Ø From where we get ELECTRICITY ?
• TRANSFORMER on the street end.
Ø From where the transformer is getting the supply ?
• SUBSTATION
Ø From where the substation is getting to supply ?
• Power station
Ø What is the device which generates Electrical Energy ?
• Generator
Ø What are the basic requirements generating EMF ?
• Conductor
• Magnetic Field
• Relative Speed
How EMF is Induced ?
Ø When ever a conductor cuts the Magnetic field an EMF is induced in it ( Faradays Ist law of Electro magnetic induction )
What are the types of Induced EMF ?
Ø Dynamically Induced EMF
Ø Self Induced EMF
What do you mean my the word Dynamics ?
Ø Dynamics is related to motion
DYNAMICALLY INDUCED EMF
Ø Emf is induced when the flux linking a conductor changes
Ø In dynamically induced emf , the conductor moves in a stationary magnetic field.
Ø The emf is induced in the conductor when it is in motion
Fig1
Dynamically Induced Emf Derivation
Ø Magnetic flux density = B wb/ m ²
Ø The conductor is moving with a velocity of ‘v’ m/s
Ø Conductor length : l m
Fig2
Ø If the conductor moves through a small distance dx in time dt seconds
Ø Area swept by the conductor = l ×dx
Ø Flux cut by conductor dø = flux density × area swept = (B l dx ) wb
Ø According to ‘faraday’s laws ,‘emf’ induced in the conductor is given by
E = N dø / dt
= N *B * l * dx / dt
= B * l * dx / dt (N=1)
= B l v volts
Ø If the conductor moves at an angle’θ‘ to the magnetic field
The EMF E = B * l * v * Sinθ
FLEMINGS RIGHT HAND RULE
Statement
Ø Stretch out the fore finger, middle finger and thumb of your right hand, so that they are at right angles to one another.
Ø Fig3
If the forefinger points in the direction of magnetic field, Thumb in the direction of motion of the Conductor and middle finger in the direction of Induced emf
FLEMINGS RIGHT HAND RULE
Thumb points motion
Forefinger points field
Middle finger points Induced Emf
Application of Flemings right hand rule
Ø Consider a conductor AB moving upwards at right angles to a uniform magnetic field as shown in Fig.2
Ø By Flemings right hand rule the direction of induced current is from B to A.
Ø If the motion of the conductor is downwards, the direction of induced current is from A to B.
Electromechanical Energy Conversion
• A machine is required to convert energy from one form to another form.
• Energy conversion is based on the production of dynamically induced e.m.f.
• Conversion is bilateral, electrical to mechanical and mechanical to electrical.
• A D.C. machine which converts mechanical energy to electrical energy is called D.C. generator.
Requirements for production of EMF
Ø Conductors
Ø Magnetic field
Ø Relation Motion of conductor or magnetic field.
Production of dynamically induced EMF
Ø Conductor moves in a stationery magnetic field
an EMF is induced in a conductor
fig4
Construction of Simple Loop Generator
Ø A simple loop generator is
l Single turn rectangular copper coil
l Placed between poles of a magnet
l Used to study the generation of emf in D.C. generator
Fig5
WORKING OF SIMPLE LOOP GENERATOR
Ø Consider a single turn rectangular copper coil is rotating in a magnetic field.
Ø The coil occupies different angular positions during its rotation.
Ø When the coil is rotated through an angle of 90° the emf induced in the coil is maximum.
Ø Rotate the coil further by an angle of 180°.The emf induced in the coil will be zero.
Ø Rotate the coil further by an angle of 270°.The emf induced in the coil is maximum in the reverse direction.
Ø We conclude that the nature of the emf induced is alternating.
D.C. GENERATOR
Fig6
PRINCIPLE OF WORKING OF D.C. GENERATOR
Principle: Based on Faraday’s laws of electromagnetic induction. That is,
Ø When ever a conductor cuts magnetic flux the dynamically induced emf is produced according to Faradays laws of electro magnetic induction.
Ø In generator, the rate of change of flux linkages is due to rotation of the conductor. That means emf generated is dynamically emf.
Ø A coil abcd is rotated clock wise between two
magnetic poles N and S.
Ø Due to cutting of magnetic flux lines emf is
induced in the coil which is alternating.
Fig7
Ø The alternating emf is converted to d.c by means of split rings. (or commutator)
Ø This emf causes current to flow if the conductor circuit is closed. (is shown in Fig.1)
Important parts of D.C. Generator
Ø Yoke
Ø Pole core and pole shoe
Ø Field coils
Ø Armature core
Ø Armature winding
Ø Commutator
Ø Brushes and brush holders
Ø Bearings
Fig8
Yoke
Ø Yoke is the outermost cylindrical frame as shown in
Ø It is made up of cast iron
Ø It provides the low reluctance path to magnetic flux
Pole core and Pole shoe
Ø Pole core is made-up of cast iron as shown in
Ø Pole shoe is made-up of cast iron laminations
Ø Pole shoe supports the field coils and spreads the magnetic flux in the air- gap
Field Coils
Ø Field coils are made-up of enamel coated copper or aluminum wire as shown in
Ø Placed on pole core
Ø Produces flux when excited
Armature core
Ø It is made up silicon steel laminations
Ø Armature core is cylindrical in shape as shown in
Ø Slots are provided on the outer periphery
Fig9
Armature winding
Ø Armature winding is an insulated copper wire.
