Hi friends, I hope all of you are fine. In today’s tutorial, we are gonna have a look at Introduction to Synchronous Generator. In electrical engineering particularly in power production there are 2 main sources of energy conversion, first is a motor and the other is a generator. The generator is a device that produces electrical energy and motor produce mechanical power. The motors and generators are further divided into the AC and DC motors and generators according to their power generation and use.
The synchronous generator is the type of AC generator. For energy generation in wind turbines, a steam turbine or hydro turbines synchronous generator is used. In today’s post we will have a look at its construction, working, excitation method, etc. so let’s get started with the introduction to a synchronous generator.
Introduction to Synchronous Generator
- The synchronous generator is also known as an alternator, it converts the mechanical power into the electrical.
- The electric energy we used in our home or industries is mostly produced by the synchronous generator.
- There are many sources of energy conversion in the world but most of the energy is converted by the synchronous generator.
- They convert mechanical energy into the electrical energy up to the 1500 megawatt.
- The synchronous generators used in our industries are constructed by a static or rotating magnetic field.
- The construction of the synchronous generator built by the static field is like the DC generator.
- In the rotating magnetic field generator, the static armature is known as the rotor.
Working Principle of Synchronous Generator
- The working of a synchronous generator is based on Faraday’s Law of electromagnetic induction.
- This law says that the rate of change of flux in any device will produce emf in that device. If a device is static and the field is rotating it will also produce field in the device.
- In case of a synchronous generator, the rotor is rotating, and it produces field in the stator.
- For an understanding of emf induced in any device study the article on the voltage induced in the loop.
Synchronous Generator Construction
- In a synchronous generator, there is no residual magnetism to produce self-excitation like the induction motor and induction generator.
- The external direct current supply is given to the rotor and it produces field in the rotor. When we rotate the rotor by mechanical way, its field link with the stator windings and produce a voltage in the stator.
- There are 2 terms we use to represent windings in the machines first one is armature winding and the other one is field winding.
- The windings that produce the main field in a machine called field winding and the windings that produce voltage is called armature.
- In case of the synchronous generator, the field windings are the rotor windings and the stator windings are the armature windings.
Stator of Synchronous Generator
- The stator is the static part of the generator it provides the covering to the internal construction of the machine.
- It is constructed by the laminated sheets of aluminum, there are slots at its inner periphery which used to hold the windings.
Rotor of the Synchronous Generator
- The rotor of the generator is an electromagnet, it is connected with the external DC source. The external source produces a voltage in the rotor, the field of the rotor induced a voltage in the stator.
- There are 2 main types of synchronous generator.
- Salient Pole Rotor
- Non-Salient Pole Rotor
Salient Pole Rotor
- This type of rotor has many no of poles that are manufactured on the wheel like arrangements.
- These poles are constructed from steel and are laminated.
- The windings of the rotor are wound on these poles and at the corners, windings are controlled by the pole shoe.
- The dia of the salient pole rotor is higher and its axis is short.
- The salient pole rotor has 4 or large no of the pole.
- The given diagram shows the salient pole rotor.
Non-Salient Pole Rotor
- The word ‘salient’ means to stick out, Non-salient pole is such a pole that is manufactured with the surface of the rotor they are not out of the surface like the salient poles.
- This type of rotor is used where 4 or more poles are required at the stator.
DC Excitation of Synchronous Generator
- As we discussed that the synchronous generator is not a self-start machine. It must connect with the external source.
- To excitation of the generator, the DC supply is connected with the circuitry of the rotor.
- As the rotor rotates so there is some precaution that we should keep in mind, connecting rotor with the DC source.
- Try to connect the windings of the rotor with the DC source through the slip ring and graphite made brushes, if you connect the windings directly with the dc source, it causes serious spark and motor will damage.
- Connect such dc source with the generator that remains permanently connected with the rotor.
- The slip ring is rings made by some metal, they are mounted on the shaft of the generator and have some insulation.
- Every end of the rotor’s windings is joined with the slip rings and the static brushes are mounted on the slip rings.
- The brushes always mounted on the slip ring because they are made from graphite which has less resistance.
- If the one terminal of the DC source is joined with one carbon brush, then the other will be connected with the second brush.
- The important thing you should note that the dc voltage you provided to the generator should have same value irrespective of the variation in the speed and angular position of the generator.
Problems of Slip Ring and Brushes in Synchronous Generator
- As we discussed that we use slip ring and carbon brushes to provide the dc supply to the windings of the rotor. These two components cause some difficulties.
- As the brush is made from the carbon that is soft material nature, so their condition must be monitored after some time and maybe they should be replaced after some time. This process increases the maintenance cost of the machine.
- There is some loss of voltage at the brushes which increases the field current and power loss at the field windings.
- For a smaller synchronous machine this method of the voltage is used because it is a cheap method for these machines.
Brushless Exciters of Synchronous Generator
- The slip ring and brush technique do not work for the larger motor and generator. For dc supply to the rotor, they used brushless exciters.
- The brushless exciter is itself an ac generator, as any machine has 2 circuitries, first is armature and other is field.
