1. ELECTRICAL ENGINEERING AND INSTRUMENTATION
Regulation : R2013
Subject code : EE6352

2. Syllabus
UNIT I DC MACHINES 9 hours Three phase circuits, a review. Construction of DC machines – Theory of operation of DC generators – Characteristics of DC generators- Operating principle of DC motors – Types of DC motors and their characteristics – Speed control of DC motors- Applications.

3. Unit – 1 DC MACHINES

4. Three phase circuits
Three phase circuits is a polyphase system when 3 phases are used together from the generator to the load
Each phase are having a phase difference of 120°
If the load is single phase, then one phase can be taken from the three phase circuit and the neutral can be used as ground to complete the circuit.

5. Waveform

6. Why three phase preferred over single phase?
Single phase system
Three phase system
The conductor needed for a single phase circuit is very much less.
The instantaneous power falls to zero when the single phase supply is disconnected.
The conductor needed in three phase circuit is 75% that of conductor needed in single phase circuit.
The instantaneous power exists even after one phase is disconnected as the net power from all the phases gives a continuous power to the load.

7. Types of connections
Star Connection
Delta Connection

8. Star Connection Vph = VL / Iph = IL
3 phase wires and 1 neutral wire from the star point.
Preferred for long distances.

9. Delta Connection Vph = VL Iph = IL/ 3 wires from the phases are used.
No Neutral wire
Preferred for short distances.

10. Types of currents
Balanced current – Equal current flows through all the 3 phases.
Unbalanced current - Unequal current flowing through all the 3 phases.

11. Y-Δ arrangement Star-Star connection Delta-Delta connection
Star – Delta connection
Delta – Star connection

12. Simple loop generator

13. Principle of Operation
Faraday’s Law of Electromagnetic induction:- When a current carrying conductor is placed in a magnetic field an EMF is induced. Generator :- Fleming’s Right hand rule Motor :- Fleming’s Left hand rule

14. DC Machines
DC machines convert electrical energy to mechanical energy and vice versa.
This process of conversion is called as electromechanical energy conversion.
If the conversion is from mechanical to electrical energy, the machine is said to act as a generator.
If the conversion is from electrical to mechanical energy, the machine is said to act as a motor.

15. DC machines Construction Types Theory of operation Characteristics
Speed control of DC motor
Applications

16. DC machine Construction

17. DC machine Construction

18. DC machine Construction

19. DC machine Construction

20. DC machine Construction

21. Armature of a DC machine

22. Armature of a DC machine

23. Armature of a DC machine

24. Dismantled view of a DC machine

25. Dismantled view of a DC machine

26. Dismantled view of a DC machine

27. Parts of a DC machine Yoke Pole core & Pole shoes Pole coils
Armature core
Armature windings
Commutator
Brushes & Bearings

28. Yoke Outermost frame Purpose: Provides mechanical support
Carries the magnetic flux produced by the poles.
Materials used:
Small machines – Made of Cast Iron
Larger machines – Made of Cast steel or rolled steel

29. Pole cores & Pole shoes Spreads out the flux in the air gap
Reduces the reluctance of the magnetic path
Supports the exciting coils or field coils

30. Pole coil or field coil or exciting coil
Copper wires or strips wound around the pole core.
When current is passed thorugh these coils they produce magnetic flux surrounding the armature conductors.
Materials used:
Made up of copper

31. Armature core

32. Armature windings
Usually former wound in the form of flat rectangular coils.
Various conductors of the coil are insulated from each other
Conductors are placed in the armature slots
Slot insulation is folded over the armature conductors.
Materials used:
Made up of copper

33. Commutator
Facilitates collection of current from the armature conductors
The commutator & brushes arrangement converts AC to DC
It is of cylindrical structure and built up of wedge-shaped segments of high-conductivity hard-drawn or drop forged copper.
These segments are insulated from each other by
thin layers of mica.
The number of segments is equal to the number of armature coils.
Each commutator segment is connected to the armature conductor by means of a copper
lug or riser.

34. Brushes & bearings Collects current from the commutator
They are housed in a brush holder.
The brush holder is mounted on a spindle and can slide in the rectangular box open at both ends.
The number of brushes per spindle depends on the magnitude of the current to be collected from the commutator.

35. Pole Pitch Distance between 2 adjacent poles.
It is equal to the number of armature conductors (or armature slots) per pole.
If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12.

36. Conductor
The length of a wire lying in the magnetic field and in which an emf is induced, is called a conductor (or inductor) as, for example, length AB or CD in the following figure.

37. Coil & Winding element
The two conductors AB and CD along with their end connections constitute one coil of the armature winding.
The side of the coil is called the winding element.
No. of winding elements = 2 x No. of Coils
The coil may be single turn coil or multi-turn coil.
Multi-turn coil may have many conductors per coil side.
The group of wires or conductors constituting a coil side of a multi-turn coil is wrapped with a tape as a unit and is placed in the armature slot.

