DCMachineConstruction

Cutaway view of a dc motor Stator with poles visible.

ConstructionofDCmachine

segments

brushes

ConstructionofDCmachine Armature

Rotor of a dc motor

Commutatorconnections

Commutatorconnectionsalongwithbrush

DCMachinecompletestructure

SimplifiedSchematicdiagram

schematicofaDCmachine

CompletestructureofaDCMachine

GeneralarrangementofadcmachineBrushesandcompensatingwinding

DCmachine..

The stator of the dc motor has poles, whichare excited by dc current to producemagnetic fields.

In the neutral zone, in the middle betweenthe poles, commutating poles are placed toreduce sparking of the commutator. Thecommutating poles are supplied by dccurrent.

Compensating windings are mounted onthe main poles. These shortcircuitedwindings damp rotor oscillations.

Differentpartsofadcmachine

Yoke Polescores&poleshoe Fieldwinding Armaturecore Armaturewinding i)LapwindingII)Wavewinding Commutator brushesandbearings

YokeMechanical strength

Support the field poles

It carries magnetic flux ( acts asreturn path for flux lines)

Statorframe

DCmachine

The poles are mounted on an iron core that provides aclosed magnetic circuit.

The motor housing supports the iron core, the brushes andthe bearings.

The rotor has a ringshaped laminated iron core with slots. Coils with several turns are placed in the slots. Thedistance between the two legs of the coil is about 180electric degrees.

Polecore&poleshoe

Pole core is laminated to reduce core lossesPole shoe is in concave shape to maintain uniform airgapand to reduce the leakage flux

Fieldwinding

FIELDWINDINGSMostDCmachinesusewoundelectromagnetstoprovidethemagneticfield.OrPermanentmagnetsareusedinPMDCmachineTwotypesoffieldwindingsareused

seriesfield shuntfield

FIELDWINDINGS(Cont)SeriesfieldwindingsTheyareconnectedinserieswiththearmatureTheymadewithrelativelyfewwindingsturnsofverylargewireandhaveaverylowresistance usuallyfoundinlargehorsepowermachineswoundwithsquareorrectangularwire.Theuseofsquarewirepermitsthewindingstobelaidclosertogether,whichincreasesthenumberofturnsthatcanbewoundinaparticularspace

FIELDWINDINGS(Cont)

Square wire permits more turns than round wire in the same area

Square wire contains more surface than round wire

Square and rectangular wire can also be made physicallysmaller than round wire and still contain the same surface area

FIELDWINDINGS(Cont)

Shunt field windings constructed with relatively many turns ofsmall wire, thus, it has a much higherresistance than the series field.

intended to be connected in parallel with,or shunt, the armature.

high resistance is used to limit current flowthrough the field.

FIELDWINDINGS(Cont..) When a DC machine uses both series and shunt fields, each pole piece will contain

both windings. The windings are wound on the pole pieces in such a manner that when current

flows through the winding it will produce alternate magnetic polarities.

ARMATURE More loops of wire = higher rectified voltage In practical, loops are generally placed in slots of an iron core The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux

to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature. The windings of armatures are connected in different ways depending on the requirements of the

machine.

Loops of wire are wound around slot in a metal core DC machine armature

Armature laminations and slot shapes

Armaturecore

Armaturewinding

Armaturewinding

Lap Wound Armatures are used in machines designed for low voltageand high current

armatures are constructed with large wirebecause of high current

The windings of a lap wound armature areconnected in parallel. This permits the currentcapacity of each winding to be added andprovides a higher operating current

No of current path, C=2p ; p=no of poles

Lap wound armatures

Wave Wound Armatures are used in machines designed for highvoltage and low current

their windings connected in series When the windings are connected in series,the voltage of each winding adds, but thecurrent capacity remains the same

No of current path, C=2

Wave wound armatures

Frogleg Wound Armatures the most used in practical nowadays designed for use with moderate currentand moderate armatures voltage

the windings are connected in seriesparallel.

Most large DC machines use frogleg woundarmatures.

