Electrical Devices

DC Generator

Electrical generators are standalone machines that provide electricity when power from the local grid is unavailable. These generators supply backup power to businesses and homes during power outages. Generators do not create electrical energy, but they convert mechanical or chemical energy into electrical energy.

What Is a DC Generator?

A DC generator is an electrical machine whose main function is to convert mechanical energy into electricity. When the conductor slashes magnetic flux, an emf will be generated based on the electromagnetic induction principle of Faraday’s Laws. This electromotive force can cause a flow of current when the conductor circuit is closed.

Parts of a DC Generator

A DC generator can also be used as a DC motor without changing its construction. Therefore, a DC motor, otherwise a DC generator, can be generally called a DC machine. Below we have mentioned the essential parts of a DC Generator.


 

Stator

The main task of the stator is to create magnetic fields at the point of rotation of the coil. The stator contains two magnets with opposite poles. These magnets are positioned to fit the rotor area.

 Rotor

The rotor of a DC machine contains slotted iron layers, the slots of which are stacked to form a cylindrical armature core. The function of lamination is to reduce the loss caused by eddy current.

 Armature coils

 The armature windings are closed loop and connected in series to increase the amount of current produced in parallel.

Yoke

The external structure of the DC generator is known as the yoke. It is made of either cast iron or steel. It provides the necessary mechanical power to move the magnetic flux through the poles.

Poles

 The function of the mast is to keep the field coils closed. These windings are wound on poles and are connected either in series or in parallel with the armature windings. No shoes The mast shoe is mainly used to separate the magnetic flux to prevent it from falling out.

Commuter The commutator acts as a rectifier, converting AC voltage to DC voltage in the armature winding. It is designed with copper segments and each copper segment is shielded from each other by mica plates. It is located on the axis of the machine.

Brushes

Brushes can be used to secure the electrical connections between the commutator and the external load circuit.

How does a DC generator work?

 According to Faraday's law of electromagnetic induction, we know that when a current-carrying conductor is placed in a changing field, an emf is induced in the conductor. According to Fleming's right-hand rule, the direction of the induced current changes whenever the direction of motion of the conductor changes. Note that the anchor rotates clockwise and the left driver moves up. When the armature makes a half turn, the direction of movement of the driver turns downwards. Thus, the direction of current in each armature is alternating. But with a split-ring commutator, the connections of the armature wires are reversed when the current is reversed. Hence we get unidirectional current to the terminals.

E.M.F Equation of DC generator

The emf equation of the DC generator is given by the equation:


where

  • Z is the total number of armature conductor
  • P is the number of poles in a generator
  • A is the number of parallel lanes within the armature
  • N is the rotation of armature in r.p.m
  • E is the induced e.m.f in any parallel lane within the armature
  • Eg is the generated e.m.f in any one of the parallel lane

N/60 is the number of turns per second

The time for one turn will be dt=60/N sec

Losses in DC Generator

The input power is not fully transformed into the output power in a DC machine. Some part of input power gets wasted in various forms. In a DC machine, the losses are broadly classified into four types:

 

Copper Loss

Copper loss takes place when the current flows through the winding. These losses occur due to the resistance in the winding. The copper loss is categorized into three forms armature loss, the field winding loss and brush contact resistance loss.

Core Losses or Iron Losses

Some losses in the iron core occur when the armature rotates in the magnetic field. These losses are known as core losses. These losses are categorized into two losses as Hysteresis loss and Eddy current loss.

Types of DC generator

The DC generator can be classified into two main categories as separately excited and self-excited.

Types of DC Generator

Separately Excited

The field coils are energized from an independent exterior DC source in a separately excited type generator.

Self Excited

In a self-excited type, the field coils are energized from the generated current within the generator. These types of generators can further be classified into a series of wounds, shunt-wound, and compound wound.

Applications of DC Generators

A few applications of DC generators are:

The separately excited type DC generators are used for power and lighting purposes.

The series DC generator is used in arc lamps for lighting, stable current generator and booster.