Ø They are formerly would in the form of rectangular coils
Ø They are placed in the slots as shown
Ø Fig10
Commutator
Ø It is cylindrical in shape
Ø It is made-up of high conductivity hard drawn copper
Ø It facilitates the collection of current from the armature conductors as shown in Fig.11
Brushes and Brush holders
Ø Brushes are made-up of high grade carbon as shown in
Ø Main function of the brush is to collect current from the commutator and supply it to the external load
BEARINGS
The function of the bearings is to reduce friction between the rotating and the stationery part of the machine
Ø Types of bearings:
(a)Bush bearings
(b)Ball bearings
(c)Roller bearings
Fundamental parts of a direct current machine
Fig12
D.C. Generator
Ø D.C. Generator is a machine which converts mechanical energy into electrical energy
Ø The conversion process is based on the dynamically induced e.m.f
Basic requirements for EMF
Ø Conductor
Ø Magnetic flux
Ø Relative motion between the conductor and flux
Conductors
Ø Conductors are armature windings placed on the periphery of the armature core
Magnetic Flux
Ø The field windings are placed on the pole core
Ø Field windings are supplied with d. c. current, produces magnetic flux.
Field Excitation means giving d. c. current to the field winding.
Rotating of Armature
Ø Armature is rotated by means of prime mover
Ø Prime mover may be a turbine or diesel engine
Production of EMF
Ø Armature is rotated in the magnetic flux an emf produces e.m.f. is produced.
Ø EMF is produced according to faradays laws of electromagnetic induction
The basic difference between a Simple loop generator and D.C generator is slip ring and split ring (commutator)
Fig13
WORKING OF D.C. GENERATOR
Ø A coil abcd is rotated clock wise between two
magnetic poles N and S.
Ø Due to cutting of magnetic flux lines emf is
induced in the coil which is alternating.
Ø The alternating emf is converted to d.c by means of split rings. (or commutator)
Ø This emf causes current to flow if the conductor circuit is closed. (is shown in Fig.1)
Important Terms
Ø Pole pitch
Ø Conductor
Ø Coil
Ø Coin span or coil pitch
Ø Full pitched coil
Ø Fractional pitch
Ø Pitch of the winding
Ø Back pitch
Ø Front pitch
Ø Resultant pitch
Ø Commutator pitch
Pole Pitch-YP
Ø It is The distance between the two adjacent poles
Ø It is also equal to the number of armature conductors per pole
Example
Ø If there are 48 conductors and 4 poles
Ø The pole pitch
Ø Yp = Z / P = 36 / 4 = 9
Conductor
Ø It is a wire of certain length lying in a magnetic field
Ø In which an e.m.f is induced
Coil
Ø The two conductors AB and CD
Ø Their end connection make one coil of armature winding.
Fig14
Coil Pitch or Span-YS
Ø It is defined as the distance measured in terms of armature slots between two sides of a coil as shown in Fig .2
Ø The coil Pitch or Span of coil “A” is ( 5 – 1) =4
Ø The Span of coil “B” is ( 6 – 3 ) = 3
Fig15
Full pitched coil
Ø If the coil pitch is equal to pole pitch, the winding is called full pitched winding
Ø For example:-
Ø If there are 16 conductors and 4 poles then Pole pitch = 16 / 4 = 4
Ø The coil span of coil “A” = ( 5 - 1)
= 4
This winding is called Full pitched winding
Fig16
Fractional pitch coil
If the coil pitch is less than the pole pitch the winding is called fractional pitch coil
In this, the coil span is180 electrical degrees.
Ex: If there are 36 slots and 4 poles
Coil pitch=36/4=9slots
Back pitch YB
Ø It is distance measured in terms of the number of armature conductors between the two coil sides of a coil measured around the back of the armature
Fig17
Front pitch-YF
Ø It is the distance between two coil sides connected to the same commutator segment
Fig18
Pitch of winding Y
Ø It is defined as the distance between the beginning of two consecutive turns
Ø Y=YB-YF ---------------Lap winding
Ø Y=YB+YF ---------------Wave winding
Resultant pitch-YR
Ø It is defined as the distance in terms of number of coil sides between the start of one coil and start of the next coil, coil to which it is connected
Fig19
Commutator pitch -YC
Ø It is defined as the distance measured In terms of commutator segments between the segments to which The two ends of a coil are connected
Fig20
Single layer winding
Ø It is the winding in which one conductor or one coil side is placed in each
armature slot as shown in Fig.3
fuig21\
Two layer winding
It is the winding in which two conductors or coil sides per slot arranged in two layersAs shown
Fi22
Types of windings
Lap winding
Wave winding
Fig23
Simplex Lap winding
Ø In a lap winding the finishing end of one coil
Ø Is connected via the commutator segment
Ø To the starting end of the adjacent coil Situated under the same pole
Ø In this way all the coils are connected
Simplex wave winding
Ø In a wave winding a coil side under one pole is connected to a second coil side
Ø Which occupies approximately the same position
Ø Under the next pole through back connection
Ø Thus the wave winding progresses in
Ø One direction round the armature in a series of
What do you mean by excitation
Excitation means giving d.c supply to the field winding which in turn produces magnetic flux
Classification of DC Generators are classified as
(a) Separately excited d. c generators
(b) self excited d.c generators
Shunt generators
Series generators
Compound generators
Classification of Compound generators
Long shunt compound generator
Short shunt compound generator
Separately excited DC Generators
In this type, the field winding is excited by a separate source of d.c current
The field current can be varied by a variable resistance connected in series as shown in Fig.1
Self excited DC Generators
Energized by the current produced by the generator itself.
Initially, there is always some residual magnetism present in the poles
DC Shunt Generator
In this generator,field windings are connected in parallel with the armature. as shown in
Fig25
Generated e.m.f equation for DC shunt Generator is
Generated EMF, E g=V+I a R a+ Brush drop
DC Series Generator
In this generator,field windings are connected in series with the armature.
