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Transcript of DC Machines Presentation
7/23/2019 DC Machines Presentation
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DC MACHINES
Engr. Jonathan C. PacaldoREE
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References:
1.Electrical Machines, By: Siskind2.Electricity 4, By: Jef el!ik".#C C$rrent Machinery, By:loefer%erchner%Brenne&an
4.Electric Machines and 'rans(or&er, By)nderson%Macneill
Grading System:
*$i++es 2-a/oratory%Co$rse0ork 2-Class Particiation 1-
Midter& Ea& 23-
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Introduction
)ltho$gh a (ar greater ercentage o( the electrical&achines in ser5ice are a.c. &achines, the d.c.&achines are o( considera/le ind$strial i&ortance.
'he principal advantage o( the d.c. &achine,
artic$larly the d.c. &otor, is that it provides ane control of speed. S$ch an ad5antage is notclai&ed /y any a.c. &otor. 6o0e5er, d.c. generatorsare not as co&&on as they $sed to /e, /eca$sedirect c$rrent, 0hen re7$ired, is &ainly o/tained
(ro& an a.c. s$ly /y the $se o( recti8ers.9e5ertheless, an $nderstanding o( d.c. generator isi&ortant /eca$se it reresents a logicalintrod$ction to the /eha5io$r o( d.c. &otors. ndeed
&any d.c. &otors in ind$stry act$ally oerate as d.c.generators (or a /rie( eriod. n this chater, 0e
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•Most o( the electrical &achine in ser5ice are
)C tye.•#C &achine are o( considera/le ind$striali&ortance.•#C &achine &ainly $sed as #C &otors and
the #C generators are rarely $sed.•#C &otors ro5ides a 8ne control o( the seed0hich
can not /e attained /y )C &otors.•#C &otors can de5eloed rated tor7$e at allseeds
(ro& standstill to rated seed.
•#e5eloed tor7$e at standstill is &any ti&esreater
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DC Mac!ines Applications
• 'he d.c. &achine can oerate as either a&otor or a generator, at resent its $se asa generator is li&ited /eca$se o( the
0idesread $se o( ac o0er.•arge d.c. &otors are $sed in &achine tools,rinting resses, (ans, $&s, cranes, aer&ill, traction, tetile &ills and so (orth.
•S&all d.c. &achines ;(ractional horseo0errating< are $sed ri&arily as controlde5ice=s$ch as tachogenerators (or seedsensing and ser5o&otors (or osition andtracking.
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Advantages and Disadvantages ofDC Motors
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Magnetism
Magnetism is a class o( hysicalheno&enon that incl$des (orces eerted/y &agnets on other &agnets. t has itsorigin in electric c$rrents and the($nda&ental &agnetic &o&ents o(ele&entary articles. 'hese gi5e rise toa &agnetic 8eld that acts on other c$rrents
and &o&ents. )ll &aterials are in?$enced toso&e etent /y a &agnetic 8eld. 'hestrongest efect is on er&anent &agnets,0hich ha5e ersistent &agnetic &o&ents
ca$sed /y (erro&agnetis&.
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Magnetic "ield
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Magnetic"ield
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Ohm's law states that the current through a conductor
between two points is directly proportional to the potential
difference across the two points. Introducing the constant
of proportionality, the resistance, one arrives at the usual
mathematical equation that describes this relationship:
where I is the current through the conductor in units
of amperes, V is the potential difference
measuredacross
the conductor in units of volts, andR
isthe resistance of the conductor in units of ohms. More
specifically, Ohm's law states that the R in this relation is
constant, independent of the current.
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Series and Parallel Circuits
With simple series circuits, all components are connected
end-to-end to form only one path for electrons to flowthrough the circuit:
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Series Circuits:
•Voltage drops add to equal total voltage•!ll components share the same "equal# current•$esistances add to equal total resistance
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With simple parallel circuits, all components areconnected %etween the same two sets of electricallycommon points, creating multiple paths for electrons
to flow from one end of the %attery to the other:
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Parallel Circuits:
•!ll components share the same "equal# voltage
•&ranch currents add to equal total current•$esistances diminish to equal total resistance
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Generator #rinciple:
)n electric generator is a &achine that con5erts&echanical energy into electrical energy. )nelectric generator is /ased on the rincile that0hene5er ?$ is c$t /y a cond$ctor, an e.&.(. is
ind$ced 0hich 0ill ca$se a c$rrent to ?o0 i( thecond$ctor circ$it is closed. 'he direction o(ind$ced e.&.(. ;and hence c$rrent< is gi5en /yle&ing@s right hand r$le. 'here(ore, theessential co&onents o( a generator are:
;a< a &agnetic 8eld;/< cond$ctor or a gro$ o( cond$ctors;c< &otion o( cond$ctor 0.r.t. &agnetic 8eld.
