Dynamo Parts

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The direct current (dc) machine can be

used as a motor or as a generator.

The major advantages of dc machines arethe easy speed and torque regulation.

However, their application is limited tomills, mines and trains. As examples,trolleys and underground subway cars

may use dc motors.

In the past, automobiles were equippedwith dc dynamos to charge their batteries.


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Even today the starter is a series dcmotor

However, the recent development ofpower electronics has reduced the use ofdc motors and generators.

The electronically controlled ac drives aregradually replacing the dc motor drives infactories.

Nevertheless, a large number of dc motorsare still used by industry and severalthousand are sold annually.


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The parts of a dynamo or related equipment can be

expressed in either mechanical termsor electrical terms.

Mechanical

Rotor: The rotating part of an alternator, generator, dynamo

or motor.

Stator: The stationary part of an alternator, generator,

dynamo or motor.

ElectricalArmature: The power-producing component of an

alternator, generator, dynamo or motor. The

armature can be on either the rotor or the

stator.Field: The magnetic field component of an alternator,

generator, dynamo or motor. The field can be on

either the rotor or the stator and can be either an

electromagnet or a permanent magnet.


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A DC machine consists of the

following essential parts

1. Magnetic frame or Yoke

2. Pole-cores and Pole-Shoes

3.Field Poles4. Field Coils

5. Armature Core

6. Armature Windings orConductors

7.Commutator

8. Brushes and Bearings


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Yoke

Yoke is the outer frame. It serves two purposes.

(i) It provides mechanical support for the poles and acts as a

protective cover for the whole machine. and(ii) It carries the magnetic flux produced by the poles.


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Pole Cores and Pole Shoes

The field magnet consist of pole cores and pole shoes.

They have two purposes:

(i) they spread out the flux in the air gap and also, being

larger cross section, reduce the reluctance of the magneticpath

(ii) they support the exciting coils (field coils)

The pole cores can be made from solid steel castings

or from thin laminations of highly magnetic steel alloy.


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Field Coils

The field coils are those windings, which are located on the

poles and set up the magnetic fields in the machine.They also usually consist of copper wire are insulated from

the poles.

The field coils may be either shunt windings (in parallel with

the armature winding) or series windings (in series with the

armature winding) or a combination of both.


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Armature Core

The armature core is made up thin magnetic steel

laminations stamped from sheet steel with a blanking die.

Slots are punched in the lamination with a slot die.

The laminations are welded, riveted, bolted or bonded

together.

It houses the armature conductors or coils, and causes

them to rotate and hence cut the magnetic flux of the field

magnets.

Air Gap

The space between the armature and the pole shoes.

(from top of teeth to pole face)


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Armature Winding

The armature winding fits in the armature slots and is

eventually connected to the commutator.

It either generates or receives the voltage depending onwhether the unit is a generator or motor.

The armature winding usually consists of copper wire, either

round or rectangular and is insulated from the armature stack.


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Gramme -Ring armature

The old Gramme-Ring armature, now obselete is

shown below.

ARMATURE and its WINDINGS


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Drum-type armature :

A drum-type armature is shown in figure.

The armature windings are placed in slots cut in a

drum-shaped iron core.


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In drum-type armature: each winding

completely surrounds the core so that the

entire length of the conductor cuts the mainmagnetic field.

Therefore, the total voltage induced in the

armature is greater than in the Gramme-ring.The drum-type armature is much more

efficient than the Gramme-ring.

This accounts for the almost universal use of

the drum-type armature in moderndc

generators.
http://powerelectrical.blogspot.com/2007/03/armature-and-its-windings.htmlhttp://powerelectrical.blogspot.com/2007/03/armature-and-its-windings.html


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Commutator

The commutator is the mechanical rectifier, which changes

the AC voltage of the rotating conductors to DC voltage.

It consists of a number of segments normally equal to the

number of slots.

The segments or commutator bars are made of silver bearing

copper and are separated from each other by mica insulation


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Brushes

The brushes, whose function is to collect current

from commutator, are usually made of carbon or

graphite and are in the shape of a rectangular

block.

These brushes are housed in brush-holders

usually of the box type variety.


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Pole-pitch

The periphery of the armature divided by the

number of poles of the generator; the distance between two adjacent poles

It is equal to the number of armature conductors

per pole


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Pole arc

Radial length of the pole face measured in

inches, cm., number of slots, number of

conductors, or mechanical degrees.

Slot pitch or Slot span Sp)

This is the distance from the center of one slotto the center of an adjacent slot, measured on the

surface of the armature.


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Conductor

The length of a wire

lying in the magnetic

field and in which an

e.m.f is induced is

called a conductor,

(as,for example

length AB or CD infigure).

Coil and Winding Element

With reference to the

figure, the two conductorsAB and CD along with

their end connections

constitute one coil of the

armature winding. The coil may be single

turn or multi-turn coil.

A single turn coil wil have

two conductors.But amulti-turn coil may have

many conductors per coil

side.


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Coil-span or Coil-pitch

It is the distance, measured in terms of armature slots,

between two sides of a coil.

It is, in fact, the periphery of the armature spanned by the two

sides of the coil.

If the coil-pitch is equal to the pole-pitch,then winding is

called full-pitched. It means that coil span is 180 electrical

degrees.

If the coil-pitch is less than the pole-pitch, then the winding isfractional-pitched.

Back Pitch (YB)

The distance, measured in

terms of the armature

conductors, which a coil

advances on the back of the

armature is called back pitch.

Commutator Pitch (YC)

It is the distance betweenthe segments to which the

two ends of a coil are

connected.

f
http://powerelectrical.blogspot.com/2007/03/pole-pitchcoil-pitchback-pitchfront.htmlhttp://powerelectrical.blogspot.com/2007/03/pole-pitchcoil-pitchback-pitchfront.html


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Two types of windings:

1. Lap Winding - This type of winding is used in dc generators designed

for high-current applications

2. Wave Winding - This type of winding is used in dc generators employed

in high-voltage applications.

The difference between the two is merely due to the different

arrangement of the end connections at the front or commutator end of

armature .


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Each winding

can be arranged progressively or retrogressively, and

connected in simplex, duplex and triplex.

In simplex lap winding, the ends of each coil areconnected to adjacent commutator segments. In this way, all the coils are connected in series.

In a duplex lap winding, there are in effect two separate sets of coils,

each set connected in series.

Similarly, in a triplex lap winding, there are in effect three separatesets of series-connected coils.

In a wave winding, the ends of each coil are connected

to commutator segments two pole spans apart.

If, after passing once round the armature, the winding falls

in a slot to the left of its starting point then winding is said to

beretrogressive. If, however, it falls one slot to the right,

then it isprogressive.


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GENERATED EMF, EG

a

PZNE

G 60

Where, EGgenerated emf

N speed in rpm of the armature rotation

P number of generator poles

Z - total number of armature conductors

flux per pole in Weber

a number of parallel paths

a = mP .for lap winding

a = 2m .for wave winding

m multiplicity; m=1 for simplex

= 2 for duplex

= 3 for triplex


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PROBLEMS:

1. A conductor of active length 30 cm carries a current of 100A and lies at right

angles to a magnetic field of strength 0.4 Wb/m2 Calculate the force in

Newton exerted on it.

2. If the force causes the conductor on problem no. 1 to move at a velocity of

10 m/s, calculate the emf induced in it.

3. Find the induced emf of an 8-pole, wave-wound dynamo having 4mWb per

pole and rotating at 600 rpm. The armature has 123 slots, and there are 3

coils per slot, each having 2 turns.

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