Basic Electricity Final

8/8/2019 Basic Electricity Final

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FUNDAMENTAL OFFUNDAMENTAL OF

ELECTRICITYELECTRICITY


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OHMS LAW, POWER ANDOHMS LAW, POWER ANDENERGYENERGY

Ohms Law as Applied to a ResistiveCircuit.


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OHMS LAWOHMS LAW

EMF ( electro-motive force ) is the forcewhich causes electrons to flow through the

connected circuit. This force orEMF

ismeasured in volts and is referred to as voltage. Symbol is V orE. In this lecture,the symbol V will be used so as not toconfuse Voltage with Energy .


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OHMS LAWOHMS LAW

A battery supplying a large number of voltsexerts a greater force on the electrons,

causing them to flow at a greater ratethrough the circuit.

The rate at which electrons flow through acircuit is termed CURRENT and ismeasured inAMPERE. It is denoted bysymbol I.


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OHMS LAWOHMS LAW

Resistance- or the opposition to the flowof electric current. Measured in Ohms,

and is assigned the letterR, it isabbreviated with the Greek letter, capital Omega ( )

The larger the ohmic value of the resistor,the greater the resistance to the flow ofelectric current.


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OHMS LAWOHMS LAW

A linear relationship exists between theapplied voltage and the resulting current,

with an increase in voltage producing aproportional increase in current. That is forany given value of resistance, doubling thevoltage applied will double the current.

This relationship is known as Ohms Lawand is given by the following expression :


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OHMS LAWOHMS LAW

Voltage (V) = Current (I) xResistance (R);

I= V/R and R= V/I


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OHMS LAWOHMS LAW

SAMPLE PROBLEMS TO SOLVE

1. If the applied voltage is 12 Volts and the

resistance is 48 Ohms, calculate theCurrent in Amperes flowing through the48 Ohms resistor.

2. Calculate the voltage required to producea current of 0.75 Amp in the circuit ofQuestion No. 1


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OHMS LAWOHMS LAW

What value of resistance isrequired to produce a current of

0.5 Ampere using a 12-Voltbattery?


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ELECTRICAL POWERELECTRICAL POWER

Electrical Power is defined as the product ofvolts and amperes and is measured inWatts ( W ). The common equations forcalculating power are :

P= EI; P= IR and P=E/R

Where P is Power inWatts

E= Voltage in Volts, and

I= Current in Amperes


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ELECTRICAL POWERELECTRICAL POWER

A Watt is a basic unit of power, but is verysmall. In the electrical utility business,Kilowatts ( KW ) and Megawatts ( MW ) are

the most common units.Putting Power Into Perspective

One Kilowatt ( KW) = 1000Watts

Hardware store portable generators haveratings such as 750W, 2.5KW, or 3.5 KW.A typical roaster oven is 1.5KW.


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Cont.Cont.-- Putting Power IntoPutting Power Into

PerspectivePerspective

One Horsepower (HP )= 746Watts

With 100 HP=74.6KW, a 75 KVAtransformer is needed to run a 100 HPmotor. However, in reality a motor is onlyabout 80% efficient, and a 100 KVAtransformer would be needed.


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Cont.Cont.-- Putting Power IntoPutting Power Into


One Megawatt ( MW )= 1000 Kilowatts

Some of the older hydraulic stations onsmall rivers have generating unit rated atabout 1MW.

E

achM

itsubishi Genset inM

alakalPP

has arating of 3.40 MW while eachWartsilla has1.75MW capacity.


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Formula for PowerFormula for Power

ForPower ( in Watts ) ;

P= IR

P= EIP=E/R


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Formula For CurrentFormula For Current

For Current ( in Amperes )

I = P/E

I = E/RI = P/R


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Formula for VoltageFormula for Voltage

For Voltage ( in Volts )

E = IRE = P/I

E = PR


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Formula for ResistanceFormula for Resistance

For Resistance ( in Ohms )

R = E/IR = E/P

R = P/I


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Typical Problem SolvingTypical Problem Solving

1. What is the current required if the voltagespecified is 120V and the power is 1200watts.


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ELECTRICAL ENERGYELECTRICAL ENERGY

Electrical energy is the product measured ata customers meter. Electrical energy isactually power x time. In other words,electrical energy and electrical power arenot the same thing.

