Generator and Transformers

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Generator and Transformer: An Introduction

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These two devices work based on Faraday’s law of electromagnetic induction principle. The “Generators” generate current, and transformers convert between current and voltage.


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


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Fig.: Working principle of AC generator


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:

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


(b) Synchronous Generators: These are large size generators which 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 stator end. The current in 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 as 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 the 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 machine.

  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 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 armature.

  5. N is the rotational-speed of armature in round per minutes (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 costing 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 which 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:

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


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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 to the secondary coil, i.e. Np <Ns.

  1. Step-Down Transformer:

These transformers convert a high voltage when 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. Air blast 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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