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KKKF1063 Introduction to ElectricalEngineering
LECTURE 11
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2
WK TOPICS LECT
9 Polyphase System BB
10 Digital Logic Circuits MBI
11 Applications - Digital System MBI
12 Applications Power Supply,
Transformer, Electric Machines
BB
13 Measurement andInstrumentation
BB
14 Circuit Disturbance Noise,
EMI
MBI
15 Other Related TechnologiesIC, Optics, GPS, Acoustic
BB/MBI
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Badariah Bais KKKF163 Introduction to EESem II 2006/07
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Reading Assignments
HambleyCh 10 p. 460 464Ch 15 p. 712 725
Ch 16 p. 735 777Ch 17 p. 787 824RizzoniCh 9 p. 513 524
Ch 18 p. 936 940Ch 19 p. 971 1027Ch 20 (Selected refer to lecture notes)
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Contents
1. Power Supply2. Transformer3. Electric Machines
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Introduction
Power supply is a group of circuits thatconvert ac energy to dc energy.
Two types:
linear power supply: provides constantcurrent path between its input and itsload.
- switching power supply: providesintermittent current path between itsinput and its output.
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Linear Power Supply
Basic components:1. Rectifier diode circuit that converts the ac to what is
called a pulsating dc.2. Filter circuit that reduces the variations in the output
of the rectifier.3. Voltage regulator maintain a constant power supply
output voltage.
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Rectifier
2 types:
Half-wave rectifier
Full-wave rectifier
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Half-wave Rectifier
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Full-wave Rectifier
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Full-wave Rectifier
Also known as a full-wave bridge rectifier
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Full-wave Rectifier - operation
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Filter
Vr=ripple voltage
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Basic Capacitive Filter
RC
Time constant
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Filter output
Note:Ripple is minimized for higher values of RL or CF
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Surge Current
Surge current can be resolved by incorporating a seriescurrent limiting resistor, Rsurge.
Current limiting resistor are usually a low resistance, highwattage component.
The disadvantage reduces the output voltage from thecircuit.
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Filter Output VoltageVpk: peak rectifier output
voltageVdc : average (or dc) value
Vr : ripple
Example:
Assuming the linefrequency is 60 Hz, thetime between chargingpeak for half-wave rectifier,
t= 1/f= 1/60 = 16.7 ms
For full-wave rectifier, thefrequency is 2x60 = 120Hz. Therefore,
t= 1/120 = 8.33 ms
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Badariah Bais KKKF163 Introduction to EESem II 2006/07 17
RippleVoltage
Given by the equation:
C
tIV Lr
where IL= the dc load current
t = the time between charging peaks
C = the capacitance (in Farads)
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The full-wave rectifier has approximately half the ripple outputproduced by the half-wave rectifier. This is due to the shortened timeperiod between capacitor charging pulses. Therefore, full-wave rectifierare typically used in power supplies.
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Zener Voltage Regulator
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Zener Reduction of Ripple Voltage
ppppppr
SLZ
LZ
outr mVVVVRRZ
RZV 129)5.1(
518.4
8.4)5.1(
511205
1205
)(
)()(
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Putting It All Together
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Transformer
Made up of inductors.
Not electrically connected.
An ac voltage applied to the primary induces an ac voltage in the secondary.
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Types of Transformer
Step-up transformer
- provides a secondary voltage that isgreater thanthe primary voltage.
Step-down transformer
- provides a secondary voltage that isless thanthe primary voltage.
Isolation transformer
- provides a secondary voltage that is equaltothe primary voltage.
- to isolate the power supply electrically fromthe power line, which serves as a
protection.
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Transformer secondary voltage
The turns ratio of a transformer is equal to the voltage ratio of the component:
)(
)(
1
2
1
2
tv
tv
N
N
or )()(1
1
2
2 tvN
N
tv
For example:acac VVtv
N
Ntv 30)120(
4
1)()(
1
1
2
2
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Transformer secondary current
Assuming the transformer is 100% efficient, then
or
12PP
)()()()(1122titvtitv
)()(
)()( 1
2
1
2 titv
tvti )()( 1
2
12 ti
N
Nti
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ExampleConsider the source, transformer, and load shown in the circuit below. Determinethe rms values of the currents and voltages (a) with the switch open and (b) withthe switch closed.
VrmsV 110)(1
Solution
Voltage applied to the primary,
VrmsVN
NrmsV 22)110(
5
1)()(
11
2
2
(a) With the switch open, the secondary current is zero. Hence, the primarycurrent is also zero.
(b) With the switch closed, AR
rmsVrmsI
L
2.210
22)()( 22
ArmsIN
NrmsI 44.0)2.2(
5
1)()( 2
1
21
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Badariah Bais KKKF163 Introduction to EESem II 2006/07 28
Transformer Rating
The rating of a transformer is statedas Volt Ampere (VA) that it cantransform without overheating.
The transformer rating can becalculated as either V1I1 or V2I2where I2 is the full load secondary
current.
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Impedance Transformation
LZ
2
2
I
V
LZ
N
N
N
N
11
2
12
1
I
V
LL ZN
NZ
2
2
1
1
1'
I
V
The phasor current and voltage in the secondary are related to the loadimpedance by
Then, The impedance seen by the source,
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ExampleConsider the circuit shown below. Find the phasor currents and voltages. Also, findthe power delivered to the load.
