my ppt dc machines

8/3/2019 My Ppt Dc Machines

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Dc Machines


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ContentsWorking principle of DC machine as a generator anda motor.

Types and constructional features.emf equation of generator,

relation between emf induced and terminal voltageenumerating the brush drop and drop due to armature

reaction.Illustrative examples.

DC motor working principle,

Back emf and its significance,

torque equation.Types of D.C. motors, characteristics andapplications.

Necessity of a starter for DC motor.

Illustrative examples on back emf and torque.


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Overview of Direct CurrentMachines

Direct-current (DC) machines are divided into dcgenerators and dc motors.

DC generators are not as common as they used tobe, because direct current, when required, is mainly

produced by electronic rectifiers. While dc motors are widely used

in many applications.


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DCGenerator

A dc generator is a machinethat converts mechanicalenergy into electrical energy

(dc voltage and current) byusing the principle ofmagnetic induction.

In this example, the ends ofthe wire loop have been

connected to two slip ringsmounted on the shaft, whilebrushes are used to carrythe current from the loop tothe outside of the circuit.

Principle of magnetic induction in DC machine

The simplest DC rotating machine consists of a singleloop of wire rotating about a fixed axis. The magneticfield is supplied by the North and South poles of the

magnet. Rotor is the rotating part. Stator is the stationarypart.


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Principle operation ofGenerator Whenever a conductor is moved within a

magnetic field in such a way that the conductorcuts across magnetic lines of flux, voltage isgenerated in the conductor.

The AMOUNT of voltage generated depends on:

i. the strength of the magnetic field,

ii. the angle at which the conductor cuts themagnetic field,

iii. the speed at which the conductor is moved,and

iv. the length of the conductor within themagnetic field


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THE ELEMENTARY

GENERATOR The simplest elementary generator

that can be built is an ac generator.Basic generating principles aremost easily explained through theuse of the elementary ac generator.For this reason, the ac generatorwill be discussed first. The dcgenerator will be discussed later.

An elementary generator consistsof a wire loop mounted on the

shaft, so that it can be rotated in astationary magnetic field. This willproduce an induced emf in theloop. Sliding contacts (brushes)connect the loop to an externalcircuit load in order to pick up oruse the induced emf. Elementary Generator


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THE ELEMENTARY GENERATOR(Cont)

The pole pieces (marked N and S) provide the magnetic field.The pole pieces are shaped and positioned as shown toconcentrate the magnetic field as close as possible to the wireloop.

The loop of wire that rotates through the field is called theARMATURE. The ends of the armature loop are connected torings called SLIP RINGS. They rotate with the armature.

The brushes, usually made of carbon, with wires attached to

them, ride against the rings. The generated voltage appearsacross these brushes. (These brushes transfer power from the battery tothe commutator as the motor spins discussed later in dc elementarygenerator).


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THE ELEMENTARY GENERATOR (A)

An end view of the shaft andwire loop is shown. At thisparticular instant, the loop of

wire (the black and whiteconductors of the loop) isparallel to the magnetic linesof flux, and no cutting actionis taking place. Since thelines of flux are not being cut

by the loop, no emf isinduced in the conductors,and the meter at this positionindicates zero.

This position is called the

NEUTRAL PLANE.

00 Position (Neutral Plane)


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THE ELEMENTARY GENERATOR (B)

The shaft has been turned 900

clockwise, the conductors cutthrough more and more lines of flux,and voltage is induced in theconductor.

at a continually increasing angle ,the induced emf in the conductorsbuilds up from zero to a maximumvalue or peak value.

Observe that from 00 to 900, theblack conductor cuts DOWNthrough the field. At the same timethe white conductor cuts UPthrough the field. The induced emfsin the conductors are series-adding.This means the resultant voltageacross the brushes (the terminal

voltage) is the sum of the twoinduced voltages. The meter atposition B reads maximum value.

900 Position


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THE ELEMENTARY GENERATOR (c)

After another 900 of rotation,the loop has completed 1800

of rotation and is again

parallel to the lines of flux.As the loop was turned, thevoltage decreased until itagain reached zero.

Note that : From 00 to 1800

the conductors of thearmature loop have beenmoving in the same directionthrough the magnetic field.Therefore, the polarity of theinduced voltage has

remained the same

1800 Position


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THE ELEMENTARY GENERATOR (D)

As the loop continues to turn, theconductors again cut the lines ofmagnetic flux.

This time, however, the conductorthat previously cut through theflux lines of the south magneticfield is cutting the lines of thenorth magnetic field, and vice-versa.

