Transcript RIKI PAUL

Electrical Machine
Submitted by:


Riki paul(130390111009)
Himanshu Patel(130390111006)
DC Machines
1. Dc Generators
2. Dc Motors
DC Generator
Windings in a DC Generator
Principle Of Operation Of a Dc
Generator.

Fleming's Right Hand Rule

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.
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 90-180, 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.
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.
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.
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.
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.
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.
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.
Armature Core(rotor)
Commutator
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.
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
LAP WINDING & WAVE WINDING
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.
Symbolic representation of dc
generators
Separatly excited DC Generator
Characteristics of Separately
Excited DC Generator
The characteristics is separately excited d.c. generator are divided into two types,
1) Magnetization and
2) Load characteristics.
1.1 Magnetization or Open Circuit Characteristics
The arrangement to obtain this characteristics is shown in the Fig. 1.
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.
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.
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.
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 1.in separately excited generator an
the excitation voltage from the voltage
external supply is must for supplying
generated in it after a time period but not in exciting voltage to the coil
the starting itself,
2.self excited generator works on its own
feedback
2.separately excited generator need a
external source to work
Self Excited Generator
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.
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 serieswound generator. In actual use, the output voltage in a dc shuntwound 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).
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.
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.
Compound Generators
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.
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.
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.
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.
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.
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.
DC MOTOR
Basics of a Electric Motor
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Basics of a Electric Motor
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A Two Pole DC Motor
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A Four Pole DC Motor
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Operating Principle of a DC Machine
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Fleming’s Left Hand Rule Or
Motor Rule
FORE FINGER = MAGNETIC FIELD
900
900
900
MIDDLE FINGER= CURRENT
FORCE = B IAl
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Fleming’s Right Hand Rule Or
Generator Rule
FORE FINGER = MAGNETIC FIELD
900
900
900
MIDDLE FINGER = INDUCED
VOLTAGE
VOLTAGE = B l u
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Action of a Commutator
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Armature of a DC Motor
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Generated Voltage in a DC Machine
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Armature Winding in a DC Machine
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Lap Winding of a DC Machine
Used in high current
low voltage circuits

Number of parallel paths
equals number of brushes
or poles

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Wave Winding of a DC Machine
Used in high voltage
low current circuits

Number of parallel paths
always equals 2

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Magnetic circuit of a 4 pole DC Machine
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Magnetic circuit of a 2 pole DC Machine
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Summary of a DC Machine

1.
2.


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
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Deriving the induced voltage in a
DC Machine
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Deriving the electromagnetic torque in a
DC Machine
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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
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Interaction of Prime-mover DC Generator
and Load
Tdev
Ia
+
m
DC Generator Ea
-
Tpm
Ea is Generated voltage
VL is Load voltage
Tpm is the Torque generated by Prime Mover
Tdev is the opposing generator torque
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Load
Prime-mover
(Turbine)
+
-
VL
Interaction of the DC Motor
and Mechanical Load
+
Ia
Tload
+
VT
- -
m
Ea DC Motor
-
Tdev
Mechanical
Load
(Pump,
Compressor)
Ea is Back EMF
VT is Applied voltage
Tdev is the Torque developed by DC Motor
Tload is the opposing load torque
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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
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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)
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Magnetization Curve
Ea Ka
m

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

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Separately Excited DC Machine
RA
+
Vf
-
Armature
Field Coil
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Shunt Excited DC Machine
Shunt Field Coil
Armature
RA
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Series Excited DC Machine
RA
Armature
Series Field Coil
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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
excited machine

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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

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Effects of Armature Reaction

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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.
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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.

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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.
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Separately Excited DC Generator
Ra
Rf
If
Vf
+
+
RL
Ea
-
Field Coil
Armature
-
-
Field equation: Vf=RfIf
+
Vt
Ia
Armature equation: Vt=Ea-IaRa
Vt=IaRL, Ea=Kam
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Shunt Generators
If
Ia – If
Ia
+
Ea
Shunt Field Coil
-
Field coil has Rfw :
Implicit field resistance
+
Armature
RL
Ra
-
Rfc
Field equation: Vt=Rf If
Armature equation: Vt=Ea-Ia Ra
Rf=Rfw+Rfc
Vt=(Ia – If) RL, Ea=Kam
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Vt
Voltage build-up of shunt generators
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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

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Separately Excited DC Motor
Ra
Rf
If
+
Vf
+
+
Ea
-
Field Coil
Armature
-
Field equation: Vf=RfIf
Ia
Armature equation: Ea=Vt-IaRa
Ea=Kam
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Vt
-
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Separately Excited DC Motor
Torque-speed Characteristics
RA
+
+
Vf
Armature
Mechanical Load
-
-
Field Coil
m
V
R
t
a



T
m
2
K
(K

)
a
a
T
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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
m
T1
T2
V1<V2<V3
T3
V
R
t
a



T
m
2
K
(K

)
a
a
V1
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V2
V3
VT
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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.

T1<T2< T3
m
1
T1
2
 1>  2>  3
V
R
t
a



T
m
2
K
(K

)
a
a
T2
T3
3

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Regions of operation of a Separately
Excited DC Motor
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Separately excited dc motor –Example 2
A separately excited dc motor with negligible armature resistance
operates at 1800 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
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Series Excited DC Motor
Torque-Speed Characteristics
Ra
Rsr
Rae
+
Armature
Series Field Coil
-
V
R

R

R
t
a
sr
ae


m
K
K
T
sr
sr

T
m
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Losses in dc machines
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Losses in dc machines-shunt motor
example
If
Ia – If
Ia
+
+
Vt
Ea
Shunt Field Coil
-
Armature
Field coil has Rfw :
Implicit field resistance
Ra
Rfc
Field equation: Vt=Rf If
Armature equation: Vt=Ea+Ia Ra
Rf=Rfw+Rfc
Ea=Kam
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Mechanical Load
Losses in DC machine
1. Copper Losses or Electrical losses.
2. Iron Losses or Core Losses.
3. Brush Losses.
4. Mechanical Losses.
5. Stray Load Losses.
The End