Frogleg wound armatures

Armaturewindingwithcommutatorconnections

Dcmachinewindingsoverview

Winding

LapC=2p

WaveC=2

SeparatelyExcited

Frogleg

Self excited

armature field

series shunt compound

Commutatorconnections

Commutatorandcoilconnections

Commutator. The coils are connected in series through the commutatorsegments.

The ends of each coil are connected to a commutatorsegment.

The commutator consists of insulated copper segmentsmounted on an insulated tube.

Two brushes are pressed to the commutator to permitcurrent flow.

The brushes are placed in the neutral zone, where themagnetic field is close to zero, to reduce arcing.

BrushesandbearingsBrushesareusedtocollectthecurrentfromcommutator.

Materialsusedarecarbonorgraphite.

Bearings

Bearings

GenerallyRollerbearingsareusedindcmachines.

MagneticcircuitofDCmachine

DismantledDCmachine

Statorwithfieldpoles

Bearings

ArmatureandCommutator

Brushgearsystem

PrincipleoperationofGenerator

Whenever a conductor is moved within a magnetic fieldin such a way that the conductor cuts across magneticlines of flux, voltage is generated 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 the magnetic field,iii. the speed at which the conductor is moved, andiv. the length of the conductor within the magnetic field

Principleofoperation(Cont)

FlemingsRighthandrule(GeneratorRule) To determine the direction of the induced emf/current of a conductormoving in a magnetic field.

The POLARITY of the voltage depends on the direction of themagnetic lines of flux and the direction of movement of theconductor.

THEELEMENTARYGENERATOR

The simplest elementary generator that can bebuilt is an ac generator.

Basic generating principles are most easilyexplained through the use of the elementary acgenerator.

For this reason, the ac generator will be discussedfirst. The dc generator will be discussed later.

An elementary generator consists of a wire loopmounted on the shaft, so that it can be rotated in astationary magnetic field.

This will produce an induced emf in the loop. Sliding contacts (brushes) connect the loop to anexternal circuit load in order to pick up or use theinduced emf.

Elementary Generator

The pole pieces (marked N and S) provide the magnetic field. The pole pieces areshaped and positioned as shown to concentrate the magnetic field as close aspossible to the wire loop.

The loop of wire that rotates through the field is called the ARMATURE. The endsof the armature loop are connected to rings called SLIP RINGS. They rotate withthe armature.

The brushes, usually made of carbon, with wires attached to them, ride againstthe rings. The generated voltage appears across these brushes. (These brushestransfer power from the battery to the commutator as the motor spins discussed later in dc elementary generator).

THEELEMENTARYGENERATOR(Cont)

THEELEMENTARYGENERATOR(A) An end view of the shaft and wire loop isshown.

At this particular instant, the loop of wire(the black and white conductors of theloop) is parallel to the magnetic lines offlux, and no cutting action is takingplace.

Since the lines of flux are not being cutby the loop, no emf is induced in theconductors, and the meter at thisposition indicates zero.

This position is called the NEUTRALPLANE.

00 Position (Neutral Plane)

THEELEMENTARYGENERATOR(B) The shaft has been turned 900 clockwise, theconductors cut through more and more lines offlux, and voltage is induced in the conductor.

at a continually increasing angle , the induced emfin the conductors builds up from zero to amaximum value or peak value.

Observe that from 00 to 900, the black conductorcuts DOWN through the field.

At the same time the white conductor cuts UPthrough the field.

The induced emfs in the conductors are seriesadding.

This means the resultant voltage across thebrushes (the terminal voltage) is the sum of thetwo induced voltages.

The meter at position B reads maximum value.

900 Position

THEELEMENTARYGENERATOR(C)

After another 900 of rotation, the loophas completed 1800 of rotation and isagain parallel to the lines of flux.

As the loop was turned, the voltagedecreased until it again reached zero.

Note that : From 00 to 1800 theconductors of the armature loop havebeen moving in the same directionthrough the magnetic field.

Therefore, the polarity of the inducedvoltage has remained the same

1800 Position

THEELEMENTARYGENERATOR(D) As the loop continues to turn, the conductorsagain cut the lines of magnetic flux.

This time, however, the conductor thatpreviously cut through the flux lines of the southmagnetic field is cutting the lines of the northmagnetic field, and viceversa.