The field windings consist of less number of turns with thick wire as shown in
EMF Equation of Series Generator
Fig24
Generated EMF, E g = V+I a(Ra+R se)+ Brush drop
DC Compound Generator
It is a combination of series and shunt windings
Long shunt DC compound Generator
In this generator ,the shunt field is connected parallel to both armature and series field as shown
Fig26
E.M.F Equation for Long shunt Compound DC Generator
Generated EMF, E g=V+I a(Ra+R se)+ Brush drop
Short Shunt Compound Generator
In this generator, the shunt field is connected across the armature terminals only as shown in
Fig27
E.M.F Equation For Short Shunt Dc Compound Generator
Generated EMF, E g=V+I a Ra+I se R se + Brush drop
EMF Equation of a DC Generator
Let us Assume:
f = Flux per pole in webers
Z =Total number of armature conductors
N=Armature rotation in rpm
P=No of poles
A=No of parallel paths
Eg= e.m.f induced in any one of the parallel paths of armature
Average e.m. f generated per conductor = N df / dt
Flux cut per conductor = fP webers
No. of revolutions per second = N/60
Time for one revolution = dt = 60/N
Rate of change of flux linkage
= Flux cut per conductor/sec
df / dt = (fP/60) / N = (fPN) / 60 wb/sec
EMF generated per conductor per second
= (fPN) / 60 volts
EMF generated for Z conductors = ( fPNZ ) / 60volts
No. of parallel paths = A
For lap winding, A = P
For wave winding, A = 2
\Generated e. m. f, Eg = (fNZP) / (60*A)volts
DC GENERATOR WINDINGS
ARMATURE WINDING
• Armature winding is the collection of conductors connected in some fashion.
SHUNT FIELD WINDING
• Shunt field winding has many number of turns
• Shunt winding has high resistance compared to other windings of dc generator
• Shunt winding must be connected in parallel to the armature
SERIES FIELD WINDING
• Series field winding has less number of turns
• Series field winding has low resistance
• Series field winding must be connected in series with the armature
INTERPOLE WINDING
• Interpole winding has very less number of turns
• Interpole winding has very low resistance compared to series and shunt field windings
• Interpole winding must be connected in series with the armature
Voltage drops in a DC Generator
• In any electrical circuit voltage drop occurs in series elements only
• In DC generator voltage drop occurs in series elements only
SERIES ELEMENTS OF A
DC GENERATORS
• Armature
• Brushes
• Series field
• Load
ARMATURE VOLTAGE DROP
• When armature current (Ia) flows through the armature winding having a resistance (Ra), some voltage drop occurs in the armature winding.
• Armature voltage drop = armature current X armature resistance
• Va = Ia.Ra
SERIES FIELD VOLTAGE DROP
• When a series field current (Ise) flows through the series field winding (Rse),series field drop occurs in the series field winding
• Series field drop = series field current × series field resistance
• Vse = Ise Rse
BRUSH CONTACT VOLTAGE DROP
• It is the voltage drop due to brush contact resistance
Brush contact drop depends on
• Value of contact resistance
• Amount of armature current
Ø But brush contact voltage drop is considered as constant for all loads
Brush contact for different materials
• For metal graphite brushes, 0.5V
• For carbon brushes, 2V
Shunt Generator
Fig28
Field winding connected in parallel to armature
• Ish =
• Ia = Ish + I
• Eg = V + IaRa
Field winding connected in parallel to armature
Field winding is connected in series with armature
• Ia = Ise = I
• Eg = V + IaRa + IsRse
Long Shunt Compound Generator
Both series and shunt windings are present
• Ia = Ise = I
• Eg = V + IaRa + IseRse
Short Shunt Compound Generator
Both series and shunt windings are present
• Vsh = V+Ise Rse
• Ia = Ise +Ise
• Ise = I
• Ish =
• Eg = Vsh + IaRa + Brush drop
Total Losses In D.C Machines
COPPER LOSS
Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other electrical devices
Copper loss occur in three ways :
1. Armature copper loss
2. Field copper loss
3. Brush contact copper loss
ARMATURE COPPER LOSS
Ø Armature COPPER LOSS = Ia2Ra
Ø Where ‘Ra’ = resistance of armature
Ø This loss is up to 30 to 40% of full load losses
FIELD COPPER LOSS
IN SHUNT GENERATORS :
Field copper loss = Ish2Rsh
= V Ish
Where Rsh is the shunt field resistance
SHUNT FIELD COPPER LOSS IS PRACTICALLY CONSTANT
IN SERIES GENERATORS :
• Field copper loss =I2se * Rse
• Where Rse is the resistance of the series field winding
• Field copper loss is about 20 to 30 % of full load losses
BRUSH CONTACT POWER LOSS
Ø This loss is due to brush contact resistance
Ø It is usually included in the armature copper loss
IRON LOSS
• Due to rotation of the iron core in the magnetic field some losses take place in the armature core and they are known as iron or core losses .