DC generators are used to reimburse the voltage drop within Feeders.

DC generators are used to provide a power supply for hostels, lodges, offices, etc.

This was a comprehensive explanation about DC generators. From the information above, we can conclude that the main advantages of a DC generator are its simple construction and design

Alternating current Generator:

Electricity is everywhere, from turning on a switch to heating a snack in the microwave. Now that you think about it, you might wonder how this important source of energy is created and how it reaches your doorstep. Electricity is produced in power plants with turbines and generators. A turbine transforms available energy into rotation, while electric generators transform rotation into electricity. Based on the electrical output of generators, they are divided into two types, AC generators and DC generators

 AC Generator:

AC generator is a machine that converts mechanical energy into electrical energy. The AC Generator’s input supply is mechanical energy supplied by steam turbines, gas turbines and combustion engines. The output is alternating electrical power in the form of alternating voltage and current.

AC generators work on the principle of Faraday’s law of electromagnetic induction, which states that electromotive force – EMF or voltage – is generated in a current-carrying conductor that cuts a uniform magnetic field. This can either be achieved by rotating a conducting coil in a static magnetic field or rotating the magnetic field containing the stationary conductor. The preferred arrangement is to keep the coil stationary because it is easier to draw induced alternating current from a stationary armature coil than from a rotating coil.The generated EMF depends on the number of armature coil turns, magnetic field strength, and the speed of the rotating field.

AC Generator Parts and Function


The various parts of an AC generator are:

 

·         Field

·         Armature

·         Prime Mover

·         Rotor

·         Stator

·         Slip Rings

The following are the functions of each of these components of an AC generator.

Field

The field consists of coils of conductors that receive a voltage from the source and produce magnetic flux. The magnetic flux in the field cuts the armature to produce a voltage. This voltage is the output voltage of the AC generator.

 

Armature

The part of an AC generator in which the voltage is produced is known as an armature. This component primarily consists of coils of wire that are large enough to carry the full-load current of the generator.

 

Prime Mover

The component used to drive the AC generator is known as a prime mover. The prime mover could either be a diesel engine, a steam turbine, or a motor.

 

Rotor

The rotating component of the generator is known as a rotor. The generator’s prime mover drive the rotor.

 

Stator

The stator is the stationary part of an AC generator. The stator core comprises a lamination of steel alloys or magnetic iron to minimise the eddy current losses.

 

 

Slip Rings

Slip rings are electrical connections used to transfer power to and fro from the rotor of an AC generator. They are typically designed to conduct the flow of current from a stationary device to a rotating one.

Working of an AC Generator

When the armature rotates between the poles of the magnet upon an axis perpendicular to the magnetic field, the flux linkage of the armature changes continuously. As a result, an electric current flows through the galvanometer and the slip rings and brushes. The galvanometer swings between positive and negative values. This indicates that there is an alternating current flowing through the galvanometer. The direction of the induced current can be identified using Fleming’s Right-Hand Rule.

Advantages of AC Generators over DC Generators

Following are a few advantages of AC generators over DC generators:

«  AC generators can be easily stepped up and stepped down through transformers.

«  The transmission link size in AC Generators is thinner because of the step-up feature.

«  Losses in AC generators are relatively lesser than in DC machines

«  The size of an AC generator is smaller than a DC generator

«  Most of us begin our study with Direct Current, but eventually, we learn that direct current is not the only type of current we come across. There are sources of electricity that produce voltages and currents which are alternating in nature. This type of current is called an alternating current or an AC.

Motor Starter:

What is a Motor Starter?

A motor starter is an electrical device that is used to start & stop a motor safely. Similar to a relay, the motor starter switches the power ON/OFF & unlike a relay, it also provides a low voltage & overcurrent protection.

The main function of a motor starter is;

To safely start a motor

To safely stop a motor

To reverse the direction of a motor

To protect the motor from low voltage & over current.


