Electrical Theory I - The Basics

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Electrical Theory I - The Basics

• Let there be light.

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(No Transcript)
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Introduction

• Basic Terminology
• Ohms Law
• Kirchhoffs Laws Applications
• Basic Circuit Analysis
• Transformers Rectifiers

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Basic Terminology

• Electromotive Force (E or V)
• Force which causes electrons to move from one
  location to another
• Known as emf, potential difference, or voltage
• Unit is volt (V)
• Source
• Generator
• Battery
• Like pump that moves water through pressure

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Basic Terminology

• Current (I)
• Flow of electric charges - electrons (or holes) -
  through a conductor or circuit per increment of
  time
• Unit is ampere (number of charged particles
  passing a point each second)
• 1 amp 1 coulomb/sec 6.02x1023 electrons/sec
• Like rate of flow of water through a pipe

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Basic Terminology

• Resistance (R)
• An electrical circuits opposition to the flow of
  current through it
• Measured in ohms (W)
• Conductor
• All materials will conduct electricity, but at
  varying resistances
• Good conductors have little resistance (ie
  silver, copper, aluminum, iron)

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Basic Terminology

• Insulator
• Substances which offer high resistance to current
  flow (ie wood, rubber, plastics)
• Circuits made of wires covered with insulator
• Power (P)
• Rate at which work is performed
• Measured in watts (W)

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Basic Terminology

• Direct Current (DC)
• Current flow is unidirectional and of constant
  magnitude (battery)
• Alternating Current (AC)
• Magnitude direction of current flow
  periodically change
• Each sequence called a cycle
• Frequency is cycles per second (Hz)

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Electrical Devices

• Rectifier
• Converts AC DC
• Designed to have small resistance to current flow
  in one direction large resistance in opposite
  direction
• Typically called a diode or rectifier

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Electrical Devices

• Transformer
• Device w/o moving parts that transfers energy
  from one circuit to another by electromagnetic
  induction
• Consists of ferromagnetic core sets of windings
• Step-up Vin Vout
• Step-down Vin Vout
• Only works with AC

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Ohms Law Applications

• Law current of a circuit is directly
  proportional to the applied voltage and inversely
  proportional to circuit resistance
• I a V, I a 1/R V IR

• Power
• P VI P (IR)I I2R

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Applications

• Resistors in Series
• RT R1 R2 R3 . . .
• Resistors in Parallel
• 1/RT 1/R1 1/R2 1/R3 . . .
• Examples should be able to find total current
  flow in circuit, current flow through each
  resistor, voltages, power dissipated, etc.

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Kirchhoffs Laws

• Kirchhoffs Current Law (KCL)
• A node is any junction in a circuit where two or
  more elements meet
• Currents into a node sum to zero OR
• Current entering a junction is equivalent to the
  current leaving a junction

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Kirchhoffs Laws

• Kirchhoffs Voltage Law (KVL)
• A loop is any path in a circuit that current can
  take so that it meets back up to where it starts
• Voltages around a CLOSED loop sum to zero

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Questions?
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Electrical Theory II The Applications

• Harnessing the Power

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• of

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Introduction

• Electromagnetic Induction
• DC
• Generators
• Motors
• AC
• Generators
• Motors
• Three-phase AC

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How is Electricity Produced?

• Friction static electricity from rubbing
  (walking across a carpet)
• Pressure piezoelectricity from squeezing
  crystals together (quartz watch)
• Heat voltage produced at junction of dissimilar
  metals (thermocouple)
• Light voltage produced from light striking
  photocell (solar power)
• Chemical voltage produced from chemical reaction
  (wet or dry cell battery)
• Magnetism voltage produced using electromotive
  induction (AC or DC generator).

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Electromagnetic Induction

• Faraday (1831)
• Showed that an emf is induced in a conductor if a
  magnet passes by a conductor

• When pole of magnet entered coil, current
  flowed in one direction
• When direction of magnet reversed, current
  flowed in opposite direction

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Electromagnetic Induction

• Results in
• Generator action generator converts mechanical
  to electrical energy
• Motor action motor converts electrical to
  mechanical energy

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Generator Action

• For emf/current (electricity)
• Magnetic Field
• Conductor
• Relative Motion b/t the two
• Voltage produced induced emf/voltage

• Current produced induced current
• Left-hand rule for generator action

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Motor Action

• For motor action (torque/motion)
• Magnetic Field
• Conductor
• Current flow in conductor

• Torque produced induced torque
• Right-hand rule for motor action

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Electromagnetic Induction

• Magnitude of induced current can be increased by
• Increasing strength of magnetic field
• Increasing speed of relative motion
• Positioning of field conductor to increase
  number of magnetic lines of flux cut
• Magnetic field usually produced by electromagnet

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Electromagnet

• Soft iron core wound with coils of wire
• When current present (excitation current), core
  becomes magnetized
• Field strength determined by number of turns and
  magnitude of current
• B a NIDC

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Standard Terminology

• Stator stationary housing of the generator or
  motor
• Rotor rotating shaft inside the stator
• Field windings conductors used to produce
  electromagnetic field
• Armature windings conductors in which output
  voltage is produced (or input is provided)

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DC Generators

• Basic Principle rotate a conductor within a
  magnetic field to induce an EMF
• Field windings located on stator receive
  current from outside source

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DC Generators

• Armature windings on rotor
• Commutator rings used to mechanically reverse the
  armature coil connection to the external circuit
• EMF developed across the brushes becomes a DC
  voltage/current (pulsating and unidirectional)

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DC Motors

• Essentially the same in construction as DC
  generator
• Based on principle that current carrying
  conductor placed at a right angle to a magnetic
  field tends to move in a direction perpendicular
  to magnetic lines of flux
• Only need to change relative voltage to go
  between generator motor

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AC Generators

• Most electrical power used is AC made by AC
  generators
• Basic principle rotating magnetic field cutting
  through a conductor
• Regardless of size, all AC generators work on
  same principle
• Two types
• Revolving armature (NOT used)
• Revolving field (Used in SSTGs, GTGS, DG)

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AC Generators

• Two types
• Revolving armature (NOT used)
• Revolving field (Used in SSTGs, GTGS, DG)

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AC Generators

• Field windings on rotor
• DC current provided for field via slip rings and
  brushes (vice commutator rings)
• Rotor turned by prime mover creates
  rotating magnetic field

• Armature windings on stator
• As field rotates, AC current produced in
  armature
• Since stationary contacts, no arc-over

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AC Generators

• Determining speed of AC machine
• f P(RPM)/120 RPM 120f/P
• Must maintain constant 60Hz output use speed
  governor to maintain constant RPM (independent of
  loading)
• Must also regulate voltage output
• Since constant RPM, must control field excitation
  (DC current) to control output voltage

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AC Motors

• Use AC current as input to produce work
• Many different types depending on number of
  phases of AC input construction
• Ex induction motor
• Input AC current on stator produces rotating
  field
• Current produced in conductors on rotor produces
  torque

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Three Phase (3f) AC Power

• Phases number of sets of armature windings on
  stator
• 3f has three sets of armature windings
• Voltage induced is 120o out of phase for each
• Output 3 sinusoidal voltages and currents
• Allows more power to be delivered with a smaller
  design generator

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Three Phase (3f) AC Power
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Questions?