Transcript Slide 1
SEE 3433 ELECTRICAL MACHINES Classification of DC machines DC Generators - Separately excited - Armature reaction CLASSIFICATION OF DC MACHINES Several possible connections for field and armature circuits - classification of DC machines is determined by the way they are connected Separately excited dc machine + Ea CLASSIFICATION OF DC MACHINES Shunt dc machine + Ea 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 Mechanical output Electrical input ELECTRICAL MACHINES - Motor - Electrical output Mechanical input ELECTRICAL MACHINES - Generator - DC GENERATORS Power low from mechanical to electrical is rotated by a prime mover at constant speed ElectricalRotor output Armature terminal is connected to a load Mechanical input ELECTRICAL MACHINES - Generator - DC GENERATORS Separately excited DC generator + Ea DC GENERATORS Separately excited DC generator Ra + vf Lf Rfc Rfw La + + Ea Vt Lf - Field winding inductance Ra – Armature winding resistance Rfc – External field resistance La - Armature winding inductance LL – External Load Rfw – Field winding resistance RL DC GENERATORS Separately excited DC generator Ra + vf Lf Rfc Rfw La + + Ea Vt Under steady state condition Lf and La can be ‘removed’ RL DC GENERATORS Separately excited DC generator Ra + vf Rfc Rfw + + Ea Vt RL Under steady state condition Lf and La can be ‘removed’ from the circuit DC GENERATORS Separately excited DC generator Ra If + Ia It Rfc Rfw vf Vf = (Rfc + Rfw)If + + Ea Vt RL Ea = IaRa + Vt Ea = Ka m Vt = Ia RL , also It = Ia DC GENERATORS Separately excited DC generator Terminal characteristic Load characteristic Vt Ea IaRa Operating point Ea = IaRa + Vt Vt = Ia RL It DC GENERATORS Armature Reaction Flux due to field winding alone Airgap flux density 0 2 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 Eao= Vto without load current With Ia, AR causes reduction in Ea since flux per pole decreases Ea = Vt + IaRa Ifield DC GENERATORS Armature Reaction Vt = Ea - IaRa As Ia increases, Ea reduces due to AR DC GENERATORS Armature Reaction Vt = Ea - IaRa Ea = Vt + IaRa 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) 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