A synchronous motor is an AC (alternating current) electric motor that operates at a fixed, synchronous speed determined by the frequency of the AC power supply and the number of poles in the motor. Unlike asynchronous motors (induction motors), which run at a speed slightly below synchronous speed, synchronous motors maintain a constant speed in AC applications.
Here's how a synchronous motor maintains constant speed:
Synchronization with AC frequency: The synchronous motor is designed with a specific number of poles in its construction. The number of poles and the frequency of the AC power supply determine the synchronous speed of the motor. For example, a 4-pole motor operating on a 60 Hz power supply has a synchronous speed of 1800 RPM (revolutions per minute) in the United States.
Magnetic field synchronization: When the AC power supply is connected to the motor, it creates a rotating magnetic field in the stator (the stationary part of the motor). The rotor (the rotating part of the motor) is designed with electromagnets and is excited by a direct current (DC) to create its magnetic field. The rotor's magnetic field aligns with the rotating magnetic field in the stator, resulting in synchronization of the rotor with the rotating magnetic field.
Zero slip operation: The synchronous motor operates with zero slip, meaning that its rotor speed is precisely equal to the synchronous speed of the rotating magnetic field in the stator. As a result, the rotor moves at a constant speed, matching the speed of the rotating magnetic field.
Load and excitation control: To maintain a constant speed under varying loads, the excitation of the rotor's electromagnets is adjusted. By increasing or decreasing the excitation, the strength of the rotor's magnetic field is controlled, ensuring that it stays in sync with the rotating magnetic field in the stator, and hence, maintaining a constant speed.
Prime mover or power source: Synchronous motors are typically used in applications where a constant and precise speed is required. In many cases, they are driven by a prime mover, such as a steam turbine or a diesel engine, which provides a stable AC power supply at the required frequency.
It's worth noting that synchronous motors require precise control of their excitation to maintain constant speed, and this control is typically achieved through advanced control systems and feedback mechanisms. These motors are commonly used in applications like power generation, industrial processes, and synchronous clocks, where accurate and constant speed is essential.