The angle found by incremental encode interface is reset to zero by the index signal. In perfect situation, the mounting of the encoder should guarantee that the index happens at the very instant of having the corresponding stator and rotor phases magnetically aligned. Under this condition, the angle read from the encoder truly expresses the angle of the rotor magneitc axis with respect to the stator magnetic axis (e.g. magnetic axis of rotor phase a with respect to magnetic axis of stator phase A, b with B and c with C).

The mounting of the encoder with the rotor shaft doesn't necessarily follow this perfect assumption especially in laboratory setups for DFIG research. Therefore, It can result in random position of the index signal with respect to the alignment position and consequently incorrect readings of the angle enclosed between stator and rotor magnetic axes of the wound rotor induction machine.

The question is:

What is the procedure to be followed to quantify the possible angular offset by encoder and thereby nullify this offset?

Taking into consideration that the following measurements are available: three instantaneous stator voltage, three instantaneous rotor voltages, direct encoder angle reading