dc machine

7/21/2019 DC Machine

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D.C. MACHINES .Working principle of D.C.Machine as generator and motor, constructional

features, EMF equation of generator and simple problems, back emf and torque

equation of DC motors, simple problems, types of DC motors, characteristics and

applications, necessity of starter, 3point starter.

______________________________________________________________________ A machine which works on direct current is defined as a D.C.Machine.

D.C.Machines are of two types. !i" D.C.#enerator and !ii" D.C.Motor.

Sl.No. D.C. #enerator D.C.Motor

Definition$

A !enerator is a rotatin!

machine which con"ertsmechanical ener!y into

electrical ener!y

Definition$

A motor is a machine which

con"erts electrical ener!y intomechanical ener!y

# %rinciple$$hene"er a coil is rotated

in a ma!netic field ane.m.f. will %e induced in

this coil

and is !i"en %y

e&'l"Sin( "olts)coil sidewhere* '&+he flu, density

in +esla* l&the acti"e len!th

of the coil side in meters*"&the "elocity with which

the coil is mo"ed inmeters)sec and ( is thean!le %etween the direction

of the flu, and the

direction of rotation of the

coil side.

%rinciple$$hene"er a current coil is

placed under a ma!netic fieldthe coil e,periences a

mechanical force* and is !i"en

%y -& 'IlSin( Newtons)coil

side.$here* I is the current throu!h

the coil in ampere.

+he direction of the emf

induced is fi,ed %yapplyin! the -lemin!/s

ri!ht hand rule

+he direction of the force

actin! is fi,ed %y applyin! the-lemin!/s left hand rule.

C&'()*+C)&' &F - D.C.M-C'E.

Salient parts of a D.C.machine are0

• -ield system 1poles2

• Coil arran!ement 1armature2

• Commutator

• 'rushes



• 3oke

-i! shows the details of a four pole D.C. machine with %oth shunt and series fieldwindin!s.

Field system0 +his is made of electroma!nets* wherein a iron laminated core is

wound with well insulated enameled copper wire. +he core is laminated tominimi4e the eddy current loss. Each lamination is dipped in "arnish and dried. A

pole shoe is attached to the pole face to direct the flu, to concentrate radially on to

the armature there%y reducin! the leaka!e and frin!in! flu,. 5oles are fi,ed to theyoke %y means of %olts.

-rmature0 +his is the rotatin! part of the machine made of laminated iron core

cylindrical in structure with slots on its periphery. Insulated copper coils are laid in

these slots* and these coils are connected for lap or wa"e connection. +he corelaminations are firmly mounted on a shaft fitted with smooth %earin!s on either side

for smooth rotation.

Comparison of lap and /a0e /indings01-% W-2E

Num%er of armature parallel paths

is e6ual to the num%er of poles.

Num%er of parallel paths is always

e6ual to two.

5referred when lar!e current at

lesser "olta!e is the re6uirement.

5referred when lar!e "olta!e with

lesser current is the re6uirement.

Commutator$ As the induced e.m.f. in the armature is alternatin! commutator

con"erts alternatin! e.m.f. into unidirectional e.m.f.+his is cylindrical in structure made of copper se!ments with mica insulation

%etween them and is firmly fi,ed on to the shaft carryin! the armature and the

armature coil free ends are %ra4ed to the commutator se!ments.rushes$ +hese are current collectin! de"ices placed on the %ody of the

commutator with a holder. 'rushes are made of car%on* copper or !raphite.

4oke$ +his is the outer most part of the machine made of cast steel which is the

mechanical enclosure for the machine to protect it from dust and moisture and also pro"ides the return path for the ma!netic flu, and carries half the flu, per pole.

E.M.F. Equation$

7et the D.C. machine has % num%er of poles* 5 num%er of armature conductors

arran!ed in - num%er of parallel paths. 7et 6 %e the flu, per pole and ' is the

speed of rotation in re"olutions per minute.

Consider one North 5ole of the machine under which a !roup of armatureconductors all %ein! connected in series. 7et 7 %e the spacin! %etween any two

nei!h%orin! conductors ant t %e the time taken to mo"e throu!h this distance of 7.

+he total flu, per pole 6 is made of se"eral lines and one line of flu, is cut %y oneconductor when it mo"es throu!h a distance of 7 in t seconds.

+herefore the induced emf in the st conductor when cut %y the flu, of 68

ise & 689t "olts



Similarly in the #nd conductor e# & 6:9t "olts* and so on.

+herefore the total emf induced in all the conductors under one pole is the sum ofall these emf/s.

E& e 8 e# 8 e 8 e9 8 ::..E& 689t ; 6:9t ; 639t ; 6<9t ; ==..

E& 69t "olts)pole. -or all the 5 num%er of poles E& %69t "olts+he speed is defined as ' re"olutions per minute*

' re"olutions in one minute or >? seconds.

re"olutions will %e in time of >?9' seconds* and as one re"olution corresponds to

all the 5 num%er of conductors the time t for a tra"el of distance 7 can %e written as

t@ >?9'5 seconds.

+herefore the induced EM- E@ %69t @ %69>?9!'5"@%5'69>?.

As the ; num%er of conductors are arran!ed in A num%er of parallel paths*

+he induced e.m.f per parallel path is

E@ %5'69>?- "olts.

As %, 5, - are fi,ed the induced e.m.f is mainly dependent on the flu, and the

speed* and hence we write that the induced e.m.f E is proportional to the product of

the speed ' and the flu, 6.

dc machine

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