1. Electrical Machine Submitted by: Riki paul(130390111009)
Himanshu Patel( )

2. 1. Dc Generators 2. Dc Motors
DC Machines
1. Dc Generators
2. Dc Motors

3. DC Generator

5. Windings in a DC Generator

6. Principle Of Operation Of a Dc Generator.
Fleming's Right Hand Rule

7. Magnitude of Induced EMF
This diagram shows a single turn rectangular coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. Collecting brushes are made of copper or carbon and press against the slip rings. This function is to collect current (induced from the coil to the load resistor R.

8. Principle of Operation
Let’s assume the coil is rotating in clockwise direction. As the coil assumes position in the field, the flux linked with it changes. Hence, the induced emf is proportional to the rate of change of flux linkages.ie
When the plane of the coil is at right angles to the lines of flux i.e. when it is in position 1, then the flux linked with the coil is maximum but rate of change of flux linkages is minimum. It is so because in this position, the coil sides AB and CD do not cut or shear the flux, rather they slide along them i.e. they move parrarell to them. This is the starting position. Angle of rotation or time will be measured from this position.
As the coil rotates further, the rate of change of flux linkage (and hence emf in it) increases till position 3 where it is 90.here the coil plane is horizontal i.e. Parrarell to the lines of flux.
As seen, the flux linked with the coil is minimum but the rate of flux linkage is maximum. Hence maximum emf is induced in the coil when in this position.
In the next quarter revolution i.e. from , flux linked gradually increases but the rate of change of flux linkage decreases. Hence the induced emf decreases gradually till position 5 of the coil.it is reduced to zero value.

9. Flemming’s Right-Hand Rule gives the direction of induced emf from A to B and C to D. Hence the direction of current flow is ABMLCD. The current through the load resistance R flows from M to L during the first revolution of the coil.
In the next half revolution i.e. from 180 to 360, the variations in the magnitude of emf are similar to those ij the first half revolution. Its value is maximum when the coil is in position 7 and minimum when in position 1.But the direction of the induced current is from D to C and B to A. Hence, the path of current flow is along DCLMBA.
Therefore we find that the current obtained from a simple generator reversed its direction after every half revolution aka Alternating Current. It should be noted that A.C not only reverses its direction, it does not even keep its magnitude constant while flowing in any one direction.
For making the flow of current in the external circuit. The slip ring are replaced by split-rings. The split-rings are made of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulated material.

10. It is seen that in the first half revolution current flows along ABMNLCD i.e. Brush 1 is in contact with segment ‘a’ acts as the +ve end of the supply and ‘b’ acts as the –ve end. in the next half revolution, the direction of the induced current in the coil has reversed. But at the same time, the positions of segment ‘a’ and ‘b’ have also reversed such that No1 touches the segment which is positive i.e. ‘b’ hence the current is unidirectional but not continues like pure direct current.
Another important point worth remembering is that even now the current induced in the coil sides is alternating as before. it is only due to the rectifying action of the split-rings (also called commutator) that it become unidirectional in the external circuit. Hence it should be clearly understood that even in the armature of a DC generator, the induced voltage is AC.

11. Single Turn Alternator
Generation Of AC Voltage
The working principle of alternator is very simple. It is just like basic principle of DC generator. It also depends upon Faraday's law of electromagnetic induction which says the electric current is induced in the conductor inside a magnetic field when there is a relative motion between that conductor and the magnetic field.
For understanding working of alternator let's think about a single rectangular turn placed in between two opposite magnetic pole as shown above.

14. Say this single turn loop ABCD can rotate against axis a-b
Say this single turn loop ABCD can rotate against axis a-b. Suppose this loop starts rotating clockwise. After 90° rotation the side AB or conductor AB of the loop comes in front of S-pole and conductor CD comes in front of N-pole. At this position the tangential motion of the conductor AB is just perpendicular to the magnetic flux lines from N to S pole. Hence rate of flux cutting by the conductor AB is maximum here and for that flux cutting there will be an induced electric curraent in the conductor AB and direction of the induced electric current can be determined by Flemming's right hand rule. As per this rule the direction of this electric current will be from A to B. At the same time conductor CD comes under N pole and here also if we apply Fleming right hand rule we will get the direction of induced electric current and it will be from C to D.

