What is unique about synchronous motors compared to induction motors?

Synchronous motors run at exactly synchronous speed (Ns = 120f/P) with no slip, unlike induction motors. They require DC excitation for the rotor field and are not self-starting - requiring starting methods like damper windings or VFDs. Uniquely, synchronous motors can operate at leading power factor by over-excitation, supplying reactive power to the grid. They're used for large constant-speed loads and power factor correction.