DC Generator

1. -Mr. M.N. Mestri
Department of Electrical Engineering,
ATS’S Sanjay Bhokare Group of Institutes,
Miraj.
mnmestri@gmail.com /
mestrimn@sbgimiraj.org
Electrical Machine-I

2. I. Single Phase Transformer. (7 Hours) (7/7)
II. Three Phase Transformers. (8 Hours) (5/8)
III. Electromechanical Energy Conversion Principles. (6 Hours) (6/6)
IV. DC Generators. (9 Hours)
V. DC Motors. (9 Hours)
VI. Special Machines. (6 Hours)
Syllabus Contents
-Mr. M.N. Mestri 2

3. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 1st
DC Generators (9 Hours)
-Mr. M.N. Mestri 3

4. Introduction: Types of DC Machines
DC Machine is a division of Generator and Motor.
The Generator works on Generator Action and Motor Works on
Motoring Action.
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-Mr. M.N. Mestri
DC Machines
DC Generator DC Motor

5. Generator Action
Generators:
 Mechanical Input: Kinetic Energy obtained from IC Engines, Wind Power Plant, Hydro
Power Plant, Steam Power Plant and so on.
 Prime Movers: Turbines, Blades, Any Other Electrical Machines.
 Generator: Convert Mechanical Energy into Electrical Energy.
 Electrical Output: In form of AC or DC Power depending on Generator Type.
Mechanical
Input
Prime Mover
AC or DC
Generator
Electrical
Energy
Output
5
-Mr. M.N. Mestri

6. Motoring Action
Motor:
 Electrical Input: AC or DC Power.
 Motor: To convert Electrical Energy into Mechanical Energy.
 Motor Shaft: To deliver Mechanical power to load.
 Mechanical Output: Its Mechanical load like, Belt, Conveyor, Chain, Blades,
Compressors and so on.
Electrical
Input
AC or DC
Motor
Mechanical
Power at
Motor Shaft
Mechanical
Output at
Load
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-Mr. M.N. Mestri

7. Main Parts:
Field Side: Yoke/ Frame, Poles and Interpoles, Field Winding, Pole Shoe and Terminal Box.
Armature Side: Armature Core, Armature Winding, Commutator, Brushes and Shaft.
Construction of DC Machines
(Generators/Motors)
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-Mr. M.N. Mestri

8. Construction of Armature and Field Sides
Generator Windings:
1. Field Winding: It is wounded on Stator Side of Generator, its
Stationary Part.
2. Armature Winding: It is wounded on Rotor Side of Generator
with connection to Commutator, it is Rotating Part.
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-Mr. M.N. Mestri

9. Field Winding
Field Winding is wound on Poles and Interpoles which is Stationary
(Stator) Part of DC Machines.
Field Core/Poles is laminated and made of low reluctance material
to avoid eddy current and hysteresis losses.
Field Winding is made up of Copper material.
Insulation is placed in between Winding and Poles, Interpoles to
avoid shock at Yoke.
In a Generator Field winding is connected across DC Supply as
Excitation Winding.
As DC Supply Starts flowing the Magnetic Field is produced
between Field and Armature Side in Air Gap, but motor doesn't
get started as Armature side is not connected to DC Supply.
Field Winding is used to Produce Magnetic Field in Air Gap.
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-Mr. M.N. Mestri

10. Construction of Field Winding
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-Mr. M.N. Mestri

11. Armature Winding
Armature Winding is wound on Armature Core which is on Shaft
which is Rotational (Rotor) Part of DC Machines.
Armature Core is laminated and made of low reluctance material to
avoid eddy current and hysteresis losses.
Armature Winding is made up of Copper material.
Insulation is placed in between Winding and Armature Core to
avoid Shock at Shaft.
In a Generator Armature through Shaft is connected to Prime
Mover.
Connection of Armature Winding is taken out and connected to
Electric Load.
Basically DC Generator Generates AC Power at Armature Winding
in Generating Action, Further Commutator Converts it into DC
Power and Feeds out to Electrical Load.
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-Mr. M.N. Mestri

