What is Synchronous Generator

In this article, we will explore synchronous generator, an electrical machine that converts rotational mechanical energy into alternating current electricity.

Definition of Synchronous Generator

A synchronous generator is a type of electric generator that is used to convert mechanical energy into AC electrical energy. It is also known as an alternating current generator or AC generator or alternator.

A synchronous generator is basically a rotating electrical machine that converts mechanical energy from sources like steam turbines, hydro turbines, diesel engines, etc. into electrical energy through the principle of electromechanical energy conversion.

It is called a synchronous generator because its rotor must be rotated at a constant speed called synchronous speed (N_s). This is because the speed of the alternator defines the supply frequency of the electrical power system.

Construction of Synchronous Generator

A synchronous generator is a rotating electrical machine consisting of two main parts namely, stator and rotor. The stator is a static or non-moving part of the alternator and the rotor is the rotating or moving part of the alternator.

Stator of Synchronous Generator:

It is a hollow cylindrical shaped part of the generator having slots cut on the inner periphery. It has three main parts namely, the outer cover (yoke), stator core, and stator windings.

The outer cover also called a yoke, is generally made up of cast iron. It provides a casing to the machine and protects the sensitive internal parts of the alternator against external mechanical and environmental threats.

The stator core is a cylindrical structure made up of thin laminations of high-grade silicon steel. It has several slots cut on its inner periphery to hold stator windings. It acts as a housing for stator winding and also provides a low reluctance path for the magnetic field.

The stator winding, also called armature winding, is made up of copper conductors. The voltage is generated in this winding when the generator is operated. Depending on the generator type, this stator winding can be a single-phase or three-phase winding.

Rotor of Synchronous Generator:

In a synchronous generator, the rotor is a mechanically moving part of the machine, which is rotated by some external force like a turbine or engine. In the case of a synchronous generator, the rotor acts as the magnetic field system that produces working magnetic flux in the machine.

It has four main components namely, rotor core, rotor winding, rotor shaft, and exciter.

Based on construction, there are two types of synchronous generator rotors namely, cylindrical rotor and salient-pole rotor.

(1). Cylindrical Rotor:

The construction of the cylindrical rotor of a synchronous generator is depicted in the following figure.

It has a cylindrical rotor core with slots for rotor winding cut on its outer periphery. The field winding or rotor winding is uniformly distributed in rotor slots. The portion of the rotor core without slots acts as the pole faces.

The rotor core is mounted on a rotor shaft and is locked with a lock key. Also, the electrical connection between the exciter and rotor winding is provided through an assembly of slip rings and carbon brushes mounted on the rotor shaft.

The cylindrical rotor synchronous generator is usually used in high-speed applications, where rotation speed varies in the range of around 1500 RPMs to 3000 RPMs, as in the case of steam turbine generators.

(2). Salient-Pole Rotor:

The construction of a salient-pole rotor is shown in the following figure.

It consists of a rotor core with project pole faces mounted on a rotor shaft. The field windings are placed on the salient-pole cores as shown in the figure. These field windings are excited from an external exciter through an arrangement of slip-rings and carbon brushes.

The salient-pole rotor is generally used in low-speed synchronous generators like in hydropower plants, where the speed of rotation varies from 100 RPMs to 500 RPMs.

Working Principle of Synchronous Generator

The working principle of synchronous generator is based on the principle of electromagnetic induction, i.e.

`\v=N ((dϕ)/dt)`

This means a time-varying magnetic field can induce a voltage or emf in a conductor or coil.

This electromagnetic phenomenon is utilized to generate a voltage in a synchronous generator which is explained below.

Operation of Synchronous Generator

Firstly, the rotor winding, also called field winding, of the synchronous generator is excited from an external DC supply, called an exciter. Due to this, the magnetic poles are developed on the rotor and produce a constant magnetic field in the machine.

Next, the rotor is rotated by a prime mover like a steam turbine or hydro-turbine. The rotor’s magnetic field also rotates at the same speed as the rotor and cuts the armature conductors. Due to this, there is a relative motion between the rotor magnetic field and the armature’s stationary conductors causing electromagnetic induction and inducing a voltage in the armature winding.

Since the rotor magnetic poles of different polarity cut the armature conductors in a complete cycle, hence generating alternating voltage in the armature. This alternating voltage is then collected from the alternator's terminal box.

Frequency of Generated Voltage:

The rotor of the synchronous generator is usually rotated at a constant speed called synchronous speed. Hence, a synchronous generator produces an alternating voltage of constant frequency, which is given by,

`\f=(N_s P)/120" Hz"`

Where N_s is the synchronous speed, f is the frequency of generated voltage, and P is the number of poles in the generator.

Applications of Synchronous Generator

Synchronous generators or alternators are widely used in the following applications:

• For electric power generators at power generating stations.
• These are also used in automotive charge batteries.
• They are also used as portable generators.

Hence, this is all about the synchronous generator and its operation. In conclusion, synchronous generators are the most widely used type of electric generator for electricity generation. They are most commonly used in power plants like steam power plants, hydropower plants, etc. to produce electrical energy.