1. Instructor: Dr. ABDAL-RAZAK SH. Materials covered from Chapter 2: Direct Current Generators  Working principle of a generator  Practical DC generators  Energy conversion in a DC generator  Armature reaction in a DC generator  Classification of DC generators  Equivalent circuit of DC generators  No-load characteristics of DC generators  Load-characteristics and voltage regulation of DC generators D.C. GENERATOR

2. DC machines n Dc machines are either motors or generators.

3. DC motors applications

4. DC motor applications

5. Generating an AC Voltage n Fig. shows an elementary ac generator composed of a coil that revolves at a constant rpm between the N, S poles of a permanent magnet n The rotation is due to an external driving force, such as a motor, called prime mover n The coil is connected to two slip rings mounted on the shaft n The slip rings are connected to an external electric load by means of two stationary brushes x and y n As the coil rotates, a voltage is induced according Electromagnetic principle n This voltage appears across the load via slip ring and brush n The amount of flux cut by the conductors depends on their position in between the poles n The polarity voltage across the load changes as the conductors move from N pole to S pole

6. Direct-current Generator n How can we get constant polarity voltage? n In order to get a constant polarity voltage, a commutator is used n A commutator in its simplest form is composed of a slip ring that is cut in half, with each segment insulated from the other as well as from the shaft n Now the voltage across the load is still fluctuating but always has same polarity

7. Practical DC Generators n In a practical generator, there are a number of conductors n This conductors are housed in the slot of the armature n The terminals of the conductor are connected to the terminals of other conductors such that the connected conductors create an winding in the armature. Sometime armature conductors are also called armature coils n Usually there are two ways to connect the terminals – One is called lap winding – The other is wave winding n The front terminals of the conductors are connected to the commutator’s segment

8. Elements of Armature Winding in DC Machines Elements of an armature windings A turn – two conductors connected to an end by an end connector

9. A coil – several turns connected in series

10. A winding – several coils connected in series

13. The angle between centers of adjacent poles is 180 o (electrical) N N SS 360 o electrical = 180 o mech 180 o electrical = 90 o mech

14. The angle between centers of adjacent poles is 180 o (electrical) N N SS If coil sides are placed 180 o electrical apart, the coil is said to be full-pitch a b 180 o elec ab

15. The most common ways of connecting coils for armature windings: Lap winding Wave winding Ends of the coils are connected to the commutator bars In DC machines most of the coils are full-pitch.

16. Coil span or coil pitch(Ys): Is the distance measured in terms of number of armature slots (or armature conductors) spanned by a coil.

17. FUTHER ARMATYRE WINDING TERMINOLOGY n BACK PITCH(Y B ): Is the distance measured in in terms of armature conductors between two sides of a coil at the back of the armature. n FRONT PITCH(Y F ): Is the distance measured in in terms of armature conductors between two sides attached to any one commutator segment. n RESULTANT PITCH(Y R ): Is the distance measured in terms of armature conductors between the beginning of one coil and the beginning of the next coil to which it is connected. n Progressive winding: Y B >Y F ; Y C = +1 n Retrogressive winding: Y F >Y B ; Y C = -1

18. RELATIONS BETWEEN PITCHES FOR SIMPLEXLAP WINDING

19. Commutator bar Lap winding One coil between adjacent commutator bars 1/p of total coils are connected in series No. of poles  no. of brushes  no. of parallel paths FURTHER ARMATYRE WINDING TERMINOLOGY

21. Wave winding p/2 coils in series between adjacent commutator bars ½ of all coils between brushes Regardless of no. of poles, there are always 2 parallel path The distance between end coils (commutator pitch) is 2(N C  1)/p where N C is the no. of commutator bars

22. SIMPLEX WAVE WINDING

24. Value of the Induced Voltage: n The voltage induced in a dc generator having a lap winding is given by the equation – E g =ZnΦ/60 Volt – Where – E g =voltage between the brushes [V] – Z=Total number of conductors on the armature – Φ=Flux per pole [Wb]

25. Generator Under Load: The Energy conversion Process n When a load (e.g., a resistor) is connected between two- terminals of a DC generator, some fundamental flux and current relationships take place n The current delivered to the load also flows through all the armature conductors. n The current in the conductors under N pole will have opposite direction of the current in the conductors under S pole n Since these current carrying conductors are in a magnetic field, there will be force on conductors according to Lorentz’s law. n The individual forces F on different conductors all act clockwise n In effect they produce torque that acts opposite to the direction in which the generator is driven by a driver e.g., a turbine. n Therefore, the prime mover or driver tends to slow down and need more torque to keep the generator going on in order to overcome this opposing torque. This is conversion process helps to convert mechanical energy to electrical energy

26. Excitation and Classification of DC Generators n Permanent Magnet Generator-PM is used to create magnetic field- Difficult to have very strong PM and magnetic strength decays with time, usually used for low power and high efficient Gen. n Excited Generator-Instead of PM, field coil is used to create the magnetic field – Self Excited-Current in the field coil comes from the generator itself n Shunt-The field coil is connected in parallel with the armature n Series-The field coil is connected in series with the armature n Compound-It has two field coils; one is connected in series with armature and the other is connected in parallel with the armature n Differential Compound-In this case the series and shunt fields are connected as such the magnetic flux from these field coils cancel each other. – Separately excited-Current in the field coil come from different source other than the generator itself

27. Equivalent Circuits of DC Generators: n A voltage source with a value of E 0 which is determined by the rotational speed, number of conductors and flux per pole. n Armature conductors or coils, commutators and Carbon brush are made of finite resistive element, there is also losses in these elements. Therefore, armature and its conductors are represented by a resistor R a n The magnetic field for excited generator is obtained from a field coil. The coil has a resistance, R f n Do not confused with winding and inductance in case of DC machines!

