Electrical Electrical Theory I - The Theory I - The

BasicsBasicsLet there be Let there be

light….light….

IntroductionIntroduction• Basic TerminologyBasic Terminology• Ohm’s LawOhm’s Law• Kirchhoff’s Laws & Kirchhoff’s Laws &

ApplicationsApplications• Basic Circuit AnalysisBasic Circuit Analysis• Transformers & RectifiersTransformers & Rectifiers

How is Electricity How is Electricity Produced?Produced?

• Friction:Friction: “static electricity” from rubbing “static electricity” from rubbing (walking across a carpet)(walking across a carpet)

• Pressure:Pressure: piezoelectricity from squeezing piezoelectricity from squeezing crystals together (quartz watch)crystals together (quartz watch)

• Heat:Heat: voltage produced at junction of voltage produced at junction of dissimilar metals (thermocouple)dissimilar metals (thermocouple)

• Light:Light: voltage produced from light striking voltage produced from light striking photocell (solar power)photocell (solar power)

• Chemical:Chemical: voltage produced from chemical voltage produced from chemical reaction (wet or dry cell battery)reaction (wet or dry cell battery)

• Magnetism:Magnetism: voltage produced using voltage produced using electromotive induction electromotive induction (AC or DC (AC or DC generator). generator).

Basic TerminologyBasic Terminology• Electromotive Force (E or V)Electromotive Force (E or V)

• Force which causes electrons to move Force which causes electrons to move from one location to anotherfrom one location to another

• Known as emf, potential difference, or Known as emf, potential difference, or voltagevoltage

• Unit is volt (V)Unit is volt (V)• Source: Source:

• GeneratorGenerator• BatteryBattery

• Like pump that moves water through Like pump that moves water through “pressure”“pressure”

Basic TerminologyBasic Terminology

• Current (I)Current (I)• Flow of electric charges - electrons Flow of electric charges - electrons

(or holes) - through a conductor or (or holes) - through a conductor or circuit per increment of timecircuit per increment of time

• Unit is ampere (number of charged Unit is ampere (number of charged particles passing a point each particles passing a point each second)second)

• 1 amp = 1 coulomb/sec = 6x101 amp = 1 coulomb/sec = 6x101818 electrons/secelectrons/sec

• Like rate of flow of water through Like rate of flow of water through a pipea pipe

Basic TerminologyBasic Terminology• Resistance (R)Resistance (R)

• An electrical circuit’s opposition to An electrical circuit’s opposition to the flow of current through itthe flow of current through it

• Measured in ohms (Measured in ohms ())• ConductorConductor

• All materials will conduct All materials will conduct electricity, but at varying electricity, but at varying resistancesresistances

• Good conductors have little Good conductors have little resistance (ie: silver, copper, gold) resistance (ie: silver, copper, gold)

Basic TerminologyBasic Terminology• InsulatorInsulator

• Substances which offer high Substances which offer high resistance to current flow (ie: resistance to current flow (ie: wood, rubber, plastics)wood, rubber, plastics)

• Circuits made of wires covered Circuits made of wires covered with insulatorwith insulator

• Power (P)Power (P)• Rate at which work is performedRate at which work is performed• Measured in watts (W)Measured in watts (W)

Ohm’s Law & Ohm’s Law & ApplicationsApplications

• Law: current of a circuit is Law: current of a circuit is directly proportional to the directly proportional to the applied voltage and inversely applied voltage and inversely proportional to circuit resistanceproportional to circuit resistance• I I V, I V, I 1/R V =IR1/R V =IR

• PowerPower• P = VIP = VI P = (IR)I = I P = (IR)I = I22RR

Kirchhoff’s LawsKirchhoff’s Laws• Kirchhoff’s Current Law (KCL)Kirchhoff’s Current Law (KCL)

• A node is any junction in a A node is any junction in a circuit where two or more circuit where two or more elements meetelements meet

• Currents into a node sum to zero Currents into a node sum to zero OROR

• Current entering a junction is Current entering a junction is equivalent to the current leaving equivalent to the current leaving a junctiona junction

Kirchhoff’s LawsKirchhoff’s Laws• Kirchhoff’s Voltage Law (KVL)Kirchhoff’s Voltage Law (KVL)

• A loop is any path in a circuit A loop is any path in a circuit that current can take so that it that current can take so that it meets back up to where it startsmeets back up to where it starts

• Voltages around a CLOSED loop Voltages around a CLOSED loop sum to zerosum to zero

ApplicationsApplications• Resistors in SeriesResistors in Series

• RRTT = R = R11 + R + R22 + R + R33 + . . . + . . .• Resistors in ParallelResistors in Parallel

• 1/R1/RTT = 1/R = 1/R11 + 1/R + 1/R22 + 1/R + 1/R33 + . . . + . . .• Examples: should be able to Examples: should be able to

find total current flow in find total current flow in circuit, current flow through circuit, current flow through each resistor, voltages, power each resistor, voltages, power dissipated, etc.dissipated, etc.