38. Coil span or Coil pitch (YS)
Coil span is the distance between 2 sides of the coil.

39. Full pitched winding
If coil span or coil pitch = pole pitch, the winding is called full pitched wdg.
Full pitch means coil span is 180 ͦ electrical degrees.
In full pitch, coil sides lie under opposite poles and their induced emf is scalar sum.
So maximum emf is induced in full pitch winding

40. Chorded wdg or Short pitched wdg or Fractional pitched wdg
The pitch of the winding is less than pole pitch

41. Single Layer Winding : Winding in which one conductor or one coil side is placed in each armature slot Double Layer Winding : Two conductors or coil sides per slot is placed in each armature slot

42. Types of windings
Lap winding
Wave winding

43. Lap winding Laps back with its succeeding coils.
For Simplex lap winding, No.of parallel paths = No. of poles
For duplex winding, No. of parallel paths = 2x No. of poles

44. Lap Winding

45. Different pitches for Lap winding
Back Pitch (YB) – No of coil sides or slots spanned by the back end connections. (Z/P)
Front Pitch (YF) - No of coil sides or slots spanned by the front end connections.
Resultant Pitch (YR) – Distance between the beginning of one coil to the beginning of next coil to which it is connected.
Commutator Pitch (YC) – Distance between the segments to which the two ends of the coil are connected. For Lap, (YC = YB- YF) & for Wave, (YC = YB+YF)

46. Equalizer rings or Equalizer connections
A thick copper conductor connecting the equipotential points of lap winding for equalizing the potential of different parallel paths.
Avoids unequal distribution of current at the brushes thereby heling to get sparkless commutation.

47. One Equalizer ring for a pole pair is used.

48. Interpoles or Compoles
Small poles placed in between the main poles.
As their polarity is same as that of main poles, they induce emf in the coil which helps in current reversal. The induced emf is called commutating emf or reversing emf which opposes the reactance emf thereby making commutation sparkless.
Cross magnetising effect due to armature reaction is also neutralised.

49. Design of Lap Winding

50. Lap Winding Advantages:
This winding is necessarily required for large current application because it has more parallel paths.
2. It is suitable for low voltage and high current generators.
Disadvantages:
It gives less emf compared to wave winding. This winding is required more no. of conductors for giving the same emf, it results high winding cost.
2. It has less efficient utilization of space in the armature slots.

51. Wave winding
The end of one coil is not connected to the beginning of the same coil but is connected to the beginning of another coil of the same polarity as that of the first coil

53. Simplex wave winding – Circular form

54. LAP & WAVE WINDING

55. Cross-sectional view of Armature

56. Lap & Wave windings with pitches

57. Types of DC machines
DC Generator
DC motor

58. DC generator

59. Losses

60. Separately excited DC generator

61. Self excited DC generator

62. Compound generator

63. Characteristics of DC generator
Open circuit characteristics or Magnetization characteristics or No load characteristics
Load Characteristics
Internal or Total characteristics
External characteristics

64. Open Circuit Characteristics
Eo Vs If
The data is obtained by operating the generator at no load and keeping speed constant
Field current is varied and the corresponding terminal voltage is recorded

65. Relation b/n Eg and Speed

66. Internal & External Characteristics

67. Internal & External Characteristics

68. Characteristics of DC compound generator

69. Types of DC motor

70. Characteristics of DC motor
Electrical or Internal characteristics
Mechanical or External characteristics

71. Characteristics of DC series motor

72. Characteristics of DC shunt motor

73. Characteristics of DC Compound motor

74. Speed control of DC motor The speed of the DC motor is given by the relation,

75. Speed of DC shunt motor can be controlled by varying
(i) Flux/pole, Φ (Flux Control) (ii) Resistance Ra of armature circuit (Rheostatic Control) and (iii) Applied voltage V (Voltage Control).

76. Flux control method

77. Armature control method

78. Voltage control method

79. Speed of DC series motor can be controlled by
Flux Control method
Variable resistance method

80. Flux control method Field diverters Armature diverters
Trapped Field control method
Paralleling Field coils

81. Field diverters

82. Armature diverters

83. Trapped Field control method

84. Paralleling field coils

85. Variable resistance control

86. Applications
DC series motor – Paper machines, diesel electric propulsion of ships, in steel rolling mills, hoists, cranes, trolley cars, conveyer belts
DC shunt motor – Lathes, centrifugal pumps, reciprocating pumps, fans, blowers, conveyors, wood working machines, machine tools, printing press, spinning & weaving machines.
DC compound motor – Crushes, ice-making machines, compressors, rolling mills.