Direct current generators || Part-01 ~ MONU TUTORIALS ACADEMY

Basic structure of electric machine

A rotating electric machine has two main parts stator and rotor comma separated by the air gap.

the stator of the machine does not move and normally is the outer frame of the machine.
The rotor is free to move and normally is the inner part of the machine.

Both stator and rotor are made up of ferromagnetic materials. Slot are cut on the inner periphery of the stator and the outer periphery of the rotor. Conductors are placed in the slot of the stator and rotor. They are interconnected to form windings full stop the windings in which voltage is induced is called the armature winding.

The winding through which a current is passed to produce the main flux is called the field winding.

Permanent magnet are used in some machine to provide the main flux of the machine.

There are two types of DC machines, the DC generator and the DC motor. The DC generator converts mechanical energy into electrical energy. the DC based on the principle that when conductor is rotated in a DC magnetic field, a voltage will generated in the conductor.

DC generator construction

A DC generator consists of three main parts

Magnetic field system
Armature
Commutator and brush gear

1. Magnetic field system

The magnetic field system is the stationary part of the machine.

It produced the main magnetic flux. the outer frame or yoke is Hollow cylindrical of cast steel or rolled Steel. And even number of pole corse are bolted to the Yoke.

The yoke serve the following two purpose

Itsupport the pole core and act a protecting cover to the machine.
IT form a part of the magnetic circuit.

Since the poles project in word they are called salient poles. Each pole has a pole shoe having a curved surface.

The pole shoe serve two purpose

It support the field coil
it increase the cross section area of the magnetic circuit and reduce it reluctance.

the pole core are made up of Steel laminations that are insulated from each other and riveted together. The poles are laminated to reduce Eddy current losses.

Armature

The rotating part of the DC machine is called the armature. The armature consists of a soft upon which laminated cylinder, called armature core, is mounted. The armature core has groovesaur slot on its outer surface. The lamination are insulated from each other and tightly clamped together. In small machine the lamination are keyeddirectly to the shop in large machine they are mounted on a spider the purpose of using lamination is to reduce Eddy current losses.

Dance related conductor are put in the slot of the armature court. The conductor are wedge and bands of Steel wire are first round the court to prevent them flying under centrifugal forces. the conductor are suitable connected. This connected arrangement of conductor is called armature winding two types of winding are used wave and lap.

Commutator and brush gear

alternating voltage is produced in a coil rotating in a magnetic field. To obtain direct current in the external circuit a commutator is needed. the commutator, which rotate with the armature, is made from number of wedge shaped hard drawn copper bus or segment insulated from each other and form the shaft. The segments form ring around the support of the armature. Is commutator segment is connected to the ends of the armature coils.

current is collected from the armature winding by means of two or more carbon brushes mounted on the commutator. Each brush is supported in a metal box called a brush box or brush holder. the pressure exerted by the brushes on the commutator can be adjusted and is maintained at constant value by means of spring. current produced in the armature winding is passed on to the commutator and then to the external circuit by means of brushes.

Magnetic circuit of a DC generator

The magnetic circuit of a 4 pole DC generator is shown in figure broken lines indicate the main flux paath. Flux produced by the field winding of a generator who is stabilized in the pole course, air gap armature core and Yoke. The air gap is the space between the armature surface and the pole face. This space is kept as a smallest possible..

Equivalent circuit of a DC Machine armature

the armature of a DC generator can be represented by an equivalent electric circuit. It can be represented by three series connected elements E, Ra and Vb. The element E is the generated voltage, Ra is the armature resistance, Vb is the press contact voltage drop. the equivalent circuit of the armature of a DC generator is shown in figure and that of a DC motor is shown in figure case of DC motor E is the back EMF.

Types of DC machines

There are two methods of excitation separate excitation and self excitation. In separate excitation the field coil are and raised by a separate DC source. in self excitation the current flowing through the field winding is supplied by the machine itself.

Direct current machine are named according to the connection of the field winding with the armature.

The principal types of DC machines are;

Separately excited DC machine.
Shunt wound or shunt machine.
Series wound or series machine.
Compound wound or compound machine.

Separately excited DC machine

As the name implies, the field coil and rise by a separate DC source. The connection showing the separately excited DC Machine are given in figure.

Shunt wound DC machine

a machine in which the field coil are connected in parallel with the armature is called as shunt machine. Since the shunt field receive the full output voltage offer generator for the supply voltage of motor. It is made up of large number of turns of fine wire carrying a small field current. in figure show the connection diagram of DC shunt generator and DC shunt motor.

Series wound DC machine

a DC machine in which the field coil are connected in series with the armature is called as series machine. The series field winding carries the armature current and since the armature current is large, the series field winding consists of few turns of wire of large cross section area in figure show the connection of DC series generator and DC series motor.

Compound wound DC machine

A DC Machine having both sentence series field is called the compound machine. Each field pole of the machine carries two winding. the Saint winding has many turns of fine wire and the series winding has few turns of large cross sectional area.

The compound machine may be connected in two ways. if the shunt field is connected in parallel with the armature alone the machine is called the short shunt compound machine. Such a machine is shown in figure

if the shunt field is in parallel with both armature and series field the machine is called the long shunt compound machine.

if the magnetic flux produced by the series winding assist the flux produced by shunt field winding, the machine is said to be cumulatively compounded.if the series field flux oppose the shunt field flux, the machine is said to be differentially compounded. The type may be long shunt or short shunt connected.

EMF equation of DC machine

As the armature rotates, a voltage is generated in the coils. In case of generator, EMF of rotation is called the generated EMF or armature EMF and Er=Eg.

In case of motor, the EMF of rotation is known as back EMF and Er=Eb. The expression, however, is the same for both condition of operation in this figure

Since n revolution are made in one second, one revolution will be made in 1/n second. Therefore, the time for one revolution of the armature is 1/n second.

The generated voltage Eis determined by the number of armature conductor in series in any one path between the brushes. Therefore, the total voltage generated in this figure

Lap and wave winding

armature coil can be connected to the commutator to form either lab or wave winding.

Lap winding

the ends of each armature coil are connected to adjustment segment on the commutator so that the total number of parallel path is equal to the total number of poles. That is, for lap winding A=P this may be remember by the letter A and p in lap.

Wave winding

the ends of each armature coil are connected to commutator segments some distance apart, so that only two parallel paths are provided between the positive and negative brushes coma that is, for wave winding A=2.

in general, the lap winding is used in low voltage, high current machine and the wave winding is used to high voltage low current machines.

General procedure for solving problem on generated voltage and armature current

the following general procedure may be used conventionally to solve problems on generated voltage and armature current in a DC machine;

Draw the equivalent circuit for the particular machine.
Mark symbol for Electrical quantities on the circuit diagram at proper places.
Write down the Kcl, kvl and power equation for the given machine.

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