Since the conductors are cuttingthe flux lines of oppositemagnetic polarity, the polarity ofthe induced voltage reverses.After 270' of rotation, the loop hasrotated to the position shown, andthe maximum terminal voltage will

be the same as it was from A to Cexcept that the polarity is

2700 Position


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THE ELEMENTARY GENERATOR (A)

After another 900 of rotation,the loop has completed onerotation of 3600 and returned

to its starting position. The voltage decreased from

its negative peak back tozero.

Notice that the voltageproduced in the armature is

an alternating polarity. Thevoltage produced in allrotating armatures isalternating voltage.

3600 Position


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Elementary Generator(Conclusion)

Observes The meter

direction

- Theconductorsof thearmature

loop Direction of

the current

flowOutput voltageof an elementary generatorduring one revolution


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THE ELEMENTARY DC GENERATOR


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The output current isalternating and changesdirection each halfrevolution.


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Basic Electrical Engineering

DC Generator

Simple Loop Generator:

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DC Machine, Materials and Principle of Operation

cont'd ...Animation
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Basic Electrical Engineering

DC Generator

Consider rectangular coil ABCD with coil sides AB and CD being rotated in the magnetic field. The

ends of the two coil sides are connected to two slip rings (S1 and S2). The two rings rotate along

with the conductors. Two brushes (B1 and B2) make contact to these two slip rings to collect the

current. When the coil starts rotating in anti-clockwise direction, conductor AB is under the

influence of North pole and CD is under the influence of South pole. By Flemings right hand rule,

the direction of the current through the load resistance is from M to N.

After the coil rotates through 1800, the conductor CD comes under the influence of North pole and

the conductor AB under the influence of South pole. Hence, again emf is induced in the coil sides.

As a result, the current flows through load resistance from N to M (reversed). This is shown in

figure.

Note that, e.m.f generated in the loop is an alternating emf hence the current also. The alternating

current in the load can be converted into direct current by commutator.

Simple Loop Generator:

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DC Machine, Materials and Principle of Operation
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(Armature)

The armature has an axle, and thecommutator is attached to the axle.In the diagram to the right, you can

see three different views of thesame armature: front, side and end-on.

In the end-on view, the winding iseliminated to make the commutator

more obvious. You can see that thecommutator is simply a pair ofplates attached to the axle. Theseplates provide the two connectionsfor the coil of the electromagnet.

Armature with commutator view
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(Commutator & Brushes work together)

The diagram at the right shows how thecommutator and brushes work togetherto let current flow to the electromagnet,

and also to flip the direction that theelectrons are flowing at just the rightmoment. The contacts of thecommutator are attached to the axle ofthe electromagnet, so they spin with themagnet. The brushes are just two

pieces of springy metal or carbon thatmake contact with the contacts of thecommutator.

Through this process the commutatorchanges the generated ac voltage to apulsating dc voltage which also knownas commutation process.

Brushes and commutator
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THE ELEMENTARY DC

GENERATOR

The loop is parallel tothe magnetic lines of

flux, and no voltage isinduced in the loop

Note that the brushesmake contact with both

of the commutatorsegments at this time.The position is calledneutral plane.

00

Position (DC Neutral Plane)
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THE ELEMENTARY DC

GENERATOR As the loop rotates, the

conductors begin to cut throughthe magnetic lines of flux.

The conductor cutting throughthe south magnetic field isconnected to the positive brush,and the conductor cuttingthrough the north magnetic fieldis connected to the negative

brush.

Since the loop is cutting lines offlux, a voltage is induced intothe loop. After 900 of rotation,the voltage reaches its mostpositive point.

900

Position (DC)
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THE ELEMENTARY DC

GENERATOR As the loop continues to

rotate, the voltagedecreases to zero.

After 1800 of rotation, theconductors are againparallel to the lines of flux,and no voltage is induced inthe loop.

Note that the brushes againmake contact with bothsegments of the commutatorat the time when there is noinduced voltage in theconductors

1800

Position (DC)
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THE ELEMENTARY DC

GENERATOR During the next 900 of rotation, the

conductors again cut through themagnetic lines of flux.

This time, however, the conductor that

previously cut through the southmagnetic field is now cutting the flux linesof the north field, and vice-versa. .

Since these conductors are cutting thelines of flux of opposite magneticpolarities, the polarity of induced voltageis different for each of the conductors.The commutator, however, maintains thecorrect polarity to each brush.

The conductor cutting through the northmagnetic field will always be connectedto the negative brush, and the conductorcutting through the south field will alwaysbe connected to the positive brush.

Since the polarity at the brushes hasremained constant, the voltage willincrease to its peak value in the same

direction. 2700

Position (DC)
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THE ELEMENTARY DC

GENERATOR As the loop continues to rotate,

the induced voltage againdecreases to zero when theconductors become parallel to themagnetic lines of flux.

Notice that during this 3600

rotation of the loop the polarity ofvoltage remained the same forboth halves of the waveform. Thisis called rectified DC voltage.

