Final answer:
The principal difference between a synchronous machine and an induction machine lies in how they generate magnetomotive force and produce a rotating magnetic field.
Step-by-step explanation:
The principal difference between a synchronous machine and an induction machine lies in the way they generate magnetomotive force (mmf) and produce a rotating magnetic field. In a synchronous machine, the mmf is generated by direct current supplied to the rotor winding, which creates a magnetic field that is synchronized with the stator field. This synchronization allows the rotor to lock into step with the rotating magnetic field, resulting in a constant speed of rotation. On the other hand, an induction machine relies on the concept of induction, where the stator windings are energized to produce a rotating magnetic field. This rotating field induces voltage in the rotor windings, which in turn produces rotor current and creates another magnetic field. The interaction between the two fields causes the rotor to rotate.