Since the conductors are cutting the flux lines ofopposite magnetic polarity, the polarity of theinduced voltage reverses.

After 270' of rotation, the loop has rotated tothe position shown, and the maximum terminalvoltage will be the same as it was from A to Cexcept that the polarity is reversed.

2700 Position

THEELEMENTARYGENERATOR(A)

Afteranother900 ofrotation,theloophascompletedonerotationof3600 andreturnedtoitsstartingposition.

Thevoltagedecreasedfromitsnegativepeakbacktozero.

Noticethatthevoltageproducedinthearmatureisanalternatingpolarity.Thevoltageproducedinallrotatingarmaturesisalternatingvoltage. 3600 Position

ElementaryGenerator(Conclusion) Observes

The meter direction The conductors of the armature loop Direction of the current flow

THEELEMENTARYDCGENERATOR Since DC generators must produce DC current instead of ACcurrent, a device must be used to change the AC voltageproduced in the armature windings into DC voltage.

This job is performed by the commutator. The commutator is constructed from a copper ring split intosegments with insulating material between the segments (Seenext page).

Brushes riding against the commutator segments carry thepower to the outside circuit.

The commutator in a dc generator replaces the slip rings ofthe ac generator. This is the main difference in theirconstruction.

The commutator mechanically reverses the armature loopconnections to the external circuit.

THEELEMENTARYDCGENERATOR(Armature)

The armature has an axle, and the commutator isattached to the axle.

In the diagram to the right, you can see three differentviews of the same armature: front, side and endon.

In the endon view, the winding is eliminated to makethe commutator more obvious.

We can see that the commutator is simply a pair ofplates attached to the axle.

These plates provide the two connections for the coilof the electromagnet.

Armature with commutator view

THEELEMENTARYDCGENERATOR(Commutator&Brushesworktogether)

The diagram at the right shows how the commutator andbrushes work together to let current flow to theelectromagnet, and also to flip the direction that theelectrons are flowing at just the right moment.

The contacts of the commutator are attached to the axle ofthe electromagnet, so they spin with the magnet.

The brushes are just two pieces of springy metal or carbonthat make contact with the contacts of the commutator.

Through this process the commutator changes the generatedac voltage to a pulsating dc voltage which also known ascommutation process.

Brushes and commutator

THEELEMENTARYDCGENERATOR

The loop is parallel to the magneticlines of flux, and no voltage is inducedin the loop

Note that the brushes make contactwith both of the commutator segmentsat this time. The position is calledneutral plane.

00 Position (DC Neutral Plane)

THEELEMENTARYDCGENERATOR

As the loop rotates, the conductors begin to cutthrough the magnetic lines of flux.

The conductor cutting through the south magneticfield is connected to the positive brush, and theconductor cutting through the north magnetic field isconnected to the negative brush.

Since the loop is cutting lines of flux, a voltage isinduced into the loop.

After 900 of rotation, the voltage reaches its mostpositive point.

900 Position (DC)

THEELEMENTARYDCGENERATOR As the loop continues to rotate, thevoltage decreases to zero.

After 1800 of rotation, the conductorsare again parallel to the lines of flux, andno voltage is induced in the loop.

Note that the brushes again makecontact with both segments of thecommutator at the time when there isno induced voltage in the conductors 180

0 Position (DC)

THEELEMENTARYDCGENERATOR During the next 900 of rotation, the conductors again cut

through the magnetic lines of flux. This time, however, the conductor that previously cut

through the south magnetic field is now cutting the flux lines of the north field, and vice-versa. .

Since these conductors are cutting the lines of flux of opposite magnetic polarities, the polarity of induced voltage is different for each of the conductors. The commutator, however, maintains the correct polarity to each brush.

The conductor cutting through the north magnetic field will always be connected to the negative brush, and the conductor cutting through the south field will always be connected to the positive brush.

Since the polarity at the brushes has remained constant, the voltage will increase to its peak value in the same direction.

2700 Position (DC)

THEELEMENTARYDCGENERATOR

As the loop continues to rotate, the inducedvoltage again decreases to zero when theconductors become parallel to the magnetic linesof flux.

