d-axis q-axis
Copied from "Electric Machinery" by Fitzgerald (pg. 282, 6th edition):
Note that, by definition, the field winding produces flux which is oriented along the rotor d-axis. Thus, when phasor diagrams are drawn, the field-winding mmf and its corresponding flux are found along the rotor d-axis. The generated internal voltage is proportional to the time derivative of the field-winding flux, and thus it's phasor leads the flux by 90 deg. Since by convention the q-axis leads the d-axis by 90 deg, we see that the generated voltage phasor lies along the q-axis...
Questions
1) Which theory that motivated the author came to this comment: "...by convention, the q-axis leads the d-axis by 90 deg"? Why not the other way round?
2) The generated internal voltage is proportional to the time derivative of the field-winding flux, and thus it's phasor leads the flux by 90 deg. From mathematical point of view, how does the 'derivative' suggest that the voltage phasor leads the flux?
Thanks