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Electrical DC Generators

Electrical DC Generators

Generator Principle

An electrical DC generators are the machines that converts mechanical energy into DC electrical energy. An electric generator is based on the principle that whenever flux is cut by a conductor, an e.m.f is induced within the conductor that causes a current to flow in it if only when the circuit is closed. The direction of that induced e.m.f can be given by Fleming’s right hand rule.

Fleming's right hand rule for Electrical DC Generator
Fleming’s right hand rule

Therefore, the most essential components of a generator are:

  1. A magnetic field
  2. Conductors
  3. Motion of conductor w..r.t magnetic field

According to law of conservation of energy, no machine is 100 % efficient. Similarly, the DC generator is not 100 % efficient because there are also some energy losses in the machine.

Construction Of Electrical DC Generator/Motor

The general construction of electrical DC generators and DC motors are same. In fact, when the machine is being assembled, the constructor do not know whether it is a DC generator or motor. It can be said that DC generator can be used as a generator or motor. All DC machines have five basic principle components.

Electrical DC Generator Construction

  1. Field system
  2. Armature core
  3. Armature winding
  4. Commutator
  5. Brushes

Field System

The field system function is produce uniform magnetic field within which the armature rotates. It consist of even number of silent poles bolted to the inside of the circular frame known as yoke. The yoke is usually made of cast steel whereas the pole pieces are composed of stacked lamination. Filed coils are mounted on the poles and carry the DC exciting current. Those field coils are placed in such a way that adjacent coils experiences opposite polarity.

Armature Core

The armature core is connected to the shaft of the machine that rotates between the field poles. It consist of slotted soft iron lamination that are placed together such that it forms a cylindrical shape core. These lamination are coated with insulator films so that they do not come in contact with each other. The reason for insulating the lamination is to reduce eddy current loss. The lamination are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to cross between the pole face and the armature teeth.

Armature Winding

The slots of the armature core hold insulated conductors that are connected in a suitable manner is known as armature winding. Working e.m.f is induced within the same winding. Armature conductors are connected in series-parallel ( in series to increase the voltage and in parallel to increase the current ). The conductors of a DC machine are being connected in a symmetrical manner forming a closed loop or series of closed loops.

Commutator

A commutator within the a machine is a mechanical rectifier that converts generated alternating voltage within the armature winding in to direct voltage across the brushes. The commutator is mounted on the shaft of the machine made of copper segments insulated from each other by mica sheets. The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. Depending upon the connection manners, there are two types of armature winding in a DC machine.

  • Lap Winding
  • Wave Winding

Lap Winding

In this arrangement, the armature coils are connected in series through commutator segments in such a way that the armature winding is divided in to as many parallel paths as the number of poles of the machine.

In simple words:

  • There are as many parallel paths as the number of poles (P) of the machine
  • Each parallel path has Z/P conductors in series where Z and P  are the total number of armature conductors and poles respectively
  • Generated e.m.f = e.m.f/parallel path
  • Total armature current, Ia = P×(current/parallel path)

Wave Winding

In this arrangement, the armature coils are connected in series through commutator segments in such a way that the armature winding is divided in to parallel paths irrespective of the number of the poles of the machine.

In simple words:

  • There are two parallel parts irrespective of the number of poles of the machine
  • Each parallel path has Z/2 conductors in series where Z is the total number of armature conductors
  • Generated e.m.f = e.m.f/parallel path
  • Total armature current, Ia = 2×(current/parallel path)

Choice of Armature Winding

The limit to the number of armature coils which can be accommodated in a machine of give size depends mainly on the number of segments that can be accommodated in the commutator. In multi-polar machines, the coil number in series in each parallel path is less in lap than in the wave-wound armature. Therefore, current carrying capacity in lap winding is greater while the terminal voltage of wave winding will be larger.

  1. In small machines, the current carrying capacity is not generally critical. Therefore, in order to obtain suitable voltages, wave winding are often used.
  2. In large machines, suitable voltage are easily obtained because of the relatively large number of armature conductors available and current capacity is more critical. Therefore, large machines generally use lap winding.

Brushes

The brushes are install within the machine to ensure electrical connection between the rotating commutator and external load circuit. It is made up of carbon and are placed on commutator. The pressure of the brushes on the commutator to ensure connectivity is adjusted by mean of adjustable springs. (If the pressure of the brushes on the commutator is too large, the friction between them produces heating while if the pressure to too weak, the imperfect contact may produce sparking)

E.M.F EQUATION OF D.C GENERATOR

The equation for the e.m.f generated in a DC generator can be calculated as

Let us assume that

Φ = flux/pole in Weber (Wb)

Z = total number of armature conductors

P = number of poles

A = number of parallel paths               (2 for wave winding and P for lap winding)

N = speed of armature in r.p.m

Eg = e.m.f of generator = e.m.f / parallel paths

Flux cut by single conductor in one revolution of the armature is

dΦ = P×Φ Wb

Time required to complete one revolution is

dt = (60/N) seconds

e.m.f generated per conductor = dΦ/dt = P×Φ/(60/N) = (P×Φ×N/60) volts

e.m.f of generator, E = e.m.f per parallel path

=  (P×Φ×N/60) × (Z/A)

E =  (P×Φ×N×Z/60×A)     (Where N=2 for wave winding and N=P for lap winding)

Armature Resistance

The resistance offered by the armature circuit is known as armature resistance (Ra) and includes:

  • Resistance of armature winding
  • Resistance of brushes

The armature resistance depends upon the construction of the machine. Except for small machines, its value is generally less than 1 Ω.

