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

2. CLASSIFICATION OF DC MACHINES + E a  Several possible connections for field and armature circuits - classification of DC machines is determined by the way they are connected Separately excited dc machine

3. CLASSIFICATION OF DC MACHINES + E a  Shunt dc machine

4. CLASSIFICATION OF DC MACHINES Series dc machine + E a 

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

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

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

8. 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 ELECTRICAL MACHINES - Generator - Mechanical input Electrical output

9. DC GENERATORS Separately excited DC generator + E a 

10. DC GENERATORS Separately excited DC generator L f - Field winding inductance R fc – External field resistance R fw – Field winding resistance R a – Armature winding resistance L a - Armature winding inductance L L – External Load R a L a + E a  L f R fc R fw R L + v f  + V t 

11. DC GENERATORS Separately excited DC generator Under steady state condition L f and L a can be ‘removed’ R a L a + E a  L f R fc R fw R L + v f  + V t 

12. DC GENERATORS Separately excited DC generator R a + E a  R fc R fw R L + v f  + V t  Under steady state condition L f and L a can be ‘removed’ from the circuit

13. DC GENERATORS Separately excited DC generator R a + E a  R fc R fw R L + v f  + V t  V f = (R fc + R fw )I f E a = I a R a + V t E a = K a   m V t = I a R L , also I t = I a I a I f I t

14. DC GENERATORS Separately excited DC generator E a = I a R a + V t V t = I a R L Terminal characteristic Load characteristic Operating point V t I t E a I a R a

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

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. DC GENERATORS Armature Reaction E ao = V to without load current With I a , AR causes reduction in E a since flux per pole decreases E a = V t + I a R a E a I field

19. DC GENERATORS Armature Reaction V t = E a - I a R a As I a increases, E a reduces due to AR

20. DC GENERATORS Armature Reaction Since AR causes a decrease in E a , the effect of AR can be considered as a reduction in field current I f(eff) = I f(actual) – I f(AR) V t = E a - I a R a E a = V t + I a R a

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