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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:

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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.