Iron losses consists of
. Hysteresis losses
. Eddy current loss
Hysteresis Loss
Hysteresis meansLag behind
Hysteresis loss is a heat loss caused by the magnetic properties of the armature
• This loss is due to reversal of magnetization of armature core
• When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field
When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction
• This in turns produces heat so the power is lost in the form of heat
HYSTERESIS LOSS DEPENDS ON
Volume of iron core (v)
2) Maximum volume of flux density (Bmax )
3) Frequency of magnetic reversals (f)
4) Grade of iron
HYSTERESIS LOSS GIVEN BY STEINMETZ FORMULA
Hysteresis loss Wh =ŋ* B1.6max * f * v watt
Where
v = Volume of iron in m3
ŋ=Steinmetz hysteresis co-efficient
TO REDUCE HYSTERESIS LOSS
• Silicon steel laminations are used
• Annealing process also reduces the hysteresis loss
Eddy current loss
• When the armature core rotates it cuts the magnetic flux
• Hence an emf is induce in the body of the core due to laws of electromagnetic induction
• This emf sets up large current in the body of the core due to its small resistance
• This current is known as eddy current
• The power loss due to flow of eddy current is known as eddy current loss
• Eddy current loss is considerably high if solid iron core is used.
• Eddy current loss can be reduced by using laminated core.
EDDY CURRENT LOSS DEPENDS ON
1. Volume of Iron core
2. Maximum value of flux density
3. Frequency of magnetic reversals
4. Thickness of each lamination
EDDY CURRENT LOSS FORMULA
We=K * B2max f² * t² * v watt
Where
Bmax=maximum flux density
f=frequency of magnetic reversals
t=thickness of each lamination
v=volume of armature core
CORE LOSSES
• Hysteresis and eddy current losses are practically constant for shunt and compound wound generators, as the field current is approximately constant.
• Iron loss is about 20 to30% of full load losses.
MECHANICAL LOSS
This loss consist of 1.Friction loss
2.Windage lossMechanical loss is about 10 to 20% of full load losses.
FRICTION LOSS
• Friction loss occur at bearings and commutator or brushes.
• Friction losses depends on
1. Brush pressure
2. Coefficient of friction
3. Type of bearing
4. Lubrication
5. Speed
• Friction loss is approximately proportional to speed.
WINDAGE LOSS
OR
WIND FRICTION LOSS
• It is the power required to circulate air through the machine and ventilating ducts.
• Windage loss is approximately proportional to the square of the speed.
STRAY LOSSES
Ø Magnetic( iron) and Mechanical losses are collectively know as stray losses
Ø Stray losses can also be called as rotational losses
CONSTANT LOSSES
or
STANDING LOSSES
• Field copper losses is constant for shunt and compound generators
Stray losses and shunt copper loss are together known as constant losses or standing losses ( Wc )
VARIABLE LOSSES
• Armature copper loss (Ia2Ra) is known as variable loss because it varies with load current
TOTAL LOSS IN A D.C GENARATOR
Total loss =variable loss + constant loss
INPUT POWER
• DC generator input power is mechanical power
• DC generator is generally coupled to a driving engine
• The driving engine supplies mechanical power to the dc generator power
• Mechanical power input supplied to the dc generator, Some power is lost in the form of iron & frictional losses.
• The remaining power appears in armature as electrical power
• This power can also be called as electrical power developed in the armature
Electrical power developed in armature
• Electrical power developed in armature =EgIa watt
• Electrical power developed in armature =
[mechanical power input] - [iron & frictional losses]
• Out of this Electrical power developed in the armature , some power is lost in the armature winding resistance and this loss is called as armature copper loss
• The remaining power appears as electrical power output
• Electrical power output =
[Electrical power developed]-[Armature copper loss]
Electrical power output
• Electrical power output is the power that appears across the load
• Electrical power output = VI watt
where
V= is the terminal voltage
I= is the load current
Condition for maximum efficiency
Generator output = VI
Generator input = output + losses
= output + (armature cu loss + constant losses)
= VI + Ia2 Ra + Wc
= VI + (I + Ish)2 Ra+Wc
Ish is negligible as compared to load current , then Ia=I
Generator output = VI + I2 Ra +Wc
Efficiency , = output
input
= VI
VI + I2Ra + Wc
Divide numerator and denominator by VI
1
=
1+ IRa + Wc
V VI
Efficiency is maximum when denominator is minimum,
I,e when d IRa + Wc = 0
dI V VI
Ra - Wc = 0
V VI2
I2Ra = Wc
Variable loss = Constant loss
Hence , generator efficiency is maximum when
variable loss = constant loss
Example: 1
A shunt generator delivers 450A at 230V and the
resistance of the shunt field and armature are 50Ω and
0.03Ω respectively. Calculate the generated emf ?
Solution:
Given data:
Load current, IL= 450 A
Voltage, V= 230 V
Shunt field, resistance, Rsh= 50
Armature resistance, Ra= 0.03
Current through shunt field winding ,Ish = 230/50
= 4.6A
Load current, I = 450A
Armature current ,Ia = I + Ish
= 450 + 4.6 A
= 454.6 A
Armature voltage drop = Ia Ra = 454.6×0.03
= 13.6 V
Generated emf, Eg = V + Ia Ra
= 230 + 13.6 V
= 243.6 V
Example: 2
A long-shunt compound generator delivers a load current of 50A at 500V and has armature, series, and shunt field resistances of 0.05Ω, 0.03Ω and 250Ω respectively. Calculate the generated voltage and the armature current. Allow 1V per brush for contact drop
Solution:
Given data:
Load constant, IL= 50 A
Load voltage, V= 500 volts
Series field resistance, Rse= 0.03
Armature resistance, Ra= 0.05
Shunt field resistance, Rsh= 250
Brush drop, = 1 volt per brush.
Shunt field current, Ish= V
Rsh
= = 2 A
Armature current, Ia= IL + Ish
= 50+2 = 52 A
Generated emf ; Eg = V + Ia (Ra + Rse) + Brush drop
= 500 + 52(0.05 + 0.03) + 2
= 506.16 volts
• TRANSFORMER on the street end.