12. Construction of Armature Winding
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-Mr. M.N. Mestri

13. Types of Armature Winding.
Armature Windings:
1. Lap Winding: Connected in Continuous Manner with
commutator. Used for High Current Low Voltage
Applications.
2. Wave Winding: Basic Appearance is like Waveform.
Used for Low Current High Voltage Applications.
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-Mr. M.N. Mestri
Armature Winding
Lap Winding Wave Winding

14. Lap Winding
As Number of A increases, Current Rating Increases.
Number of Parallel Path is equal to Number of Poles.
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-Mr. M.N. Mestri
Lap Winding
Simplex Winding Duplex Winding Triplex Winding

15.  In Wave Winding Number of Parallel Path (A) is not equal to Number of Pole
(P).
 Number of Parallel Path is equal to Number of Brushes is always equal to 2
(A=Brush=2).
Wave Winding
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-Mr. M.N. Mestri
Wave Winding
Simplex Winding
Progressive
Winding
Retrogressive
Winding

16. Applications of Windings
Lap Winding Applications:
1. Welding Generator,
2. Generator at Furnace,
3. Heavy Lighting Loads.
Wave Winding Applications:
1. Low Lighting,
2. Low Current Applications.
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-Mr. M.N. Mestri

17. Commutator is nothing but a Dynamic Rectifier.
It is used to convert Generated AC Power into DC Power as a
Single Phase Full Wave Rectifier Output.
Commutator is made up of Copper Material and mounted on Motor
Shaft with Mica Insulation to avoid shock on Shaft.
Commutator is connected to Armature Winding and to Electrical
Load in Generation Mode.
Commutator
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-Mr. M.N. Mestri

18. Construction of DC Machines
18
-Mr. M.N. Mestri

19. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 2nd
DC Generators (9 Hours)
-Mr. M.N. Mestri 19

20. DC Generators are Worked on Principle of Dynamically Induced
EMF
The Faraday’s Law of Electromagnetic Induction can State
Dynamically Induced EMF
We will see Dynamically Induced EMF Rule in Next Slide Video
Working of DC Generator
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-Mr. M.N. Mestri

21. Working (Faraday’s Law of Electromagnetic Induction and Static and Dynamic Induced EMF)
21
-Mr. M.N. Mestri

22. From Dynamically Induced EMF Rule we will Examine our DC
Generator Working
Magnetic Flux Creating in Stationary Coil is our Field Winding
(Stator Side) in DC Generator
To have Mechanical Displacement we will have Prime Mover
Here, displacement is Rotational achieved by Prime Mover
Displacement is done in Armature Coil (Rotor Side) with Prime
Mover
As Flux Linkage in Moving Coil takes place, due to Dynamic Action
it will generate EMF.
Working of DC Generator
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-Mr. M.N. Mestri

23. Fleming's right-hand rule is used for electric generators. Since
neither the direction of motion nor the direction of the magnetic
field (inside the motor/generator) has changed, the direction of
the electric current in the motor/generator has reversed.
When a current-carrying conductor is placed in an external
magnetic field, the conductor experiences a force perpendicular to
both the field and to the direction of the current flow.
Working (Fleming’s Right Hand Rule of Electromagnetic Induction)
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-Mr. M.N. Mestri

24. Working of DC Generator
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-Mr. M.N. Mestri

25. Types of DC Machines
25
-Mr. M.N. Mestri

26. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 3rd
DC Generators (9 Hours)
-Mr. M.N. Mestri 26

