Dcp 702 Electrical Theory Web

8/3/2019 Dcp 702 Electrical Theory Web

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Fundamental ElectricalFundamental ElectricalTheoryTheory


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ObjectivesObjectives Basic electrical theory including ohm's lawBasic electrical theory including ohm's law

and its derivationsand its derivations Generator theoryGenerator theory

Generator construction and controlGenerator construction and control

mechanisms including prime movers andmechanisms including prime movers andpower ratingspower ratings


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ObjectivesObjectives Fundamentals of electric motor theoryFundamentals of electric motor theory

including construction, power rating, usage,including construction, power rating, usage,

and control mechanisms.and control mechanisms.

Compare the uses for AC and DCCompare the uses for AC and DC

electric power and their transmissionelectric power and their transmission

methods.methods.


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References

References

INEINE, Chapter 16,, Chapter 16, pp. 307 pp. 307--318318

PNEPNE, Chapter 20,, Chapter 20, pp. 20pp. 20--11 -- 2020--2020


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DefinitionsDefinitions

Current (I):Current (I): flow of electric charges per unit time or flowflow of electric charges per unit time or flow

rate, measured in amperes or amps (A)rate, measured in amperes or amps (A)

Electromotive Force (emf) (E):Electromotive Force (emf) (E): a potential difference ora potential difference or

electric pressure which drives the flow of charges,electric pressure which drives the flow of charges,measured in volts (V)measured in volts (V)

Resistance (R):Resistance (R): an electrical circuits opposition to currentan electrical circuits opposition to current

flow, measured in ohms (flow, measured in ohms (;;))

Conductor:Conductor: a material which offers little resistance toa material which offers little resistance tocurrent flow, e.g. silver, copper, iron, etccurrent flow, e.g. silver, copper, iron, etc

Insulator:Insulator: a material which offers high resistance to currenta material which offers high resistance to current

flow, e.g. wood, paper, plastic, etc...flow, e.g. wood, paper, plastic, etc...


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ELECTRICAL THEORYELECTRICAL THEORY


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Direct Current (DC)Direct Current (DC)

Current flow is unidirectional and ofCurrent flow is unidirectional and of

constant magnitude (Batteries)constant magnitude (Batteries)

Ohms Law: current in a circuit is directlyOhms Law: current in a circuit is directlyproportional to the applied voltage andproportional to the applied voltage and

inversely proportional to the circuitinversely proportional to the circuit

resistanceresistance

E = I RE = I R

P = I E = IP = I E = I22 RR (W)


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Basic CircuitBasic Circuit

PropertiesProperties Electrons flow (Electrons flow (--) to (+)) to (+)

Kirchoffs Law of VoltagesKirchoffs Law of Voltages

Sum of all voltages in a complete ckt is zeroSum of all voltages in a complete ckt is zero Choose arbitrary current flowChoose arbitrary current flow

If current encounters (+) terminal 1st, then (+)If current encounters (+) terminal 1st, then (+)

voltagevoltage

Voltage drops preceded by (Voltage drops preceded by (--) sign if in the) sign if in the

same direction as electron flowsame direction as electron flow

Sum of current into and out of a node isSum of current into and out of a node is

always zero and constantalways zero and constant


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Basic CircuitBasic Circuit

PropertiesProperties SeriesSeries

Current is constant (flowrate)Current is constant (flowrate)

Voltage drops across each resistor (pressure)Voltage drops across each resistor (pressure)

R = RR = R11 + R+ R22 + R+ R33 + etc+ etc

ParallelParallel

Voltage drop constantVoltage drop constant

IIinin = I= I11 + I+ I22 + I+ I33 +etc+etc

1/R = 1/ R1/R = 1/ R11 + 1/R+ 1/R22 + 1/R+ 1/R33 + etc+ etc


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Example Problem #1Example Problem #1Determine VDetermine V11, V, V22, V, V33, V, V44, and I., and I.

V1

V2

V4

V3

I

90V

+-

20;5;10;10;


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Example Problem #2Example Problem #2Determine IDetermine I11, I, I22, I, I33, I, I44 and total circuitand total circuit

resistance.resistance.