Ø From where the transformer is getting the supply ?
• SUBSTATION
Ø From where the substation is getting to supply ?
• Power station
Ø What is the device which generates Electrical Energy ?
• Generator
Ø What are the basic requirements generating EMF ?
• Conductor
• Magnetic Field
• Relative Speed
How EMF is Induced ?
Ø When ever a conductor cuts the Magnetic field an EMF is induced in it ( Faradays Ist law of Electro magnetic induction )
What are the types of Induced EMF ?
Ø Dynamically Induced EMF
Ø Self Induced EMF
What do you mean my the word Dynamics ?
Ø Dynamics is related to motion
DYNAMICALLY INDUCED EMF
Ø Emf is induced when the flux linking a conductor changes
Ø In dynamically induced emf , the conductor moves in a stationary magnetic field.
Ø The emf is induced in the conductor when it is in motion
Fig1
Dynamically Induced Emf Derivation
Ø Magnetic flux density = B wb/ m ²
Ø The conductor is moving with a velocity of ‘v’ m/s
Ø Conductor length : l m
Fig2
Ø If the conductor moves through a small distance dx in time dt seconds
Ø Area swept by the conductor = l ×dx
Ø Flux cut by conductor dø = flux density × area swept = (B l dx ) wb
Ø According to ‘faraday’s laws ,‘emf’ induced in the conductor is given by
E = N dø / dt
= N *B * l * dx / dt
= B * l * dx / dt (N=1)
= B l v volts
Ø If the conductor moves at an angle’θ‘ to the magnetic field
The EMF E = B * l * v * Sinθ
FLEMINGS RIGHT HAND RULE
Statement
Ø Stretch out the fore finger, middle finger and thumb of your right hand, so that they are at right angles to one another.
Ø Fig3
If the forefinger points in the direction of magnetic field, Thumb in the direction of motion of the Conductor and middle finger in the direction of Induced emf
FLEMINGS RIGHT HAND RULE
Thumb points motion
Forefinger points field
Middle finger points Induced Emf
Application of Flemings right hand rule
Ø Consider a conductor AB moving upwards at right angles to a uniform magnetic field as shown in Fig.2
Ø By Flemings right hand rule the direction of induced current is from B to A.
Ø If the motion of the conductor is downwards, the direction of induced current is from A to B.
Electromechanical Energy Conversion
• A machine is required to convert energy from one form to another form.
• Energy conversion is based on the production of dynamically induced e.m.f.
• Conversion is bilateral, electrical to mechanical and mechanical to electrical.
• A D.C. machine which converts mechanical energy to electrical energy is called D.C. generator.
Requirements for production of EMF
Ø Conductors
Ø Magnetic field
Ø Relation Motion of conductor or magnetic field.
Production of dynamically induced EMF
Ø Conductor moves in a stationery magnetic field
an EMF is induced in a conductor
fig4
Construction of Simple Loop Generator
Ø A simple loop generator is
l Single turn rectangular copper coil
l Placed between poles of a magnet
l Used to study the generation of emf in D.C. generator
Fig5
WORKING OF SIMPLE LOOP GENERATOR
Ø Consider a single turn rectangular copper coil is rotating in a magnetic field.
Ø The coil occupies different angular positions during its rotation.
Ø When the coil is rotated through an angle of 90° the emf induced in the coil is maximum.
Ø Rotate the coil further by an angle of 180°.The emf induced in the coil will be zero.
Ø Rotate the coil further by an angle of 270°.The emf induced in the coil is maximum in the reverse direction.
Ø We conclude that the nature of the emf induced is alternating.
D.C. GENERATOR
Fig6
PRINCIPLE OF WORKING OF D.C. GENERATOR
Principle: Based on Faraday’s laws of electromagnetic induction. That is,
Ø When ever a conductor cuts magnetic flux the dynamically induced emf is produced according to Faradays laws of electro magnetic induction.
Ø In generator, the rate of change of flux linkages is due to rotation of the conductor. That means emf generated is dynamically emf.
Ø A coil abcd is rotated clock wise between two
magnetic poles N and S.
Ø Due to cutting of magnetic flux lines emf is
induced in the coil which is alternating.
Fig7
Ø The alternating emf is converted to d.c by means of split rings. (or commutator)
Ø This emf causes current to flow if the conductor circuit is closed. (is shown in Fig.1)
Important parts of D.C. Generator
Ø Yoke
Ø Pole core and pole shoe
Ø Field coils
Ø Armature core
Ø Armature winding
Ø Commutator
Ø Brushes and brush holders
Ø Bearings
Fig8
Yoke
Ø Yoke is the outermost cylindrical frame as shown in
Ø It is made up of cast iron
Ø It provides the low reluctance path to magnetic flux
Pole core and Pole shoe
Ø Pole core is made-up of cast iron as shown in
Ø Pole shoe is made-up of cast iron laminations
Ø Pole shoe supports the field coils and spreads the magnetic flux in the air- gap
Field Coils
Ø Field coils are made-up of enamel coated copper or aluminum wire as shown in
Ø Placed on pole core
Ø Produces flux when excited
Armature core
Ø It is made up silicon steel laminations
Ø Armature core is cylindrical in shape as shown in
Ø Slots are provided on the outer periphery
Fig9
Armature winding
Ø Armature winding is an insulated copper wire.