15. Elementary DC Generator
Role of Commutator
The commutator mechanically rectifies the current. When the current switches direction as the armature (the wire coil) rotates, the commutator is stationary, so it connects the positive terminal and the negative terminal to the same parts of the circuit every time. This reverses the negative voltage you would normaly see in an AC current.

17. Constructional features of DC Machine
Yoke: The outer frame of a generator or motor is called as yoke. Yoke is made up of cast iron or steel. Yoke provides mechanical strength for whole assembly of the generator (or motor). It also carries the magnetic flux produced by the poles.
Poles: Poles are joined to the yoke with the help of screws or welding. Poles are to support field windings. Field winding is wound on poles and connected in series or parallel with armature winding or sometimes separately.
Pole shoe: Pole shoe is an extended part of the pole which serves two purposes, (i)to prevent field coils from slipping and (ii)to spread out the flux in air gap uniformly.

19. Armature Core(rotor)

20. Commutator

21. Armature core (rotor)
Armature core: Armature core is the rotor of a generator. Armature core is cylindrical in shape on which slots are provided to carry armature windings.
Commutator and brushes: As emf is generated in the armature conductors terminals must be taken out to make use of generated emf. But if we can't directly solder wires to commutator conductors as they rotates. Thus commutator is connected to the armature conductors and mounted on the same shaft as that of armature core. Conducting brushes rest on commutator and they slides over when rotor (hence commutator) rotates. Thus brushes are physically in contact with armature conductors hence wires can be connected to brushes.

22. Armature Windings LAP WINDING WAVE WINDING
1. lap winding is high current, low voltage
1.wave winding is low current, high voltage.
2.IN LAP winding,if connection is in started form conductor in slot,then connections overlap each other as winding proceeds till startng point is reached again.
2.IN WAVE type of connection winding always travels ahead avoiding overlapping.it travels like a progressive wave
3.IN LAP winding, no. of parallel paths = holes i.e,A=P=4
3.IN WAVE winding,no. of parallel paths=2
4.LAP winding is preferrable for high current low voltage capacity generator
4.WAVE winding is preferrable for high voltage low current capacity generator
5.Lap windings are also used for applications requiring lower voltages at higher currents
5.Wave windings are used for applications requiring higher voltages at lower currents

23. LAP WINDING & WAVE WINDING

24. Classification of DC generators(methods of excitation)
when the current is passing through the winding then it is called "Excitation".
Types of Excitation:
(1)seperately excited generator.
(2)self excited generator.
Self Generator is classified into 3 types.
1.shunt generator.
2.series generator.
3.compound generator.
Compoud Generator is again classified into 2 types.
1.short shunt generator.
2.long shunt generator.

25. Symbolic representation of dc generators

26. Separatly excited DC Generator

27. Characteristics of Separately Excited DC Generator
The characteristics is separately excited d.c. generator are divided into two types,
1) Magnetization and ) Load characteristics.
1.1 Magnetization or Open Circuit Characteristics
The arrangement to obtain this characteristics is shown in the Fig. 1.

28. The rheostat as a potential driver is used to control the field current and the flux. It is varied from zero and is measured on ammeter connected.
Eo = (ΦPNZ)/(60A)
As If is varied, then Φ change and hence induced e.m.f. Eo also varies. It is measured on voltmeter connected across armature. No Load is connected to machine, hence characteristics are also called no load characteristics which is graph of Eo against field current If as shown in the Fig. 2. As If increases, flux Φ increases and Eo increases. After point A, saturation occurs when Φ becomes constant and hence Eo saturates.

