Generator and Transformers

Dhristi JEE 2022-24

An Introduction to Generators and Transformers

These two devices work based on Faraday’s law of electromagnetic induction principle. The “Generators” generate current, and transformers convert between current and voltage. Here, we discuss in detail the important concepts of generators and transformers. 


Table of Content

  • Generator and Transformer: An Introduction

  • What is an Electric Generator?

  • Working Principle of an Electric Generator

  • Types of Generator

  1. AC generator- types, applications

  2. DC generator- types, applications

  • What is a Transformer?

  • Working principle of a Transformer

  • Types of transformer 

  • Application of transformer


What is a Generator?

A generator is defined as a machine that, with the help of magnetic induction, changes the mechanical energy into electrical energy.This is possible due to the revolution of coils in a magnetic field, i.e., a generator consisting of exterior fields also maybe because of the revolution of two electromagnets around a fixed coil, i.e. a generator consisting of internal fields.

 

An Electric Generator: Working Principle

The generator is made of a rectangle-shaped coil having several copper wires which wound over an iron core. This coil is called the armature. The function of this armature is used to increase the magnetic flux. A strong permanent magnet is being placed, and the armature rotates in between these magnets. Here the magnetic lines produced are perpendicular to the armature's axis. There are two slip rings also connected to the armature’s arms. These rings are used for providing movable contact, and two metallic brushes are also connected to the slip rings, which help in passing current from the armature to the slip rings. Finally, the current is passed through a load resistance that is connected across the two slip-rings.

 

The position of the armature keeps changing at different time gaps. At the stage when the magnetic field lines are positioned perpendicular to the coil, the coil is then rotated in the magnetic field to increase the induced e.m.f produced. It occurs in this position as the number of intercepting-magnetic field lines are maximum here.

 

Types of Generators:

The generators are classified further into two types as AC generators and DC generators:

  1. AC Generators:

AC-generators are also known as alternators. The principle of its working is based on electromagnetic induction.

AC generators are classified into two types:

  1. Induction Generator: It does not require any DC excitation, frequency control or regular control. The induction concepts happen when inductor coils turn in the magnetic field, producing a current and a voltage.

  2. Synchronous Generators: These are large size generators that are generally used in power plants. These are considered as rotating field or armature types. In the rotating armature type, the armature is positioned at the rotor and the field is at the stator end. The current in the rotor armature is taken through brushes and slip rings. These generators are used for low power requirement applications.

However, the Rotating field type of alternator is widely used due to its high power generation capability, and it does not require slip rings and brushes.

  1. Two-phase or Three Phase-Generators:

The two-phase generator generates two different voltages, and each voltage is considered a single-phase voltage. However, both the generated voltages are not entirely dependent on each other.

The three-phase alternator has 3 single-phase windings present apart in such a way that 120º displaces the voltage generated in any of the phases from the other two.

These generators are used in applications like naval, oil and gas extraction, wind power plants and mining machinery etc.

 

Application Advantages of AC Generator:

  1. As they do not require brushes, these Generators are generally maintenance-free.

  2. These generators are small in size in comparison to DC  generators.

  3. Losses are relatively less than DC machines.

  4. AC Generator breakers are relatively small in size than DC breakers.

 

DC Generators:

The DC generator is used for converting mechanical energy into direct current electricity.

It is typically found in off-grid type applications. These generators give a continuous power supply directly into electric storage machines and DC power grids without the use of novel equipment. In the case of the DC generator also, the working principle is based on Faraday’s law of electromagnetic induction.

 

When the conductor is placed in the varying field, an electromagnetic force is induced in the conductor. The magnitude of this emf, i.e. induced, can be determined with the help of the emf - equation used for DC generators. Induced current circulation takes place within its closed path. According to Fleming’s right-hand rule, the direction of induced current can be determined.

 

Emf- equation of the DC generator is given as:

 

Eg = P Ф NZ / 60 A

 

Where

  1. P is the number of field poles.

  2. Φ is the flux produced / pole in Weber.

  3. Z is the total no.’s of armature conductors.

  4. A is the no.’s of parallel paths in the armature.

  5. N is the rotational speed of armature in round per minute (rpm)

 

Types of DC Generators:

There are three main types of DC Generators:

  1. Permanent Magnet DC Generator:

There is no need for external field excitation in Permanent magnet type DC generators as it has permanent magnets for producing the flux.

Application: These may be used for low power applications like dynamos etc.

  1. Separately Excited DC Generator:

This separately-excited DC generator requires external field excitation for producing the magnetic flux. Here we can also vary the excitation for getting variable output-power.

Application: These are used in the electroplating process and electrorefining applications etc.

  1. Self-Excited DC Generator:

Self-excited DC generators can produce their magnetic field when it is as they have residual magnetism in the poles of the stator. These are very simple in design, and there is no requirement of the external circuit to vary the field excitation.