Ø They are formerly would in the form of rectangular coils
Ø They are placed in the slots as shown
Ø Fig10
Commutator
Ø It is cylindrical in shape
Ø It is made-up of high conductivity hard drawn copper
Ø It facilitates the collection of current from the armature conductors as shown in Fig.11
Brushes and Brush holders
Ø Brushes are made-up of high grade carbon as shown in
Ø Main function of the brush is to collect current from the commutator and supply it to the external load
BEARINGS
The function of the bearings is to reduce friction between the rotating and the stationery part of the machine
Ø Types of bearings:
(a)Bush bearings
(b)Ball bearings
(c)Roller bearings
Fundamental parts of a direct current machine
Fig12
D.C. Generator
Ø D.C. Generator is a machine which converts mechanical energy into electrical energy
Ø The conversion process is based on the dynamically induced e.m.f
Basic requirements for EMF
Ø Conductor
Ø Magnetic flux
Ø Relative motion between the conductor and flux
Conductors
Ø Conductors are armature windings placed on the periphery of the armature core
Magnetic Flux
Ø The field windings are placed on the pole core
Ø Field windings are supplied with d. c. current, produces magnetic flux.
Field Excitation means giving d. c. current to the field winding.
Rotating of Armature
Ø Armature is rotated by means of prime mover
Ø Prime mover may be a turbine or diesel engine
Production of EMF
Ø Armature is rotated in the magnetic flux an emf produces e.m.f. is produced.
Ø EMF is produced according to faradays laws of electromagnetic induction
The basic difference between a Simple loop generator and D.C generator is slip ring and split ring (commutator)
Fig13
WORKING OF D.C. GENERATOR
Ø A coil abcd is rotated clock wise between two
magnetic poles N and S.
Ø Due to cutting of magnetic flux lines emf is
induced in the coil which is alternating.
Ø The alternating emf is converted to d.c by means of split rings. (or commutator)
Ø This emf causes current to flow if the conductor circuit is closed. (is shown in Fig.1)
Important Terms
Ø Pole pitch
Ø Conductor
Ø Coil
Ø Coin span or coil pitch
Ø Full pitched coil
Ø Fractional pitch
Ø Pitch of the winding
Ø Back pitch
Ø Front pitch
Ø Resultant pitch
Ø Commutator pitch
Pole Pitch-YP
Ø It is The distance between the two adjacent poles
Ø It is also equal to the number of armature conductors per pole
Example
Ø If there are 48 conductors and 4 poles
Ø The pole pitch
Ø Yp = Z / P = 36 / 4 = 9
Conductor
Ø It is a wire of certain length lying in a magnetic field
Ø In which an e.m.f is induced
Coil
Ø The two conductors AB and CD
Ø Their end connection make one coil of armature winding.
Fig14
Coil Pitch or Span-YS
Ø It is defined as the distance measured in terms of armature slots between two sides of a coil as shown in Fig .2
Ø The coil Pitch or Span of coil “A” is ( 5 – 1) =4
Ø The Span of coil “B” is ( 6 – 3 ) = 3
Fig15
Full pitched coil
Ø If the coil pitch is equal to pole pitch, the winding is called full pitched winding
Ø For example:-
Ø If there are 16 conductors and 4 poles then Pole pitch = 16 / 4 = 4
Ø The coil span of coil “A” = ( 5 - 1)
= 4
This winding is called Full pitched winding
Fig16
Fractional pitch coil
If the coil pitch is less than the pole pitch the winding is called fractional pitch coil
In this, the coil span is180 electrical degrees.
Ex: If there are 36 slots and 4 poles
Coil pitch=36/4=9slots
Back pitch YB
Ø It is distance measured in terms of the number of armature conductors between the two coil sides of a coil measured around the back of the armature
Fig17
Front pitch-YF
Ø It is the distance between two coil sides connected to the same commutator segment
Fig18
Pitch of winding Y
Ø It is defined as the distance between the beginning of two consecutive turns
Ø Y=YB-YF ---------------Lap winding
Ø Y=YB+YF ---------------Wave winding
Resultant pitch-YR
Ø It is defined as the distance in terms of number of coil sides between the start of one coil and start of the next coil, coil to which it is connected
Fig19
Commutator pitch -YC
Ø It is defined as the distance measured In terms of commutator segments between the segments to which The two ends of a coil are connected
Fig20
Single layer winding
Ø It is the winding in which one conductor or one coil side is placed in each
armature slot as shown in Fig.3
fuig21\
Two layer winding
It is the winding in which two conductors or coil sides per slot arranged in two layersAs shown
Fi22
Types of windings
Lap winding
Wave winding
Fig23
Simplex Lap winding
Ø In a lap winding the finishing end of one coil
Ø Is connected via the commutator segment
Ø To the starting end of the adjacent coil Situated under the same pole
Ø In this way all the coils are connected
Simplex wave winding
Ø In a wave winding a coil side under one pole is connected to a second coil side
Ø Which occupies approximately the same position
Ø Under the next pole through back connection
Ø Thus the wave winding progresses in
Ø One direction round the armature in a series of
What do you mean by excitation
Excitation means giving d.c supply to the field winding which in turn produces magnetic flux
Classification of DC Generators are classified as
(a) Separately excited d. c generators
(b) self excited d.c generators
Shunt generators
Series generators
Compound generators
Classification of Compound generators
Long shunt compound generator
Short shunt compound generator
Separately excited DC Generators
In this type, the field winding is excited by a separate source of d.c current
The field current can be varied by a variable resistance connected in series as shown in Fig.1
Self excited DC Generators
Energized by the current produced by the generator itself.
Initially, there is always some residual magnetism present in the poles
DC Shunt Generator
In this generator,field windings are connected in parallel with the armature. as shown in
Fig25
Generated e.m.f equation for DC shunt Generator is
Generated EMF, E g=V+I a R a+ Brush drop
DC Series Generator
In this generator,field windings are connected in series with the armature.
