What Is a Synchronous Motor?

A synchronous motor is an alternating-current electric motor whose rotor rotates at exactly the same speed as the rotating magnetic field produced by the stator windings, a speed determined by the supply frequency and the number of magnetic poles. Unlike an induction motor, which operates with a small speed slip, the synchronous motor maintains a fixed speed that does not change with mechanical load as long as the motor remains in synchronism. This constant-speed characteristic makes synchronous motors attractive for applications where precise speed is necessary, such as drives for large compressors, ball mills, and calendar rolls in paper making. The electromagnetic fundamentals and equivalent-circuit models for synchronous motors are detailed in IEEE course materials on synchronous machine modeling.

Synchronous motor theory descends from the same analytical framework as the synchronous generator: the two machines are physically identical, distinguished only by the direction of power flow. When electrical power is supplied to the stator terminals and the machine develops mechanical torque at the shaft, it operates as a motor. This duality means that synchronous motor design follows the same standards used for generators, including IEEE Std 1110 for parameter identification.

Working Principle and Construction

The stator consists of a laminated iron core carrying three-phase distributed windings connected to the AC supply. The rotating magnetic field produced by these windings completes one revolution per electrical cycle, advancing at synchronous speed. The rotor carries a field winding supplied with direct current through slip rings, creating a fixed north-south polarity that locks into alignment with the stator's rotating field, producing the torque that turns the shaft. Salient-pole rotors, with their projecting pole faces, are common in low-speed motors because the saliency itself contributes a reluctance torque component that adds to the excitation torque. Cylindrical rotors appear in higher-speed designs such as two-pole 3,600 rpm motors driving gas compressors.

Starting Methods

A synchronous motor cannot start from rest on its own because the stator field rotates at synchronous speed while the rotor is stationary, producing an average torque of zero over a full electrical cycle. Three starting methods are in common use. First, a damper (amortisseur) winding embedded in the rotor pole faces provides induction-motor torque during run-up; once the rotor approaches synchronous speed, DC field excitation is applied and the rotor pulls into synchronism. Second, a variable-frequency drive (VFD) ramps the supply frequency from zero to rated frequency while the machine accelerates, maintaining synchronism at all speeds without a damper winding. Third, a pony motor mechanically accelerates the rotor to near-synchronous speed before the stator is energized. The VFD approach has become standard for large permanent-magnet synchronous motors because it also enables precise speed and torque control. Detailed guidance on these methods appears in Columbia University's power engineering materials on synchronous machines.

Speed Control and Power Factor Correction

Once running, a synchronous motor's speed is invariant for a given supply frequency. Speed variation requires changing the supply frequency, which is accomplished with a VFD in adjustable-speed applications. A notable advantage of synchronous motors over induction motors is their ability to control power factor by adjusting rotor field excitation. With under-excitation the motor absorbs reactive power (lagging power factor); with over-excitation it supplies reactive power (leading power factor). Large over-excited synchronous motors are therefore installed deliberately at industrial plants to compensate the lagging power factor of induction motor loads, reducing reactive demand charges and improving voltage regulation. This compensating role is discussed in Iowa State University's power systems engineering literature.

Applications

Synchronous motors have applications in a wide range of sectors, including:

• Large industrial compressors and pumps requiring constant-speed operation
• Ball mills, crushers, and rolling mills in the mining and metals industries
• Paper-machine calendar rolls and winding machinery
• Power-factor correction on industrial and utility distribution buses
• Electric vehicle traction systems using permanent-magnet synchronous motor designs