Rotor: Constant Magnetic field
Rotor is excited by a D.C power supply, magnetic field produced around the rotor coil by DC excitation is shown below. It is clear that the rotor acts like a permanent magnet due to such magnetic field. Alternatively rotor can also be made of permanent magnet.
Interaction of Rotor and RMF is interesting. Assume you are giving an initial rotation to the rotor, with same direction of RMF. You can see that opposite poles of RMF and Rotor will attract each other and they will get locked magnetically. This means that rotor will rotate at the same speed of RMF, or rotor will rotate at synchronous speed.