Rotating Diodes are diodes fitted in the rotor of the synchronous machine between the excitor and the main field winding. The function of the Rotating Diodes is to rectify the AC output of the excitor into DC which 
can be used to magnetise the main field windings and poles.
These diodes are connected in the shape of a normal three phase rectifier with six diodes (sometimes twelve diodes are used). The Diodes are called Rotating diodes as they are mounted on the rotor which is rotating. The diodes do not rotate by themselves.
The DC output from
the rectifier assembly is usually collected from two rings in the assembly. The Diodes are protected by a Varistor against voltage spikes during sudden load fluctuations.