design of dc motor

8/9/2019 Design of Dc Motor

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Cross section of DC machine


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Constructional

Elements(i) Armature

(1)winding

(2) armature core

(3) commutator

(ii) Field

(1) winding

(2) pole core(3) pole shoe

(iii) Frame


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Consider a conductor of length . L carr%ing a current of I4ameres5

If the conductor is mo+ed in a uniform magnetic "eld of 6u7

densit% /a+(#2m*then the (or8 done is gi+en #%

9or8 : ; /a+L I4

9here 7 . is the distance thro< (hich the conductor is mo+ed

9hen the conductor is mo+ed thro< a distance 7 the conductorcuts thro< a magnetic flu7 = : 7 / L

9or8 : = I4

Conductors are laced in armature5 In one re+olution of thearmature) each conductor mo+es thro< a total 6u7 of = (#

(or8 done in one re+olution is gi+en #%

9or8 : = 1> I4

9here) . no5 of oles

? . 6u7 er ole

> . total no5 of armature conductors

Magnetic & Electric loadings


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Magnetic & Electric loadingsTotal Magnetic loading: It is de"ned as the total 6u7 around the

armature eriher% at the air ga5

Magnetic loading:,num#er ofoles1,6u7 er oleTotal magnetic loading:'=

9here) ':No5 of oles

?:$lu7 er oleTotal Electric Loading: It is de"ned as the total no5 of amere

conductoraround the armature eriher%5 Electric loading:,num#er of

conductors1,current er conductor Total Electric loading:I44

9here) I4:Current through one armature

conductor

>:Total No5 of Armature conductor


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Importance of specic loadings

There are t(o t%es of loadingsla%s an imortant role in the design

of electrical machines) such are5Seci"c magnetic loading

*5 Seci"c electrical loading

!pecic magnetic loading:

i /ma7 in the iron art of themagnetic circuit

/ma7 B ma7imum allo(a#le 6u7

densit%

ii Magneti4ing current

large /a+e high magneti4ing current large core loss

Seci"c magnetic loading

/a+e : 6u7 er ole2 area under a ole

: 6u7 er ole2 ole itch 1 lengthof armature

: ,9#2m*

F D L

'

: ' ,9#2m*

F D L

9here)

/a+e- Seci"c magneticloading

'- No5 of oles

?-$lu72ole

D- Diameter of the armature

L- Length of the armature - 'ole itch


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Importance of specic loadings

!pecic electric loading:

It is de"ned as the total num#er ofamere conductors er unit of the

circumference of the armature5 iTemerature rise,H

H deends on ,losses)(hich in turndeends on ac5

Allo(a#le H deends on insulatingmaterial used5

i Cooling coecient ,C

H is also roortional to the coolingcoecientJ

a machine (ith a #etter +entilationhas a lo(er C and then higher ac can#e used5

iiiOerating +oltage ,0

In high +oltage machines) the slotsace factor) Sf is less and so onl%

smaller ac can #e used5 It also deendson the shae of the conductors)circular or rectangular in cross section5

i+ Current Densit% ,K

choice of K deends on coolingJ

Seci"c Electric loading

ac : Total armature amereconductors2 armature

eriher% at air ga

: I4 > ,am5conductor2m

F D

9here) ac- Seci"c electrical loading

Ia- armature current

>- Num#er of armatureconductors D- Diameter of thearmature


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Output equation

The output of a machine can ee!pressed in terms of its maindimensions" speci#c magnetic andelectric loadings and speed$

%a&Co'2n Let)

'a- 'o(er de+eloed #% thearmature in 9

' . Outut o(er or rating of themachines in 9

E - Generated Emf in +olts

0- Terminal +oltage in +olts

' . No of oles

? . $lu7 er ole in 9#

Ia- Armature current in Am

I4 . Current in each conductor inam

A . No5 of arallel ath

>- Total no of armatureconductor

n- Seed in rm

N . Seed in rs

D . Armature diameter in meter

L - Armature length in meter 'ole itch

DC Machines output equation

Total Electric loading : J

Total Magnetic loading : I4 >

Seci"c Electric loading : ac : I4 >

,am5conductor2m F D

Seci"c magnetic loading : /a+e : ,9#2m*

F DL

Outut e!uation

'a : E Ia7 -: ,2A,>N2 I4 A -

: PF* /a+e ac 7- QD* L n : Co D*

L n

(here Co: outut coecient


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Choice of speci#cloading

Choice of speci#c

magnetic loading

(a*)'epends on"

1.Flux density in teeth

2.Frequency of ux

reversal

5Si4e of machine

,i $lu7 densit% Large +alues of/a+ in teeth increases "eldmmf5 Righer mmf results inincrease of iron loss) cu loss &

cost of cu5 /a+ does not e7ceed*5*(#2m*

,ii $re!uenc% of 6u7 re+ersal If f ishigh then iron losses in arm5Core & teeth (ould #e high5 So

high +alue of /a+ is not used5

,iii Si4e of machine If si4eincreases /a+ also increases5As the dia increases the (idth

of the tooth also increases)ermitting increased +alue of/g (ithout saturation5


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Choice of seci"c electricloading ,ac

Deends on)

5 Temerature rise

*5 Seed of machine5 0oltage

35 Si4e of machine

5 Armature reaction5 Commutation

,i Temerature rise Righer acresults in high tem rise of(dgs5 Tem rise deends ont%e of enclosure & coolingtechni!ues emlo%ed5 E75 In

m2cs5 sing class $ insulation(hich can (ithstand a tem5 of


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35 Choice of seci"celectric loading

Deends on)

5 Temerature rise

*5 Seed of machine

5 0oltage

35 Si4e of machine

5 Armature reaction5 Commutation

,i+ Si4e of m2c In large si4e m2cs it iseasier to "nd sace for accommodatingconductors5 Rence ac can #e increased(ith increase in dimensions5

,+ Armature reaction 9ith high ac) arm5

Veaction (ill #e se+ere5 To counter thisthe "eld mmf is increased & so costgoes high5

,+i Commutation Ac:I41> D1W Righ+alue of ac (ill ha+e either ,i Large >OV ,ii small dia D

,i M2c ha+e large > ha+ing large no5 ofturnsJ L is roortional to s!uare of no5of turnsJ so large ac

,ii If small dia) it is not ossi#le to use(ide slots #ecause other(ise the saceleft for teeth (ill #ecome smaller gi+ingrise to high / in them5 Onl% (a% is us

ting deeer slots to use5 /ut the deeerslots increases the L +alue5

Increased L increases reactance+oltage (hich dela%s the commutation5Righ ac (orsens the commutationcondition in m2cs5 $rom commutation

oint of +ie( small ac is desira#le5 Aclies #et5 ) to )am5cond2m


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Magnetic loading

The total 6u7 in the air ga of a machine is calledmagnetic loading5It is measured in (e#ers5This

loading sets the limit on (or8ing 6u7 (ithoutsaturation5

Magnetic loading:,num#er of oles1,6u7 er ole

Electric loadingThe total num#ers of amere conductors around the

eriher% is called electric loading5It is measured inamere5This loading sets the limit on (or8ing

current (ithout e7ceeding the temerature rise5Electric loading:,num#er of conductors1,current

er conductor

design of dc motor

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