Notice that during this 3600 rotation of the loop thepolarity of voltage remained the same for bothhalves of the waveform. This is called rectified DCvoltage.

The voltage is pulsating. It does turn on and off, butit never reverses polarity. Since the polarity foreach brush remains constant, the output voltage isDC.

00 Position (DC Neutral Plane)

THEELEMENTARYDCGENERATOR

Observes Themeterdirection Theconductorsofthearmatureloop Directionofthecurrentflow

Effectsofadditionalturns To increase the amount of output voltage, it iscommon practice to increase the number of turnsof wire for each loop.

If a loop contains 20 turns of wire, the inducedvoltage will be 20 times greater than that for asingleloop conductor.

The reason for this is that each loop is connectedin series with the other loops. Since the loopsform a series path, the voltage induced in theloops will add.

In this example, if each loop has an inducedvoltage of 2V, the total voltage for this windingwould be 40V(2V x 20 loops = 40 V).

Effects of additional turns

Effectsofadditionalcoils When more than one loop is used, the average outputvoltage is higher and there is less pulsation of therectified voltage.

Since there are four segments in the commutator, anew segment passes each brush every 900 instead ofevery 1800.

Since there are now four commutator segments in thecommutator and only two brushes, the voltagecannot fall any lower than at point A.

Therefore, the ripple is limited to the rise and fallbetween points A and B on the graph. By adding morearmature coils, the ripple effect can be furtherreduced. Decreasing ripple in this way increases theeffective voltage of the output.

Effects of additional coils

ThePracticalDCGenerator The actual construction and operation of a practical dcgenerator differs somewhat from our elementarygenerators

Nearly all practical generators use electromagnetic polesinstead of the permanent magnets used in ourelementary generator

The main advantages of using electromagnetic poles are:(1) increased field strength and(2) possible to control the strength of the fields. Byvarying the input voltage, the field strength is varied.By varying the field strength, the output voltage ofthe generator can be controlled.

Four-pole generator (without

armature)

Topicstobecovered/notyetcompletedingenerators

principleofoperationofCommutator,Armaturereaction,effectofinterpolesandcompensatingwinding.

WindingDiagrams

WindingDiagrams

(i)DCWindingdiagrams

(ii)ACWindingDiagrams

Terminologiesusedinwindingdiagrams Conductor:Anindividualpieceofwireplacedintheslotsinthemachineinthemagneticfield.

Turn:Twoconductorsconnectedinseriesandseparatedfromeachotherbyapolepitchsothattheemf inducedwillbeadditive.

Coil:Whenoneormoreturnsareconnectedinseriesandplacedinalmostsimilarmagneticpositions.Coilsmaybesingleturnormultiturncoils.

Coilgroup:Oneormorecoilsinglecoilsformedinagroupformsthecoilgroup.

Winding:Numberofcoilsarrangedincoilgroupissaidtobeawinding.

PolePitch:Distancebetweenthepolesintermsofslotsiscalledpolepitch.

Coilsorturnsrepresentation

Schematicofmultiturnandsingleturn

Fullpitchedandshortpitchedcoils

Singleanddoublelayerwindings

Singlelayerwinding:Onlyonecoilsideplacedinoneslot. Doublelayerwinding:Twocoilsidesareplacedinasingleslot.Singleanddoublelayerwindings

Singleanddoublelayerwindings

Definitions

PolePitch The pole pitch is defined as peripheral distance between center of two

adjacent poles in dc machine. This distance is measured in term of armatureslots or armature conductor come between two adjacent pole centers.

This is naturally equal to the total number of armature slots divided bynumber of poles in the machine.

If there are 96 slots on the armature periphery and 4 numbers of poles in themachine, the numbers of armature slots come between two adjacent polescenters would be 96/4 = 24. Hence, the pole pitch of that dc machine wouldbe 24.

pole pitch is equal to total numbers of armature slots divided by totalnumbers of poles, this can alternatively referred as armature slots per pole

CoilSpanorCoilPitch Coil of dc machine is made up of one turn or multi turns of the

conductor. If the coil is made up of single turn or single loop ofconductor, it is called single turn coil.