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Magnetic"ield
Magnetic #oles
Rotation
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RIGH$ HAND R%&E
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Generated'oltage
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9
Magnetic"lu(
Conductor
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Construction of a DC Generator
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"IE&D S)S$EM
'he ($nction o( the 8eld syste& is torod$ce $ni(or& &agnetic 8eld 0ithin0hich the ar&at$re rotates. t consists o( an$&/er o( salient oles ;o( co$rse, e5en
n$&/er< /olted to the inside o( circ$lar(ra&e ;generally called yoke<. 'he yoke is$s$ally &ade o( solid cast steel 0hereasthe ole ieces are co&osed o( stackedla&inations. ield coils are &o$nted onthe oles and carry the d.c. ecitingc$rrent. 'he 8eld coils are connected in
s$ch a 0ay that ad!acent oles ha5e
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"rame
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Armature Core *Rotor+
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Armature Core
'he ar&at$re core is keyed to the &achine
sha(t and rotates /et0een the 8eld oles. tconsists o( slotted so(t=iron la&inations;a/o$t .4 to .A && thick< that are stackedto (or& a cylindrical core. 'he la&inations areindi5id$ally coated 0ith a thin ins$lating 8l&so that they do not co&e in electrical contact0ith each other. 'he $rose o( la&inating
the core is to red$ce the eddy c$rrent loss. 'he la&inations are slotted to acco&&odateand ro5ide &echanical sec$rity to thear&at$re 0inding and to gi5e shorter air ga
(or the ?$ to cross /et0een the ole (ace
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Armature ,indings
'he slots o( the ar&at$re core hold ins$lated
cond$ctors that are connected in a s$ita/le&anner. 'his is kno0n as ar&at$re 0inding.
'his is the 0inding in 0hich 0orking e.&.(.is ind$ced. 'he ar&at$re cond$ctors are
connected in series=arallelD the cond$ctors/eing connected in series so as to increasethe 5oltage and in arallel aths so as to
increase the c$rrent. 'he ar&at$re 0indingo( a d.c. &achine is a closed=circ$it 0indingDthe cond$ctors /eing connected in asy&&etrical &anner (or&ing a closed loo or
series o( closed loos.
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Commutator
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Commutator
) co&&$tator is a &echanical recti8er0hich con5erts the alternating 5oltagegenerated in the ar&at$re 0inding intodirect 5oltage across the /r$shes. 'he
co&&$tator is &ade o( coerseg&ents ins$lated (ro& each other /y&ica sheets and &o$nted on the sha(t
o( the &achine. 'he ar&at$recond$ctors are soldered to theco&&$tator seg&ents in a s$ita/le&anner to gi5e rise to the ar&at$re
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-rus!es
'he $rose o( /r$shes is to ens$reelectrical connections /et0een the rotatingco&&$tator and stationary eternal load
circ$it. 'he /r$shes are &ade o( car/on andrest on the co&&$tator. 'he /r$sh ress$reis ad!$sted /y &eans o( ad!$sta/le srings.( the /r$sh ress$re is 5ery large, the
(riction rod$ces heating o( the co&&$tatorand the /r$shes. n the other hand, i( it istoo 0eak, the i&er(ect contact 0ith theco&&$tator &ay rod$ce sarking.
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C t ti i D C G t
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Commutation in a D.C Generator
ommutation is the positioning of the !generator brushes so that the commutator
segments change brushes at the same time
the armature current changes direction. Moresimply stated, commutation is the mechanical
conversion from " to ! at the brushes of a
! machine.
Commutator Action
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Commutator Action
#he commutator converts the " voltage generated
in the rotating loop into a ! voltage. It also serves
as a means of connecting the brushes to the rotating
loop. #he purpose of the brushes is to connect thegenerated voltage to an e$ternal circuit. In order to
do this, each brush must ma%e contact with one of
the ends of the loop. &ince the loop or armature
rotates, a direct connection is impractical. Instead,the brushes are connected to the ends of the loop
through the commutator.