27. Generator Action
Generators:
 Mechanical Input: Kinetic Energy obtained from IC Engines, Wind Power Plant, Hydro
Power Plant, Steam Power Plant and so on.
 Prime Movers: Turbines, Blades, Any Other Electrical Machines.
 Generator: Convert Mechanical Energy into Electrical Energy.
 Electrical Output: In form of AC or DC Power depending on Generator Type.
Mechanical
Input
Prime Mover
AC or DC
Generator
Electrical
Energy
Output
27
-Mr. M.N. Mestri

28. As we know, DC Generator Converts Mechanical Energy into
Electrical Energy as we have seen in accordance to Faraday’s Law
of Induction
As per Law, Armature Conductors are Rotated in a free plane
cutting Magnetic Flux Produced by Field Section and EMF is
Induced in it.
This Produced EMF is Electrical Energy which we take Output from
DC Generator
EMF Equation of a DC Generator
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-Mr. M.N. Mestri

29. While Deriving EMF Equation we must consider some Parameters of
Generator
P= Number of Poles of the Generator
φ= Flux Produced by Each Pole in Weber
N= Speed in r.p.m. at which the Generator is Driven
Z= Number of Conductor of Armature Winding
A= Number of Parallel paths of Armature Winding
Then by Faraday’s Law of Induction we can Derive Equation for
EMF
EMF Equation of a DC Generator
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-Mr. M.N. Mestri

30. EMF induced in DC Generator is in Proportion to Flux Created by Field
Winding with Respect to Time.
Therefore,
But we know, to have EMF induced in Armature Winding, the Armature
Winding must Rotated and it must Cut the Flux Produced by Field
This Flux is Cut by Respective Pole of DC Generator
Thus, we can say that Flux Cut by Conductor will be (Pφ)
To Cut this Flux the Time Required to Complete One Full Rotation will be
(Time in sec per min/ Speed of Generator per min)
Therefore, Time required will be (60/N) sec
EMF Equation of a DC Generator
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-Mr. M.N. Mestri

31. Depending on Flux Cut by Armature and Time Required for one
rotation in sec. We can rewrite EMF Equation as,
Therefore, =>
As per Armature Windings we know, Armature Winding has its own
Parallel Paths while Winding
Thus, we can say that Conductors Wound in Armature Winding are
Divided in Number of Parallel Paths
We need to Consider these Number of Windings and Number of
Parallel Paths in a Armature Winding to Determine EMF Equation
EMF Equation of a DC Generator
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-Mr. M.N. Mestri

32. Therefore EMF Equation will be,
Thus, Above Equation is Final Equation for EMF in DC Generator
EMF Equation of a DC Generator
32
-Mr. M.N. Mestri

33. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 4th
DC Generators (9 Hours)
-Mr. M.N. Mestri 33

34. The armature reaction simply shows the effect of armature field on the
main field. In other words, the armature reaction represents the impact
of the armature flux on the main field flux. The armature field is
produced by the armature conductors when current flows through them.
And the main field is produced by the magnetic poles
Armature Reaction takes Place only When DC Generator is Connected to
Load, as We Connect Load Ia will Introduce in Armature Winding and it
will Produce Armature Flux which will result in Armature Reaction
The armature flux causes two effects on the main field flux.
1. Cross-Magnetizing Effect.
2. Demagnetizing Effect.
Armature Reaction
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-Mr. M.N. Mestri

35. Armature Reaction
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-Mr. M.N. Mestri

36. From previous figure we can say that as Electrical Load is
Connected to Armature Winding of DC Generator it will Produce
Armature Current and Armature Flux
As per Fleming’s Right Hand Rule we can observe that, Conductor
A-B will create Clockwise Armature Flux and Conductor C-D will
create Anti-Clockwise Armature Flux
As Conductor A-B are taking Current Inside and Conductor C-D
are taking Current Outside
We can Observe Modified Figure on these Basis
Armature Reaction
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-Mr. M.N. Mestri