If the coil is made up of more than one turn of conductor, it is referredas multi turn coil.

A single turn coil will have one conductor per side of the coil whereasin multi turns coil, there will be multiple conductors per side of thecoil.

Whatever may be the number of conductors per side of the coil, eachcoil side is placed inside one armature slot only. That means allconductors of one side of a particular coil must be placed in one singleslot only. Similarly, all conductors of other side of the coil are placedin another single armature slot.

Coilspan Coil span is defined as peripheral distance between two sides of a coil,

measured in terms of number of armature slots between them. That means, after placing one side of the coil in a particular slot, after how many conjugative slots, the other side of the same coil is placed on the armature. This number is known as coil span.

If the coil span is equal to the pole pitch, then the armature winding is said to be full - pitched. At this situation, two opposite sides of the coil lie under two opposite poles. Hence emf induced in one side of the coil will be in 180 phase shift with emf induced in the other side of the coil. Thus, total terminal voltage of the coil will be nothing but the direct arithmetic sum of these two emfs.

If the coil span is less than the pole pitch, then the winding is referred as fractional pitched. In this coil, there will be a phase difference between induced emfs in two sides, less than 180. Hence resultant terminal voltage of the coil is vector sum of these two emfs and it is less than that of full -pitched coil.

PitchofArmatureWinding

Back Pitch (Yb)

A coil advances on the back of the armature. This advancement is measured in terms of armature

conductors and is called back pitch. It is equal to the number difference of the conductor connected to a

given segment of the commutator.

Front Pitch (Yf)

The number of armature conductors or elements spanned by a coil on the front is called front pitch.

Alternatively, the front pitch may be defined as the distance between the second conductor of the next coil which are

connected together at the front i.e. commutator end of the armature. In other words, it is the number difference of the

conductors connected together at the back end of the armature. Both front and back pitches for lap and wave windings

are shown in the figure below.

CommutatorPitch

Resultant Pitch (Y)

It is the distance between the beginning of one coil and the beginning of the next coil to which it is

connected. As a matter of precautions, it should be kept in mind that all these pitches, though

normally stated in terms of armature conductors, are also times of armature slots or commutator bars

Commutator pitch is defined as the distance between two commutator segments which

two ends of same armature coil are connected. Commutator pitch is measured in terms of

commutator bars or segment.

Classificationofwindings

Closed type and open type winding Closed type windings: In this type of winding there is a closed path

around the armature or stator. Starting from any point, the winding path can be followed through all the turns and starting point can be reached. Such windings are used in DC machines.

Open windings: There is no closed path in the windings. Such windings are used in AC machines.

Photographsofacoilorcoils

DCWindings

DCWindings:Twotypesofwindings(a)Lapwinding(b)Wavewinding Thesetwotypesofwindingsdifferintwoways(i)numberofcircuitsbetweenpositiveandnegative

brushes,(ii)themannerinwhichthecoilendsareconnected.Howeverthecoilsofbothlapandwavewindingsareidenticallyformed.

WindingPitches Back Pitch: The distance between top and bottom coil sides of a coil

measured around the back of the armature is called back pitch and isdesignated as yb. Back pitch is approximately equal to number of coil sidesper layer. Generally back pitch is an odd integer.

Front Pitch: The distance between two coil sides connected to the samecommutator segment is called the front pitch and is designated as yf.

Winding Pitch: The distance between the starts of two consecutive coilsmeasured in terms of coil sides is called winding pitch and is designated asY.

Y = yb yf for lap winding Y = yb + yf for wave winding Commutator pitch: The distance between the two commutator segments to

which the two ends of a coil are connected is called commutator pitch and isdesignated as yc and is measured in terms of commutator segments

SchematicofaLapwinding

Lapwinding

The winding in which successive coils overlap each other hence it is called lap winding. In this winding end of one coil is connected to the commutator segment and start of the adjacent coil situated under the same pole as shown in fig.

Lap winding is further divided as simplex and Duplex lap winding. Simplex lap winding: In this type of winding finish F1 of the coil 1 is

connected to the start S2 of coil 2 starting under the same pole as start s1 of coil 1.

