Download EE6352_Unit_1

ELECTRICAL ENGINEERING
AND INSTRUMENTATION
Regulation : R2013
Subject code : EE6352
Syllabus
UNIT I
DC MACHINES
9
hours
Three phase circuits, a review. Construction of
DC machines – Theory of operation of DC
generators – Characteristics of DC generatorsOperating principle of DC motors – Types of
DC motors and their characteristics – Speed
control of DC motors- Applications.
Unit – 1
DC MACHINES
Three phase circuits
• Three phase circuits is a polyphase system
when 3 phases are used together from the
generator to the load
• Each phase are having a phase difference of
120°
• If the load is single phase, then one phase can
be taken from the three phase circuit and the
neutral can be used as ground to complete the
circuit.
Waveform
Why three phase preferred over
single phase?
Single phase system
• The conductor needed for a
single phase circuit is very
much less.
• The instantaneous power
falls to zero when the single
phase supply is
disconnected.
Three phase system
• The conductor needed in
three phase circuit is 75%
that of conductor needed in
single phase circuit.
• The instantaneous power
exists even after one phase
is disconnected as the net
power from all the phases
gives a continuous power to
the load.
Types of connections
• Star Connection
• Delta Connection
Star Connection
• Vph = VL /
• Iph = IL
• 3 phase wires and 1
neutral wire from the
star point.
• Preferred for long
distances.
Delta Connection
• Vph = VL
• Iph = IL/
• 3 wires from the phases
are used.
• No Neutral wire
• Preferred for short
distances.
Types of currents
• Balanced current – Equal current flows
through all the 3 phases.
• Unbalanced current - Unequal current flowing
through all the 3 phases.
Y-Δ arrangement
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Star-Star connection
Delta-Delta connection
Star – Delta connection
Delta – Star connection
Simple loop generator
Principle of Operation
Faraday’s Law of Electromagnetic induction:When a current carrying conductor is placed in
a magnetic field an EMF is induced.
Generator :- Fleming’s Right hand rule
Motor :- Fleming’s Left hand rule
DC Machines
• DC machines convert electrical energy to
mechanical energy and vice versa.
• This process of conversion is called as
electromechanical energy conversion.
• If the conversion is from mechanical to electrical
energy, the machine is said to act as a generator.
• If the conversion is from electrical to mechanical
energy, the machine is said to act as a motor.
DC machines
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Construction
Types
Theory of operation
Characteristics
Speed control of DC motor
Applications
DC machine Construction
DC machine Construction
DC machine Construction
DC machine Construction
DC machine Construction
Armature of a DC machine
Armature of a DC machine
Armature of a DC machine
Dismantled view of a DC machine
Dismantled view of a DC machine
Dismantled view of a DC machine
Parts of a DC machine
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Yoke
Pole core & Pole shoes
Pole coils
Armature core
Armature windings
Commutator
Brushes & Bearings
Yoke
• Outermost frame
Purpose:
• Provides mechanical support
• Carries the magnetic flux produced by the poles.
Materials used:
• Small machines – Made of Cast Iron
• Larger machines – Made of Cast steel or rolled
steel
Pole cores & Pole shoes
• Spreads out the flux in
the air gap
• Reduces the reluctance
of the magnetic path
• Supports the exciting
coils or field coils
Pole coil or field coil or exciting coil
• Copper wires or strips wound around the pole
core.
• When current is passed thorugh these coils
they produce magnetic flux surrounding the
armature conductors.
Materials used:
• Made up of copper
Armature core
Armature windings
• Usually former wound in the form of flat
rectangular coils.
• Various conductors of the coil are insulated from
each other
• Conductors are placed in the armature slots
• Slot insulation is folded over the armature
conductors.
Materials used:
• Made up of copper
Commutator
• Facilitates collection of current from the armature conductors
• The commutator & brushes arrangement converts AC to DC
It is of cylindrical structure and built up of wedge-shaped segments of high-conductivity
hard-drawn or drop forged copper.
• These segments are insulated from each other by
thin layers of mica.
•The number of segments is equal to the number of armature coils.
Each commutator segment is connected to the armature conductor by means of a copper
lug or riser.
Brushes & bearings
• Collects current from the commutator
• They are housed in a brush holder.
• The brush holder is mounted on a spindle and
can slide in the rectangular box open at both
ends.
• The number of brushes per spindle depends on
the magnitude of the current to be collected
from the commutator.
Pole Pitch
• Distance between 2 adjacent poles.
• It is equal to the number of armature
conductors (or armature slots) per pole.
• If there are 48 conductors and 4 poles, the pole
pitch is 48/4 = 12.
Conductor
The length of a wire lying in the magnetic field and in
which an emf is induced, is called a conductor (or inductor)
as, for example, length AB or CD in the following figure.
Coil & Winding element
• The two conductors AB and CD along with their end
connections constitute one coil of the armature
winding.
• The side of the coil is called the winding element.