37. Armature Reaction
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-Mr. M.N. Mestri
From Figure Cross Symbol shows Current Flow Inside the
Generator as Conductor A-B in Previous Figure and Dot Symbol
Shows Current Flow Outside the Generator as we seen in
Conductor C-D
MNA= Magnetic Neutral Axis, GNA= Geometrical Neutral Axis

38. Armature Reaction
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-Mr. M.N. Mestri
From reference to First Figure we can Co-relate following Figure
and Observe Creation of Armature Flux in Loading Condition
In Brush Axis we can Observe all Armature Fluxes are going
Downwards from Top
Thus we can draw Phasor between Main (Field) Flux and Armature
Flux

39. Armature Reaction
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-Mr. M.N. Mestri
From Phasor we can Observe that, Field Flux and Armature Flux
are in 900 Phase shift to each other
Thus, we get Resultant Flux will be Tilted Flux
If we Increase Load the Magnitude of Resultant Flux will be more
towards Armature Flux
The Relation between Field and Armature Flux will create in Field
Flux which will be known as “Cross Magnetizing Effect”

40. Armature Reaction
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-Mr. M.N. Mestri
Due to Tilted Resultant Flux, we can Observe Title in Field Flux
and in MNA
Due to this Tilt we can say that Magnetic Neutral Axis will Shift
its Position and it can Observe as in Below Figure
But, GNA will remain Constant as Brushes Position is Constant

41. Armature Reaction
41
-Mr. M.N. Mestri
From tilting of MNA we can say that Quantity of Field Flux gets
changed in Field Poles
At Tg (Tailing Point) of Pole Flux Density will get Increased as
Compared to Lg (Leading Point) of Pole

42. Armature Reaction
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-Mr. M.N. Mestri
Due to which we can say that Weakening Flux will get Created at
Lg Point.
Weakening Flux means Strength of Flux will get Reduced
Thus this effect is known as “Demagnetizing Effect”

43. Armature Reaction
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-Mr. M.N. Mestri
Thus in Phasor we can Observe “Cross Magnetizing Effect” and
“Demagnetizing Effect” with Reference to MNA and Field Flux
To Determine Effects with MMF then we only need to Replace
Magnetic Quantities of Flux into Forces
Hence, Armature Flux will be Noted as Fa and Field Flux will be
Noted as Ff

44. Armature Reaction
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-Mr. M.N. Mestri
Hence, Armature Flux will be Noted as Fa and Field Flux will be
Noted as Ff
Thus, Ff is in 900 to Fa in Old MNA so it will be “Cross
Magnetizing Effect” and Ff is Opposite to Fa*Sin(β) in New MNA
so it will be “Demagnetizing Effect”

45. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 5th
DC Generators (9 Hours)
-Mr. M.N. Mestri 45

46. Armature Reaction
46
-Mr. M.N. Mestri
Effects Due to Armature Reaction:
1. Demagnetizing Effect
2. Sparking in Commutation Process due to Brush Shifting
3. Insulation Breakdown
4. Poor Commutation
5. Iron Losses Increases

47. Armature Reaction
47
-Mr. M.N. Mestri
Effects Due to Armature Reaction:
1. Demagnetizing Effect
 We know due to Shifting of MNA
the Field Flux will get changed in
Poles
 In result at Leading Point Flux
Density will get Reduced
 This Reduction in Flux is known as
“Demagnetizing Effect”
 It is created due to Cross
Magnetizing Effect due to
Armature Reaction

48. Armature Reaction
48
-Mr. M.N. Mestri
 Effects Due to Armature Reaction:
2. Sparking in Commutation Process due to Brush
Shifting:
 We can Observe in Below Figure that shifting of
MNA will take place due to Cross Magnetizing
Effect.
 Due to this effect we can observe that Neutral
Conductor will shift from Old MNA to New MNA
point as Magnetizing Axis is changed
 In this Process we can Observe now Conductor
Coming Under Brush is now Conducting Conductor
and not a Neutral Conductor
 Hence, when Commutator will Rotate it will have
contact with brush and as there is Conducting
Conductor under Brush it will Produce Sparking
between Brush and Commutator

