synchronous-motor
I am looking for an explanation for the fact that over excited synchronous motor has leading power factor. If induced voltage due to the rotor is greater that the supply voltage, wouldn't that being a generator?
How is a motor with windings and magnetic fields act as a capacitor?
Perhaps your difficulty is that the frame of reference or the "space" containing the magnetic flux and force vectors is rotating rather than fixed to points on the structure of the machine. In induction motors, the frame of reference rotates with respect to both the stator and the rotor, moving at the synchronous speed with respect to the stator and at the slip speed with respect to the rotor. In synchronous motors, the frame of reference is fixed with respect to the rotor, but rotates at the synchronous speed with respect to the stator. Thus the vector representations of the fluxes and forces have fixed positions with respect to the phase-vector representations of the AC voltages and currents.
I believe that this representation is developed by first examining the vectors with respect to the windings in the machine without considering the motion of the rotor and then examining the machine operating at a steady-state speed.
Equivalent Circuits
The analysis and control of electric machines makes use of equivalent electric circuits that are mathematically analogous to electromagnetic machines. They have a basis in physics, but that basis may not be as firm and direct as you might like. Engineers generally use the tools that will provide answers with a level of accuracy that is suitable for the ultimate purpose. Scientists strive to explain the laws of nature as thoroughly and accurately as humanly possible. Engineers strive to design products that are useful to human endeavors and economically affordable.
Terminology
Not all engineering terminology is carefully thought out. Even terminology that is carefully created is often simplified in everyday use. As a result, some terminology can have more than one meaning depending on context. I suspect that engineers are more tolerant of imprecise terminology than scientists may be.
This information is not related to synchronous motor but rather to an induction motor with wound rotor and slip rings.
Adding a resistance in rotor circuit affects the torque vs. speed characteristics, so you can get higher starting torque. Note that this doesn't increase the available torque, it only shifts the max. torque point to the zero speed.
Best Answer
The operation of a synchronous motor with a leading power factor is explained by analyzing the phasor diagram. It has a leading power factor when the excitation is higher than the excitation required for operation with a unity power factor. That is the definition of "overexcited." Leading power factor operation is explained mathematically using the phasors derived from the equivalent circuit. There may not be an explanation that is intuitively satisfying.