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SEE 3433ELECTRICAL MACHINESClassification of DC machines
DC Generators - Separately excited - Armature reaction
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CLASSIFICATION OF DC MACHINES+
Ea
Several possible connections for field and armature circuits -
classification of DC machines is determined by the way they are
connectedSeparately excited dc machine
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CLASSIFICATION OF DC MACHINES+
Ea
Shunt dc machine
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CLASSIFICATION OF DC MACHINESSeries dc machine+
Ea
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CLASSIFICATION OF DC MACHINESCompound dc machine - short
shunt+
Ea
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CLASSIFICATION OF DC MACHINESCompound dc machine long shunt+
Ea
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DC GENERATORSELECTRICALMACHINES- Motor -ELECTRICALMACHINES-
Generator -Electrical inputMechanical outputMechanical
inputElectrical output
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DC GENERATORSPower low from mechanical to electricalRotor is
rotated by a prime mover at constant speedArmature terminal is
connected to a load
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DC GENERATORSSeparately excited DC generator+
Ea
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DC GENERATORSSeparately excited DC generatorLf - Field winding
inductanceRfc External field resistanceRfw Field winding
resistanceRa Armature winding resistanceLa - Armature winding
inductanceLL External Load
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DC GENERATORSSeparately excited DC generatorUnder steady state
condition Lf and La can be removed
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DC GENERATORSSeparately excited DC generatorRa+
Ea
RfcRfwRL+
vf
+
Vt
Under steady state condition Lf and La can be removed from the
circuit
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DC GENERATORSSeparately excited DC generatorRa+
Ea
RfcRfwRL+
vf
+
Vt
Vf = (Rfc + Rfw)IfEa = IaRa + VtEa = Ka mVt = Ia RL , also It =
IaIaIfIt
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DC GENERATORSSeparately excited DC generatorEa = IaRa + VtVt =
Ia RLTerminal characteristicLoad characteristicOperating point
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DC GENERATORSArmature ReactionFlux due to field winding
alone
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DC GENERATORSArmature ReactionWhen armature current flows (i.e.
terminal is connected to the load), armature produces MMFMMF
produces results in flux which will disturb field flux
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DC GENERATORSArmature ReactionFlux at one side of the pole may
saturate Zero flux region shifted Flux saturation, effective flux
per pole decreases
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DC GENERATORSArmature ReactionEao= Vto without load currentWith
Ia, AR causes reduction in Ea since flux per pole decreasesEa = Vt
+ IaRa
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DC GENERATORSArmature ReactionVt = Ea - IaRa
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DC GENERATORSArmature ReactionSince AR causes a decrease in Ea ,
the effect of AR can be considered as a reduction in field
currentIf(eff) = If(actual) If(AR)Vt = Ea - IaRaEa = Vt + IaRa
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DC GENERATORSArmature ReactionThe mmf produced by rotor can be
neutralized using compensating windingArmature current flows in
compensating winding will cancel out mmf produced by armature
winding
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DC GENERATORSCompensating windingExpensive and normally
installed in large machines
