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 generators- Operating principle of
DC motors Types of DC motors and their characteristics Speed
control of DC motors- Applications.
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Unit 1 DC MACHINES
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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.
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Waveform
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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.
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Types of connections Star Connection Delta Connection
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Star Connection V ph = V L / I ph = I L 3 phase wires and 1
neutral wire from the star point. Preferred for long
distances.
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Delta Connection V ph = V L I ph = I L / 3 wires from the
phases are used. No Neutral wire Preferred for short
distances.
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Types of currents Balanced current Equal current flows through
all the 3 phases. Unbalanced current - Unequal current flowing
through all the 3 phases.
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Y- arrangement Star-Star connection Delta-Delta connection Star
Delta connection Delta Star connection
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Simple loop generator
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Principle of Operation Faradays Law of Electromagnetic
induction:- When a current carrying conductor is placed in a
magnetic field an EMF is induced. Generator :- Flemings Right hand
rule Motor :- Flemings Left hand rule
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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.
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DC machines Construction Types Theory of operation
Characteristics Speed control of DC motor Applications
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DC machine Construction
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Armature of a DC machine
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Dismantled view of a DC machine
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Parts of a DC machine Yoke Pole core & Pole shoes Pole
coils Armature core Armature windings Commutator Brushes &
Bearings
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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
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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
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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
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Armature core
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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
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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.
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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.
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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.
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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.
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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.
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Coil span or Coil pitch (YS) Coil span is the distance between
2 sides of the coil.
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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
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Chorded wdg or Short pitched wdg or Fractional pitched wdg The
pitch of the winding is less than pole pitch
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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
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Types of windings Lap winding Wave winding
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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
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Lap Winding
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Different pitches for Lap winding Back Pitch (Y B ) No of coil
sides or slots spanned by the back end connections. (Z/P) Front
Pitch (Y F ) - No of coil sides or slots spanned by the front end
connections. Resultant Pitch (Y R ) Distance between the beginning
of one coil to the beginning of next coil to which it is connected.
Commutator Pitch (Y C ) Distance between the segments to which the
two ends of the coil are connected. For Lap, (Y C = Y B - Y F )
& for Wave, (Y C = Y B +Y F )
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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.
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One Equalizer ring for a pole pair is used.
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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.
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Design of Lap Winding
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Lap Winding Advantages: 1.This winding is necessarily required
for large current application because it has more parallel
paths.current 2.2. It is suitable for low voltage and high current
generators.voltagecurrent 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.
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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
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Simplex wave winding Circular form
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LAP & WAVE WINDING
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Cross-sectional view of Armature
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Lap & Wave windings with pitches
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Types of DC machines DC Generator DC motor
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DC generator
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Losses
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Separately excited DC generator
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Self excited DC generator
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Compound generator
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Characteristics of DC generator Open circuit characteristics or
Magnetization characteristics or No load characteristics Load
Characteristics Internal or Total characteristics External
characteristics
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Open Circuit Characteristics E o Vs I f 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
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Relation b/n Eg and Speed
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Internal & External Characteristics
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Characteristics of DC compound generator
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Types of DC motor
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Characteristics of DC motor Electrical or Internal
characteristics Mechanical or External characteristics
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Characteristics of DC series motor
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Characteristics of DC shunt motor
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Characteristics of DC Compound motor
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Speed control of DC motor The speed of the DC motor is given by
the relation,
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Speed of DC shunt motor can be controlled by varying (i)
Flux/pole, (Flux Control) (ii) Resistance R a of armature circuit
(Rheostatic Control) and (iii) Applied voltage V (Voltage
Control).
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Flux control method
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Armature control method
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Voltage control method
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Speed of DC series motor can be controlled by Flux Control
method Variable resistance method
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Flux control method Field diverters Armature diverters Trapped
Field control method Paralleling Field coils
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Field diverters
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Armature diverters
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Trapped Field control method
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Paralleling field coils
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Variable resistance control
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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.