Construction And Working Principle Of DC Generator With Types

INTRODUCTION OF D.C GENERATOR

The take a look at the electrical engineering, basically includes the analysis of the strength switch from one shape to another. An electrical system deals with the strength transfer either from mechanical to electrical shape or electric to mechanical form. This process is called as electromechanical energy conversion.

An electrical device that converts mechanical energy into electrical power is known as an electric generator. While the electrical system which converts electric energy into mechanical energy is known as the electrical motor,

Such an electrical system may be related to electrical energy of an alternating type called a.c machines or can be associated with electrical energy of direct type referred to as d.c machines.

Cross Section of DC Machine

CONSTRUCTIONAL DETAIL

The D.C generator consists of

• Yoke
• Pole core, pole shoes and inter poles
• Armature
• Commutator
• Brushes & Bearings

Whether a machine is a d.c generator or a DC motor, the construction basically remains the same as shown.

Yoke

It is also called a magnetic frame. It serves two purposes.

• It acts as a protecting cover for the DC machine and provides mechanical support for the poles.

• It carries the magnetic flux produced by the poles.

The material used:  Yokes are made of cast iron. This is suitable only for small machines but for large machines usually cast steel is employed. Since the permeability of steel is double that of cast iron.

 

Pole Cores, Pole Shoes & Interpoles

The field magnet consists of pole cores and pole shoes.

Functions of pole shoes:

• They spread out the flux in the air gap and also reduce the reluctance of the magnetic path.

• They support the exciting coils or field coils.

Pole coils are made up of copper wire when current is passed through these coils the pole becomes an electromagnet and starts establishing a magnetic field in the machine

Interpoles are provided to improve commutation. Commutating poles have exciting
Coils which are connected in series with the armature

Material Used: For small machines, the poles are made up of cast iron. For larger machines, cast steel is used. The poles are laminated by using sheet steel to reduce eddy current losses.

Armature

Armature Winding - dc generator

The Armature consists of an armature core and armature windings.

Armature core is cylindrical in shape and it is mounted on the shaft. It consists of slots on its periphery and the air ducts to permit airflow through armature which is used for the purpose of cooling,

• Armature core provides a house for armature conductors.

• It is used to provide a low reluctance path to the magnetic flux produced by the field winding.

It is made from magnetic material like cast iron or cast steel. It is made up of laminated construction. The reason for using lamination is to lessen the loss due to eddy currents. Thinner the lamination more are the resistance provided to the prompted emf, smaller the current and therefore lesser the copper loss in the core.



Armature winding is the interconnection of the armature conductors, positioned inside the slots provided on the armature core-periphery. When the armature is rotated, it cuts the magnetic flux and emf prompted in them.

It is made of copper.

Functions

• The generation of emf takes place in the armature winding.

• To carry the current supplied in d.c motors. 

Armature winding is commonly a former wound. These are first wound within the form of flat rectangular coils and are then pulled into their proper shape

These are two types of armature winding

Lap winding: The number of parallel paths is equal to the number of poles and the total current is equally divided among them. It is suitable for large current and low voltage machines

Wave winding: Irrespective of the number of poles of machines, the armature has always two parallel paths. Wave winding is suitable for high voltage and low current machines.

Commutator

Functions: Collection of current from the armature conductors and also it converts the alternating emf into unidirectional emf.

ie, it converts the alternating current induced in the armature conductors into direct current in the external load circuit.

The material used: It is made up of wedge-shaped segments or drops forged copper, which are insulated from each other by thin layers of mica.

The number of segments is equal to the number of armature coils.

Brushes and Bearings

Function of Brushes

It is used to collect current from commutator and it is made up of carbon or graphite. They are rectangular in shape. These brushes are housed in brush-holders usually of the box type variety

Function of Bearings

The Bearings are fitted inside the cover and their purpose is to provide free and smooth rotation to the armature.