I1

75V

+-

I4

I3

I2

20;

30;

20;


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Navy Generators

Navy Generators

GeneratorGenerator-- machine used to convertmachine used to convert

mechanical energy into electrical energy.mechanical energy into electrical energy.

Generator is comprised of the essentialGenerator is comprised of the essential

elements of Faradays Law to produceelements of Faradays Law to produce

electrical powerelectrical power


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Induction of VoltageInduction of Voltage

(Faraday)(Faraday) ThreeThree thingsthings mustmust bebe presentpresent inin orderorder toto

produceproduce electricalelectrical currentcurrent::

MagneticMagnetic fieldfield

ConductorConductor

RelativeRelative motionmotion

ConductorConductor cutscuts lineslines ofof magneticmagnetic flux,flux, aavoltagevoltage isis inducedinduced inin thethe conductorconductor

Direction/SpeedDirection/Speed importantimportant


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ElectromagneticElectromagnetic

InductionInduction

MAGNET

RELATIVE MOTION

COIL (CONDUCTOR)

VOLTMETER

INDUCED CURRENT

INDUCED CURRENT


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Direction ofInduced emfDirection ofInduced emf

B

MOTION OF

CONDUCTOR

INDUCED

CURRENT

N S

LEFT HAND

GENERATOR RULE(electron flow)


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Generator PartsGenerator Parts Prime moverPrime mover: mechanical work which turns the rotor, may: mechanical work which turns the rotor, may

be a steam turbine, gas turbine, diesel engine...be a steam turbine, gas turbine, diesel engine...

Armature windingsArmature windings: the conductor in which the output: the conductor in which the output

voltage is inducedvoltage is induced

Field windingsField windings: the conductors used to produce the: the conductors used to produce theelectromagnetic field (needs a DC power supply), theelectromagnetic field (needs a DC power supply), the

magnetmagnet

Stator Stator: stationary housing of the generator, contains the: stationary housing of the generator, contains the

magnet (field windings)magnet (field windings)

Rotor Rotor: rotates inside the stator, moved by a prime mover: rotates inside the stator, moved by a prime mover

(steam turbine, gas turbine, diesel), contains the(steam turbine, gas turbine, diesel), contains the

conductor (armature windings)conductor (armature windings)

PolesPoles: one set of armature windings is called a pole in the: one set of armature windings is called a pole in the

generatorgenerator


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DC GeneratorDC Generator


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ElectroElectro--MagnetMagnet

BBEE (N x I)(N x I)


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D

C MotorsD

C Motors Similar in construction to DC generatorsSimilar in construction to DC generators

A DC generator may be made to act as a DCA DC generator may be made to act as a DC

motor by applying a suitable voltage across itsmotor by applying a suitable voltage across itsoutput terminals (a DC motor acts as a DCoutput terminals (a DC motor acts as a DC

generator operating in reverse)generator operating in reverse)

Operates based on the principle that a currentOperates based on the principle that a current

carrying conductor placed in, and at right anglescarrying conductor placed in, and at right anglesto, a magnetic field tends to move in a directionto, a magnetic field tends to move in a direction

perpendicular to the magnetic lines of force (rightperpendicular to the magnetic lines of force (right--

hand rule)hand rule)


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AC PowerAC Power


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Alternating CurrentAlternating Current

(AC) Theory(AC) Theory

ACAC-- The magnitude and direction ofThe magnitude and direction of

current flow in an AC circuit willcurrent flow in an AC circuit willchange periodically (called a cycle).change periodically (called a cycle).

The frequency (Hz) of an AC circuit isThe frequency (Hz) of an AC circuit is

the number of cycles per second.the number of cycles per second.