The field windings consist of less number of turns with thick wire as shown in
EMF Equation of Series Generator
Fig24
Generated EMF, E g = V+I a(Ra+R se)+ Brush drop
DC Compound Generator
It is a combination of series and shunt windings
Long shunt DC compound Generator
In this generator ,the shunt field is connected parallel to both armature and series field as shown
Fig26
E.M.F Equation for Long shunt Compound DC Generator
Generated EMF, E g=V+I a(Ra+R se)+ Brush drop
Short Shunt Compound Generator
In this generator, the shunt field is connected across the armature terminals only as shown in
Fig27
E.M.F Equation For Short Shunt Dc Compound Generator
Generated EMF, E g=V+I a Ra+I se R se + Brush drop
EMF Equation of a DC Generator
Let us Assume:
f = Flux per pole in webers
Z =Total number of armature conductors
N=Armature rotation in rpm
P=No of poles
A=No of parallel paths
Eg= e.m.f induced in any one of the parallel paths of armature
Average e.m. f generated per conductor = N df / dt
Flux cut per conductor = fP webers
No. of revolutions per second = N/60
Time for one revolution = dt = 60/N
Rate of change of flux linkage
= Flux cut per conductor/sec
df / dt = (fP/60) / N = (fPN) / 60 wb/sec
EMF generated per conductor per second
= (fPN) / 60 volts
EMF generated for Z conductors = ( fPNZ ) / 60volts
No. of parallel paths = A
For lap winding, A = P
For wave winding, A = 2
\Generated e. m. f, Eg = (fNZP) / (60*A)volts
DC GENERATOR WINDINGS
ARMATURE WINDING
• Armature winding is the collection of conductors connected in some fashion.
SHUNT FIELD WINDING
• Shunt field winding has many number of turns
• Shunt winding has high resistance compared to other windings of dc generator
• Shunt winding must be connected in parallel to the armature
SERIES FIELD WINDING
• Series field winding has less number of turns
• Series field winding has low resistance
• Series field winding must be connected in series with the armature
INTERPOLE WINDING
• Interpole winding has very less number of turns
• Interpole winding has very low resistance compared to series and shunt field windings
• Interpole winding must be connected in series with the armature
Voltage drops in a DC Generator
• In any electrical circuit voltage drop occurs in series elements only
• In DC generator voltage drop occurs in series elements only
SERIES ELEMENTS OF A
DC GENERATORS
• Armature
• Brushes
• Series field
• Load
ARMATURE VOLTAGE DROP
• When armature current (Ia) flows through the armature winding having a resistance (Ra), some voltage drop occurs in the armature winding.
• Armature voltage drop = armature current X armature resistance
• Va = Ia.Ra
SERIES FIELD VOLTAGE DROP
• When a series field current (Ise) flows through the series field winding (Rse),series field drop occurs in the series field winding
• Series field drop = series field current × series field resistance
• Vse = Ise Rse
BRUSH CONTACT VOLTAGE DROP
• It is the voltage drop due to brush contact resistance
Brush contact drop depends on
• Value of contact resistance
• Amount of armature current
Ø But brush contact voltage drop is considered as constant for all loads
Brush contact for different materials
• For metal graphite brushes, 0.5V
• For carbon brushes, 2V
Shunt Generator
Fig28
Field winding connected in parallel to armature
• Ish =
• Ia = Ish + I
• Eg = V + IaRa
Field winding connected in parallel to armature
Field winding is connected in series with armature
• Ia = Ise = I
• Eg = V + IaRa + IsRse
Long Shunt Compound Generator
Both series and shunt windings are present
• Ia = Ise = I
• Eg = V + IaRa + IseRse
Short Shunt Compound Generator
Both series and shunt windings are present
• Vsh = V+Ise Rse
• Ia = Ise +Ise
• Ise = I
• Ish =
• Eg = Vsh + IaRa + Brush drop
Total Losses In D.C Machines
COPPER LOSS
Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other electrical devices
Copper loss occur in three ways :
1. Armature copper loss
2. Field copper loss
3. Brush contact copper loss
ARMATURE COPPER LOSS
Ø Armature COPPER LOSS = Ia2Ra
Ø Where ‘Ra’ = resistance of armature
Ø This loss is up to 30 to 40% of full load losses
FIELD COPPER LOSS
IN SHUNT GENERATORS :
Field copper loss = Ish2Rsh
= V Ish
Where Rsh is the shunt field resistance
SHUNT FIELD COPPER LOSS IS PRACTICALLY CONSTANT
IN SERIES GENERATORS :
• Field copper loss =I2se * Rse
• Where Rse is the resistance of the series field winding
• Field copper loss is about 20 to 30 % of full load losses
BRUSH CONTACT POWER LOSS
Ø This loss is due to brush contact resistance
Ø It is usually included in the armature copper loss
IRON LOSS
• Due to rotation of the iron core in the magnetic field some losses take place in the armature core and they are known as iron or core losses .