Questions?Questions?

Electrical Electrical Theory II –Theory II –

The The ApplicationsApplicationsHarnessing the Harnessing the

Power…Power…

Power GenerationPower Generation• Chemical CellsChemical Cells

• ElectrodesElectrodes• ElectrolyteElectrolyte• ConductorConductor• Wet/Dry BatteriesWet/Dry Batteries

• MagnetismMagnetism

Electromagnetic Electromagnetic InductionInduction

• Faraday (1831):Faraday (1831):• Showed that an emf is induced in a Showed that an emf is induced in a

conductor if a magnet passes by a conductor if a magnet passes by a conductorconductor

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

Electromagnetic Electromagnetic InductionInduction

• Magnitude of induced current Magnitude of induced current can be increased by:can be increased by:• Increasing strength of magnetic Increasing strength of magnetic

fieldfield• Increasing speed of relative motionIncreasing speed of relative motion• Positioning of field & conductor to Positioning of field & conductor to

increase number of magnetic lines increase number of magnetic lines of flux cutof flux cut

• Magnetic field usually produced Magnetic field usually produced by electromagnetby electromagnet

ElectromagnetElectromagnet• Soft iron core wound Soft iron core wound

with coils of wirewith coils of wire• When current When current

present (excitation present (excitation current), core current), core becomes becomes magnetizedmagnetized

• Field strength Field strength determined by determined by number of turns and number of turns and magnitude of magnitude of current:current:

B B NI NIDCDC

Electromagnetic Electromagnetic InductionInduction

• Results in:Results in:• Generator action:Generator action: generator generator

converts mechanical to electrical converts mechanical to electrical energyenergy

• Motor action:Motor action: motor converts motor converts electrical to mechanical energy electrical to mechanical energy

Generator ActionGenerator Action• For emf/current (electricity):For emf/current (electricity):

• Magnetic FieldMagnetic Field• ConductorConductor• Relative Motion b/t the twoRelative Motion b/t the two

• Voltage produced: “induced Voltage produced: “induced emf/voltage”emf/voltage”

• Current produced: Current produced: “induced current” “induced current”• Left-handLeft-hand rule for rule for generator actiongenerator action

Motor ActionMotor Action• For motor action For motor action

(torque/motion):(torque/motion):• Magnetic FieldMagnetic Field• ConductorConductor• Current flow in conductorCurrent flow in conductor• Torque Torque

produced: produced: “induced “induced torque”torque”• Right-handRight-hand rule for rule for motor actionmotor action

Standard TerminologyStandard Terminology• Stator:Stator: stationary housing of stationary housing of

the generator or motorthe generator or motor• Rotor:Rotor: rotating shaft inside the rotating shaft inside the

statorstator• Field windings:Field windings: conductors used conductors used

to produce electromagnetic fieldto produce electromagnetic field• Armature windings:Armature windings: conductors conductors

in which output voltage is in which output voltage is produced (or input is provided)produced (or input is provided)

Basic TerminologyBasic Terminology• Direct Current (DC)Direct Current (DC)

• Current flow is Current flow is unidirectional and of unidirectional and of constant magnitude constant magnitude (battery)(battery)

• Alternating Current (AC)Alternating Current (AC)• Magnitude & direction of Magnitude & direction of

current flow periodically current flow periodically changechange

• Each sequence called a Each sequence called a cyclecycle

• Frequency is cycles per Frequency is cycles per second (Hz)second (Hz)

AC GeneratorsAC Generators• Most electrical power used is AC Most electrical power used is AC

made by AC generatorsmade by AC generators• Basic principle: rotating magnetic Basic principle: rotating magnetic

field “cutting through” a conductorfield “cutting through” a conductor• Regardless of size, all AC generators Regardless of size, all AC generators

work on same principlework on same principle• Two types:Two types:

• Revolving armature (NOT used)Revolving armature (NOT used)• Revolving field (Used in SSTG’s, GTGS, Revolving field (Used in SSTG’s, GTGS,