The voltage is pulsating. It doesturn on and off, but it neverreverses polarity. Since thepolarity for each brush remainsconstant, the output voltage isDC.

00

Position (DC Neutral Plane)
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THE ELEMENTARY DC

GENERATOR Observes

The meter

direction

Theconductors ofthe armatureloop

Direction ofthe currentflow

Effects of commutation

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Effects of additional turns

To increase the amount ofoutput voltage, it is commonpractice to increase thenumber of turns of wire foreach loop.

If a loop contains 20 turns ofwire, the induced voltage willbe 20 times greater than thatfor a single-loop conductor.

The reason for this is thateach loop is connected inseries with the other loops.Since the loops form a seriespath, the voltage induced inthe loops will add.

In this example, if each loophas an induced voltage of 2V,the total voltage for this

winding would be 40V(2V x 20 loops = 40 V).Effects of additional turns


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Effects of additional coils

When more than one loop is used, theaverage output voltage is higher and there isless pulsation of the rectified voltage.

Since there are four segments in thecommutator, a new segment passes eachbrush every 900 instead of every 1800.

Since there are now four commutatorsegments in the commutator and only twobrushes, the voltage cannot fall any lowerthan at point A.

Therefore, the ripple is limited to the rise andfall between points A and B on the graph. Byadding more armature coils, the ripple effect

can be further reduced. Decreasing ripplein this way increases the effective voltageof the output.

Effects of additional coils


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The Practical DC Generator

The actual construction and operation of apractical dc generator differs somewhatfrom our elementary generators

Nearly all practical generators useelectromagnetic poles instead of thepermanent magnets used in ourelementary generator

The main advantages of usingelectromagnetic poles are:

(1) increased field strength and(2) possible to control the strength of the

fields. By varying the input voltage, thefield strength is varied. By varying the fieldstrength, the output voltage of thegenerator can be controlled.

Four-pole generator (without armature)


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A dc generator consists of

i) Field system (stationary)ii) Armature (rotating)

iii) Armature is having the following parts

a)Armature core

b)Armature winding

c)Commutator

d)Brushes

e)Shaft and bearings


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(i) Field system: The main

function of the field system is to

produce uniform magnetic field

within which the armature

rotates. It consists of

(a) Yoke (or frame): Yoke forms

the outermost cover for the

machine. Its functions are:(i) Giving mechanical protection

to the generator and

(ii) to provide path for the flux.

For small generators, yoke is

made of cast iron; for largegenerators, it is made of silicon

steel.


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Pole core, pole shoes and pole

coils:The main poles are made of steel of high

relative permeability. The pole core is

made of thin laminations to reduce

eddy current loss. The poles are fixedto the yoke with bolts and nuts.

The pole shoe performs the following

functions.

(i) It supports the field winding.

(ii) It spreads out the flux

uniformly in the air gap and

also reduces the reluctance of

the magnetic path.

The field coils (or field winding) are

mounted on the poles and carry the

d.c. exciting current.

The field coils are made of copper.

pole coils:

(ii) Armat re Core: It is a


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(ii) Armature Core: It is a

cylindrical drum like structure

made of thin laminations of

silicon steel. Each lamination

is insulated to reduce theeddy current loss. Silicon steel

is used for the core to reduce

hysteresis loss.

(iii) Armature Winding: Theouter periphery of the

armature is cut into number

of slots to hold insulated

conductor called armature

winding.

(show movie)

There are t o t pes of indings:


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There are two types of windings:

a) Lap winding average pitch= Z/P

b) Wave winding average pitch= (Z2)/P (+ for progressive and for

retrogressive)


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Commutator: The function of the

commutator is to convert,

alternating current to direct

current. The commutator is

made up of hard drawn copper

segments insulated from each

other by mica sheets and

mounted on the shaft.

Brushes: The function of brushes is

to collect the direct current

from the commutator

segments and supply it to the

external circuit. The brushes

are made of carbon. Carbon is

having negative temperature

coefficient and is very soft.

Shaft and Bearings: For small

generators, ball bearings are

used. For large rating

generators, roller bearings are

used.


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Let, = Flux / pole in webers

Change in flux d = P webers

Z = Total number armature conductors

= Number of slots x Number of conductors per slot

P = Number of poles

A = Number of parallel paths in the armature.

N = Rotational speed of armature in revolutions per minute

(r.p.m)

Time taken to complete one revolution = 60/N sec.

E = e.m.f induced / parallel path in armature.

Generated e.m.f Eg = e.m.f generated /parallel path


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d PNE.M.F generated per conductor dt 60 volts

ZNumber of armature conductors per parallel path

A

gE e.m.f generated per conductor Number of conductors ineach parallel path

g

PN ZE volts ........(i)60 A


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