The equation is that for a pendulum and suggests that anyinbalance between the system mechanical and electrical torquewill create a pendulum-like oscillation at a frequency dependenton the inverse square root of the moment of inertia J. Thisequation is not in the required form since we wish to study theexcursion in rotor or load angle due to a disturbance. Since m ismeasured with respect to a stationary reference axis on the stator,it is an absolute measurement of rotor angle. It is the rotor speedrelative to synchronous speed which is of interest, and therefore itis more convenient to measure the rotor angle with respect to areference axis, which rotates at synchronous speed. Therefore, wedefine șm = Ȧsmt + įm ,where Ȧsm is the synchronous speed of themachine, in mechanical radians per second and įm is the angulardisplacement of the rotor, in mechanical radians, from thesynchronously rotating reference axis.