• No. of winding elements = 2 x No. of Coils
• The coil may be single turn coil or multi-turn coil.
• Multi-turn coil may have many conductors per coil
side.
• The group of wires or conductors constituting a coil
side of a multi-turn coil is wrapped with a tape as a unit
and is placed in the armature slot.
Coil span or Coil pitch (YS)
• Coil span is the distance between 2 sides of the
coil.
Full pitched winding
• If coil span or coil pitch
= pole pitch, the winding
is called full pitched wdg.
• Full pitch means coil span
is 180 ͦ electrical degrees.
• In full pitch, coil sides lie
under opposite poles and
their induced emf is scalar
sum.
• So maximum emf is
induced in full pitch
winding
Chorded wdg or Short pitched wdg or
Fractional pitched wdg
• The pitch of the winding is
less than pole pitch
Single Layer Winding : Winding in which one conductor or one coil side is placed in
each armature slot
Double Layer Winding : Two conductors or coil sides per slot is placed in each
armature slot
Types of windings
• Lap winding
• Wave winding
Lap winding
• Laps back with its
succeeding coils.
• For Simplex lap
winding, No.of parallel
paths = No. of poles
• For duplex winding, No.
of parallel paths = 2x
No. of poles
Lap Winding
Different pitches for Lap winding
Back Pitch (YB) – No of coil sides
or slots spanned by the back
end connections. (Z/P)
Front Pitch (YF) - No of coil sides
or slots spanned by the front
end connections.
Resultant Pitch (YR) – Distance
between the beginning of one
coil to the beginning of next
coil to which it is connected.
Commutator Pitch (YC) – Distance
between the segments to which
the two ends of the coil are
connected. For Lap, (YC = YBYF) & for Wave, (YC = YB+YF)
Equalizer rings or Equalizer
connections
• A thick copper conductor
connecting the
equipotential points of lap
winding for equalizing the
potential of different
parallel paths.
• Avoids unequal
distribution of current at
the brushes thereby heling
to get sparkless
commutation.
One Equalizer ring for a pole pair is
used.
Interpoles or Compoles
• Small poles placed in between
the main poles.
• As their polarity is same as that
of main poles, they induce emf
in the coil which helps in
current reversal. The induced
emf is called commutating emf
or reversing emf which opposes
the reactance emf thereby
making commutation sparkless.
• Cross magnetising effect due to
armature reaction is also
neutralised.
Design of Lap Winding
Lap Winding
Advantages:
1. This winding is necessarily required for large current
application because it has more parallel paths.
2. 2. It is suitable for low voltage and high current
generators.
Disadvantages:
1. It gives less emf compared to wave winding. This
winding is required more no. of conductors for giving
the same emf, it results high winding cost.
2. It has less efficient utilization of space in the armature
slots.
Wave winding
• The end of one coil is
not connected to the
beginning of the same
coil but is connected to
the beginning of another
coil of the same polarity
as that of the first coil
Simplex wave winding – Circular form
LAP & WAVE WINDING
Cross-sectional view of Armature
Lap & Wave windings with pitches
Types of DC machines
• DC Generator
• DC motor
DC generator
Losses
Separately excited DC generator
Self excited DC generator
Compound generator
Characteristics of DC generator
• Open circuit characteristics or Magnetization
characteristics or No load characteristics
• Load Characteristics
Internal or Total characteristics
External characteristics
Open Circuit Characteristics
• Eo Vs If
• The data is obtained by
operating the generator
at no load and keeping
speed constant
• Field current is varied
and the corresponding
terminal voltage is
recorded
Relation b/n Eg and Speed
Internal & External Characteristics
Internal & External Characteristics
Characteristics of DC compound
generator
Types of DC motor
Characteristics of DC motor
• Electrical or Internal characteristics
• Mechanical or External characteristics
Characteristics of DC series motor
Characteristics of DC shunt motor
Characteristics of DC Compound motor
Speed control of DC motor
The speed of the DC motor is given by the
relation,
Speed of DC shunt motor can be
controlled by varying
(i) Flux/pole, Φ (Flux Control)
(ii) Resistance Ra of armature circuit (Rheostatic
Control) and
(iii) Applied voltage V (Voltage Control).
Flux control method
Armature control method
Voltage control method
Speed of DC series motor can be
controlled by
• Flux Control method
• Variable resistance method
Flux control method
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Field diverters
Armature diverters
Trapped Field control method
Paralleling Field coils
Field diverters
Armature diverters
Trapped Field control method
Paralleling field coils
Variable resistance control
Applications
• DC series motor – Paper machines, diesel electric propulsion
of ships, in steel rolling mills, hoists, cranes, trolley cars,
conveyer belts
• DC shunt motor – Lathes, centrifugal pumps, reciprocating
pumps, fans, blowers, conveyors, wood working machines,
machine tools, printing press, spinning & weaving machines.
• DC compound motor – Crushes, ice-making machines,
compressors, rolling mills.