Solution
Impedance at the secondary, )2010( jZL
Impedance reflected at the primary,
)20001000()2010(
1
1022
2
1' jjZ
N
NZ LL
Total impedance )20002000(200010001000'
1jjZRZ LS
452828SZ
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Example
Primary current and voltage:
452828SZ
A
S
S
453536.0
452828
010001
Z
VI
)20001000(453536.0
'
11 jL ZIV
V 43.186.790)43.632236(453536.0
Secondary current and voltage: AN
N
45536.3)453536.0(
1
101
2
1
2II
VN
N
43.1806.79)43.186.790(10
11
1
22 VV
)20001000(' jZL
Power delivered to the load: WRrmsIP LL 51.62)10(2
536.3)(
2
2
2
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Electric Machines
Motor and Generator
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Electric Machines
Machines that convert mechanicalenergy to and from electric energy.
- Motor convert electrical energyinto rotational mechanical energy
- Generator convert mechanicalenergy into electrical energy
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Electric Motor Basic construction
Basic components:
1. Stator stationary part
2. Rotor rotating part
3. Shaft coupled the machine to the mechanicalload.
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Electric Motor
Rotor is rotating inside the
stator and separated bymeans of an air gap.
The rotor and stator eachconsists of a magneticcore, some electrical
insulation and the windingsnecessary to establish amagnetic flux.
The windings carry theelectric currents thatgenerate the magneticfields and flow to theelectrical loads.
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Motor can be divided into
AC Motors
powered by AC sources which caneither be single phase or three phase.
- most common type is induction motorand synchronous motor.
DC Motors
powered by DC sources.
Electric Motor
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In most types of motor, a given windings canbe classified as field winding or as armaturewinding.
Field winding - to set up the magnetic fieldrequired to produce torque.
Armature windings - carry currents that varywith mechanical load. When the machine is
used as a generator, the output is taken fromthe armature windings.
Electric Motor
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Electric Motor - Basic classification
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Losses, Power Ratings, and Efficiency
The electrical input powerPin, in watts, supplied by the three-phase source isgiven by:
cos3 rmsrmsin IVP
where Vrms is the rms value of line-to-line voltage, Irms is the rms value of linecurrent and cos is the power factor.
The mechanical output powerPout, in watts, is: moutout TP
where Tout is the output torque in newton-meters, and m is the angular speed ofthe load in radians per second.
The rotational speed may be given in revolutions per minute denoted by nmorby radian per second denoted by m . These quantities are related by:
60
2 mm n
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Losses, Power Ratings, and Efficiency
The mechanical output power for a given electric motor is stated in horsepower(hp). To convert from watts to horsepower, we have
746
watts
horsepower
PP
The power rating of a motor is the output power that the motor can safely
produce on a continuous basis. Most motor can supply output power varyingfrom zero to several times their rated power, depending on the mechanical load.
The power efficiency of a motor is given by:
%100in
out
PP
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DC Machines
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DC Machines - Construction
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DC Machines - Construction
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DC Machines - Construction
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DC Motor
Can be divided into 2 types:
(a) Wound type
shunt
series
compound
(b) Permanent magnet type
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Characteristics of DC Electrical Motors
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DC motor
Shunt wound motor- field is connected in parallel with the armature
- has good speed regulation
Series wound motor
- field winding in series with the armature- very high starting torque and poor speed regulation.
Compound wound motor
- field winding has both series and shunt components
- offers better starting torque than the shunt motor butworse speed regulation
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DC motor
Permanent magnet
- field windings are replaced by permanentmagnets
- adequate starting torque- speed regulation somewhat worse than that ofthe compound wound motor
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Speed Control of DC Motors
Vary the voltage supplied to thearmature circuit while holding thefield constant.
Vary the field current while holdingthe armature supply voltageconstant.
Insert resistance in series with thearmature circuit.
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Induction motor
Widely used because of its relativesimplicity in construction
Does not require external electricalconnection to the rotor, thus slipsrings and brushes are not required
Operates at a lower speed than the
synchronous speed
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Synchronous Speed
For a P-pole machine, the angular velocity of thefield is given by:
2/Ps
This is also known as synchronous angular velocity
The synchronous speed (in rpm) is given by:
P
fns120
S i l C I d ti M t
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Squirrel-Cage Induction Motor
cross-section conductors inrotor
photograph
Views of Smokin Buckeye motor: rotor, stator and cross-section of stator
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Selection of Induction Motors
Some of the most important considerations inselecting an induction motor are:
1. Efficiency2. Starting torque3. Pull-out torque4. Power factor
5. Starting current
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Synchronous Motor
Generation of electrical energy by utilitycompanies is done almost exclusively withsynchronous machines.
Assuming a constant frequency source,the speed of a synchronous motor doesnot vary with load.
The stator windings of a synchronous
machine are basically the same as thoseof an induction machine
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Synchronous Motor
The synchronous motor can act as asource of reactive power
Proper use of synchronous motorscan lower energy costs of anindustrial plant by increasing thepower factor
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Basic Single-Phase Induction Motor
The pulsating fluxproduced by themain winding can be
resolved into twocounter-rotatingcomponents
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Auxiliary Windings
Two windings that are 90 apartphysically and carrying currents 90apart in phase produce a rotating
magnetic field.
Single-phase induction motorscontain an auxiliary winding
displaced by 90 electrical degreesfrom the main winding.
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Auxiliary Windings
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Stepper Motor
Are used for accurate, repeatablepositioning applications such as read/writeheads of a computer floppy drive or for
moving the head in an ink-jet printer By controlling the rate at which pulses are
applied to the windings of the steppermotor, speed can be varied continuously
from a standing stop to a maximum thatdepends on the motor and the load.
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Stepper Motor
Various types:
1. Variable-reluctance stepper motor
2. Permanent-magnet stepper motor
3. Hybrid stepper motor
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