29. 1.2 Load Saturation Curve
This is the graph of terminal voltage Vt against field current If. When generator is loaded, armature current Ia flows and armature reaction exists. Due to this, terminal voltage Vt is less than the no load rated voltage. On no load, current Ia is zero and armature reaction is absent. Hence less number of ampere turns are required to produce rated voltage Eo .
These ampere-turns are equal to OB as shown in the Fig. 3. On load, to produce same voltage more field ampere-turns are required due to demagnetizating effect of armature reaction. These are equal to BC as shown in the Fig. 3. Similarly there is drop Ia Ra across armature resistance. Hence terminal voltage V = E - Ia Ra . This graph OR is also shown in the Fig. 3. The triangle PQR is called drop reaction triangle. Thus OP is no load saturation curve, OQ is the graph of generated voltage on load and OR is the graph of terminal voltage on load.

30. Internal and External Characteristics External
Let be the no load rated voltage which drops to E due to armature reaction on load and further drops to Vt due to armature resistance drop Ia Ra on load.
The graph of Vt against load current IL is called external characteristics while the graph of E against load current IL is called internal characteristics. These are shown in the Fig. 4. for separately excited d.c.generator. The graphs are to be plotted for constant field current. In case of separately excited d.c. generator induced e.m.f. is totally dependent on flux Φ i.e. field current If . Hence to have control over the field current, in case of separately excited d.c. generators field regulator is necessary.

31. Difference between Self Excited and Separately Excited DC Generator
Self Excited DC Generator
Seperaty Excited DC Generator
1.In self excited dc generators the coil gets the excitation voltage from the voltage generated in it after a time period but not in the starting itself,
1.in separately excited generator an external supply is must for supplying exciting voltage to the coil
2.self excited generator works on its own feedback
2.separately excited generator need a external source to work

32. Self Excited Generator

33. Shunt Generator
A shunt generator is a method of generating electricity in which field winding and armature winding are connected in parallel, and in which the armature supplies both the load current and the field current.

34. Voltage and Current Relations
Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator. In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

35. Shunt Generator and its Characteristics
In these types of generators the field windings, armature windings and external load circuit all are connected in series as shown in figure below.

36. Series Wound DC Generator
Therefore, the same electric current flows through armature winding, field winding and the load.
Let, I = Ia = Isc = IL
Here, Ia = armature current
Isc = series field current
IL = load current
There are generally three most important characteristics of series wound DC generator which show the relation between various quantities such as series field electric current or excitation current, generated voltage, terminal voltage and load current.

37. Compound Generators

38. In compound wound DC generators both the field windings are combined (series and shunt). This type of generators can be used as either long shunt or short shunt compound wound generators as shown in the diagram below. In both the cases the external characteristic of the generator will be nearly same. The compound wound generators may be cumulatively compounded or differentially compounded (discussed earlier in the type of generators). Differentially compound wound generators are very rarely used. So, here we mainly concentrate upon the characteristic of cumulatively compound wound generators.

40. We all know that, in series wound DC generators, the output voltage is directly proportional with load current and in shunt wound DC generators, output voltage is inversely proportional with load current. The electric current in the shunt field winding produces a flux which causes a fall in terminal voltage due to armature reaction and ohmic drop in the circuit. But the electric current in the series field also produces a flux which opposes the shunt field flux and compensate the drop in the terminal voltage and try to operate the machine at constant voltage.
The combination of a series generator and a shunt generator gives the characteristic of a cumulative compound wound generator.
At no load condition there is no electric current in the series field because the load terminals are open circuited. But the shunt field electric current helps to produce field flux and excite the machine. When the dc generator supplies load, the load current increases and electric current flows through the series field. Therefore, series field also provides some field flux and emf is also increased. The voltage drop in the shunt machine is therefore compensated by the voltage rise in the series machine.

42. Applications of Separately Excited DC Generators
These types of DC generators are generally more expensive than self-excited DC generators because of their requirement of separate excitation source. Because of that their applications are restricted. They are generally used where the use of self-excited generators are unsatisfactory.
I. Because of their ability of giving wide range of voltage output, they are generally used for testing purpose in the laboratories.
II. Separately excited generators operate in a stable condition with any variation in field excitation. Because of this property they are used as supply source of DC motors, whose speeds are to be controlled for various applications. Example- Ward Leonard Systems of speed control.