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Alternating CurrentAlternating Current

(AC)(AC) Current is constantly changing in magnitudeCurrent is constantly changing in magnitude

and direction at regular intervalsand direction at regular intervals

Current is a function of time and usuallyCurrent is a function of time and usually

varies as a sine functionvaries as a sine function

I

t


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Two Types of AC GeneratorsTwo Types of AC Generators

Revolving armatureRevolving armature

rotor is an armature (conductor) which is rotating insiderotor is an armature (conductor) which is rotating inside

a stationary electromagnetic fielda stationary electromagnetic field

seldom used since output power must be transmittedseldom used since output power must be transmittedthrough slipthrough slip--rings and brushesrings and brushes

Revolving fieldRevolving field

dc current is supplied to the rotor which makes adc current is supplied to the rotor which makes a

rotating electromagnetic field (revolving magnet) insiderotating electromagnetic field (revolving magnet) insidethe stator, stator becomes the armature (conductor) ontothe stator, stator becomes the armature (conductor) onto

which electrical current is induced.which electrical current is induced.

more practical since the current required to supply amore practical since the current required to supply a

field is much smaller & there is reduced sparking andfield is much smaller & there is reduced sparking and


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Generator PartsGenerator Parts Prime moverPrime mover: mechanical work which turns the rotor, may: mechanical work which turns the rotor, may

be a steam turbine, gas turbine, diesel engine...be a steam turbine, gas turbine, diesel engine...

Armature windingsArmature windings: the conductor in which the output: the conductor in which the output

voltage is inducedvoltage is induced

Field windingsField windings: the conductors used to produce the: the conductors used to produce theelectromagnetic field (needs a DC power supply)electromagnetic field (needs a DC power supply)

Stator Stator: stationary housing of the generator: stationary housing of the generator

Rotor Rotor: rotates inside the stator, moved by a prime mover: rotates inside the stator, moved by a prime mover

(steam turbine, gas turbine, diesel)(steam turbine, gas turbine, diesel) Sliding contacts (slipSliding contacts (slip--rings and brushes)rings and brushes): used to conduct: used to conduct

the field or armature current to and from the rotorthe field or armature current to and from the rotor

CommutatorCommutator -- maintains output current in one directionmaintains output current in one direction

(DC generators)(DC generators)


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Revolving ArmatureRevolving Armature

(Low Power/Voltage)(Low Power/Voltage)


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Revolving FieldRevolving Field


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Most electrical equipment in the UnitedMost electrical equipment in the United

States operates on 60 Hz AC electricalStates operates on 60 Hz AC electricalpower (many foreign countries use 50 Hz)power (many foreign countries use 50 Hz)

How fast must a 2How fast must a 2--pole generator bepole generator be

rotating to produce a 60 Hz output?rotating to produce a 60 Hz output?

N x P = 120 x fN x P = 120 x f

NN -- rpm Prpm P -- poles f poles f -- frequency (Hz)frequency (Hz)

Relationship BetweenRelationship Between

Generator Speed andGenerator Speed andFrequencyFrequency


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Classifying ACClassifying AC

GeneratorsGenerators Number of phasesNumber of phases: most shipboard electrical: most shipboard electricalpower is 3 phase, this is more reliable plus loss ofpower is 3 phase, this is more reliable plus loss of

one phase will not cause a loss of equipmentone phase will not cause a loss of equipment

operabilityoperability FrequencyFrequency: most shipboard electrical power is 60: most shipboard electrical power is 60

Hz, some electronic equipment operate at 400 HzHz, some electronic equipment operate at 400 Hz

or higheror higher

VoltageVoltage: usually 450 V, smaller appliances use: usually 450 V, smaller appliances use

120 V120 V

PowerratingPowerrating: measured in kW, most shipboard: measured in kW, most shipboard

generators are 2,000generators are 2,000 -- 3,000 kW3,000 kW


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ThreeThree--Phase ElectricalPhase Electrical

PowerPower Uses three sets of armature windings to produceUses three sets of armature windings to produce

three separate outputsthree separate outputs

Armature windings are physically separated 120Armature windings are physically separated 120oo

from each other, and therefore, each phase is 120from each other, and therefore, each phase is 120oo

apart from anotherapart from another

More power may be generated by a generator of aMore power may be generated by a generator of a

given size and weightgiven size and weight

Provides continuous power to electrical equipmentProvides continuous power to electrical equipment

even if one phase is damagedeven if one phase is damaged


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3 Phase3 Phase


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Three PhaseThree Phase

-1.5000

-1.0000

-0.5000

0.0000

0.5000

1.0000

1.5000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 7 18 19 20 21 22 23 24 25