28. Equivalent Circuits of DC Generators: Shunt Generator Series Generator Compound Generator Text Book has used Voltage source to show generator.

29. LOSSES IN A D.C. MACHINE n 1- COPPERLOSSES  ARMATURE Cu LOSS  SHUNT FIELD Cu LOSS  SERIES FIELD Cu LOSS n 2- IRON LOSSES  HYSTERESIS LOSS  EDDY CURRENT LOSS n 3- MECHANICAL LOSSES  FRICTION  WINDAGE

30. Hysteresis Loss

31. Eddy Current Loss

32. CONSTANT AND VARIABLE LOSSES 1- COSTANT LOSSES a- iron losses b- mechanical losses c- shunt field loss 2- VARIABLE LOSSES a- copper loss in armature winding b- copper loss in series field winding

33. POWER STAGES

34. CONDITION FOR MAXIMUM EFFICIENCY

35. Armature Reaction n In a loaded generator, the current flowing in the armature coils also creates a magnetic field or flux which interacts with the poles’ magnetic field. n In fact the poles’ fields are distorted and weakened by the armature flux n This m.m.f. in the armature is called as armature reaction n End effects – Sparking in the brush – Resultant poles’ flux reduces and voltage generated by the generator decrease!

36. 36 Armature Reaction -- Generator Neutral Plane shifts in the direction of rotation

38. DEMAGNETISING AND CROSS- MAGNETISING CONDUCTOR

39. CALCULATING OF DEMAGNETISNG AMPERE-TURNS PERPOLE(AT d /POLE)

40. CROSS-MAGNETISING AMPERE-TURNS PER POLE(AT C /POLE)

41. COMPENSATING WINDING Zc = No. of compensating conductors/pole face Za = No. of active armature conductors Ia = total armature current Ia/A = current in each armature conductor Zc Ia = Za Ia/A Zc = Za/A

42. AT/pole for COMPENSATING WINDING

43. 43 Effect of Armature Inductance on Commutation – No Load Consider Coil #2 Induced Voltage changes from clockwise to counter-clockwise as the coil passes from under the South pole, through the neutral plane, to under the North pole

44. 44 Coil #2 Under the South Pole Clockwise voltage induced in coils under the South pole Counter-clockwise voltage induced in coils under the North pole

45. 45 Coil #2 in the Neutral Plane Coil #2 is shorted out by the brush No voltage is induced

46. 46 Coil #2 Under the North Pole Counter-clockwise voltage induced in the coil

47. 47 Under “Loaded” Conditions Coils #2 and #3 supply current to the load through the commutator bar #3 and the brush to LOAD

48. 48 Coil #2 in the Neutral Plane Current in Coil #2 wants to go to zero, but cannot change instantaneously!

49. 49 Coil #2 Under the North Pole As soon as commutator bar #3 slides off the brush, the current in coil #2 is forced to zero A large emf is induced between commutator bar #3 and the brush, resulting in an “arc”

50. 50 Ideal Commutation For Ideal Commutation, need the coil current in phase with the coil voltage (no delay)

51. METHODS OF IMPROVING COMMUTATION  Resistance commutation  E.M.F. commutation By brush shifting By using interpoles or compoles

52. 52 Interpoles – Generator Action n Install narrow poles in the Neutral plane n Only the coil undergoing commutation is affected n A neutralizing voltage forces the reversal of current in each coil as it moves through the region

53. No-load operation and saturation curve: n Voltage building up in a self excited DC shunt generator Initially no voltage, no currents in the field However, there is a residual flux in the field-very important ! This generates a voltage, this voltage causes current flow in the field coil If the flux due to the current in the field is in the same direction, the total field flux increases-very important! The induced voltage then increases n How much voltage will build up? Infinite amount n Determined by the field resistance as shown the graph

54. Controlling Voltage of Shunt Generator n Recall E 0 =ZnΦ/60 Volt n In order to change E 0, we can change – No. of conductors, Z, it is not possible – rotational speed, n which may not be feasible – flux per pole. This is usually used in practice. A rheostat is connected in series with the field coil. If the value of the rheostat is increased, field current decreases and consequently the induced voltage E 0 decreases.

55. Voltage Regulation of a DC Shunt Generator: n Terminal voltage of a self excited shunt generator decreases as the load current increases due to armature reaction and armature resistance drop. This is called as the voltage regulation. This is bad for most electrical loads. In a gold mine, you are driving motor and bulb from a generator. n Voltage regulation is defined as (noload voltage-full load voltage)/full load voltage

56. Compound Generators: n In order to prevent compound generator is developed to prevent voltage regulation – The series field’s flux add with shunt field flux – If the load current increases, the shunt field current decreases and thus shunt field flux – But the series field current increases and thus the series field flux – Resultant flux remains almost same if number of turn in series coil is selected properly to compensate for voltage due to armature resistance drop n Overcompound Generator-If the series coils has comparatively higher number of turns n Differential compound generators-Series field’s flux works in opposite direction of shunt field flux. It is good for limiting short circuit current as terminal voltage falls very quickly in case of short circuit. Welding applications

57. Load Characteristics of DC generators n For differential compound terminal voltage falls abruptly n For shunt terminal voltage falls quickly n For separate excitation it falls but as not sharp as the shunt one n For compound, it falls slowly, better voltage regulation and good for constant voltage application n For over compounded, it increases

58. Generator Specifications Generator Spec. – Power (how much it can deliver) – Voltage (terminal voltage) – Exciting current (shunt field current) – Temperature rise – Speed – Type (series, shunt, compound..) – Class (for insulation)