DG)DG)

AC GeneratorsAC Generators• Two types:Two types:

• Revolving Revolving armature armature (NOT used)(NOT used)

• Revolving Revolving field (Used in field (Used in SSTG’s, SSTG’s, GTGS, DG)GTGS, DG)

AC GeneratorsAC Generators• Field windings on rotorField windings on rotor

• DC current provided for field via slip DC current provided for field via slip rings and brushes (vice commutator rings and brushes (vice commutator rings)rings)

• Rotor turned by prime mover Rotor turned by prime mover creates rotating magnetic fieldcreates rotating magnetic field• Armature windings Armature windings

on statoron stator• As field rotates, As field rotates, AC current AC current produced in produced in armaturearmature• Since stationary Since stationary contacts, no arc-contacts, no arc-overover

AC GeneratorsAC Generators• Determining speed of AC machine:Determining speed of AC machine:

ff= P(RPM)/120= P(RPM)/120 RPM = 120f/P RPM = 120f/P • Must maintain constant 60Hz output Must maintain constant 60Hz output

use speed governor to maintain use speed governor to maintain constant RPM (independent of constant RPM (independent of loading)loading)

• Must also regulate voltage outputMust also regulate voltage output• Since constant RPM, must control field Since constant RPM, must control field

excitation (DC current) to control output excitation (DC current) to control output voltagevoltage

Three Phase (3Three Phase (3) AC ) AC PowerPower

• Phases: number of sets of armature Phases: number of sets of armature windings on statorwindings on stator

• 33has three sets of armature has three sets of armature windingswindings• Voltage induced is 120Voltage induced is 120oo out of phase out of phase

for eachfor each• Output: 3 sinusoidal voltages and Output: 3 sinusoidal voltages and

currentscurrents• Allows more power to be delivered Allows more power to be delivered

with a smaller design generatorwith a smaller design generator

Three Phase (3Three Phase (3) AC ) AC PowerPower

AC MotorsAC Motors• Use AC current as input to produce Use AC current as input to produce

workwork• Many different types depending on Many different types depending on

number of phases of AC input & number of phases of AC input & constructionconstruction

• Ex: induction motorEx: induction motor• Input AC current on stator produces Input AC current on stator produces

rotating fieldrotating field• Current produced in conductors on Current produced in conductors on

rotor produces torquerotor produces torque

DC GeneratorsDC Generators• Basic Principle: Basic Principle:

rotate a rotate a conductor within conductor within a magnetic field a magnetic field to induce an EMFto induce an EMF

• Field windings Field windings located on stator located on stator & receive current & receive current from outside from outside sourcesource

DC GeneratorsDC Generators• Armature windings Armature windings

on rotoron rotor• Commutator rings Commutator rings

used to mechanically used to mechanically reverse the armature reverse the armature coil connection to coil connection to the external circuitthe external circuit

• EMF developed EMF developed across the brushes across the brushes becomes a DC becomes a DC voltage/current voltage/current (pulsating and (pulsating and unidirectional)unidirectional)

DC MotorsDC Motors• Essentially the same in Essentially the same in

construction as DC construction as DC generatorgenerator

• Based on principle that Based on principle that current carrying current carrying conductor placed at a conductor placed at a right angle to a magnetic right angle to a magnetic field tends to move in a field tends to move in a direction perpendicular direction perpendicular to magnetic lines of fluxto magnetic lines of flux

• Only need to change Only need to change relative voltage to go relative voltage to go between generator between generator motormotor

AC vs DC powerAC vs DC power• Many different voltages Many different voltages

required on board shiprequired on board ship• Easier to transform AC power Easier to transform AC power

for each applicationfor each application

Electrical DevicesElectrical Devices• TransformerTransformer

• Device w/o moving parts that Device w/o moving parts that transfers energy from one circuit transfers energy from one circuit to another by electromagnetic to another by electromagnetic inductioninduction

• Consists of ferromagnetic core & Consists of ferromagnetic core & sets of windingssets of windings

• Step-up: VStep-up: Vinin V Voutout

• Step-down: VStep-down: Vinin V Voutout

• Only works with ACOnly works with AC

Electrical DevicesElectrical Devices• RectifierRectifier

• Converts AC Converts AC DCDC

• Designed to have Designed to have small resistance small resistance to current flow in to current flow in one direction & one direction & large resistance large resistance in opposite in opposite directiondirection

• Typically called a Typically called a diode or rectifierdiode or rectifier

Questions?Questions?