KKKF1063 Introduction to Electrical Engineering

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KKKF1063 Introduction to Electrical Engineering

• LECTURE 11

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(No Transcript)
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Reading Assignments

• Hambley
• Ch 10 p. 460 464
• Ch 15 p. 712 725
• Ch 16 p. 735 777
• Ch 17 p. 787 824
• Rizzoni
• Ch 9 p. 513 524
• Ch 18 p. 936 940
• Ch 19 p. 971 1027
• Ch 20 (Selected refer to lecture notes)

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Contents

• Power Supply
• Transformer
• Electric Machines

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Introduction

• Power supply is a group of circuits that convert
  ac energy to dc energy.
• Two types
• linear power supply provides constant current
  path between its input and its load.
• - switching power supply provides intermittent
  current path between its input and its output.

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Linear Power Supply

• Basic components
• Rectifier diode circuit that converts the ac to
  what is called a pulsating dc.
• Filter circuit that reduces the variations in
  the output of the rectifier.
• 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
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
  series current limiting resistor, Rsurge.
• Current limiting resistor are usually a low
  resistance, high wattage component.
• The disadvantage reduces the output voltage
  from the circuit.

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Filter Output Voltage
Vpk peak rectifier output voltage Vdc
average (or dc) value Vr ripple
Example Assuming the line frequency is 60 Hz,
the time between charging peak for half-wave
rectifier, t 1/f 1/60 16.7 ms For
full-wave rectifier, the frequency is 2x60 120
Hz. Therefore, t 1/120 8.33 ms
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Ripple Voltage
Given by the equation
where IL the dc load current t the time
between charging peaks C the capacitance (in
Farads)
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Ripple Voltage
For example
(a) Vr (ILt)/C (20mA)(16.7ms)/500?F) 668
mVpp (half-wave rectifier) (b) Vr (ILt)/C
(20mA)(8.33ms)/500?F) 333 mVpp (full-wave
rectifier)
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The full-wave rectifier has approximately half
the ripple output produced by the half-wave
rectifier. This is due to the shortened time
period between capacitor charging pulses.
Therefore, full-wave rectifier are typically used
in power supplies.
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Zener Voltage Regulator
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Zener Reduction of Ripple Voltage
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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 is greater than the primary voltage.
Step-down transformer - provides a secondary
voltage that is less than the primary voltage.

• Isolation transformer
• provides a secondary voltage that is equal to the
  primary voltage.
• to isolate the power supply electrically from the
  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
or
For example
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Transformer secondary current
Assuming the transformer is 100 efficient, then
or
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Example
Consider the source, transformer, and load shown
in the circuit below. Determine the rms values of
the currents and voltages (a) with the switch
open and (b) with the switch closed.
Solution
Voltage applied to the primary,
(a) With the switch open, the secondary current
is zero. Hence, the primary current is also zero.
(b) With the switch closed,
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Transformer Rating

• The rating of a transformer is stated as Volt
  Ampere (VA) that it can transform without
  overheating.
• The transformer rating can be calculated as
  either V1I1 or V2I2 where I2 is the full load
  secondary current.

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Impedance Transformation
The phasor current and voltage in the secondary
are related to the load impedance by
Then,
The impedance seen by the source,
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Example
Consider the circuit shown below. Find the phasor
currents and voltages. Also, find the power
delivered to the load.
Solution
Impedance at the secondary,
Impedance reflected at the primary,
Total impedance
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Example
Primary current and voltage
Secondary current and voltage
Power delivered to the load
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Electric Machines

• Motor and Generator

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Electric Machines

• Machines that convert mechanical energy to and
  from electric energy.
• - Motor convert electrical energy into
  rotational mechanical energy
• - Generator convert mechanical energy 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 mechanical load.
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Electric Motor
Rotor is rotating inside the stator and separated
by means of an air gap. The rotor and stator each
consists of a magnetic core, some electrical
insulation and the windings necessary to
establish a magnetic flux. The windings carry the
electric currents that generate the magnetic
fields and flow to the electrical loads.
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Electric Motor

• Motor can be divided into
• AC Motors
• powered by AC sources which can either be
  single phase or three phase.
• - most common type is induction motor and
  synchronous motor.
• DC Motors
• powered by DC sources.

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Electric Motor

• In most types of motor, a given windings can be
  classified as field winding or as armature
  winding.
• Field winding - to set up the magnetic field
  required to produce torque.
• Armature windings - carry currents that vary
  with mechanical load. When the machine is used as
  a generator, the output is taken from the
  armature windings.

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Electric Motor - Basic classification
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Losses, Power Ratings, and Efficiency
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Losses, Power Ratings, and Efficiency
The electrical input power Pin, in watts,
supplied by the three-phase source is given by
where Vrms is the rms value of line-to-line
voltage, Irms is the rms value of line current
and cos? is the power factor.
The mechanical output power Pout, in watts, is
where Tout is the output torque in newton-meters,
and ?m is the angular speed of the load in
radians per second.
The rotational speed may be given in revolutions
per minute denoted by nm or by radian per second
denoted by ?m . These quantities are related by
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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
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 varying
from zero to several times their rated power,
depending on the mechanical load.
The power efficiency of a motor is given by
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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 but worse speed regulation

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DC motor

• Permanent magnet
• - field windings are replaced by permanent
  magnets
• - adequate starting torque
• - speed regulation somewhat worse than that of
  the compound wound motor

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Speed Control of DC Motors

• Vary the voltage supplied to the armature circuit
  while holding the field constant.
• Vary the field current while holding the armature
  supply voltage constant.
• Insert resistance in series with the armature
  circuit.

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AC Machines
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Characteristics of AC Electrical Motors
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Induction motor

• Widely used because of its relative simplicity in
  construction
• Does not require external electrical connection
  to the rotor, thus slips rings 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 the
field is given by
This is also known as synchronous angular velocity
The synchronous speed (in rpm) is given by
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Squirrel-Cage Induction Motor
photograph
conductors in rotor
cross-section
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 in
selecting an induction motor are 1.
Efficiency 2. Starting torque 3. Pull-out
torque 4. Power factor 5. Starting current
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Synchronous Motor

• Generation of electrical energy by utility
  companies is done almost exclusively with
  synchronous machines.
• Assuming a constant frequency source, the speed
  of a synchronous motor does not vary with load.
• The stator windings of a synchronous machine are
  basically the same as those of an induction
  machine

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Synchronous Motor

• The synchronous motor can act as a source of
  reactive power
• Proper use of synchronous motors can lower energy
  costs of an industrial plant by increasing the
  power factor

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Basic Single-Phase Induction Motor

• The pulsating flux produced by the main winding
  can be resolved into two counter-rotating
  components

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Auxiliary Windings

• Two windings that are 90 apart physically and
  carrying currents 90 apart in phase produce a
  rotating magnetic field.
• Single-phase induction motors contain an
  auxiliary winding displaced by 90 electrical
  degrees from the main winding.

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Auxiliary Windings
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Stepper Motor

• Are used for accurate, repeatable positioning
  applications such as read/write heads 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 stepper motor,
  speed can be varied continuously from a standing
  stop to a maximum that depends on the motor and
  the load.

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Stepper Motor

• Various types
• Variable-reluctance stepper motor
• Permanent-magnet stepper motor
• Hybrid stepper motor