What Is a Synchronous Motor? — Definition, Working Principle & Applications

A synchronous motor is a type of AC motor that runs at a constant speed exactly synchronized with the supply frequency. Its rotor rotates at the same speed as the stator’s rotating magnetic field (RMF), with zero speed slip, which distinguishes it from asynchronous induction motors that suffer from inherent slip errors.

Core Features

1. Constant-speed operation
   Its running speed depends only on grid frequency and pole pairs, remaining stable regardless of load changes, ideal for precision speed-control applications.
2. Non self-starting
   The motor cannot start on its own. External auxiliary force is required to drive the rotor close to synchronous speed first; magnetic locking takes effect afterward to maintain steady rotation.
3. Adjustable power factor
   It can operate at lagging, unity, or leading power factors. It is widely used in industrial power systems to improve grid power factor and reduce reactive power loss, replacing static capacitors in many complex working conditions.

Main Components

A synchronous motor is a double-excitation device consisting of two core parts and auxiliary structures:

• Stator: The stationary part with three-phase windings powered by AC, generating the rotating magnetic field.
• Rotor: The rotating part with DC-excited windings or permanent magnets to produce a fixed magnetic field for magnetic locking with the stator field.
• Auxiliary parts: Exciter, slip rings and brushes, which provide continuous DC excitation for the rotor.

Working Principle

The stator’s three-phase AC produces a rotating magnetic field, while the rotor’s DC excitation generates a constant magnetic field. At static state, the alternating attraction and repulsion between the two magnetic fields offset each other, resulting in zero starting torque. After external acceleration brings the rotor to synchronous speed, the rotor and stator magnetic fields lock tightly, and the motor runs stably at a constant synchronous speed.

Classification

According to rotor excitation modes, synchronous motors are divided into two categories:

1. Non-excited synchronous motors
   No external excitation voltage is needed, including hysteresis motors (for audio equipment), reluctance motors (for precise transmission devices), and permanent magnet synchronous motors (PMSM) (high efficiency, energy-saving, widely used in industrial automation).
2. Current-excited (DC-excited) synchronous motors
   Equipped with DC excitation devices, mostly high-power industrial models, applicable to heavy-load industrial scenarios.

Typical Applications

1. Precision timing equipment: Clocks, record players, precision instrumentation;
2. Low-speed high-power industrial equipment: Mills, compressors, pumps, agitators, fans;
3. Power grid optimization: Power factor correction and grid voltage stabilization for industrial systems.

Advantages & Disadvantages

• Advantages: Ultra-high speed accuracy, adjustable power factor, stable low-speed performance, high efficiency for low-speed heavy-load operation.
• Disadvantages: Complex structure, unable to self-start, higher manufacturing and maintenance costs than induction motors。