Suvendu
22
SEE 3433 ELECTRICAL MACHINES Classification of DC machines DC Generators - Separately excited - Armature reaction
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Transcript of Suvendu
SEE 3433ELECTRICAL MACHINES
Classification of DC machines
DC Generators - Separately excited - Armature reaction
CLASSIFICATION OF DC MACHINES
+
Ea
Several possible connections for field and armature circuits
- classification of DC machines is determined by the way they are connected
Separately excited dc machine
CLASSIFICATION OF DC MACHINES
+
Ea
Shunt dc machine
CLASSIFICATION OF DC MACHINES
Series dc machine
+
Ea
CLASSIFICATION OF DC MACHINES
Compound dc machine - short shunt
+
Ea
CLASSIFICATION OF DC MACHINES
Compound dc machine – long shunt
+
Ea
DC GENERATORS
ELECTRICALMACHINES- Motor -
ELECTRICALMACHINES
- Generator -
Electrical input Mechanical output
Mechanical inputElectrical output
DC GENERATORS
ELECTRICALMACHINES
- Generator -
Mechanical inputElectrical output
Power low from mechanical to electricalRotor is rotated by a prime mover at constant speedArmature terminal is connected to a load
DC GENERATORS
Separately excited DC generator
+
Ea
DC GENERATORS
Separately excited DC generatorRa La
+
Ea
Lf Rfc
Rfw
RL
+
vf
+
Vt
Lf - Field winding inductance
Rfc – External field resistance
Rfw – Field winding resistance
Ra – Armature winding resistance
La - Armature winding inductance
LL – External Load
DC GENERATORS
Separately excited DC generatorRa La
+
Ea
Lf Rfc
Rfw
RL
+
vf
+
Vt
Under steady state condition Lf and La can be ‘removed’
DC GENERATORS
Separately excited DC generatorRa
+
Ea
Rfc
Rfw
RL
+
vf
+
Vt
Under steady state condition Lf and La can be ‘removed’ from the circuit
DC GENERATORS
Separately excited DC generatorRa
+
Ea
Rfc
Rfw
RL
+
vf
+
Vt
Vf = (Rfc + Rfw)If Ea = IaRa + Vt
Ea = Ka m
Vt = Ia RL , also It = Ia
IaIfIt
DC GENERATORS
Separately excited DC generator
Ea = IaRa + Vt
Vt = Ia RL
Terminal characteristic
Load characteristic
Vt
It
Operating point
Ea
IaRa
DC GENERATORS
Armature Reaction
Flux due to field winding alone
0 2
Airgap flux density
DC GENERATORS
Armature Reaction
When armature current flows (i.e. terminal is connected to the load), armature produces MMF
MMF produces results in flux which will ‘disturb’ field flux
DC GENERATORS
Armature Reaction
Flux at one side of the pole may saturate
Zero flux region shifted
Flux saturation, effective flux per pole decreases
DC GENERATORS
Armature Reaction
Ea
Ifield
Eao= Vto without load current
With Ia, AR causes reduction in Ea since flux per pole decreasesEa = Vt + IaRa
DC GENERATORS
Armature Reaction
Vt = Ea - IaRa
As Ia increases, Ea reduces due to AR
DC GENERATORS
Armature Reaction
Since AR causes a decrease in Ea , the effect of AR can be considered as a reduction in field current
If(eff) = If(actual) – If(AR)
Vt = Ea - IaRa
Ea = Vt + IaRa
DC GENERATORS
Armature Reaction
The mmf produced by rotor can be neutralized using compensating winding
Armature current flows in compensating winding will cancel out mmf produced by armature winding
DC GENERATORS
Compensating winding
Expensive and normally installed in large machines
