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
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