Synchronous motors stators are very similar to induction motor stators. The rotor is made of a shaft with a sandwich of steel layers (laminations) or alternately with a solid casting. The rotor requires a winding which must be excited with dc current. The rotor current or excitation electro-magnetizes the rotor.
Rotor excitation current can be achieved with normal brushes or on larger motors with a brushless exciter. The brushless exciter consists of an extra small generator on the shaft with a rectifier.
The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn at exactly 3600 RPM (3000 RPM) at 60H (50 Hz). A 4-pole motor will rotate at exactly 1800 RPM. A 6 pole motor will rotate at exactly 1200 RPM.
Synchronous motors produce torque only when they are turning at exactly synchronous speed. Slipping a pole due to torque overload is a serious problem which must trip the motor. The trick is in starting a synchronous motor since it has no torque until it gets to synch speed. For across the line starts, the rotor is shorted. This make the rotor act essentially like the rotor in an induction motor. Induced rotor current then produces torque proportional to slip and the motor accelerates. Once the speed is close to synch speed, the field excitation is applied and the motor snaps into synchronous speed.
Variable speed drives must be specified for operation with synchronous motors. Except for some servo drives, few synchronous motors are used for variable speed operation. Note that Permanent Magnet (PM) motors are a special class of synchronous motor and are becoming widespread in web-handling applications.
A large synchronous motor can have its rotor current over-excited to produce vars (Volt-Ampere Reactive) which can correct the plant electrical power factor and reduce utility charges due to poor power factor.