EEN 112 - Unit 7
EEN 112 - Unit 7
EEN 112 - Unit 7
EEN-112
Electrical Science
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Principle of Operation
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Fleming’s Right-Hand Rule
• Also termed as the rule of generator
• It indicates the direction of induced current when a
conductor is moved inside a magnetic field.
• Induced emf, in the conductor
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Fleming’s Left Hand Rule
• Also termed as the rule of motor
• It indicates the direction of force when a current
carrying conductor is placed in a magnetic field.
• Force, F on the conductor
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Basic Requirement of a Rotating
Electrical Machine
• Magnetic circuit
– Provides magnetic field/flux
– Created by electro-magnets or permanent magnets
• Electric circuit
– Provides path for electricity to flow
– Consists of winding/conductor
– In case of generator, induced emf causes current to flow, while
in motor, electricity is provided by external supply
• Relative motion between magnetic field and conductor
– Caused by prime mover (in case of generator)
– Caused by interaction of current and magnetic field (in case of
motor)
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Working of a Generator
• Consider a single turn coil ‘ABCD’ rotating clockwise in a
uniform magnetic field with a constant speed.
• When the loop rotates, the magnetic flux linking the coil sides
‘AB’ and ‘CD’ changes continuously. This change in flux linkage
induces an EMF in coil sides and the induced EMF in one coil
side adds the induced EMF in the other.
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Working of a Generator (cont.)
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Working of a DC Generator
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Working of a DC Generator (cont.)
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Working of a DC Motor
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Operation of a DC Machine
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Constructional Features
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Constructional Details of DC
Machine
• Magnetic field system
– The stationary part of the machine.
– Responsible to produce the main magnetic field flux.
• Armature
– The rotating part of the dc machine.
– Responsible to allow the flow of the induced voltages and currents in external
circuit in generating mode
– Responsible to allow the flow of the voltages and currents from external circuit in
motoring mode
• Commutator and brushgear
– A rotating mechanical converter
– Responsible to convert alternating voltages and currents induced in the armature
winding to dc form.
https://youtu.be/-xebh8wU8gY
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Constructional Details of DC
Machine (cont.)
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Constructional Details of DC
Machine (cont.)
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Constructional Details of DC
Machine (cont.)
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Magnetic Field System
• Yoke
– Outer frame in form of a hollow cylinder of cast steel or rolled steel.
– Supports the pole cores and acts as a protective cover to the machine.
– Also forms a part of the magnetic circuit.
• Poles
– Even number of poles projected inwards (called salient poles) are bolted to the
yoke.
– Each pole core has a pole shoe having a curved surface to support the field coils
and to increase the cross-sectional area of the magnetic circuit reducing its
reluctance.
– The pole cores are made of sheet steel laminations and these laminations are
insulated from each other but riveted together.
– Each pole core has one or more field coils (windings) placed over it and are
connected in series with one another such that when the current flows through
the coils, alternate north and south poles are produced in the direction of
rotation.
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Magnetic Field System (cont.)
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Armature
• Consists of a shaft upon which a laminated cylinder
(called armature core) is mounted.
• The laminations are insulated from each other but
tightly clamped together
• The armature core has grooves (or slots) on its outer
surface. Insulated conductors (usually copper) are
placed in the slots of the armature core and are
fastened round the core to prevent them flying under
centrifugal forces when the armature rotates.
• The conductors are suitably connected and this
arrangement of conductors is called armature winding.
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Armature (cont.)
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Armature Winding
• Lap winding
– Used in machines designed for low voltage and high current
– When the windings are connected in parallel, the current of
each winding adds, but the voltage remains the same
– The number of parallel paths is equal to the number of poles
• Wave winding
– Used in machines designed for high voltage and low current
– When the windings are connected in series, the voltage of
each winding adds, but the current capacity remains the
same
– The number of parallel paths of armature winding is always
two
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Lap Winding
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Wave Winding
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Commutator and Brushgear
• Made from a number of wedge-shaped hard-drawn copper bars
or segments insulated from each other as well as from the shaft.
• Each commutator segment is connected to the ends of the
armature coils. The number of segments is equal to number of
coils.
• Two or more carbon brushes are mounted on the commutator.
• Each brush is supported by a metal holder called brush holder.
The pressure exerted by the brushes on the commutator can be
adjusted through this brush holder and is maintained at a
constant value by means of springs.
• Current produced in the armature winding is passed to the
commutator and then to the external circuit through brushes.
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Commutator and Brushgear (cont.)
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Armature with Commutator and
Brushgear
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EMF and Torque Equations
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EMF Equation
• Let,
– = Flux per pole in weber
– = Total number of conductor
– = Number of poles
– = Number of parallel paths
– = Armature speed in rpm
– = emf generated in any one of the parallel path
Flux cut by 1 conductor in 1 revolution
Flux cut by 1 conductor in 60 sec
Average emf generated in 1 conductor
Number of conductors in each parallel path
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Torque Equation
• Turning or twisting force about an axis
• Let
– = Electromagnetic power
– = Electromagnetic torque
– = Armature current
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Schematic Representations of a DC
Machine
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Circuit Model of a DC Machine
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Model of a DC Generator
• Terminal voltage,
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Model of a DC Motor
• Terminal voltage,
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Types of Excitation
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Excitation in DC Machines
• Voltage Excitation
– The field winding has a large number of turns of thin wire (high resistance) and
is excited from a voltage source (self or separate). This winding is known as
shunt field winding.
