Synchronous motors

To understand the basic concept of a synchronous motor, look at Figure 2, which shows a two-pole synchronous motor. The field current IF of the motor produces a steady-state magnetic field BR. A three-phase set of voltages is applied to the stator of the machine, which produces a three-phase current flow in the windings.
A three-phase set of currents in an armature winding produces a uniform rotating magnetic field Bs. Therefore, there are two magnetic fields present in the machine, and the rotor field will tend to line up with the stator field, just as two bar magnets will tend to line up if placed near each other. Since the stator magnetic field is rotating, the rotor magnetic field (and the rotor itself) will constantly try to catch up. The larger the angle between the two magnetic fields (up to a certain maximum), the greater the torque on the rotor of the machine. The basic principle of synchronous motor operation is that the rotor "chases" the rotating stator magnetic field around in a circle, never quite catching up with it.
Synchronous motors supply power to loads that are basically constant-speed devices. They are usually connected to power systems very much larger than the individual motors, so the power systems appear as infinite buses to the motors. This means that the terminal voltage and the system frequency will be constant regardless of the amount of power drawn by the motor. The speed of rotation of the motor is locked to the rate of rotation of the magnetic fields, and the rate of rotation of the applied mechanical fields is locked to the applied electrical frequency, so the speed of the synchronous motor will be constant regardless of the load. This fixed rate of rotation is given