What is the working principle of a synchronous machine?

Short Answer:

The working principle of a synchronous machine is based on electromagnetic induction and the concept of synchronous speed. It operates by rotating the magnetic field of the stator and the rotor at the same speed, meaning the rotor turns in exact synchronism with the rotating magnetic field of the stator.

When three-phase AC supply is given to the stator, it creates a rotating magnetic field. The rotor, excited with DC, locks with this rotating field and rotates at the same speed. This synchronized movement makes the machine suitable for both generator and motor applications with constant speed operation.

Detailed Explanation:

Working principle of a synchronous machine

A synchronous machine is a type of AC machine that works with the rotor speed equal to the rotating magnetic field speed in the stator. The name “synchronous” itself means that the rotor rotates in step with the stator’s magnetic field. It is used as both synchronous motor and alternator (generator) in various electrical applications such as power stations, industrial drives, and large-scale grid systems.

Let’s break down the working principle in a very simple way.

  1. Basic Construction:

A synchronous machine has two main parts:

  • Stator: The stationary outer part. It has three-phase windings and is supplied with a three-phase AC voltage. This creates a rotating magnetic field (RMF) inside the stator.
  • Rotor: The rotating inner part. It is given DC supply through slip rings or brushes, which produces a constant magnetic field.
  1. Generation of Rotating Magnetic Field:

When a three-phase AC supply is given to the stator windings, it generates a rotating magnetic field. This field rotates at a fixed speed called synchronous speed (Ns), calculated using the formula:

Ns = (120 × f) / P
Where:

  • f = frequency of the AC supply (in Hz)
  • P = number of poles of the machine

This magnetic field continuously rotates around the stator at synchronous speed.

  1. Rotor Movement and Synchronization:

The rotor is excited with DC current, producing a constant magnetic field on it. When the machine is started (using an external motor or damper winding in case of motors), and the rotor speed matches the rotating magnetic field speed of the stator, a locking effect occurs. The rotor poles align with the rotating field poles, and the rotor starts rotating at the same speed as the stator field—this is called synchronism.

Unlike induction motors, where the rotor lags behind the stator field, synchronous machines run without slip—their speed is always constant and equal to synchronous speed.

  1. Power Conversion:
  • As a Generator (Alternator):
    When the rotor is driven mechanically (e.g., by a turbine), and DC excitation is given, it cuts the rotating magnetic field across stator windings, inducing AC voltage due to Faraday’s law.
  • As a Motor:
    When AC is supplied to the stator and the rotor is excited by DC, the rotor gets pulled into synchronism with the stator field, and mechanical torque is developed. Since there’s no relative motion between stator field and rotor, it keeps running at a constant speed regardless of the load.
  1. Applications of Synchronous Machines:
  • Power plants (as alternators)
  • Large industrial motors
  • Grid voltage regulation (synchronous condensers)
  • High-precision machines requiring constant speed
Conclusion:

The working principle of a synchronous machine is based on the interaction of a rotating magnetic field from the stator and a constant magnetic field from the rotor. The machine works only when both rotate at the same speed, making it highly stable and reliable. This synchronous operation allows the machine to be used effectively in generating and maintaining steady AC power in electrical networks and industries.