What Are Synchronous Motors?

Synchronous motors are alternating-current rotating machines that develop mechanical torque by locking the rotor into alignment with a rotating magnetic field produced by the stator windings, operating at a shaft speed that is exactly proportional to the electrical supply frequency. The speed of a synchronous motor is determined by the expression n = 120f / P, where f is the supply frequency in hertz and P is the number of magnetic poles, and it does not vary with mechanical load in the way that an induction motor's speed does. Synchronous motors belong to the broader family of rotating machines, which encompasses all devices that convert electrical energy to mechanical energy (or the reverse) through electromagnetic interaction between a stationary and a rotating member. A thorough treatment of rotating machine theory, including synchronous motors, is available in Iowa State University's IEEE-standard guide for synchronous machine modeling.

The operating principle of synchronous motors is identical to that of synchronous generators: in both cases, a rotor field produced by direct current (or permanent magnets) interacts with a stator field rotating at synchronous speed. The difference is the direction of power flow: motors draw electrical power and deliver mechanical power at the shaft.

Wound-Field Synchronous Motors

Traditional wound-field synchronous motors carry a DC excitation winding on the rotor, supplied through slip rings or a brushless exciter. The excitation level controls both the torque angle (the angular displacement between rotor field and stator field) and the machine's reactive-power exchange with the supply system. When the field current is increased beyond the level required for unity power factor operation, the motor becomes over-excited and supplies reactive (leading) current to the network, performing the function of a capacitor bank. This reactive-power correction capability made wound-field synchronous motors the preferred choice for large industrial loads at 1,000 horsepower and above throughout much of the twentieth century, with detailed selection and application guidance provided in Columbia University's power engineering course materials on AC machines.

Permanent-Magnet Synchronous Motors

Permanent-magnet synchronous motors (PMSMs) replace the wound rotor field with embedded or surface-mounted magnets, typically composed of neodymium-iron-boron alloys, eliminating rotor copper losses and the associated exciter hardware. PMSMs achieve higher efficiency and power density than wound-field designs and are widely deployed in electric vehicle traction drives, servo positioning systems, and high-efficiency industrial drives. Control of a PMSM relies on field-oriented control (FOC) or direct torque control (DTC), algorithms that resolve the stator current into torque-producing and flux-producing components in a rotating reference frame aligned with the rotor magnet, enabling fast dynamic response comparable to a separately excited DC machine. The variable-frequency inverter drive required for PMSM starting and speed control also eliminates the need for damper windings.

Starting and Speed Control

Because synchronous motors cannot self-start on a fixed-frequency supply, they require a starting strategy. Wound-field synchronous motors typically start on their damper windings, which provide induction-motor torque until the rotor approaches synchronous speed, at which point DC excitation is applied and the rotor snaps into synchronism. Motors controlled by variable-frequency drives (VFDs) start smoothly from zero frequency, maintaining synchronism throughout acceleration. VFD control also enables adjustable speed operation over a wide range, which has expanded the use of synchronous motors into applications previously served exclusively by DC or variable-speed induction motor drives. The University of Chicago's electrical engineering readings on turbine-driven generator and motor design cover how design choices between salient-pole and cylindrical rotors influence starting behavior.

Applications

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

• Large industrial compressors, pumps, and fans requiring constant speed
• Electric vehicle traction and regenerative braking systems
• Servo axes in machine tools and robotics
• Power-factor correction on utility distribution feeders
• Textile machinery and paper mills requiring precise speed coordination