electrical machines

electrical machines

1. Electrical Ma c hi nes By S. Mah esh Babu

2. General Definition An apparatus using mechanical power and having several parts, each with a definite function and together performing a particular task.

3. Basics of Ma c hi nes Magnetic Flux(ᴓ) Direction of current Ampere’s right thumb rule Cark’s screw rule Flemings left hand rule Faradays laws Lentz law

4. Classification of Electrical Ma c hi nes Classification based on supply Alternating current machines Direct current machines

5. DC Ma c hi nes

6. Construction of DC Ma c hi ne

7. TYPES OF WINDINGS

8. LAP winding No. of parallel paths = No. of poles = No. of brushes No need of dummy coils High current low voltage No. of commutator segments =No. of armature slots

9. Wave winding No. of parallel paths = 2 = No. of brushes Dummy coils are required (Mech balance) High voltage low current No. of commutator segments = No. of armature segments

10. Ma te r ials for const r uction Core --- High permeability silicon steel Commutator segments---Hard drawn copper Body --- Mild steel (or) cast iron Windings--- Copper (or) Aluminum Brushes--- Carbon , copper.

11. Classification of DC Ma c hi nes

12. Losses in DC Ma c hi nes

13. Separately excited DC Ma c hi ne

14. DC Shunt Ma c hi ne

15. DC Series Ma c hi ne

16. DC compound Ma c hi ne

17. Speed cont r ol of DC m oto r speed control of DC motor is classified as Armature control methods Field control methods.

18. Testing of DC M oto r s Efficiency By Direct Loading. Swinburne's Method ( Indirect Method ) Regenerative or Hopkinson's-Test ( Regenerative Method )

19. Efficiency By Di r ect Loading

20. Swinbu r ne's M ethod

21. Hopkinson's-Test

22. Classification of AC Ma c hi nes Rotating Stationary Synchronous Asynchronous Induction motors Transformers

23. Rotating AC Ma c hi nes

24. Induction m oto r s An induction motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.

25. Concept of R M F A rotating magnetic field is a magnetic field that has moving polarities in which its opposite poles rotate about a central point or axis. Ideally the rotation changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor.

26. Concept of R M F Sine wave current in each of the three stationary coils produces three sine varying magnetic fields perpendicular to the rotation axis. The three magnetic fields add as vectors to produce a single rotating magnetic field.

27. Classification 3- Ph Squirrel cage Double cage rotor Slip ring 1-Ph (Special motors) Ac series motor Universal motor Synchronous motor Stepper motor

28. Construction Stator(Mild Steel) Rotor(Copper bars) Core (High permeability Silicon steel ) Windings (Copper, Alluminium)

29. Squi rr el Cage I M A squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It consists of a cylinder of steel with aluminum or copper conductors embedded in its surface. An electric motor with a squirrel-cage rotor is termed a squirrel-cage motor. It consist of a stationary and a rotating parts. The rotating rotor is known as the squirrel cage rotor as it resembles a squirrel cage

30. Squirrel Cage I M

31. Squi rr el Cage I M

32. Slip R ing I M A wound-rotor motor is a type of induction motor where the rotor windings are connected through slip rings to external resistances. ... Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor.

33. …Slip Ring I M

34. Synchronous Speed The speed of Rotating M agnetic Field (R M F) is Known as synchronous speed. It is Given by N S = 120 f/P Where 120= phase shift between each phase f= Frequency P= No. of poles

35. Sl i p The relative motion between the rotating field and the rotating rotor is called the slip and is given by: S = N s - N r          N s Where S is the slip, N s is the synchronous speed in RPM, and N r is the rotor speed.

36. To r que Eqn Twisting moment of force is known as To r que

37. losses

38. Speed control The Speed of Induction Motor is changed from Both Stator and Rotor Side The speed control of three phase induction motor from stator side are further classified as : V / f control or frequency control. Changing the number of stator poles. Controlling supply voltage. Adding rheostat in the stator circuit.

39. …Speed control The speed controls of three phase induction motor from rotor side are further classified as: Adding external resistance on rotor side. Cascade control method. Injecting slip frequency emf into rotor side.

40. Advantages Squirrel Cage I M Cheaper in cost Less maintenance and rugged construction Explosion proof Better cooling high over load capacity Slip Ring I M High Starting torque with low starting current. N-can be adjusted by resistance adding r decing no abnormal heating during starting Suitable for traction apps

41. Disadvantages Squirrel Cage Poor starting torque and high starting currents Sensitive to voltage fluctuations Speed control is difficult. Total energy losses r more in case of SQI M @ starting Slip Ring Initial and maintenance cost is more Eff &pf is low Sensitive to voltage fluctuations

42. Synchronous M otors

43. Definition A synchronous motor is an AC motor in which, at steady state the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles

44. Construction Stator Stationary armature Rotating armature Rotor Salient pole Non salient pole

45. ….cons

46. Non salient

47. Synchronous condenser The no load over excited synchronous motor is known as synchronous condenser.

48. losses

49. t R ansforme R s

50. Definition T ransformer is a “ static device’’ which transforms electrical energy from one circuit to another without Changing Frequency. Key words No electrical connection. No power consumption. low losses due to absence of rotating parts & air gap between Pry & sry Windings.

51. Ideal T r ansfo r me r An ideal transformer is an imaginary transformer which does not have any loss in it Means no core losses Copper losses and any other losses in transformer. Efficiency of this transformer is considered as 100%.

52. Const r uction The transformer consists of 1. HV, LV windings 2. Magnetic Core 3. Body 4. Conservator tank (700 kva ) 5. Buchholz relay 6. Breather 7. OLTC (RCTC) 8. Cooling Fan r Radiator fins

53. construction

54. Classification of transformers Based on construction Core type, Shell type & Berry type transformer Based on wdg Two wdg, Auto transformer Based on no . of phases Single ph Three ph ….. contd

55. Co r e type

56. Shell type

57. Be rr y type

58. Phasor diagrams