What Is Synchronous Generator?

What Is Synchronous Generator?
synchronous generator

A Synchronous generator is a commonly used sources of alternating current of a constant frequency. The term synchronous refers to the fact that rotor and the magnetic field rotate with same speed.

Synchronous generator is used in power stations as AC generator to produce electrical energy. Its designed to be driven by gas turbines, internal-combustion engines or wind engines.

Synchronous Generator Working Principle

To produce electricity, We need three things:

  1. Electrical conductor.
  2. Magnetic field.
  3. Relative motion between the conductor and the field.

The simplest alternator shape is a permanent magnet, a wire and motion between them. By applying the same principle, we can build any alternator.

So, the synchronous generator working principle is the electromagnetic induction. An emf is induced in the conductors if there is a relative motion between the flux and conductors.

Synchronous Generator Construction

synchronous generator rotor
synchronous generator rotor

An alternator consists of two main parts stator and rotor.

Stator

The stator is the stationary part of the generator, It carries the winding in which the voltage is generated.

This means that the output power of the generator is taken from the stator.

Rotor

The rotor is the rotating part which produces the field flux. It has windings that are connected to a DC source, The DC current in the rotor produces field flux.

The rotor rotates with the help of a mechanical prime-mover. This rotation makes the field to rotate and cut the windings of the stator and produce voltage.

In order to reduce hysteresis and eddy-current losses, the stator core is assembled with high grade silicon content steel lamination. The rotor is the field generation part.

Description And Facts

  • Synchronous generator is the majority source of commercial electrical energy.
  • They are used to convert the mechanical power output of a prime mover which is steam turbines, gas turbines, reciprocating engines or hydro turbines into electrical power for the overhead power lines grid.
  • This generator is used in some designs of wind turbines.
  • The rotor contains the field coils.
  • The load is connected to the “stator” which is the stationary armature.
  • The rotor windings of a synchronous generator are supplied with direct current from rectifying equipment fixed and rotates with the rotor.
  • The rectifier feeding the rotor gets its power from a separate generator  which is mounted on the same shaft as the synchronous generator, this separate generator is to avoid the old issues of carbon brushes.
  • Some types of this generator has permanent magnets mounted on the rotor and are used to provide magnetic field instead of coils.

What is Carbon Brushes ?

carbon brushes
carbon brushes

The older designs  of the synchronous generator had an issue of feeding the rotor with its DC current while it’s rotating, to produce field

Using carbon brushes to feed the rotor with DC current was the solution.

Brushes need continues maintenance! so, the modification was to use separate generator on the rotor shaft.

Nowadays, brushless excitation is the solution of brushes many issues. What is brushless exciter?

Brushless Exciter

Brushless Excitation System is a system that uses no slip rings or carbon brushes to supply field current to the synchronous generator rotor.

With Brushless Excitation System, we don’t need to supply external power for the field in order to generate electricity as a static excitation system.

Static Excitation System takes power for field excitation from an external power DC power source.

Nonetheless, Brushless Excitation Systems do not require any start-up external power supply to operate. It uses the residual magnetism to build up the field of excitation.

Are you wondering, what is residual magnetism?, Let’s discuss it a little more.

What is Synchronous Generator Residual Magnetism?

The amount of magnetization that remains in a magnetized body after removing the external magnetic field from the circuit is referred to as residual magnetism.

For instance, if a current pass through a coil, the coil will get magnetized, after switching off the electric current, the coil won’t loose all its magnetization.

The synchronous generator uses this residual magnetism as an excitation in the start instead of an external power supply.

The principle is easy, When the generator is started, it has no out source of excitation, the small amount of residual magnetism produces electrical voltage at the output terminals of the stator winding.

The produces electricity feeds up the excitation system of the generator, which in turns gets more magnetization due to electric current. As a result, the generator produces more electrical energy and so on until it reaches its voltage level. Of course, all starts by the residual magnetism.  

Does A generator Lose Its Residual Magnetism With Time?

Yes, If the generator is not being used for a long time, it can lose its residual magnetism.

Residual magnetism is lost from not being used or when the load is connected to the generator when it is shut off.

Running a generator without a load for too long can also result in this failure. In this case we need to restore the magnetism in a process called flashing the generator.

