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UNIT II

5/2/2010 V.BALAJI,AP/EEE,DCE 1


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Electric Machine Electric machines can be used as motors and

generators Electric motor and generators are rotating energy-

transfer electromechanical motion devices

V.BALAJI,AP/EEE,DCE 25/2/2010

Electric motors convert electrical energy tomechanical energy

Generators convert mechanical energy to electrical

energy


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Electric Machine Electric machines can be divided into 2 types:

AC machines DC machines

Several types DC machines


eparate y exc te Shunt connected

Series connected

Compound connected

Permanent magnet


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Electric Machine All Electric machines have:

Stationary members (stator) rotating members (rotor)

Air gap which is separating stator and rotor


The rotor and stator are coupled magnetically


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Schematic representation of a DCMachine

DC Machines

+ If

NN


StatorStator

VVff

-

If

If

ff

22

SS


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Electric Machine The armature winding is placed in the rotor

slot and connected to rotating commutatorwhich rectifies the induced voltage


armature winding, ride on commutator


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DC Machines

Elementary two-pole DC Machine



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Electric Machine The armature winding consists of identical coils carried in

slots that are uniformly distributed around the periphery of the

rotor Conventional DC machines are excited by direct current, in

particular if a voltage-fed converter is used a dc voltage uf issupplied to the stationary field winding


ence t e exc tat on magnet c e s pro uce y t e ecoils

Due to the commutator, armature and field windings producestationary magnetomotive forces that are displaced by 90electrical degrees


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The field winding is placed on the statorand supplied from a DC Source.

DC Machines

NN ArmatureArmatureWindinWindin


RotorRotor

ff

22SS

xx

xx x

xx

x


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Magnetic Flux in DC machines

rotor

NN

+

a


f/2

stator

f

SS

Vf

-

.. .. ..

xx x x x x

Armature

Winding

If


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The current is induced in the RotorRotor

WindingWinding (i.e. the Armature WindingArmature Winding)since it is placed in the field (Flux

DC Machines


.

ff


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mmf produced by the armature and mmfproduced by the field winding areortho onal.

Orthogonality of Magnetic Fields inDC Machines


B

IL Fo

ILBL 90sin== BIF

Magnetic field due to

field winding

Magnetic field due to

armature winding

90o


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The force acting on the rotor, isexpressed as

DC Machines

{ {

FieldthetoDue

ArmaturethetoDue

BLIf =


ll

ff

ff

TTeeTTee = xff ll


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The Field winding is placed on the stator

and the current (voltage) is induced inthe rotor winding which is referred alsoas the armature winding.

DC Machines


In DC Machines, the mmfmmfproduced bythe field winding and the mmfmmfproduced

by the armature winding are at right-

angle with respect to each other.

The torque is produced from theinteraction of these two fields.


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8.1 DC Motor

The direct current (dc) machine can be usedas a motor or as a generator. DC Machine is most often used for a motor.


the easy speed and torque regulation. However, their application is limited to mills,

mines and trains. As examples, trolleys andunderground subway cars may use dcmotors.

In the past, automobiles were equipped withdc dynamos to charge their batteries.


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8.1 DC Motor

Even today the starter is a series dc motor However, the recent development of power

electronics has reduced the use of dc motorsand enerators.


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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8.1 Construction



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DC Machine Construction


Figure 8.1 General arrangement of a dc machine


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DC Machines

The stator of the dc motor has

poles, which are excited by dccurrent to produce magneticfields.

In the neutral zone, in themiddle between the poles,


commu a ng po es are p ace

to reduce sparking of thecommutator. The commutatingpoles are supplied by dccurrent.

Compensating windings aremounted on the main poles.

These short-circuited windingsdamp rotor oscillations. .


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DC Machines

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

The motor housing supportsthe iron core, the brushes


and the bearings. The rotor has a ring-shaped

laminated iron core withslots.

Coils with several turns are

placed in the slots. Thedistance between the twolegs of the coil is about 180electric degrees.


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DC Machines

The coils are connected in

series through the commutatorsegments.

The ends of each coil areconnected to a commutatorsegment.


The commutator consists ofinsulated copper segmentsmounted on an insulated tube.

Two brushes are pressed to thecommutator to permit currentflow.

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


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DC Machines

The rotor has a ring-shapedlaminated iron core withslots.

The commutator consists ofinsulated co er se ments


mounted on an insulatedtube.

Two brushes are pressed tothe commutator to permitcurrent flow.

The brushes are placed inthe neutral zone, where themagnetic field is close tozero, to reduce arcing.


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DC Machines

The commutatorswitchesthe current from one rotorcoil to the adjacent coil,

The switching requires theinterru tion of the coil


current. The sudden interruption of

an inductive currentgenerates high voltages .

The high voltage produces

flashover and arcingbetween the commutatorsegment and the brush.


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|

Shaft

BrushIr_dc

Ir_dc

/2

Rotation

Ir_dc

/2

12

8

Pole

winding


Copper

segment

Insulation

Rotor

Winding

N S

Ir_dc

3

4

5

6

7

Figure 8.2 Commutator with the rotor coils connections.


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Figure 8.3 Details of the commutator of a dc motor.


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Figure 8.4 DC motor stator with poles visible.


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Figure 8.5 Rotor of a dc motor.


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Figure 8.6 Cutaway view of a dc motor.


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Ch i f S ifi L di


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Choice of Specific Loadings

(i) Choice of specific magnetic loading


(II) o ce o spec c e ec r c oa n

Choice of specific magnetic


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loading

(1) Teeth Flux density

(2) Frequency


o age


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Teeth Flux density If flux density in the air gap is high, itmay lead to high flux density in

armature teeth beyond the maximumermissible limit.