49. Armature Reaction
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-Mr. M.N. Mestri
Effects Due to Armature Reaction:
3. Insulation Breakdown
 As we know Due to Cross Magnetizing
Effect and Conducting Conductor
Under Brush Sparking takes Place in
Commutation Process
 Due to this Sparking we know that
Heating will take Place in Commutator
and Brush which may Result in
Breakdown of Insulation placed
between Commutator Segments and
between Commutator and Shaft

50. Armature Reaction
50
-Mr. M.N. Mestri
Effects Due to Armature Reaction:
4. Poor Commutation
 As we know Due to Cross Magnetizing
Effect and Conducting Conductor
Under Brush Sparking takes Place in
Commutation Process
 Due to this Sparking in Commutator
the Output of Generated Electrical
Power will be not Proper as we can
Observe in Animation in Slide
 Distorted Electrical Output will take
place in Commutator, which is known as
Poor Commutation

51. Armature Reaction
51
-Mr. M.N. Mestri
 Effects Due to Armature Reaction:
5. Iron Losses Increases
 We know, Iron Losses are correlated with
Magnetic Quantities taking place in Core
of Machine/ Generator
 Here we know, due to Cross Magnetizing
Effect the Shifting of Magnetic Flux occur
 Due to this Shifted Magnetic flux we can
say that Core will have different Magnetic
Flux Density
 Thus, we can say that as Flux Density is
less at some parts of Pole, at that part of
Core Iron Losses are Increased

52. Armature Reaction
52
-Mr. M.N. Mestri
Methods to Reduce Armature Reaction:
1. Increasing Reluctance of Pole Tips
2. Increasing Air Gap Between Armature and Pole Tips
3. By Using Compensation Winding
4. By Using Interpoles

53. Armature Reaction
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-Mr. M.N. Mestri
 Methods to Reduce Armature Reaction:
1. Increasing Reluctance of Pole Tips
 We know Reluctance Opposes Flow of Flux in
Magnetic Core
 When we Increase Reluctance of Core Tips
it will Oppose flow at Tips of Poles
 Hence, in 2nd Figure we can Observe that,
Due to High Reluctance Flux will Avoid to
Flow from Tip and tries to Flow from Centre
Part of Pole, due to which MNA Shifting can
get cancelled out and we can say Armature
Reaction is reduced
 But, due to High Reluctance at Tip
Demagnetizing effect will be present at Tips
 Thus, Practically we are not using these
method

54. Armature Reaction
54
-Mr. M.N. Mestri
 Methods to Reduce Armature Reaction:
2. Increasing Air Gap Between Armature and
Pole Tips
 In this Method of Reduction, we are
increasing distance between Field and
Armature (Increasing Air Gap)
 Due to Increased in Air Gap, Flux Linked
with Armature Coil will be less
 Hence, Current Produced in Armature will
be less and Due to Less Current in
Armature, Armature Flux will be less
 Thus, Armature Reaction will get Reduced
 But, due to less Armature Generate EMF
will also Less, so this method is not
Implemented in Practical Cases

55. Armature Reaction
55
-Mr. M.N. Mestri
Methods to Reduce Armature Reaction:
3. By Using Compensation Winding
 In this Method Extra Compensation
Winding is Connected in Main Poles
 These windings are connected in
Opposite Pattern of Armature Winding
 Hence, they will cut Armature Flux
and it will reduce Armature Reaction
and makes Generator Stable
 This Method is widely used in Practical
Cases

56. Armature Reaction
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-Mr. M.N. Mestri
Methods to Reduce Armature Reaction:
4. By Using Interpoles
 In this method, Auxiliary
Interpoles are Connected in Series
with Main Poles as shown in Figure
 Due to Interpoles in Series they
will Create Opposite Flux of
Armature Winding Flux which
Cancels each other
 This Method is also used widely in
Practical

57. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: Lecture 6th
DC Generators (9 Hours)
-Mr. M.N. Mestri 57

58. Commutator is nothing but a Dynamic Rectifier.
It is used to convert Generated AC Power into DC Power as a
Single Phase Full Wave Rectifier Output.
Commutator is made up of Copper Material and mounted on Motor
Shaft with Mica Insulation to avoid shock on Shaft.
Commutator is connected to Armature Winding and to Electrical
Load in Generation Mode.
Commutator
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-Mr. M.N. Mestri

59. Commutator and Commutator Segments
59
-Mr. M.N. Mestri
1 4
10
7
1 2 3 4 5 6 7 8 n
1 2 3 4 5 6 7 8 n
1
2
1 2
Current Flow in Commutator
Brush
Commutator Segments
1 4
10
7
1
2

60. Commutation Process
60
-Mr. M.N. Mestri
Commutation Process: The Process of Reversal
of Current in a Coil When it Passes Through
MNA is know as Commutation Process
Commutation Time: The Time Taken by Brush in
a Span to Travel from One Segment to Another
Segment is Known as Commutation Time
Ideal Commutation: When Reversal of Current is
Proper in Armature and Commutator without
any Sparking is known as Linear or Successful or
Ideal Commutation
Delayed Commutation: When Reversal of Current
is Improper or lags in Armature and Commutator
and Creates Sparking Due to Reactance Voltage
then it is known as Non-Linear or Unsuccessful
or Delayed Commutation

61. Commutation Process
61
-Mr. M.N. Mestri
1 2 3 4 5 6 7 8 n
1 2
Current Flow in Commutator
Brush
Commutator Segments
 Assumptions for Commutation Process: Winding is Wave Winding, Width of
Commutator Segment and Brush is Same and Armature Current in each Coil is
20A
a b c d e f g … … and so on
Armature Winding

62. Commutation Process
62
-Mr. M.N. Mestri
1 2 3 4 5 6 7 8 n
1 2
Current Flow in Commutator
Brush
Commutator Segments
Simplified Figure from Previous Slide
a b c d e f g … … and so on
Armature Winding

63. Commutation Process
63
-Mr. M.N. Mestri
1 2 3 4 5 6 7 8 n
1 2
Current Flow in Commutator
Brush
Commutator Segments
Simplified Figure from Previous Slide
Cases to Observe Commutation Process we will Observe One by
One
a b c d e f g … … and so on
Armature Winding

64. Commutation Process
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-Mr. M.N. Mestri
Commutation Process:
1. Brush is at 7th Segment of Commutator
In this Process the Current Addition takes
Place in Commutator
In Figure we can Observe that, each Coil
has Current 20A flow in Armature and
Through Brush Current will get Added and
we will get 40A Current Output as Current
from Coil f and Coil g gets Added
Commutator Rotation
Direction
5 6 7 8
2
e f g
20A 20A 20A
40A

65. Commutation Process
65
-Mr. M.N. Mestri
Commutation Process:
2. Some Part of Brush is at 6th and 7th Segment of
Commutator (25% at 6th and 75% at 7th)
 We know, Resistance (R)= ρ*(L/A)
 Where, Rα(1/A) => Rα(1/I) => IαA
 Therefore, if Area is more Current Flow will be More
 Hence, we can say that at 7th Segment Brush is only
75% touching, resistance will Increase and Current
flow will reduce, Such as 10A current is reduced and
we will get 30A from 7th Segment and as now Brush is
touching Segment 6th we will get 10A from it
 Thus, Output will be same as 40A
5 6 7 8
2
e f g
10A 10A 20A
40A
Commutator Rotation
Direction

