotor.
The LM7805 is a voltage regulator to give a constant voltage - the power supply of the data logger can be used here instead.