-It is the product of watts and time. Onewatt times one second is watt-second.

One Kilowatt times one hour is a Kilowatt-hour.


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ENERGYENERGY

As a unit, the watt is the amount of powerbeing used at a given instant. It is alsonecessary to know how long the power isused to determine the amount a customeris charged for energy. Customer are billedbased on the kilowatt-hour ( KW-hr ),

where the kilowatt is the rate at whichenergy is used and the hour is the lengthof time the power is used.


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Putting KWPutting KW--Hr into PerspectiveHr into Perspective

One Kilowatt-hour will run a 1000-wattmicrowave oven or a 1000-watt hair dryerfor one hour.

One liter of gasoline has the energyequivalent of approximately 10 KW-hr.

One U.S. gallon of gasoline has the energyequivalent of approximately 37KW-hr ).


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Cont. Putting KWCont. Putting KW--Hr inHr in


1000 Kilowatt-HoursAn average household in North America is

considered to use 1000 KW-Hr per month,and this figure is often used whencomparing electric bills between utilities.

For residential rate, the energy rate in Palau

for the first 500 KW-Hr per month is 8cents/ KW-Hr ( excluding Fuel AdjustmentCost )


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Cont. Putting KWCont. Putting KW--Hr inHr in


One KW-Hr is a small unit. When utilitiesand even countries are compared, theamount of energy generated is shown inbillion kilowatt-hours.

Annual statistics for the sale or generation ofelectricity are expressed in billions of KW-

Hr. The total generating capacity of theworld is approx 12,000 billion KW-Hr.


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ENERGY MEASUREMENTENERGY MEASUREMENT

KW-Hr meters are installed to measure thekilowatt-hours used by a customer. Thereis a large variety of revenue metering usedto measure other variables.


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TYPES OF REVENUE METERINGTYPES OF REVENUE METERING

There are three kinds of electrical power:

1. Real power

2. Apparent power3. Reactive power


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UNIT OF MEASURE FORUNIT OF MEASURE FOR

BILLINGBILLING

Kinds of Power Power Energy

Real Power Peak KW KWh

Apparent Power Peak KVA KVAhReactive Power Peak KVAR KVARh


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METER AVAILABILITYMETER AVAILABILITY

Self-contained meters- Is a meter that can beinstalled without the use of current orpotential transformers. The meter can

carry the actual load current through itscoil and can use the actual service voltagein the meter potential coil. The S-C meterare normally 240 volts or less, but 600-voltmeters are available. Generally 200 Ampor less, although there are higher ratedmeters on the market.


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Transformer-Rated meters-When currentor voltage is too high to be carried by self-contained meter, instrument transformersare used to send a representative currentand voltage to the meter. A transformer-rated meter is used to measure a

representative current from the currenttransformer and a representative voltagefrom the potential transformer.


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Primary or Secondary Metering -Somecustomers buy power at a primary voltageand use their own transformers to step itdown to a utilization voltage. Atransformer- rated meter and itsassociated potential and current

transformers are used to send arepresentative voltage and current fromthe primary to the meter.


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ALTERNATING CURRENT (AC)ALTERNATING CURRENT (AC)

Introduction

Alternating current (AC) became the standardform of electrical power over Direct Current

( DC) in the pioneer days of electrical power.ACs Major Advantage the easy transformation

from one voltage to another. Easytransformation allows the voltage to be steppedup for efficient transmission of electrical energyover long distances.


