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SEE 3433 ELECTRICAL MACHINES

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

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SEE 3433 ELECTRICAL MACHINES

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  1. SEE 3433 ELECTRICAL MACHINES Classification of DC machines DC Generators - Separately excited - Armature reaction

  2. 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 

  3. CLASSIFICATION OF DC MACHINES Shunt dc machine + Ea 

  4. CLASSIFICATION OF DC MACHINES Series dc machine + Ea 

  5. CLASSIFICATION OF DC MACHINES Compound dc machine - short shunt + Ea 

  6. CLASSIFICATION OF DC MACHINES Compound dc machine – long shunt + Ea 

  7. DC GENERATORS Mechanical output Electrical input ELECTRICAL MACHINES - Motor - Electrical output Mechanical input ELECTRICAL MACHINES - Generator -

  8. Electrical output Mechanical input ELECTRICAL MACHINES - Generator - DC GENERATORS Power low from mechanical to electrical Rotor is rotated by a prime mover at constant speed Armature terminal is connected to a load

  9. DC GENERATORS Separately excited DC generator + Ea 

  10. Ra La Lf Rfc + vf  + Ea  + Vt  RL Rfw DC GENERATORS Separately excited DC generator Ra – Armature winding resistance Lf - Field winding inductance La - Armature winding inductance Rfc – External field resistance LL – External Load Rfw – Field winding resistance

  11. Ra La Lf Rfc + vf  + Ea  + Vt  RL Rfw DC GENERATORS Separately excited DC generator Under steady state condition Lf and La can be ‘removed’

  12. DC GENERATORS Separately excited DC generator Ra Rfc + vf  + Ea  + Vt  RL Rfw Under steady state condition Lf and La can be ‘removed’ from the circuit

  13. DC GENERATORS Separately excited DC generator Ra Ia If It Rfc + vf  + Ea  + Vt  RL Rfw Vf = (Rfc + Rfw)If Ea = IaRa + Vt Ea = Ka  m Vt = Ia RL , also It = Ia

  14. Vt Ea IaRa It DC GENERATORS Separately excited DC generator Terminal characteristic Load characteristic Operating point Ea = IaRa + Vt Vt = Ia RL

  15. Airgap flux density 0  2  DC GENERATORS Armature Reaction Flux due to field winding alone

  16. 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

  17. DC GENERATORS Armature Reaction Flux at one side of the pole may saturate Zero flux region shifted Flux saturation, effective flux per pole decreases

  18. Ea Ifield DC GENERATORS Armature Reaction Eao= Vtowithout load current With Ia, AR causes reduction in Ea since flux per pole decreases Ea = Vt + IaRa

  19. As Ia increases, Ea reduces due to AR DC GENERATORS Armature Reaction Vt = Ea - IaRa

  20. 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)

  21. 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

  22. DC GENERATORS Compensating winding Expensive and normally installed in large machines

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