43. Applications of Shunt Wound DC Generators
The application of shunt generators are very much restricted for its dropping voltage characteristic. They are used to supply power to the apparatus situated very close to its position. These type of DC generators generally give constant terminal voltage for small distance operation with the help of field regulators from no load to full load.
I. They are used for general lighting.
II. They are used to charge battery because they can be made to give constant output voltage.
III. They are used for giving the excitation to the alternators.
IV. They are also used for small power supply.

44. Applications of Series Wound DC Generators
These types of generators are restricted for the use of power supply because of their increasing terminal voltage characteristic with the increase in load current from no load to full load. We can clearly see this characteristic from the characteristic curve of series wound generator. They give constant electric current in the dropping portion of the characteristic curve. For this property they can be used as constant current source and employed for various applications.
I. They are used for supplying field excitation electric current in DC locomotives for regenerative breaking.
II. This types of generators are used as boosters to compensate the voltage drop in the feeder in various types of distribution systems such as railway service.
III. In series arc lightening this type of generators are mainly used.

45. Applications of Compound Wound DC Generators
Among various types of DC generators, the compound wound DC generators are most widely used because of its compensating property. We can get desired terminal voltage by compensating the drop due to armature reaction and ohmic drop in the in the line. Such generators have various applications.
I. Cumulative compound wound generators are generally used lighting, power supply purpose and for heavy power services because of their constant voltage property. They are mainly made over compounded.
II. Cumulative compound wound generators are also used for driving a motor.
III. For small distance operation, such as power supply for hotels, offices, homes and lodges, the flat compounded generators are generally used.
IV. The differential compound wound generators, because of their large demagnetization armature reaction, are used for arc welding where huge voltage drop and constant electric current is required.
At present time the applications of DC generators become very limited because of technical and economic reasons. Now a days the electric power is mainly generated in the form of alternating electric current with the help of various power electronics devices.

46. DC MOTOR

47. Basics of a Electric Motor
dcmotor 47

48. Basics of a Electric Motor
4848
Basics of a Electric Motor
dcmotor 48

49. 4949
A Two Pole DC Motor
dcmotor 49

50. 5050
A Four Pole DC Motor
dcmotor 50

51. Operating Principle of a DC Machine
5151
Operating Principle of a DC Machine
dcmotor 51

52. Fleming’s Left Hand Rule Or Motor Rule
5252
Fleming’s Left Hand Rule Or
Motor Rule
FORE FINGER = MAGNETIC FIELD
900
THUMB = MOTION
MIDDLE FINGER= CURRENT
FORCE = B IAl
dcmotor 52

53. Fleming’s Right Hand Rule Or Generator Rule
5353
Fleming’s Right Hand Rule Or
Generator Rule
FORE FINGER = MAGNETIC FIELD
900
900
900
THUMB = MOTION
MIDDLE FINGER = INDUCED
VOLTAGE
VOLTAGE = B l u
dcmotor 53

54. 5454
Action of a Commutator
dcmotor 54

55. 5555
Armature of a DC Motor
dcmotor 55

56. Generated Voltage in a DC Machine
5656
Generated Voltage in a DC Machine
dcmotor 56

57. Armature Winding in a DC Machine
5757
Armature Winding in a DC Machine
dcmotor 57

58. Lap Winding of a DC Machine
5858
Lap Winding of a DC Machine
Used in high current
low voltage circuits
Number of parallel paths
equals number of brushes
or poles
dcmotor 58

59. Wave Winding of a DC Machine
5959
Wave Winding of a DC Machine
Used in high voltage
low current circuits
Number of parallel paths
always equals 2
dcmotor 59

60. Magnetic circuit of a 4 pole DC Machine
6060
Magnetic circuit of a 4 pole DC Machine
dcmotor 60

61. Magnetic circuit of a 2 pole DC Machine
6161
Magnetic circuit of a 2 pole DC Machine
dcmotor 61

62. Summary of a DC Machine Basically consists of
6262
Summary of a DC Machine
Basically consists of
An electromagnetic or permanent magnetic structure called
field which is static
An Armature which rotates
The Field produces a magnetic medium
The Armature produces voltage and torque under the action
of the magnetic field
dcmotor 62