The maximum flux density in theteeth at minimum section should not

go beyond 2.2 wb/m2


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. The reasons are obvious as higher flux density

(i) Causes increased iron losses

(ii) Requires higher ampereturns for passing the


ux t roug teet ea ng to ncrease

field copper losses and increased cost of copper


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(2) Frequency

The frequency of flux reversal in thearmature is given bynp

f =


The higher frequency will result in increased ironlosses in the armature core and teeth. Therefore,

there is a limitation in choosing higher Bavfor a machine having higher frequency.


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(3) Voltage

For high voltage machine, space requiredfor insulation is comparatively more.


available for iron on the periphery leadingto narrower teeth

Therefore, lower value of Bav has to betaken. Otherwise teeth flux densityincreases beyond permissible limit.


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Usually, Bav lies between 0.45 to0.75 wb/m2


The corresponding value ofmaximum flux density in the gapBg varies from 0.64 to 1.1Wb/m2.


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Maximum flux density in the air gap Bg =Bav


Lower value of flux density for lower ratingmachines and higher values of fluxdensity, for higher rating machines is theusual choice.


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Choice of specific electricloading

1) Heating or Temperature Rise (2) Speed


(4) Size of Machine

(5) Armature Reaction

6) Commutation


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(1) Heating or Temperature Rise Using a high value of armature conductors (ac)

creates problem of heat dissipation

A high value of ac means either copper used ismore i.e., having large number of conductors


insulation thickness leading to poor heat dissipation or diameter is less

leading again to poor heat dissipation because of

reduced surface area. Both of these results in hightemperature rise in armature.


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(2) Speed

For a high-speed machine, ventilation isobviously better and greater losses couldbe dissi ated


Thus, a higher value of 'ac' can be usedfor higher speed machine.


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3) Voltage

Machines with high voltage require largespace for insulation


,

possible to reduce the space required foriron because of the limitation imposed byflux density in the teeth

Therefore, space for copper is reduced.So, lesser value of 'ac' is used in such

cases.


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(4) Size of Machine

In large size machine, there is more spacefor accommodating copper thereforehi her ac should be used.



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(5) Armature Reaction

With high value of ac, armature ampereturns also increases. Therefore armaturereaction will be severe


To counter this, field mmf is increased andso the cost of machine goes high.


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(6) Commutation

High value of ac worsens the commutationcondition in machines. From the point ofview of commutation a small value of ac is


desirable. The value of ac usually lies between

15,000 to 50,000 amp. conductors /m.


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Guide lines for selecting for number


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Guide lines for selecting for numberof poles



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Determine the main dimensions of a 10 H.P ,400 V, 1500r.p.m , dc shunt motor, the average gap density is 0.45

tesla and ampere conductors/meter are 20,000 . The maxefficiency is 85% Assume shunt field current to be 0 9 A


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efficiency is 85%. Assume shunt field current to be 0.9 Aand the diameter to length ratio is 2.7. The pole arc topole pitch is 0.7



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(1)Flux Pulsations

Flux pulsations mean changes in the air gap


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Flux pulsations mean changes, in the air gap

flux because of changes in the air gap reluctance

between the pole faces and the irregularly shaped


.

This flux pulsations give rise to eddy currentlosses in the pole-shoes and produce magnetic

noise. The flux pulsations are reduced with

increased number of armature slots.

(a) To avoid flux pulsations, the air gap

reluctance per pair of poles should be practically


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constant which is possible if the number of slots per

pair of poles is an odd integer i.e., the slots per poleis an integer plus .


(b) To prevent the flux oscillations, the air gap

reluctance under pole faces must be kept constant

for all reactive positions of pole shoes and armature

core. These conditions are approximated by

(i) Properly chamfering the tips of pole faces.


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(ii) Making the number of slots per pole shoes an

integer plus


,

should be an integer with slots per pole equal to aninteger plus 1/2.

(2)Cooling

For large of number slots, lesser number of

conductors per slot. Therefore cooling is obviouslybetter


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better.

(3) Tooth Width

For large number of slots, the slot pitch reduces and


.

reduced tooth width.(i) Flux density at the minimum section of tooth

increases causing increased iron losses

(ii) It is difficult to support the teeth at the ventilating

duct| without obstructing the ventilation

(4) Commutation|

From commutation point of view, large number

of slots smaller number of conductors per slot are


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better.

(5) Cost

A smaller number of slots are desirable


considering the as the charges for punching the slots

increase with their num'

Further with smaller number of slots, there are fewer

slots to insulate and therefore the cost of insulation

also goes down.

Guiding factors for choice of number of armature

slots

(1) Slot Pitch


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The value of slot pitch lies between 20 to

40mm. The usual limit is between 25 to 35mm

exce t in case of ver small machines where it ma


be 20min and even less.

(2) Slot Loading

The slot loading i.e. number of ampere

conductors per slot should not exceed about

15,000 A.

(3) Flux Pulsations

The number of slots per pole pair should be anodd integer in order to minimize pulsation losses.


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(4) Commutation

The number of slots per pole usually lies


.

(5) Suitability for WindingWhen selecting number of slots, we must

confirm that the number selected suits the armature

windings. The number of slots per pole should match

the value given in Table 2.5

Table 2.5. Number of Armature slots


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Number of Armature Coils

The number of turns per coil and the number

of coils are so chosen that the voltage between


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of coils are so chosen that the voltage between

commutator segments is limited to a value where

there is no possibility of flashover. For very small


machines, this limit may rise to 60V owing to their

high internal resistance. Normally, the maximum

voltage between adjacent segments at load should

not exceed 30V.Average voltage between adjacent segments

at no load


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CHOICE OF CURRENT DENSITY


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Design of shunt field winding


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