Sine

Sine + 120

Sine + 240


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AC MotorsAC Motors


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Synchronous MotorSynchronous Motor

Constructed exactly like a generatorConstructed exactly like a generator

In a synchronous motor, the field is on theIn a synchronous motor, the field is on the

statorstator


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Synchronous MotorSynchronous Motor


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Induction MotorInduction Motor

Induction motorInduction motor -- simple, reliable and cheapsimple, reliable and cheap

DifferenceDifference -- the construction of the rotorthe construction of the rotor

Rotating field generated on statorRotating field generated on stator

No slipNo slip--rings or external source of power torings or external source of power to

the rotorthe rotor

Ideal for constant speed, varying torqueIdeal for constant speed, varying torque

applicationsapplications


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Induction MotorInduction Motor

N S


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Other ElectricalOther ElectricalDevicesDevices


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BatteriesBatteries

DryDry--cell batteries: cylindrical zinccell batteries: cylindrical zinc

container, carbon electrode, and ammoniumcontainer, carbon electrode, and ammoniumchloride/water electrolytechloride/water electrolyte

WetWet--cell batteries: leadcell batteries: lead--acid battery is theacid battery is the

most common, can be charged by forciblymost common, can be charged by forcibly

changing the direction of electrical currentchanging the direction of electrical current


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LeadLead--acidBatteryacidBattery

PbPbO2

+ -

H2SO4

Pb + PbO2 + 2H2SO4 2PbSO4 + 2H20pn

Load


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TransformersTransformers

A device that transfers energy by electromagneticA device that transfers energy by electromagnetic

inductioninduction

Primary windings (receive energy from AC source) andPrimary windings (receive energy from AC source) and

secondary windings (delivers energy to the load) (insulatedsecondary windings (delivers energy to the load) (insulated

from each other electrically) are mounted on opposite sidesfrom each other electrically) are mounted on opposite sides

of a ferromagnetic coreof a ferromagnetic core

Used to raise voltage (stepUsed to raise voltage (step--up transformer) or lowerup transformer) or lower

voltage (stepvoltage (step--down transformer)down transformer) Voltage is raised when the primary winding has fewerVoltage is raised when the primary winding has fewer

turns than the secondary winding, and voltage is loweredturns than the secondary winding, and voltage is lowered

when the primary winding has more turns than thewhen the primary winding has more turns than the

secondary windingsecondary winding


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A Simple TransformerA Simple Transformer

CORE

PRIMARY

WINDING

SECONDARY

WINDING


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RectifiersRectifiers

Uses diodes to convert alternating currentUses diodes to convert alternating current

into direct currentinto direct current

Diodes have a small resistance to currentDiodes have a small resistance to current

flow in one direction and a very largeflow in one direction and a very large

resistance to current flow in the oppositeresistance to current flow in the opposite

direction (act as a conductor for half of thedirection (act as a conductor for half of thecycle and as an insulator for the other half)cycle and as an insulator for the other half)


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RectifyingDeviceRectifyingDevice

OutputOutput

t t

I I

INPUT OUTPUT

DIODE


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Voltage KillsVoltage Kills

It is the volume of the current that flowsIt is the volume of the current that flows

that kills.that kills.

0.001 amps = 1 milliamp0.001 amps = 1 milliamp TinglesTingles

0.01 amps = 10 milliamps0.01 amps = 10 milliamps Severe shockSevere shock,,

uncontrolled muscle spasmsuncontrolled muscle spasms

0.1 amps = 100 milliamps0.1 amps = 100 milliamps DEATHDEATH!!

If the current passes through vital organsIf the current passes through vital organs

such as the heart.such as the heart.


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AC vs DCAC vs DC

AC power is easier to generate and requiresAC power is easier to generate and requires

less complex equipment (smaller machines)less complex equipment (smaller machines)

AC energy can be used in transformers toAC energy can be used in transformers to

step up or step down voltages where DCstep up or step down voltages where DC

energy cannotenergy cannot

DC can be stored for reserve use, i.e. theDC can be stored for reserve use, i.e. theships battery!!!ships battery!!!


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