Generators need to run, and this helps to preserve the residual magnetism. It is recommended that when they are running, a load should be connected to them.

Having a load connected helps in generating an even stronger magnetic field. To disconnect the load from the electrical system, turn off the switch or breaker before you shut it off.

Shutting off the generator while a load is attached can essentially demagnetize the electromagnet.

What is Generator Excitation Field Flashing?

Flashing the exciter field is the process of restoring the residual magnetism of the generator exciter. It is necessary to conduct field flashing on a generator when the voltage does not build up.

The problem is usually caused due to an insufficient residual magnetism in the exciter and generator fields in the system.

In some circumstances, a generator that has been out of service for a long time will lose its residual magnetism and need flashing.

If there isn’t enough residual magnetism after the field flash, then an injection of current can be made into the rotor from another source to create full voltage.

We use a battery for this flashing process, or a housing unit may provide direct current, or alternating current might be rectified.

This type of current is required for a very short time, and it is referred to as field flashing. Even small, portable generator sets may need to undergo field flashing in the course of routine operation.

Exciter flashing steps

  • Disconnect the exciter leads F+ and F-, connect F+ to the battery positive terminal.
  • Touch F- to the battery negative terminal for about 5 seconds.
  • Reconnect the exciter leads to the voltage regulator again.
  • Start the generator. If its OK, it will build up its voltage.
  • You may need to repeat the process of flashing the exciter field if the generator fails to build voltage.

Can synchronous Generator Work Without Excitation?

A synchronous generator cannot build up voltage in the absence of excitation, of course it can run and rotate the rotor by the means of the prime mover but won’t generate electricity.

The creation of magnetic flux in a synchronous generator is essential for generating electrical energy.

As I mentioned above, the generation of electrical energy requires a conductor, magnetic field and a relative movement between them. If one parameter is missed, no electrical energy will be produced.

What Is The Function Of the Diode Bridge In A Generator Exciter?

synchronous generator excitation system
synchronous generator excitation system

The diode bridge job is to rectify ِcurrent from the exciter rotor, i.e converts it from AC to DC.

This direct current is sent straight to the primary rotor field coils, which require DC current to excite the main rotor. The automated voltage regulator adjusts the field current to keep the AC output voltage constant as the load varies.

Because the magnetic field requires DC electricity, this procedure is needed for a generator’s alternator to function.

Before the exciter’s AC output can be utilized to create electrical energy, it must be converted to DC power. This occurs within a diode rectifier.

Why Does Synchronous Generator Need Battery?

When a power outage occurs at a facility, synchronous generator batteries play a significant role in providing power to the generator engine starter, a DC motor, in order to start and rotates the rotor to produce electricity. 

The same reason your car needs a battery for the engine to start, so does the generator.

Batteries can also be used to provide the following benefits, depending on the generator system operation and configuration:

  • The panel of synchronous generators with the digital controls is also powered by batteries.
  • The batteries can provide power to the auxiliary panes, DC motors, and DC-powered electrical devices within the enclosure while the generator is being run.

Read also my detailed article : Electrical generator batteries, 6 important Answers.

Synchronous Generator vs Induction Generator

There are some differences between synchronous and induction generators, here are the main of them.

  • The frequency of the synchronous Gen. is equal to f = N*P/120 HZ , this is because the rotor rotates with the same synchronous speed, While in case of induction generator the rotor speed is not the same as the synchronous speed, so its frequency is lower than the value of the formula of the frequency.
  • The construction of the induction generator is less complicated because it needs no brushes or DC source for the excitation. The induction generator takes its excitation from the reactive power of the power system.

Below is a table of comparison between the two generators.

  Synchronous Induction
Frequency f = N * P / 120 Hz Lower than the calculated value of the formula, f = N * P / 120 Hz
Excitation A separate DC excitation source is required For field excitation, it takes reactive power from the power system
Construction Needs brushes or small generator on the rotor generator is less complicated

Is Synchronous Generator Self Starting?

A synchronous generator isn’t self-starting because, it needs a starting motor to run it to the synchronous speed.

In synchronized generators, the rotation is supplied by an external engine, a prime mover could be diesel engine, steam engine. etc .

Once that is done, it runs at synchronous rotation speed and starts converting mechanical energy to energy continuously.

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