We have back pitch yb = 2c/p k where c = number of coils in the armature, p = number of poles, k = an integer to make yb an odd integer.

ImportantrulesforLapwinding

Let Z = Number of conductorsP = number of polesYb = Back pitchYf = Front pitchYc = Commutator pitchYa = Average pole pitchYp = Pole pitchYR = Resultant pitch

Lapwinding Yb (Back pitch) and Yf (Front pitch) must be approximately equal to Yp (Pole

pitch) Yb (Back pitch) must be less or greater than Yf (Front pitch) by 2m where m is

the multiplicity of the winding. When Yb is greater than Yf the winding progresses from left to right and is known

as progressive winding. When Yb is lesser than Yf the winding progresses from right to left and is known

as retrogressive winding. Hence Yb = Yf 2m. Yb and Yf must be odd. Yb and Yf may be equal or differ by 2. + for progressive winding, - for

retrogressive winding Ya = (Yb + Yf ) / 2 = Yp YR ( Resultant pitch) is always even. Yc = m, m = 1 for simplex winding; m = 2 for duplex winding

SchematicofaWavewinding

Wavewinding

Simplexwavewinding:InthistypeofwindingfinishF1ofthecoil1isconnectedtothestartSx ofcoilxstartingunderthesamepoleasstarts1ofcoil

Inwavewindingtheendofonecoilisnotconnectedtothebeginningofthesamecoilbutisconnectedtothebeginningofanothercoilofthesamepolarityasthatofthefirstcoilasshowninfig.

Importantrulesinwavewinding

Yb (Backpitch)andYf (Frontpitch)mustbeapproximatelyequaltoYp

(Polepitch)

Yb andYf mustbeodd.

Yb andYf maybeequalordifferby2.

+forprogressivewinding, forretrogressivewinding

Yc =(Yb +Yf )/2andshouldbeawholenumber

Wavewinding..

Wavewinding..

Example1

DrawthewindingdiagramofaDCMachinewith4poles,14slots,progressive,doublelayerlapwinding.Showthepositionofbrushesanddirectionofinducedemf.

Solution:Numberofpoles=4;Numberofslots=14,Numberofconductors=14x2=28Polepitch=Numberofconductors/pole=28/4=7Wehavepolepitch=(Yb +Yf )/2=Yp

(Yb +Yf)=14 (Yb Yf )=2SolvingaboveequationsYb =8andYf =6backpitchyb =2c/p kForlapwindingbothYb andYf mustbeoddanddifferby2 SatisfyingtheaboveconditionYb =7andYf =5(Windingdiagramandringdiagramsareshownbelow)

Windingtable

Ringdiagram

Example2 Drawadevelopmentdiagramofasimpletwolayerlapwindingforafourpolegeneratorwith16coils.Discussthepropertiesoflapwinding.

Solution: Noofcommutatorsegments=16 Noofcoilsides=32 Polepitch=32/4=8 Yb =(Z/P)+1=9 Yf =(Z/P)1=7(Yb =Yf2m)m=1simplelapwinding;m=2forduplexwinding

Windingconnections

Windingconnection

Windingdiagram

Ringdiagram

Coilsidesconnections

Parallelpathsrepresentation

Ex.3DevelopthesinglelayerwindingforaDCmachinehaving32armatureconductorsand4poles.

MarkthepolesDrawthesequencediagram,indicatethepositionofthebrushesandthedirectionofinducedemf andshowtheequiliserconnections.Soln:Numberofconductors=32Polepitch=32/4=8;polepitch=(Yb +Yf )/2=YpHence(Yb +Yf)=16and(Yb Yf)=2henceYb =9andYf =7

Wavewindingexample

Develop a wave winding diagram for a DC machine having 30 armature conductors and 4 poles. Draw the sequence diagram indicate the position of the brushes, show the direction of induced emf.

Soln: Number of poles = 4, No of conductors = 30For wave winding (Yb + Yf)/2 = (Z 2)/p = 7 or 8Taking Yb = YfYb = 7 and Yf = 7

Windingtable

Windingdiagram

Ringdiagram

Coilsideconnections

Armatureparallelpathrepresentation