Iron losses consists of
. Hysteresis losses
. Eddy current loss
Hysteresis Loss
Hysteresis meansLag behind
Hysteresis loss is a heat loss caused by the magnetic properties of the armature
• This loss is due to reversal of magnetization of armature core
• When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field
When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction
• This in turns produces heat so the power is lost in the form of heat
HYSTERESIS LOSS DEPENDS ON
Volume of iron core (v)
2) Maximum volume of flux density (Bmax )
3) Frequency of magnetic reversals (f)
4) Grade of iron
HYSTERESIS LOSS GIVEN BY STEINMETZ FORMULA
Hysteresis loss Wh =ŋ* B1.6max * f * v watt
Where
v = Volume of iron in m3
ŋ=Steinmetz hysteresis co-efficient
TO REDUCE HYSTERESIS LOSS
• Silicon steel laminations are used
• Annealing process also reduces the hysteresis loss
Eddy current loss
• When the armature core rotates it cuts the magnetic flux
• Hence an emf is induce in the body of the core due to laws of electromagnetic induction
• This emf sets up large current in the body of the core due to its small resistance
• This current is known as eddy current
• The power loss due to flow of eddy current is known as eddy current loss
• Eddy current loss is considerably high if solid iron core is used.
• Eddy current loss can be reduced by using laminated core.
EDDY CURRENT LOSS DEPENDS ON
1. Volume of Iron core
2. Maximum value of flux density
3. Frequency of magnetic reversals
4. Thickness of each lamination
EDDY CURRENT LOSS FORMULA
We=K * B2max f² * t² * v watt
Where
Bmax=maximum flux density
f=frequency of magnetic reversals
t=thickness of each lamination
v=volume of armature core
CORE LOSSES
• Hysteresis and eddy current losses are practically constant for shunt and compound wound generators, as the field current is approximately constant.
• Iron loss is about 20 to30% of full load losses.
MECHANICAL LOSS
This loss consist of 1.Friction loss
2.Windage lossMechanical loss is about 10 to 20% of full load losses.
FRICTION LOSS
• Friction loss occur at bearings and commutator or brushes.
• Friction losses depends on
1. Brush pressure
2. Coefficient of friction
3. Type of bearing
4. Lubrication
5. Speed
• Friction loss is approximately proportional to speed.
WINDAGE LOSS
OR
WIND FRICTION LOSS
• It is the power required to circulate air through the machine and ventilating ducts.
• Windage loss is approximately proportional to the square of the speed.
STRAY LOSSES
Ø Magnetic( iron) and Mechanical losses are collectively know as stray losses
Ø Stray losses can also be called as rotational losses
CONSTANT LOSSES
or
STANDING LOSSES
• Field copper losses is constant for shunt and compound generators
Stray losses and shunt copper loss are together known as constant losses or standing losses ( Wc )
VARIABLE LOSSES
• Armature copper loss (Ia2Ra) is known as variable loss because it varies with load current
TOTAL LOSS IN A D.C GENARATOR
Total loss =variable loss + constant loss
INPUT POWER
• DC generator input power is mechanical power
• DC generator is generally coupled to a driving engine
• The driving engine supplies mechanical power to the dc generator power
• Mechanical power input supplied to the dc generator, Some power is lost in the form of iron & frictional losses.
• The remaining power appears in armature as electrical power
• This power can also be called as electrical power developed in the armature
Electrical power developed in armature
• Electrical power developed in armature =EgIa watt
• Electrical power developed in armature =
[mechanical power input] - [iron & frictional losses]
• Out of this Electrical power developed in the armature , some power is lost in the armature winding resistance and this loss is called as armature copper loss
• The remaining power appears as electrical power output
• Electrical power output =
[Electrical power developed]-[Armature copper loss]
Electrical power output
• Electrical power output is the power that appears across the load
• Electrical power output = VI watt
where
V= is the terminal voltage
I= is the load current
Condition for maximum efficiency
Generator output = VI
Generator input = output + losses
= output + (armature cu loss + constant losses)
= VI + Ia2 Ra + Wc
= VI + (I + Ish)2 Ra+Wc
Ish is negligible as compared to load current , then Ia=I
Generator output = VI + I2 Ra +Wc
Efficiency , = output
input
= VI
VI + I2Ra + Wc
Divide numerator and denominator by VI
1
=
1+ IRa + Wc
V VI
Efficiency is maximum when denominator is minimum,
I,e when d IRa + Wc = 0
dI V VI
Ra - Wc = 0
V VI2
I2Ra = Wc
Variable loss = Constant loss
Hence , generator efficiency is maximum when
variable loss = constant loss
Example: 1
A shunt generator delivers 450A at 230V and the
resistance of the shunt field and armature are 50Ω and
0.03Ω respectively. Calculate the generated emf ?
Solution:
Given data:
Load current, IL= 450 A
Voltage, V= 230 V
Shunt field, resistance, Rsh= 50
Armature resistance, Ra= 0.03
Current through shunt field winding ,Ish = 230/50
= 4.6A
Load current, I = 450A
Armature current ,Ia = I + Ish
= 450 + 4.6 A
= 454.6 A
Armature voltage drop = Ia Ra = 454.6×0.03
= 13.6 V
Generated emf, Eg = V + Ia Ra
= 230 + 13.6 V
= 243.6 V
Example: 2
A long-shunt compound generator delivers a load current of 50A at 500V and has armature, series, and shunt field resistances of 0.05Ω, 0.03Ω and 250Ω respectively. Calculate the generated voltage and the armature current. Allow 1V per brush for contact drop
Solution:
Given data:
Load constant, IL= 50 A
Load voltage, V= 500 volts
Series field resistance, Rse= 0.03
Armature resistance, Ra= 0.05
Shunt field resistance, Rsh= 250
Brush drop, = 1 volt per brush.
Shunt field current, Ish= V
Rsh
= = 2 A
Armature current, Ia= IL + Ish
= 50+2 = 52 A
Generated emf ; Eg = V + Ia (Ra + Rse) + Brush drop
= 500 + 52(0.05 + 0.03) + 2
= 506.16 volts