These self-excited DC generators are further classified into three, i.e. shunt, series, and compound-generators.

Application: These generators are used in applications like charging of batteries, welding, ordinary lightening-applications etc.

 

Advantages of DC Generators:

Following are the main advantages of the DC generator:

  1. In this case, the cost of cables comes to be less as there is no shielding from radiation required.

  2. Here, the fluctuations in the generator can be reduced by a constant arrangement of the coils.

  3. In the case of the DC generator, the operating features depend on the field winding etc.

 

Transformer: An Introduction

The device converts the voltage as the higher or lower voltages. There are different voltage levels, used when electrical power is generated, during the transfer.

 

A transformer is usually made of two coils, i.e. primary/field and secondary/inductance, between which are kept apart so that there is no electrical contact in between. When we allow passing a current through the primary coil, there is a generation of the magnetic field which changes. However, it maintains the same frequency. It results in generating an alternating voltage in the secondary coil at the same time. An alternating current passes through a secondary coil during the closed electrical circuit.

 

The greater the difference in between the number of windings in the primary and secondary coils, the greater will be the difference in between their voltages also, so they are directly proportional.

 

Working Principle of Transformer:

The transformer’s working principle is based on mutual inductance between the two circuits, which are linked by a common magnetic flux.

 

Types of Transformers:

Two types of transformers are there, as given below:

  1. Step-Up Transformer:

These transformers convert a low voltage into a high voltage. In this case, the number of turns in the primary coil is less than in the secondary coil, i.e. Np <Ns.

  1. Step-Down Transformer:

These transformers convert a high voltage when the current decreases into a low-voltage when the current increases, the no. of turns in the primary coil is greater than the number to the secondary coil, i.e. Np  ˃ Ns.

As per Faraday’s law of electromagnetic induction, the induced e.m.f is given by:

e  = - d Ф / dt

ep  = - d Фp / dt

es  = - d Фs / dt

By using the above equations, we get,

es = Ns x Np x ep

The ratio Ns / Np = K

Apart from this, there may be different types of transformers based on various parameters as follows :

 

Based on Design

  1. Core-type transformer

  2. Shell-type transformer

Based on the Cooling Method

  1. Oil filled self-cooled type.

  2. Oil-filled water-cooled type.

  3. Airblast type etc.

 

Applications of Transformer:

Following are three basic applications of Transformer:

  1. To step up the current and voltage.

  2. To step down the current and voltage.

  3. Prevention of DC to the next circuit in the DC transformers etc.

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FAQs (Frequently Asked Questions)

1. What are the different types of electric generators? 

Based on the electric energy used, the electric generator can be divided into two broad types which are - AC generators and DC generators.

  • The AC generators convert mechanical energy into electrical energy in form of alternative emf or alternating current. They are also known as single-phase generators and have an energy limit of 25 kW.

  •  The DC generators are divided into three categories: shunt, series, and compound-wound. The shunt generators are used in battery chargers and the series generators are used in street lights, whereas most of the DC generators are compound-wound.

2. What are the disadvantages of an electric generator? 

Every technology and invention comes with its own set of challenges. Some limitations or challenges of electric generators are as follows. 

  • The flow of electricity through generator and transformer coils produces resistance to heat and if the heat becomes extreme, it can cause an accidental power surge and start a fire. 

  • The electric generators using diesel as fuel causes severe air pollution and other air-related diseases.

  • The electric generator based on renewable sources of energy is costlier to design for example the generator based on nuclear power.

  • The generator also causes noise pollution. 

  • The DC generators cannot be used to transport energy to a longer distance. 

3. What are some of the limitations of a transformer?

Some of the limitations of a transformer are as follows. 

  • It cannot function under the DC voltage. 

  • The size of the transformer is huge and thus acquires more land area.

  • The size of a transformer depends on its use, that is more the use larger is the transformer.

  • It causes noise pollution. 

  • Transformer overload can cause abrupt disruption in the flow of energy.

  • Overheating can cause fire break-outs and major explosions. 

  • It is associated with the high cost of installation.

4. What is the main cause of noise in a transformer?

The major problem with a transformer is the noise it generates, the main reason for which is the Magnetostriction Effect. The ferromagnetic materials change upon contact with a magnetic field and the AC that flows through the transformer’s coils has a magnetic effect on its iron core. This causes the core to expand and contract alternatively, resulting in a humming or buzzing sound. It can be controlled by having a proper transformer design, assembly, and installation process. 

5. What are the precautions to be taken while installing a transformer to avoid any form of noise creation?

The following precautionary steps are recommended which might reduce the humming sound of a transformer.

  • Select a site with low traffic that is an area with minimum interference of living beings. 

  • Place the unit on a solid surface, the density of the surface will absorb certain sounds which are generated. 

  • Install the latest technologies which have worked on minimizing the sound. 

  • Making use of oil barriers will insulate the noise generated. 

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