1 d.c presentation

8/3/2019 1 D.C Presentation

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D.C Machines Constructionby

Deepti


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Introduction In this lecture weconsider various formsof

rotating electrical machines

Thesecan bedividedinto:

generators whichconvertmechanical energyinto

electrical energy

motors whichconvertelectrical energyinto

mechanical energy

Bothtypesoperatethroughtheinteraction

between a magnetic fieldand a setofwindings


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A SimpleAC Generator

We notedearlierthat Faradays law dictates

thatif a coil ofN turnsexperiences a changein

magnetic flux,then theinduced voltageV is

given byt

NV

d

d!

U[

Ucos

d

sind

d

dNBA

tNBA

t

NV !!!


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Basicsof a Electric Motor


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DC generator four-pole field.


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DC Machine Construction


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A four-pole DC generator


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Armatureof a DC Motor


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Armature


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ARMATUR

E WINDINGS


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Magnetic Induction andtheDC Generator

Faradays Law e = N d /dt

e = theinduced voltagein volts (V)

N = the numberofseries-connectedturnsof wirein turns

(t) d/dt = rateofchangein fluxin Webers/second (Wb/s)

e = B L v

B = the fluxdensityin teslas (T)

L = the lengthoftheconductorthatisin themagnetic fieldin meters (m)

v = therelative velocitybetween the wire andthe flux,in

meters/second (m/s)


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Magneticinduction in a wiremoving in a

field.


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Right-handrule formagneticinduction.


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Wire looprotating in a magnetic

field.


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Thus forthe arrangementshown below

U[U

cosd

sind

d

dNBA

tNBA

t

NV !!!


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Thereforethis arrangementproduces a

sinusoidal output asshown below


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Wiresconnectedtotherotating coil

would gettwisted

Therefore weusecircularslip rings

withsliding

contactscalled

brushes


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When t = 0o (A),thecoil isparallel tothe flux, andsodoes notcutthemagnetic flux lines and nocurrentisthereforeinducedin thecoil.

When t = 45o (B),therate at which flux lines arecutbythecoil ismaximal,butsincethe area ofthecoilpresentedtothe fluxis notyet atitsmaximum value,thecurrentin thecoil isstill rising.

When t = 90o (C),thecoil isperpendiculartothe flux, andtheinducedcurrentis at a maximum.

When t = 135o (D),therate at which flux lines arecutbythecoil decreasesmaximally,butsincethe area ofthecoil presentedtothe fluxis notyet atitsminimum value,thecurrentin thecoil isstill decreasing.

When t = 180

o

(E),thecoil is again parallel tothe flux, andsodoes notcutthemagnetic flux lines and nocurrentisthereforeinducedin thecoil.

Forthesecondhalfofthecycle,theinducedcurrent at variousstages will bein theoppositedirection tothatwhich was foundin the firsthalfofthecycle.


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AC generator withsliprings andbrushes.


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DC generator withcommutator andbrushes.


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Use of a commutator


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A simple generator with two coils


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Action of a Commutator


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DC generatoroutput waveform.


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DC generator with fieldcontrol.


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DC generatorrotor withtwocoils.


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Coil andoutput waveforms for a two-winding

rotor.


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15-4 MotorAction andtheDC

Motor F = B L I F = theresulting mechanical forcein newtons (N)

B = the fluxdensityin teslas (T)

L = theeffective lengthofthe wire (meters) in the fieldmultipliedbythe numberofturns

I = thecurrentin theconductorin amperes (A)

Ia(start) = (Vt Vb) /Ra Ia(start) = the armaturestarting currentin amperes (A)

Vt = the applied voltagein volts (V)

Vb = thebrushdropin volts (V)

Ra = the armatureresistancein ohms () Ia = (Vt Vb Vcemf) /Ra

Vcemf= theinducedcounteremfin the armature windingsin volts (V).


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Forceon a current-carrying wirein a magnetic

field.


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Fluxcompression andresulting

force.


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Simpledcmotor.


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DCmotor withelectromagnetic

field.


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ATwo PoleDC Motor


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A Four PoleDC Motor

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