– The field carries a small current, which is usually constant.
• Current Excitation
– The field is excited here by a few turns of thick wire (low resistance) connected
in series with the armature. This winding is known as series field winding.
– As it carries the armature current, the excitation varies with load.
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Excitation Methods in DC
Machines
• Separate excitation
– The shunt field winding is excited from an independent
voltage source
• Shunt excitation
– The shunt field winding is connected across the armature
• Series excitation
– The series field winding is connected in series with the
armature
• Compound excitation
– Both shunt and series field windings are connected is shunt
across the armature and in series with the armature,
respectively
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Excitation Methods in DC
Machines (cont.)
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Generator Characteristics
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Magnetization Characteristic
• The plot between induced emf and field current for a
given machine speed is the magnetization characteristic
of the machine.
• Machine is operated as a generator by a prime mover at a
constant speed (rated speed) with open-circuited
armature. The field current is varied by means of a
regulating resistance in series with the field.
• It is also called open-circuited characteristic (OCC).
• In the initial linear part of the characteristic, the air-gap
effect predominates and the dotted tangential line is
called the air-gap line. At higher values of field current ,
saturation effect in iron of the machine begins to show up.
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OCC of Separately Excited DC
Machine
Air-gap line: Tangential line to the
initial part of OCC
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OCC of Self Excited (Shunt) DC
Machine
Critical field resistance: Maximum field
resistance above which the generator fails
to excite.
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Motor Characteristics
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DC Shunt Motor
• Back emf,
• Torque,
• Speed,
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DC Series Motor
• Assuming linear magnetization,
• Torque,
• Speed,
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Speed-Torque Characteristics of DC
Motors
If the series field is connected to aid the shunt field, the motor is called
cumulative compound and if it opposes the shunt field, it is called
differential compound.
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Applications of DC Motors
• Shunt Motor
– Medium starting torque; speed regulation 5-15%
– Application: centrifugal pumps, conveyors, machine tools,
printing presses, etc.
• Series motor
– High torque at low speed and low torque at high speed, high
starting torque, good speed control possible.
– Application: traction, hoists, crane, battery-powered vehicles, etc.
• Compound motors
– Differentially compound motor is ideally suited for pulsating
loads (needing flywheel action).
– Application: Rolling mill, etc.
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Starting of DC Motors
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DC Motors during Starting
• At the time of starting the motor , back emf, is zero. So, the
motor draws an armature current of
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Two Point Starter
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Three Point Starter
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Four Point Starter
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Speed Control of DC Motors
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Speed Control of DC Motors
• According to the speed equation of a DC motor
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Speed Control of DC Shunt Motors
Field Control Method Armature Control Method
• Field resistance is adjusted (and • Armature voltage is adjusted (for
thus the field flux) large motors)
• This control can be used to • An adjustable resistance is
achieve speeds above the rated included in the armature circuit
speed. (for small motors)
• This control can be used to
achieve speeds below the base
speed
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Speed Control of DC Series Motors
• Field control method
– Tapped-field control
• The field turns are reduced by means of tap changing gear.
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Speed Control of DC Series Motors
• Field control method
– Series–parallel field control
• The field coils are divided in two equal halves and are connected in
series/parallel.
• Series connection corresponds to low speed and parallel connection
corresponds to high speed
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Speed Control of DC Series Motors
• Armature control method
– This is obtained by series/parallel connection of two
identical series motors which are mechanically coupled.
– The parallel connection with voltage across each armature
gives double the speed compared to the series connection
with voltage across each armature.
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Losses and Efficiency of DC
Machines
• A DC machine has two major losses:
– Constant Loss
• Core loss (including stray load core loss),
• Windage and friction loss (at specified speed),
• Shunt field copper loss (in a shunt machine),
• Total constant loss,
– Variable Loss
• Copper loss (inclusive of copper loss in series winding in a series motor, and
also inclusive of stray-load copper loss),
• The efficiency of a DC machine,
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Numerical Problems on DC
Machines
1. A 200 kW, 400 V, separately excited dc motor runs at 600
rpm. It has 864 lap-connected conductors. The full load
armature copper loss is 8 kW. Calculate the useful flux/pole.
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Numerical Problems on DC
Machines (cont.)
3. A 230 V DC shunt motor has an armature resistance of 0.1 Ω
and a shunt field resistance of 275 Ω. It runs at speed of 1000
rpm when drawing an armature current of 75 A. Calculate
the additional resistance to be inserted in the field circuit to
raise the motor speed to 1200 rpm at an armature current of
125 A. Assume linear magnetization characteristic.
4. A 60 kW, 250 V DC shunt motor takes 16 A, when running on
no load. The resistance of the armature and field are 0.2 Ω
and 125 Ω, respectively.
a) Estimate the motor efficiency when loaded to carry 152 A.
b) Also estimate its efficiency when operating as a generator
and delivering 152 A at 250 V.
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Suggested Readings
• Kothari D.P. , Nagrath I.J., “Basic Electrical
Engineering”, Tata McGraw Hill Education (India)
Private Limited, 4th Edition, 2022.
• Vincent Del Toro, “Electrical Engineering
Fundamentals”, Prentice Hall of India, 2002.
• Alexander C.K., Sadiku M.N.O., “Fundamentals of
Electric Circuits”, McGraw Hill, 5th Edition, 2012.
• Chapman, Stephen, J., “Electric Machinery
Fundamentals”, McGraw Hill Book Company, 1985.
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Thanks
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