 

Electrical Contactor: The purpose of the contactor is to switch ON/OFF the power supply to the motor by making or breaking the contact terminals.

Overload protection circuit: The purpose of this circuit is to protect the motor from potential harm due to the overload condition. Huge current through the rotor may damage the winding as well as other appliances connected to the supply. It senses the current & breaks the power supply.

How a Motor Starter Works?

A starter is a control device that is used for switching the motor either manually or automatically. It is used for safe ON/OFF control of electrical motors by making or breaking its contacts.

The manual starter is used for smaller motors where the hand operated lever is manually operated (move the contacts position) to the ON or OFF position. The disadvantage of these kinds of starters is that they need to switch ON after power frailer. In other words, they need manual control for each (ON or OFF) operation. Sometimes, this operation may leads to flow high currents in the motor winding which may burn the motor. This is why it is not recommended in most cases where other alternative motor starters with protection are used such as automatic starters.

 

 

 

On the other hand, the automatic starters which consist of electromechanical relays and contactors are used to switch the motor ON/OFF operation. When current passes through the contactor coils, it energizes and produces the electromagnetic field which pulls or pushes the contacts to make the connection of motor windings to the power supply.

 

The start and stop push buttons connected to the motor and starter can be used for ON and OFF operation of motors. The contactor coils can be de-energize by pushing the stop button which leads to de-energize the coil. This way, the contactor contacts move back due to spring arrangement to its normal position which leads to switch off the motor. In case of power failure or manual switch-off operation, the motor won’t start automatically until we manually start the motor by pressing the “start push button”. The following diagram shows that how a DOL motor starter operates for ON/OFF operation.

 

 

 Transformer

A transformer is a device used to transfer electrical energy. The transmission current is alternating current. It is usually used to increase or decrease the supply voltage without changing the AC frequency of the circuits. The transformer works on the basic principles of electromagnetic induction and mutual induction.


 Types of transformers

Transformers are used in various fields like power generation grid, distribution sector, transmission and electric energy consumption. There are various types of transformers which are classified based on the following factors:

  • Working voltage range
  • The medium used in the core
  • Winding arrangement
  • Installation location


Based on Voltage Levels

Commonly used transformer types, depending on the voltage, are classified as follows:

 

Step-up Transformer: They are used between the power generator and the power grid. The secondary output voltage is higher than the input voltage.

Step-down Transformer: These transformers are used to convert high-voltage primary supply to low-voltage secondary output.

Based on the Medium of Core Used

In a transformer, we will find different types of cores that are used.

Air Core Transformer: The flux linkage between primary and secondary winding is through the air. The coil or windings wound on the non-magnetic strip.

Iron Core Transformer: Windings are wound on multiple iron plates stacked together, which provides a perfect linkage path to generate flux.

Based on the Winding Arrangement

Autotransformer: It will have only one winding wound over a laminated core. The primary and secondary share the same coil. Auto means “self” in the Greek language.

Based on Install Location

Power Transformer: It is used at power generation stations, as they are suitable for high voltage application

Distribution Transformer: It is mostly used at distribution lanes for domestic purposes. They are designed for carrying low voltages. It is very easy to install and characterised by low magnetic losses.

Measurement Transformers: They are mainly used for measuring voltage, current and power.

Protection Transformers: They are used for component protection purposes. In circuits, some components must be protected from voltage fluctuation, etc. Protection transformers ensure component protection.

Working Principle of a Transformer


The transformer works on the principle of Faraday’s law of electromagnetic induction and mutual induction.

 

There are usually two coils – primary coil and secondary coil – on the transformer core. The core laminations are joined in the form of strips. The two coils have high mutual inductance. When an alternating current passes through the primary coil, it creates a varying magnetic flux. As per Faraday’s law of electromagnetic induction, this change in magnetic flux induces an EMF (electromotive force) in the secondary coil, which is linked to the core having a primary coil. This is mutual induction.