66. Commutation Process
66
-Mr. M.N. Mestri
Commutation Process:
3. When Brush is at Equally at 6th and 7th Segment
of Commutator (50% at 6th and 50% at 7th)
Here, we can say that at 7th Segment Brush is
now 50% touching, as 20A current is flowing
from 7th Segment and 20A from Segment 6th
Hence, Output will be same as 40A
Thus, Coil f will have 0A Current Flow due to
Increase in Resistance
As Coil f has 0A Current, Voltage will be
maximum which will be Reactance Voltage
5 6 7 8
2
e f g
20A 0A 20A
40A
Commutator Rotation
Direction

67. Commutation Process
67
-Mr. M.N. Mestri
Commutation Process:
4. Brush is at 6th and Some Part at 7th Segment of
Commutator (75% at 6th and 25% at 7th)
We know, Rα(1/A) => Rα(1/I) => IαA
Therefore, if Area is more Current Flow will be
More
Hence, we can say that at 7th Segment Brush is
only 25% touching, resistance will Increase and
Current flow will reduce, Such as 10A current
will flow from 7th Segment and Brush touching
Segment 6th we will get 30A from it
Thus, Output will be same as 40A
It is Similar and Reversal of Case 2nd
5 6 7 8
2
e f g
20A 10A 10A
40A
Commutator Rotation
Direction

68. Commutation Process
68
-Mr. M.N. Mestri
Commutation Process:
5. Brush is at 6th Segment of Commutator
In this Process the Current Addition takes
Place in Commutator
In Figure we can Observe that, each Coil
has Current 20A flow in Armature and
Through Brush Current will get Added and
we will get 40A Current Output as Current
from Coil e and Coil f gets Added
5 6 7 8
2
e f g
20A 20A 20A
40A
Commutator Rotation
Direction

69. Commutation Process
69
-Mr. M.N. Mestri
Commutation Process:
5 6 7 8
2
e f g
20A 20A 20A
40
A
Commutator Rotation
Direction
Commutator Rotation
Direction
5 6 7 8
2
e f g
20A 20A 20A
40
A
5 6 7 8
2
e f g
10A 10A 20A
40
A
Commutator Rotation
Direction
5 6 7 8
2
e f g
20A 0A 20A
40
A
Commutator Rotation
Direction
5 6 7 8
2
e f g
20A 10A 10A
40
A
Commutator Rotation
Direction

70. Bad Commutation
70
-Mr. M.N. Mestri
Causes of Bad Commutation:
Voltage Reactance
In 3rd Case we have observed that in Coil f
Current is Zero and there Reactance Voltage
is taken place
It is known as Reactance Voltage due to
Inductive Reactance it is Produced in Coil and
Due to this Higher Voltage Production it
results in Sparking in Commutator and Brush
as we can Observe in Animation
5 6 7 8
2
e f g
20A 0A 20A
40A
Commutator Rotation
Direction

71. Remedies for Bad Commutation
71
-Mr. M.N. Mestri
Remedies for Bad Commutation:
1. By Using Compensating Winding
We have see this process in Armature Reaction, we use Compensating
Winding in Series with Armature which will Control Armature Flux and
in Commutation Process it will Reduce Reactance Voltage by Reducing
effect of Change in Current Flow
2. By Using Higher Resistance Brush
By using Higher Resistance Brush we will Increase Reactance Voltage
Drop at Brush Contact and it will Reduce Reactance Voltage and
Sparking Created by it
This Method is Used in Practical Application by Using High Resistance
Carbon Brushes in DC Machines

72. Construction of armature and field systems,
Working, types, emf equation,
Armature windings, Characteristics and applications,
Building of emf,
Armature reaction - Demagnetizing and Cross
magnetizing mmfs and their estimation;
Remedies to overcome the armature reaction;
Commutation process, Causes of bad commutation
and remedies
Chapter 4th: End Chapter
DC Generators (6/9 Hours)
-Mr. M.N. Mestri 72

73. -Mr. M.N. Mestri 73
Thank You…