Types of Electrical DC Generators

In electrical DC generators, magnetic field is usually produced by electromagnets instead of permanent magnets. Electrical DC generators are usually classified in to their categories by the method of their field excitation. On this basis of excitation, DC generators are divided into following two classes:

  1. Separately excited electrical DC generators
  2. Self-excited electrical DC generators

Separately Excited Electrical DC Generators

An electrical DC generators in which the field winding is excited by an independent external DC source is called separately excited generator. The output generated e.m.f depends upon the speed of rotation of armature and the field current ( E =  (P×Φ×N×Z/60×A) ). Greater the rotation speed and field current, greater the generated output. This type of generators are rarely used in practice. 

Separately-Excited-Electrical-DC-Generators

Here,

Terminal voltage V = Eg – IaRa

Power developed = EgIa

Power delivered to load = EgIa – I2aRa

= Ia (Eg – IaRa ) = VIa  

 Self Excited Electrical DC Generators

An electrical DC generators in which current is supplied to the field magnet winding from the output of the generator itself is called a self excited generator. Depending upon the manner in which the field winding is connected to the armature is classified in to following three types.

  1. Series Generator
  2. Shunt Generator
  3. Compound Generator

Series Generator

In the series-wound generator, the field winding is connected in series with armature winding so the whole armature current flows through the field winding and the load. As the field winding carries the whole current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes. e.g. as boosters.

series generatorArmature current  Ia = Ise  = I

Terminal voltage V = Eg – I( Ra + Rse )

Power developed in armature = EgIa

Power developed to load = EgIa – I2a(Ra + Rse) = Ia ( Eg – Ia(Ra + Rse) ) = VIa or VIL 

Shunt Generator

In a shunt generator, the field winding is connected in parallel with the armature winding so the terminal voltage is applied across it. The shunt winding has many turns of fine wire having high resistance so that only a part of armature current flows through shunt field winding and the rest flow through the load.

shunt generator

Shunt field current Ish = V/Rsh   

Armature current Ia = IL + Ish

Terminal voltage V = Eg – IaRa

Power developed in armature = EgIa

Power delivered to load = VIL  

Compound Generator

In a compound-wound generator, there are two sets of field wingdings on each pole – one is in series and the other in parallel with the armature. A compound wound generator may be:

Short Shunt: in which only shunt field winding is in parallel with the armature winding.

Long Shunt: in which shunt field winding is in parallel with both armature and series field winding.

Short Shunt Equation: short shunt

Series field current Ise = IL

Shunt field current Ish = V+Ise+Rse / Rsh

Terminal voltage V = Eg-IaRa-IseRse

Power developed in armature = EgI

Power delivered to load = VIL

Long Shunt Equation: long shunt

Series field current Ise = Ia = IL+Ish

Shunt field current Ish = V / Rsh

Terminal voltage V = Eg-Ia(Ra+Rse)

Power developed in armature = EgI

Power delivered to load = VIL

Losses In DC Machine

The losses in DC machines are divided into three classes Losses In DC Machine

  • Copper losses
  • Iron losses
  • Mechanical losses

All those losses appear as a heat and thus raises the temperature of the machine. They also lower the efficiency of the machine.

Copper losses occur due to current in various winding of the machine

  1. Armature copper loss= I2aRa
  2. Shunt field copper loss= I2shRsh
  3. Series field copper loss= I2seRse

Iron losses occur due to the rotation of armature in the magnetic field of the poles in the armature of the DC machine

  1. Hysteresis loss
  2. Eddy current loss

Mechanical losses occur due to the friction and wind-age within the machine

  1. Friction loss i.e. bearing friction, brush friction
  2. Wind-age loss i.e. air friction of rotating armature

Mechanical losses depends upon the speed of the machine.

Note: Iron losses and mechanical loss together are called stray losses 

Power Stages in DC Generator

The various power stages of DC generator are shown in below given figure

Power Stages in DC Generator

Here,

A – B = Iron and friction losses

B – C = Copper losses

Mechanical Efficiency: ηm = B/A = EgIa/Mechanical power input

Electrical Efficiency: ηe = C/B = VIL/EgIa

Overall Efficiency: ηc = C/A = VIL/Mechanical power input

or    ηc = ηm × ηe

or    ηc = C/A = Output/Input = ( input – losses /Input )

Condition For Maximum Efficiency

The DC Generator efficiency is not constant but varies with the load. Consider a shunt generator delivering a load current IL at a terminal voltage V.