63. Deriving the induced voltage in a DC Machine
6363
Deriving the induced voltage in a
DC Machine
dcmotor 63

64. Deriving the electromagnetic torque in a DC Machine
6464
Deriving the electromagnetic torque in a
DC Machine
dcmotor 64

65. Voltage and Torque developed in a DC Machine
6565
Voltage and Torque developed in a
DC Machine
Induced EMF, Ea = Kam (volts)
Developed Torque, Tdev = KaIa (Newton-meter or Nm)
where m is the speed of the armature in rad/sec.,  is the flux per pole in weber (Wb)
Ia is the Armature current
Ka is the machine constant
dcmotor 65

66. Interaction of Prime-mover DC Generator and Load
6666
Interaction of Prime-mover DC Generator
and Load
Tdev Ia + +
Prime-mover
(Turbine) m Ea VL
DC Generator
Load -
Tpm -
Ea is Generated voltage
VL is Load voltage
Tpm is the Torque generated by Prime Mover
Tdev is the opposing generator torque
dcmotor 66

67. Interaction of the DC Motor and Mechanical Load
6767
Interaction of the DC Motor
and Mechanical Load Ia
Tload +
Mechanical
Load
(Pump,
Compressor) + VT
DC Motor m Ea - - -
Tdev
Ea is Back EMF
VT is Applied voltage
Tdev is the Torque developed by DC Motor
Tload is the opposing load torque
dcmotor 67

68. Power Developed in a DC Machine
6868
Power Developed in a DC Machine
Neglecting Losses,
Input mechanical power to dc generator
= Tdev m= KaIam =Ea Ia
= Output electric power to load
Input electrical power to dc motor
= Ea Ia= Ka m Ia = Tdev m
= Output mechanical power to load
dcmotor 68

69. Equivalence of motor and generator
6969
Equivalence of motor and generator
In every generator there is a motor (Tdev opposes Tpm)
In every motor there is a generator (Ea opposes VT)
dcmotor 69

70. Magnetization Curve 7070 Flux is a non-linear
function of field current and
hence Ea is a non-linear
function of field current
For a given value of flux Ea
is directly proportional to m
dcmotor 70

71. Separately Excited DC Machine
7171
Separately Excited DC Machine RA +
Armature Vf -
Field Coil
dcmotor 71

72. Shunt Excited DC Machine
7272
Shunt Excited DC Machine
Shunt Field Coil
Armature RA
dcmotor 72

73. Series Excited DC Machine
7373
Series Excited DC Machine RA
Armature
Series Field Coil
dcmotor 73

74. Compound Excited DC Machine
7474
Compound Excited DC Machine
Series Field Coil
Shunt Field Coil
Armature RA
If the shunt and series field aid each other it is called a cumulatively
excited machine
If the shunt and series field oppose each other it is called a differentially
dcmotor 74

75. Armature Reaction(AR)
7575
Armature Reaction(AR)
AR is the magnetic field produced by the
armature current
AR aids the main flux in one half of the
pole and opposes the main flux in the
other half of the pole
However due to saturation of the pole
faces the net effect of AR is demagnetizing
dcmotor 75

76. Effects of Armature Reaction
7676
Effects of Armature Reaction
The magnetic axis of the AR is 900 electrical (cross) out-of-phase with the main flux. This causes commutation problems as zero of the flux axis is changed from the interpolar position.
dcmotor 76

77. Minimizing Armature Reaction
7777
Minimizing Armature Reaction
Since AR reduces main flux, voltage in generators and torque in motors reduces with it. This is particularly objectionable in steel rolling mills that require sudden torque increase.
Compensating windings put on pole
faces can effectively negate the effect
of AR. These windings are connected
in series with armature winding.
dcmotor 77

78. Minimizing commutation problems
7878
Minimizing commutation problems
Smooth transfer of current during
commutation is hampered by
a) coil inductance and
b) voltage due to AR flux in the interpolar axis. This voltage is called reactance voltage.
Can be minimized using interpoles. They
produce an opposing field that cancels out the AR in the interpolar region. Thus this winding is also connected in series with the armature winding.
Note: The UVic lab motors have interpoles in them. This should be connected in series with the armature winding for experiments.
dcmotor 78

