Electrical Degree and Mechanical degree in Synchronous machines

Angle in Electrical and Mechanical Units

Consider a synchronous machine with two magnetic poles. The idealized radial distribution of the air gap flux density is sinusoidal along the air gap. When the rotor rotates for one revolution, the induced emf, which is also sinusoidal, varies for one cycle as illustrated by the waveforms in the diagram below. If we measure the rotor position by physical or mechanical degrees or radians and the phase angles of the flux density and emf

by electrical degrees or radians, in this case, it is ready to see that the angle measured in mechanical degrees or radians is equal to that measured in electrical degrees or radians, 

i.e.

                                           θ=θ_m

where θ   is the angle in electrical degrees or radians and θ_m   the mechanical angle.