Lets,  Generator Output = VIL

Generator Input = Output+Losses

= VIL+I2aRa+Wc

= VIL+(IL+Ish)2Ra+Wc        » (Ia=IL=Ish)

As, in shunt generator the field current Ish is kept very small as compared to load current IL and therefore, can be neglected.

Generator Intput = VIL+I2LRa+Wc

η = Output/Input = VIL/(VIL+I2LRa+Wc) = 1 / 1+( (IL+Ra/V)+(Wc/VIL) )

Here, the efficiency will be maximum when the denominator is minimum.

d/dIL( ILRa/V + Wc/VIL ) = 0

Ra/V – Wc/VI2L = 0

Ra/V = Wc/VI2L

I2LRa = Wc

Variable loss = Constant loss

The load current corresponding to maximum efficiency is given by

IL = √ (Wc/Ra)

Hence, the efficiency of a DC generator will be maximum when the load current i such that variable loss is equal to the constant loss.

ARMATURE REACTION

So far, we have studied that the only flux acting in a DC machine is that due to the main poles called main flux. However, due to current flow in armature conductors also creates a magnetic flux known as armature flux that distorts and weakens the main flux. This distortion and field weakening takes place in both motors and generators. This action of armature flux on main flux is called armature reaction.

armature reaction

The above shown diagram shows the phenomenon of armature reaction in a DC generator. When the generator is on no-load, the armature flux is too small due to small armature current flowing and that does not appreciably effect the main flux Φ1 coming from the poles as shown in part (i). In part (ii) when the generator is loaded, the current flowing through armature conductors sets up its own flux Φ2. The direction of armature flux is in opposite direction to that of the main flux thus results in distortion of main flux as shown in part (iii) forming resulting flux Φ3.

Referring to above schematic diagram, it is clear that the flux density at the pole tip (point B) is increased while at the pole tip (point A) it is decreased. The unequal distribution produces the following two effects.

  1. The main flux is dstorted
  2. Due to higher flux density at pole tip B, saturation sets in. Consequently, the increased in flux at pole B is less that the decrease in flux under pole tip A. Thus, flux Φ3 at full load is less than the flux Φ1 at no load.

Problems Of Armature Reaction

As due to armature reaction in the DC machine, the main flux distorts and weakens. This results in following two problems

  1. Due to armature reaction, the main flux decreases by 10% and that causes generated e.m.f to fall with increase in load current.
  2. To ensure good commutation, the brushes are preferred to placed in neutral zone. Due to distortion of flux, neutral zone shifted to another place and if the load current varies frequently, the positioning of brushes is not practicable.

Commutation

In the below given figure, the schematic diagram of 2 pole lap wound generator is shown. There are two parallel paths between the brushes. Therefore, each coil of the winding carries one half Ia/2 of the total current Ia entering and leaving the armature circuit. In the below given figure, we can see that the currents in the coils connected to the brushes are either all towards the brush (positive brush) or all directs away from the brush (negative brush). Therefore, current in the coil will reverse as the coil )a brush. This reversal of current in the coil as the coil passes the brush is called commutation.

COMMUTATION-IN-DC-MACHINE

 

The complete reversal of current in a coil at a uniform rate as the coil passes the brush is called ideal commutation.

During commutation, the coil undergoing commutation is short circuited by the brush. For Ideal commutation, the following assumptions are made:

  1. The commutator segments insulation is of negligible thickness as compared to commutator width.
  2. The armature coil resistance is negligible comped to carbon brush resistance when in contact with copper segment.
  3. The armature coil has negligible inductance.

Electrical DC Generator Characteristics

The DC machines speed when acting as an electrical DC generators are fixed by the prime mover. The prime mover are equipped with a speed governor so that the sped of an electrical DC generators are practically constant. Under those conditions, the performance of the generator deals primarily with the relation between excitation,terminal voltage and load. These relations can be best exhibited graphically by means of curves known as generator characteristics. The following are the three most important characteristics of an electrical DC generators.

Open Circuit Characteristics (O.C.C)

This curve shows the relation between the generated e.m.f on no load (E0) and the field current (If) at constant speed. It is also known as magnetic characteristic or no load saturation curve. Its shape is practically same for all types of electrical DC generators whether separately excited or self excited generators.

Internal Or Total Characteristics (E/Ia)

This curve shows the relation between the generated e.m.f on load (E) and the armature current (Ia). The generated output e.m.f will be less than E0 due to armature reaction effect. Therefore, this curve will lie below the open circuit characteristic curve.

External Characteristic (V/IL)

This curve shows the relation between the terminal voltage (V) and the load current (IL). The terminal voltage will be less than E due to voltage drop in the armature circuit. Therefore, this curve will lie below both the open circuit characteristic curve and internal characteristic curve.

characteristic curves of electrical DC generators

 

 

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