79. Separately Excited DC Generator
7979
Separately Excited DC Generator Ra If Rf + + RL Vt + Vf Ea
Armature -
Field Coil - - Ia
Field equation: Vf=RfIf
Armature equation: Vt=Ea-IaRa
Vt=IaRL, Ea=Kam
dcmotor 79

80. Shunt Generators Shunt Field Coil Armature 8080 If Ia Ia – If + Ea + -RL Vt
Field coil has Rfw :
Implicit field resistance Ra -
Rfc
Field equation: Vt=Rf If
Rf=Rfw+Rfc
Armature equation: Vt=Ea-Ia Ra
Vt=(Ia – If) RL, Ea=Kam
dcmotor 80

81. Voltage build-up of shunt generators
8181
Voltage build-up of shunt generators
dcmotor 81

82. Example on shunt generators’ buildup
8282
Example on shunt generators’ buildup
For proper voltage build-up the
following are required:
Residual magnetism
Field MMF should aid residual magnetism
Field circuit resistance should be less than critical
field circuit resistance
dcmotor 82

83. Separately Excited DC Motor
8383
Separately Excited DC Motor Ra If Rf + + + Vf Ea
Armature Vt -
Field Coil - - Ia
Field equation: Vf=RfIf
Armature equation: Ea=Vt-IaRa
Ea=Kam
dcmotor 83

84. Separately Excited DC Motor Torque-speed Characteristics
8484
Separately Excited DC Motor
Torque-speed Characteristics RA + +
Armature Vf
Mechanical Load - -
Field Coil m T
dcmotor 84

85. Speed Control of Separately Excited DC Motor(2)
8585
Speed Control of Separately Excited
DC Motor(2)
By Controlling Terminal Voltage Vt and keeping If or 
constant at rated value .This method of speed control is applicable
for speeds below rated or base speed.
T1<T2< T3
V1<V2<V3 m T1 T2 T3 V1 V2 V3 VT
dcmotor 85

86. Speed Control of Separately Excited DC Motor
8686
Speed Control of Separately Excited
DC Motor
By Controlling(reducing) Field Current If or  and keeping
Vt at rated value. This method of speed control is applicable
for speeds above rated speed.
 1>  2>  3
T1<T2< T3 m
 1 T1
 2 T2 T3
 3 
dcmotor 86

87. Regions of operation of a Separately Excited DC Motor
8787
Regions of operation of a Separately
Excited DC Motor
dcmotor 87

88. Separately excited dc motor –Example 2
8888
Separately excited dc motor –Example 2
A separately excited dc motor with negligible armature resistance
operates at rpm under no-load with Vt =240V(rated voltage).
The rated speed of the motor is 1750 rpm.
i) Determine Vt if the motor has to operate at 1200 rpm under no-load.
ii) Determine (flux/pole) if the motor has to operate at 2400 rpm
under no-load; given that K = 400/.
iii) Determine the rated flux per pole of the machine.
Solution on Greenboard
dcmotor 88

89. Series Excited DC Motor Torque-Speed Characteristics
8989
Series Excited DC Motor
Torque-Speed Characteristics Ra
Rsr
Rae +
Armature
Series Field Coil - T m
dcmotor 89

90. 9090
Losses in dc machines
dcmotor 90

91. Losses in dc machines-shunt motor example
9191
Losses in dc machines-shunt motor
example If Ia
Ia – If + + Vt Ea
Shunt Field Coil - -
Armature
Mechanical Load
Field coil has Rfw :
Implicit field resistance Ra
Rfc
Armature equation: Vt=Ea+Ia Ra
Ea=Kam
Field equation: Vt=Rf If
Rf=Rfw+Rfc
dcmotor

92. Losses in DC machine Copper Losses or Electrical losses.
Iron Losses or Core Losses.
Brush Losses.
Mechanical Losses.
Stray Load Losses.

93. The End