Introduction to 3-Phase Synchronous Machines

Quiz

An electromechanical energy conversion device (or electrical machine) which operates on synchronous speed (i.e. speed of rotating magnetic field) is termed as a synchronous machine. A synchronous machine an AC machine, i.e., it requires AC supply to work.

Based on the energy conversion, synchronous machines may be classified into two types −

Synchronous Generator
Synchronous Motor

The synchronous machines are the most extensively used electrical machines in power system applications like power generator, power factor correction, driving constant speed mechanical load, etc.

The synchronous machine which converts mechanical energy into alternating current electricity is called a synchronous generator or alternator. While the synchronous machine which converts alternating current electricity into mechanical energy is called a synchronous motor.

The synchronous machines, used in most practical applications, are three-phase AC machines. However, there are single-phase synchronous machines also exist but they are used in special applications.

A synchronous machine (generator or motor) always operate at a constant speed called synchronous speed. The synchronous speed is given by the following relation,

$$\mathrm{\mathit{N_{s}}\:=\:\frac{120\mathit{f}}{\mathit{p}}\cdot \cdot \cdot (1)}$$

Where,

f is the supply frequency,
P is the number of poles in the machine.

The synchronous speed is measured in revolution per minute (RPM).

For satisfactory operation, a synchronous machine always maintains the expression given in Equation-1. If the synchronous machine fails to maintain the above relationship of Equation-1. The machine will stop to operated, and this condition is known as loss of synchronism or out of synchronism of the machine. Hence, this proves that the synchronous machine is designed to operate at a constant speed.

Working Principle of Synchronous Machine

The working principle of a synchronous machine is based on the law of electromagnetic interaction and law of magnetic interlocking.

According to the law of electromagnetic interaction, when there is a relative motion between a conductor and a magnetic field, an EMF is induced in the conductor. On the other hand, when a current carrying conductor is placed in a magnetic field, a force acts on the conductor that tends to move it.

According to the law of magnetic interlocking, two different magnetic fields (field of stator and field of rotor) are locked together and rotate at the same speed. This phenomenon is called magnetic interlocking.

These two principles explain the working of a synchronous machine. The synchronous machine is first started by the electromagnetic interaction, and then the magnetic fields of rotor and stator are locked together to rotate at the synchronous speed.

Three-Phase Synchronous Generator

A synchronous machine that converts mechanical energy into 3-phase electrical energy through the process of electromagnetic induction is known as a 3-phase synchronous generator or alternator.

A 3-phase alternator consists of an armature winding and a field winding, where the EMF is induced in the armature winding, while field winding produces the working magnetic field. In case of a 3-phase alternator, the armature winding is provided on the stator part of the machine while the field winding is provided on the rotor. The major advantage of stationary armature winding is that there is no need of commutator as required in the DC generators.

The 3-phase synchronous generators are most widely used for generation of electric power in power generating plants.

Three-Phase Synchronous Motor

A synchronous machine that converts three-phase electricity into mechanical energy is known as three-phase synchronous motor.

Like any other electric motor, a synchronous motor also consists of two major parts namely stator and rotor. The stator carries a three-phase armature winding, whereas the rotor carries a field winding which is excited from a dc supply to produce a certain number of fixed magnetic poles.

A unique feature of a synchronous motor is that it can run at a constant speed, called synchronous speed. Although, the major disadvantage of a three-phase synchronous motor is that it does not have self-starting torque. Therefore, in order to start a 3-phase synchronous motor, it is brought up almost to its synchronous speed by some auxiliary mean.

Being a constant speed motor, the load on the motor does not exceed the limiting value. If the load on the motor exceeds the limiting value, the motor will immediately stop. The three-phase synchronous motors are used for − driving mechanical loads at constant speed, improving power factor of a system, etc.

Features of Synchronous Machines

The following are the key features of synchronous machines (motor or generator) −

Synchronous motors do not self-starting torque.
Synchronous machine is a doubly-excited machine because it requires two input supplies one on the stator and the other on the rotor.
Synchronous machines operate at constant speed, called synchronous speed.
Synchronous generators can produce a voltage of constant magnitude and frequency.
A synchronous machine can be operated at lagging, leading or unity power factor just by changing the excitation.
Synchronous motors have relatively high starting torque as compared to induction motors.
Synchronous motors are suitable for driving constant and slow speed (usually less than 300 RPM) loads.
Synchronous machines are expensive.

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