- When this exciter relates to any synchronous machine, its field circuitry resides on the static part of the synchronous machine and armature circuitry mounted on the shaft of the machine.
- As the output of the exciter is 3 phase ac which is then converted to dc by the rectification, this rectification circuitry is also connected on the shaft of the synchronous generator.
- Then the output of the rectifier sent to the field circuitry of the rotor. By varying the field current of the exciter, we can easily control the field current of the synchronous generator.
- As there is no physical connection among the stator and rotor of the generator, so the exciter needs a very lesser amount of repairing then slip ring and brushes.
- The arrangement of the brushless exciter with a generator is shown in a given diagram.
Pilot exciter of the Synchronous Generator
- To make the construction of the synchronous generator simplest and excitation of the generator independent from the exterior circuitry a pilot exciter introduced on the machine.
- Pilot exciter is also an alternating current generator it has permanent magnet instead of the armature circuitry which connected with the shaft and its 3 phase windings are joined with the stator.
- It generates the power for the exciter’s field circuitry, this power then governs the field circuitry of the generator.
- If a pilot exciter relates to the shaft of the rotor then there will be no need of the exterior power supply to operate the generator.
- Most of the synchronous generator also has a slip ring and brushes with the brushless exciter, in case of an emergency backup power supply is exists.
- A given diagram shows the pilot exciter circuit.
Speed of Rotation of a Synchronous Generator
- As we have discussed that the rotor of the synchronous generator is an electromagnet, it joined to the dc source.
- The direction of the field of the rotor will be the direction of the rotation of the rotor.
- The speed rotation of the field in the ac machine has a relation with the frequency at the stator, is given as.
- fe in this equation is the frequency of the stator.
- nm it is the speed of the field.
- P is the no of poles
- As the speed of the rotation of the rotor is equal to the speed of the field, this equation shows the relation of the rotor speed to the frequency of the stator.
- The electrical energy generated at the fifty or sixty-hertz frequencies, so the generator should move at the constant speed.
- For example, to produce the sixty-hertz energy in a 2-pole machine the speed of the rotor should be 3600 revolutions per minute.
- And to produce the fifty-hertz energy in a 4-pole machine the speed of the rotor should be 1500 revolution per minute.
Synchronous Generator vs Induction Generator
- These are some similarities and dissimilarities between synchronous and induction generators that are described here.
- In synchronous generator, the speed of rotation of the rotor is equal to the speed of the rotation of the field at the stator.
f = (Nx P)/120
- But in the case of a synchronous generator, the frequency of output voltage is controlled by the power system with which it linked.
- The synchronous generator is not self-excited there is a need of special direct current source to connected with it.
- While there is no need of a special external source for the induction generator, as it is self start the machine.
- The presence of carbon brushes and separate dc sources make complicated construction of a synchronous generator and increase its price.
- But the induction generator is self start and there is no need of carbon brushes and slip ring so its construction is simplest and its maintenance price is also less.
Cooling of Synchronous Generator
- When the generator is operated at heavy load then huge amount of heat produces in the generator that can be caused of dangerous for the internal structure of the synchronous generator.
- To reduce or minimize heat in the generator there are several methods are used that are described here.
Radial Flow Ventilation System
- In this type of colling method cool air is passed through the stator with the ducts and from another side of the stator it comes out.
Advantages of Radial Ventilation
- In this method of colling power losses for ventilation is less.
- It can be used for both less ratings and high rating generators.
Hydrogen Cooling of a Synchronous Generator
- In this method, hydrogen is used for the colling of a generator. Before its use for colling its ratio with air must be controlled and keep (9/1) hydrogen to air.
- As if the air exists in environments where hydrogen is performed it can create expulsion.
Difference between Synchronous Motor and Synchronous Generator
- The synchronous generator is such a device that transforms mechanical energy provided by the prime mover to electrical power, but motor transforms electrical into mechanical energy.
- But these to the motor are similar in physical structure.
- Any synchronous machine either motor or a generator can provide active power to or get active power from a system connected and provide reactive power to and get reactive power from the system.
- All these 4 possibilities of these machines are shown in a given figure in the shape of the phasor diagram.
- You can note from the given figure that.
- The unique feature of a synchronous generator (providing P) is that internal generated voltage EA lies ahead Vø, but in case of a motor internal generated voltage, EA lies behind Vø.
- The different feature a machine (either generator or motor) providing reactive power Q is that EAcosδ> Vø, irrespective of whether the machine is acting as a generator or as a motor. A machine that is taking Q has EAcosδ< Vø.
Applications of Synchronous Generator
- These are some applications of synchronous generators that are described here.
- It mostly used in such a system where constant speed is required.
- They also maintain the power factor of the system.
- Almost all power generation plants use synchronous generators due to constant frequency providing the capability.
You can also read some related topics to synchronous generator that are listed here.
So, friends, it is a detailed article on the synchronous generator I have mentioned each and everything related to a synchronous generator in this tutorial. See you next tutorial Equivalent Circuit of Synchronous Generator.