The different types of bearings used are:

•  Bush bearings,

•  Ball bearings and

•  Roller bearings

PRINCIPLE OF OPERATION

An electrical generator is a rotating machine that converts mechanical strength into Electrical power.

         

This power conversion is based on the principle of electromagnetic induction.
According to Faraday's laws of electromagnetic induction, "whenever a conductor is moved in a magnetic field, i.e., whenever a conductor cuts magnetic flux, dynamically induced emf is produced within the conductor. This emf reasons a modern to float if the Conductor circuit is closed. Thus the mechanical electricity that is given in the shape of motion to the conductor is converted into electric power.

The important parts of the generator are:

• Conductor

• Magnetic area

Imagine the coil to be rotating in a clockwise direction
As the coil assumes successive positions inside the field, the flux linked with it changes. Hence an emf is brought about in it that's proportional to the charge of the exchange of flux linkages. 

whilst the plane of the coil is at right angles to lines of flux i.e., When it is in function 1 then flux related with the coil is maximum but the rate of change of flux linkages is
minimum. Hence, there may be no caused emf inside the coil. The angle of rotation may be measured from this role.

As the coil maintains rotating further, the rate of trade of flux linkages increases, until function three is reached wherein θ = 90°. As seen the flux connected with the coil is the minimum however rate of exchange of flux linkages is most. Hence most emf is induced about in the coil. 

In the next quarter revolution i.e., from 90° to 180°, the flux connected with the coil progressively increases but the rate of flux linkages decreases. Hence the induced emf decreases regularly until in role five of the coil, it is decreased to zero value.

The direction of this induced emf can be located via making use of Fleming's Right-hand rule which offers its direction from A to B and C to B. Hence the direction of current flow is ABMLCD

In the subsequent half revolution i.e., from 180° to 360°, the variations in the magnitude of emf are just like those in the first half of the revolution. Its value is maximum while the coil is in function 7 and minimum when the coil is in function 1. But it may be discovered that the direction of the induced current is from D to C and B to A as proven. Hence the direction of current flow is alongside DCLMBA that's simply the reverse of the previous direction of flow.

Therefore, we find that the current which we gain from any such easy generator reverses its direction after each half of the revolution. Such a current present process periodic reversals are referred to as alternating current.

It must be referred to that alternating current, not handiest reverses its direction, it does not preserve its magnitude constant. Two half-cycles may additionally be referred to as positive and negative half-cycles respectively.

To convert alternating current into unidirectional current
the slip rings are replaced by split rings

 

First half revolution

Current flows alongside ABMLCD i.e. Brush no. 1 in contact
with section 'a' acts as the positive end of the supply and 'b' as the negative end

Next half revolution

The direction of the induced current within the coil has reversed at the equal time the positions of segments a' and b' have also reversed with the result that brush no.1 comes in contact with that phase which is +ve (i.e.,) 'b'

Hence current again flows from M to L. The waveform of the current is shown. This current is unidirectional.

                                       

Direction of EMF

The direction of induced emf is determined by way of using Fleming's right-hand rule.

According to this rule, if we keep our thumb, index finger or forefinger and center finger of the right hand at proper angles to one another as proven in Fig 

Motion of the conductor

The first finger represents the direction of magnetic lines of flux, the thumb indicates the

The direction of motion of the conductor, then the center finger will indicate the direction of induced emf (current). This rule is utilized in generators.

 

BASIC EQUATION OR EMF EQUATION OF DC GENERATOR

Let,

Ф = Flux/pole in weber 

N = Speed of armature in RPM 

p = Number of poles 

N/60 = Speed of armature in RPS 

z = Total number of armature conductors 

E = EMF induced in any parallel path in the armature in volts 

A = Number of parallel paths 

According to Faraday's Law, the induced emf is proportional to the rate of change of the magnetic flux.

i.e.,

e= -dФ/dt


Let us consider a single conductor moving during one revolution.

dФ = Ф x p weber

Number of revolutions per second = N/60 seconds

Time is taken to complete one revolution,

dt = 60/N seconds

According to Faraday's laws of electromagnetic induction,

EMF generated/conductor = dФ/dt ………………….(2.1)