Overall, a transformer carries out the following operations:

  • Transfer of electrical energy from one circuit to another
  • Transfer of electrical power through electromagnetic induction
  • Electric power transfer without any change in frequency
  • Two circuits are linked with mutual induction

Formation of magnetic flux lines around a current-carrying wire


The figure shows the formation of magnetic flux lines around a current-carrying wire. The normal of the plane containing the flux lines is parallel to the normal of a cross-section of a wire.


Formation of varying magnetic flux lines around a wire

 

The figure shows the formation of varying magnetic flux lines around a wire wound. The interesting part is that the reverse is also true; when a magnetic flux line fluctuates around a piece of wire, a current will be induced in it. This was what Michael Faraday found in 1831, which is the fundamental working principle of electric generators, as well as transformers.

  1. Parts of a Single-phase Transformer
  2. Parts of a Single-phase Transformer

The major parts of a single-phase transformer consist of

1. Core

 

The core acts as a support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic flux. The winding is wound on the core, as shown in the picture. It is made up of a laminated soft iron core in order to reduce the losses in a transformer. The factors, such as operating voltage, current, power, etc., decide core composition. The core diameter is directly proportional to copper losses and inversely proportional to iron losses.

 

2. Windings

 

Windings are the set of copper wires wound over the transformer core. Copper wires are used due to the following:

 

  • The high conductivity of copper minimises the loss in a transformer because when the conductivity increases, resistance to current flow decreases.
  • The high ductility of copper is the property of metals that allows it to be made into very thin wires.

There are mainly two types of windings:

Primary windings and secondary windings.

 

  • Primary winding: The set of turns of windings to which the supply current is fed.
  • Secondary winding: The set of turns of winding from which output is taken.

The primary and secondary windings are insulated from each other using insulation coating agents.

 

3. Insulation Agents

 

Insulation is necessary for transformers to separate windings from each other and to avoid short circuits. This facilitates mutual induction. Insulation agents have an influence on the durability and stability of a transformer.

 

The following are used as insulation mediums in a transformer:

 

  • Insulating oil
  • Insulating tape
  • Insulating paper
  • Wood-based lamination

Ideal Transformer

The ideal transformer has no losses. There is no magnetic leakage flux, ohmic resistance in its windings and no iron loss in the core.

EMF Equation of Transformer


  • N1 – Number of turns in the primary
  • N2 – Number of turns in the secondary
  • Φm – Maximum flux in the weber (Wb)
  • T – Time period. It is the time taken for 1 cycle.

The flux formed is a sinusoidal wave. It rises to a maximum value of Φm and decreases to a negative maximum of Φm. So, flux reaches a maximum in one-quarter of a cycle. The time taken is equal to T/4.

Average rate of change of flux = Φm/(T/4) = 4fΦm

Where, f = frequency

T = 1/f

Induced EMF per turn = Rate of change of flux per turn

 

Form factor = RMS value / average value

RMS value = 1.11  (4fΦm) = 4.44 fΦm [form factor of a sine wave is 1.11]

RMS value of EMF induced in winding = RMS value of EMF per turn x No. of turns

Primary Winding

RMS value of induced EMF = E1 = 4.44 fΦm * N1

Secondary Winding

RMS value of induced EMF = E2 = 4.44 fΦm * N2

Rms value of induced emf


This is the EMF equation of the transformer.

For an ideal transformer at no load condition,

E1 = Supply voltage on the primary winding

E2 = Terminal voltage (theoretical or calculated) on the secondary winding

Voltage Transformation Ratio


 

K is called the voltage transformation ratio, which is a constant.

Case 1:If N2> N1, K>1, it is called a step-up transformer.

Case 2: If N2< N1, K<1, it is called a step-down transformer.

Transformer Efficiency

Comparing system output with input will confirm transformer efficiency. The system is called better when its efficiency is high.


Applications of Transformer

·         The transformer transmits electrical energy through wires over long distances.

·         Transformers with multiple secondaries are used in radio and TV receivers, which require several different voltages.

·         Transformers are used as voltage regulators.