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Characteristics of ACCharacteristics of AC

AC BasicsCurrent flows in a conductor as long asthere is a potential difference present. Tohave a potential difference, one end of thecircuit is at an opposite pole ( polarity ) tothe other end. These polarities are labeledas positive and negative. The direction ofthe current flow in a circuit is determinedby the polarity of the source terminals.


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AC and DC ComparisonAC and DC Comparison

With DC, the polarity does not change andthe current flows in one direction only.With AC, the polarity at the sourcealternates between positive and negativeand the current direction changes withevery change of the source polarity.


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ACAC-- FrequencyFrequency

In North America, alternating currentsupplied by electrical utilities travels 60times in each direction in one second. Insome other parts of the world, 50 cyclesper second is common. The term cyclesper second has been replaced by the

international standard term for frequency,which is hertz and is represented by thesymbol Hz.


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ACAC -- FrequencyFrequency

Unlike voltage or current, the frequency in a circuitstays constant right from the generator to thecustomer.When the frequency starts to drop, it

is an indicator that the generator supplying theelectrical system is overloaded and slowing. Asmall reduction in frequency will trigger theelectrical system to trip out of service. A typical

range is from 59.97 to 60.03 Hz. Some systemsare set-up to start load shedding when thefrequency reaches 59.3 Hz.


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REACTANCE IN AC CIRCUITSREACTANCE IN AC CIRCUITS

There are three kinds of loads fed by an ACCircuit:

1. Heating and lighting are resistive loads. A

resistive load can do work. The amount ofwork being done can be measured by akilowatt-hour meter.

2. The energy used to magnetize a motor or a

transformer is an inductive load. An inductiveload does not generate heat or light and is notmeasured by the kilowatt-hour meter.


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1. A resistive load opposes the flow ofcurrent, but so do inductive and capacitiveloads. All three types of loads oppose theflow of current and add to the totalopposition to current flow in a circuit. Thisdisturbance imposed by inductive and

capacitive loads in a circuit is calledreactance. The symbol for reactance is X.


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Reactance can be inductive reactance( XL ) caused by loads such as motors,transformers, fluorescent lights andcomputers, or it can be capacitivereactance (XC ) caused by capacitors orparalleling conductors.


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IMPEDANCE IN AN AC CKTIMPEDANCE IN AN AC CKT

Resistance is the only opposition to the currentflow in a DC circuit. In an AC circuit, theopposition to current flow consists of resistance

and reactance. This combination of resistanceand reactance opposing the current flow isreferred to as impedance. Impedance ismeasured in ohms and is represented by the

symbol Z. Impedance can be usedinterchangeably with resistance in calculationusing Ohms Law.


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IMPEDANCE IN AN AC CKTIMPEDANCE IN AN AC CKT

The formula is :

Z=IR , Z=E/I and I=E/Z

where E= effective voltage in volts

I= effective current in amperes

Z= total impedance in ohms.


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AC POWERAC POWER

Active Power-The total power produced in a DC circuit iscalculated by multiplying total voltage by total

amperes. Because there is no reactance in a DCcircuit, all of the power the circuit supplies isuseful power oractive power.

Active Power is also referred to as effective

power, true powerorreal powerbecause thisis the power that gives light, gives heat and turnsmotor. Real power can be expressed as voltagex resistive loads.


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ACTIVE POWERACTIVE POWER

In AC circuits, power alternates at the samerate as the voltage and current arealternating. The power measured at acustomers meter is referred to as activepower. Therefore, the total active powersupplied by a circuit is equal to total

effective current x total effective voltage,or P= IE


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ACTIVEPOWERACTIVEPOWER

Active Power is measured at a customermeter and is measured as watts or inblocks of1000 watts, which is kilowatts.

It is simply abbreviated as KW.


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APPARENT POWERAPPARENT POWER

The total power supplied by a circuit iscalled apparent power, because in an ACcircuit all of the power does not performactual work. Apparent poweris acombination ofactive and reactivepower.

AP = ( active power)2+( reactive power )2

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Basic Electricity Final