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 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.

After generation in large power plants electrical transformers are used to step up voltage then power is transferred through OHTL or power lines.

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

An alternator consists of two main parts stator and rotor.

  1. 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
  2. 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.

Rotor: the rotor is the field generation part

Description And Facts

  • Synchronous generator is the majority source of commercial electrical power
  • 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 OHTL grid
  • This generator is used in some designs of wind turbines
  • The rotor contains the field coils, and 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 rotating rotor with its DC current 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?

What is The Brushless Exciter, and what Purpose of It in the Generator?

Brushless Excitation System is a system that uses no slip rings or carbon brushes to supply field current to the synchronous generator rotor. Because this technique of excitation, i.e., Brushless Excitation System, does not require carbon brushes, it has no carbon brushes contact resistance losses.

Also, with Brushless Excitation System, we don’t need to supply external power in order to start up the synchronous generator as with a static excitation system.

Static Excitation System takes power for field excitation from the output of the Synchronous Generator terminal via CT and PT.

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 asking, what is residual magnetism? Let’s discuss it a little more.

What is Synchronous Generator Residual Magnetism?

The amount of magnetization that remains a magnetized body after removing the external magnetic field from the circuit is referred to as residual magnetism. The synchronous generator uses this residual magnetism as an excitation in the start instead of an external power supply.

Magnetization happens when a current is applied in one direction, and the flux density is raised until the saturation point is achieved. To demagnetize the magnetic ring, the magnetizing force is reversed by reversing the direction of the current flow.

When the magnetizing force is applied in the opposite direction, the flux density decreases until it reaches zero. This remaining magnetism of the magnetic material is eliminated by providing a magnetizing force (oc) known as the coercive force in the opposite direction.

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 when it lost for any reason. 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. You can restore residual magnetism by flashing the field and increasing the generator’s current.

The voltage regulator can be comprised of either solid-state or semiconductor components. Make sure to follow the manufacturer’s instructions when flashing the field to prevent equipment damage. 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. The creation of magnetic flux in a synchronous generator is accomplished by passing current via the field winding. A direct current is required to excite the field winding on the synchronous generator’s rotor.

As mentioned above, if the exciter lost its residual magnetism, the generator will need field flashing to restore it and build up its voltage. 

Small-scale Excitation System

A DC source is an exciter that supplies direct current to the rotor field winding of a small synchronous generator. The exciter is attached to the synchronous machine’s shaft. The exciter’s direct current output is routed to the synchronous generator’s field winding via brushes and slip rings.

Medium-scale Excitation System

AC exciters are used instead of DC exciters in medium-sized synchronous generators. The AC exciters are three-phase alternating current sources. The output of the AC exciter is rectified and sent to the synchronous generator’s rotor field, winding through brushes and slip rings.

Large-scale Excitation System

Excitation needs for big synchronous generators grow incredibly high. The challenge of transmitting such a considerable quantity of electricity via high-speed sliding contacts becomes significant. Brushless excitation methods are therefore employed in large-scale synchronous generators.

A brushless exciter is a modest direct-coupled alternating current source with its field circuit on the stator and its armature circuit on the rotor. The AC exciter’s three-phase output is rectified using a solid-state rectifier.

The rectified output is linked directly to the field winding of the synchronous generator, eliminating the need for brushes and slip rings. The brushless excitation system needs minimal maintenance due to the lack of brushes and slide rings. It also employs a solid-state rectifier, reducing power loss.

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

The diode bridge job is to rectify current from the exciter rotor. This direct current power 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 the automated voltage regulator of a generator.

The regulator matches the exciter output with the required power output, preventing the generator from producing more power than is required at the moment. This helps to protect components like generator diodes from wearing out.

The diodes of an automated voltage regulator are grouped as rectifier diodes. There are an equal number of diodes that are biased forward and backward. This enables generators to use both sides of an alternating current sine wave. When electricity flows in one way, it passes through forward-biased diodes.

The reverse-based diodes carry the other half of the current’s sine wave. The rectifier diodes allow the magnetic field to create energy using all of the alternating current power rather than just half of it.

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 in order to ensure that the generator starts. 

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.
  • In an enclosure, if a secondary or redundant battery set is used, a battery backup can be provided by the primary battery.

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

How Does A Synchronous Generator Build Up Voltage?

A synchronous generator or alternator works on the principle of electromagnetic induction, which means an EMF is induced when the flux linking a conductor change.

If the armature winding of the alternator is exposed to the rotating magnetic field, the voltage will build up in the armature winding as it rotates.

In order to develop the alternate N and S poles on the rotor surface of the alternator, the rotor field will have to be energized by the DC exciter.

As the rotor rotates in the anticlockwise direction by means of a prime mover, the armature conductors placed on the stator are being cut by the magnetic field of the rotor poles.

Therefore, the armature conductors induce EMF through electromagnetic induction. As the N and S poles of the rotor pass alternately through the armature conductors, this induces EMF.

In order to determine the direction of the generated EMF, we can use Fleming’s law and to calculate the frequency:

F=NsP ÷ 120

DC field excitation current and rotation speed determine the magnitude of the generated voltage. When the windings are balanced, the generated voltage on each phase is exactly the same but varies by 120 degrees electrically.

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 near the synchronous speed at the start. It doesn’t run independently because it has no starting winding, so it isn’t self-starting. Regarding the excitation field, as mentioned above, the generator depends on the residual magnetism to build up its voltage.

A synchronous generator operates according to its supply frequency or rotating field speed. However, if it can start automatically, its function would be identical to an induction generator, and the purpose of the synchronous generator (leading power factor) would be disregarded. So, that is the reason that the synchronous generator is NOT self-starting.

In synchronized generators, the rotation is supplied by an external motor before reaching the speed required for creating a rotating magnetic field. After that, the rotor is locked to the magnetic field.

Once that is done, it runs at synchronous rotation speed and starts converting mechanical energy to energy continuously. The Synchronous generator rotor has been made very heavy to achieve better performance.

When it starts, it should not be interrupted by the small resistance that may occur (Low of inertia). Therefore, synchronous generators cannot be manufactured as self-starting generators due to the above reasons.

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