Lets clarify this with example of a induction motor(Referred as IM) for easy understanding.
Things to know before-
1.Synchronous speed (Ns) is 120*f/P.
2.Relative speed concept-Speed of object when seen from other reference.
3.IM has a stator(static) and rotor(rotating or static).
4.Stator magnetic field - Field generated by stator.
5.Rotor magnetic field -Field generated by induced currents in rotor.
In case of induction motor during normal operation.
Stator magnetic field speed = Ns.
Rotor magnetic field speed =Ns.
Speed of Rotor magnetic field w.r.t Stator M.F or vice versa =Ns-Ns=0.
Speed of stator = 0 (static).
speed of Rotor = N (actual speed).
speed of stator field w.r.t rotor= Ns-N.
In general speed of X w.r.t Y , we just compute by = Vx-Vy