Substituting, dФ = Ф x p and dt = 60/N in equation (2.1) gives

=Фxp/60/N ……………….(2.2)

EMF generated/conductor = ФNp/ 60 volts

Number of conductors in one path of armature = z/A


∴ Emf generated/path = θpN/60 x z/A volts


Eg = ФzN/60 x p/A ... General equation

For wave winding, A=2


∴ Emf generated/path = ФzNp/60x2 = ФzNp/120 volts

Eg= ФzN/120 p volts

For lap winding, A =p

∴ Emf generated/path = ФzNp/60p = ФzN/60 volts

E = ФzN/60 volts

Types of DC Generator

It is classified,
According to their methods of field excitation,

• Separately excited d.c generators

• Self-excited d.c generators.

a self excited generator can be classified depending upon how the field winding is connected to the armature. There are three types.

1. Series generator

2. Shunt generator

3. Compound generator

       1. Long shunt

             1.1. Cumulative

             1.2. Differential

       2. Short shunt

              2.1. Cumulative

              2.2. Differential

Separately excited DC generator

If the field winding is excited by a separate d.c supply, then the generator is called

separately excited d.c generator.

Eg = V +Ia Ra + V Brush

Self-excited DC generator

If the field winding is excited by the emf induced in the generator itself, is known

As self-excited d.c generator.

Series generator

Eg = V +Ia Ra + Ia Rse + V Brush

Where,

V = Terminal voltage in volts

Ia Ra = Voltage drop in the armature resistance

Ia Rse = Voltage drop in the series field winding resistance

V Brush = Brush drop i.e., the voltage drop at the contacts of the brush.

Shunt Generator

Eg = V + Ia Ra

Long shunt compound generator

Eg = V + Ia (Ra + Rse) + V Brush

Short shunt compound generator

Eg = V +Ia Ra +Ise Rse + V Brush

APPLICATIONS OF DC GENERATOR

DC shunt generators

It is used where a high starting torque is not required.

(1) Because of highly dropping characteristics, it is suited for charging the
batteries.

(2) it is also used for lighting, power supply purposes and light machine tools.

DC series generators

(i) Because of its raising characteristics, it is used as boosters and it is also used for supplying the        field current for regenerative braking of DC locomotive. 

(ii) Series are lightning,

(iii) Series incandescent lightning.

(iv) Constant current generators for welding purposes.

DC compound generators

It is used to drive heavy machine tools such as

(i) Punching machines 

(ii) Elevator motors

(iii) Railway circuits

(iv) Motors of electrified steam railroads and

(v) Incandescent lamps etc.

PROBLEMS IN DC GENERATORS

Example 1:

A 4-pole, wave wound generator has 40 slots and 10 conductors are placed per slot. Find the generated emf when the generator is driven at 1200 r.p.m and flux is 0.02 Wb.

Given Data:

p = 4

z = Number of slots x Conductors per slot

= 40 x 10 = 400

Ф = 0.02 Wb

N = 1200 r.p.m

To find:

(i) Generated emf (Eg)

Solution: 

Generated emf, Eg = p Ф z N / 60 A

∵ [For wave wound, A=2]

Eg = 4 x 0.02 x 400 x 1200 / 60 x 2

Eg = 320v

Example 2

An 8-pole DC generator has a simplex wave wound armature containing 32 coils of 6 turns each. Its flux per pole is 0.06 Wb. The machine is running at 250 r.p.m. Calculate the induced armature voltage.

Given Data:

p = 8

z = 2 x 32 x 6 = 384

Ф = 0.06 Wb

N = 250 r.p.m.

To find:

(i) Induced armature voltage (Eg)

Solution:

Eg = Ф z N / 60 x p/A

[∵ For wave wound, A = 2]

Eg = 0.06 x 384 x 250 / 60 x 8/2

Eg = 384 V