Electrotec - Introduction

8/14/2019 Electrotec - Introduction

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Electrotechnology- DC System

Introduction


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Subject General ObjectiveTo provide students with knowledge and skill to

operate and maintain ship board electricalsystem safely.


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General Teaching navigationElectrotechnology

Application of Circuit laws

Electrical measuring Instruments

Marine Electrical Practice

High voltage Generator and System Electronics


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Electrotechnology GLO

Understand circuit law and Apply concept to solve problems inDC circuit

SLO

Apply circuit law to solve problem in dc network- series, parallel,

combined dc and parallel circuitsSolve problem to find power and efficiency in electromechanical

system and consideration required when apply the laws in

network problemsSolve problem regarding resistance required to extent range of

ammeter and voltmeter and verify the experiments


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Key Terms and Phrases Electric circuit

battery - temperature coefficient of resistance

electrodes -Ohm's law

electromotive force -electric power

emf -kilowatt-hour (KWH)

conventional current -Joule heating

electron current -direct current (dc)

ampere (amp) -alternating current (ac)

resistance -electric generator

resistivity

ohm


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Terms of Electric Circuit All atoms consist ofprotons, neutrons and electrons. The protons, which

have positive electrical charges, and the neutrons, which have no electricalcharge, are contained within the nucleus.

Removed from the nucleus are minute negatively charged particles called

electrons. All atoms are bound together by powerful forces of attraction existing

between the nucleus and its electrons.

Electrons in the outer shell of an atom, however, are attracted to their nucleus

less powerfully than are electrons whose shells are nearer the nucleus.

Conductors are materials that contain electrons that are loosely connected tothe nucleus and can easily move through the material from one atom toanother.

Insulators are materials whose electrons are held firmly to their nucleus.

The unit used to measure the quantity of electrical charge Q is called thecoulomb C where 1 coulomb = 6.24 x 1018 electrons quantity of electricalcharge transferred


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Terms of Electric Circuits

What is Electrical Current?

Electrical current is the flow of electronsthrough a conductor.

What is a Conductor?

A material that allows electrons to flow through itis a conductor. Wires are conductors. Electricity flows through a wire much like water

through a hose. Electricity flows through the human body more

like water through a sponge.


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Terms of Electric Circuits

What is an Insulator? Insulators resist the flow of electricity.

Glass, rubber, plastic, and dry wood areinsulators.

What is Resistance?

Resistance opposes electron flow. Electricity flows through any available path,

but more of it flows through the path of leastresistance.


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Electric Circuit

What is an Electrical Circuit?

Current flows in a loop or a circuit. Circuitsare AC or DC. AC is alternating current. DCis direct current.

DC current flows from NEGATIVE toPOSITIVE. Most AC current flows from HOTto NEUTRAL.

Most circuits in a typical home orconstruction site are AC.


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Components Electric Circuit


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Components Electric CircuitA simple AC circuit has five parts:

1. electrical SOURCE;

2. HOT wire that sends electricity;3. CONSUMING DEVICE a tool, appliance, or light that

is powered by electricity;4. NEUTRAL wire that returns electricity, and

5. earth or GROUND. When a circuit works right, current flows through the

HOT wire to the CONSUMING DEVICE. It then returns to the SOURCE through the NEUTRAL

wire. When something goes wrong with a circuit, it iscalled a faulted circuit or electrical fault.


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The Electric Battery A BATTERY is a source of

electric energy.

A simple battery contains

two dissimilar metals,

called ELECTRODES, and

a solution called the

ELECTROLYTE, in which

the electrodes are

partially immersed.


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The Electric Battery An example of a simple battery would

be one in which zinc and carbon are

used as the electrodes, while a diluteacid, such as sulfuric acid (dilute),acts as the electrolyte.

The acid dissolves the zinc and causes

zinc ions to leave the electrode. Each zinc ion which enters the

electrolyte leaves two electrons on thezinc plate.

The carbon electrode also dissolvesbut at a slower rate.

The result is a difference in potentialbetween the two electrodes.


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The Dry CellThe Dry cell is relatively inexpensive

and quite portable.

It has many uses such as in flashlights

and radios.

The anode consists of a Zinc can in

contact with a moist paste of ZnCl2 and

NH4Cl.A carbon rod surrounded by MnO2 and

filler is the cathode.

The cell reaction appears to vary withthe rate of discharge, but at low power

the probable reactions are as follows:


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Lead Storage CellThe basic features of the lead

storage cell are electrodes of

lead and lead dioxide, dippinginto concentrated sulfuric acid

Both electrode reactions produce lead sulfate, which adheres to the electrode.

When the cell discharges, sulfuric acid is used up and water is produced.

The state of the cell can be determined by measuring the density of the

electrolyte solution (the density of water is about 70% that of the sulfuric acid

solution).


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Electric Current

An electric CURRENT exists

whenever electric charge flows

through a region, e.g., a simplelight bulb circuit.

The magnitude of the current is

measured in AMPERES

(Amps/A), where

1 ampere = 1coulomb/second

I = Q/ t.


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CONVENTIONAL CURRENT vs ELECTRON CURRENT

The direction of

CONVENTIONAL CURRENT is

in the direction in which positivecharge flows.

In gases and liquids both positive

and negative ions move. Only

negative charges, i.e., electrons,move through solids and this is

referred to as ELECTRON

CURRENT.

For historical reasons,

conventional current is used in

referring to the direction of

electric charge flow.


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Ohm`s Law

The magnitude of the electric current that flows through a closed circuit

depends directly on the voltage between the battery terminals and

inversely to the circuit resistance. The relationship that connects current, voltage and resistance is known as

OHM'S LAW and is written as follows:

I = V/R or V = IR The current is measured in amperes, the voltage in

volts and the resistance in ohms ().


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Alternating Current

In a DIRECT CURRENT (dc) circuit thecurrent flows in one direction only.

In an ALTERNATING CURRENT (ac)circuit the direction of current flow through

the circuit changes at a particular frequency

(f).

The frequency used in Malaysia and on the

ship is 60 cycles per second or 60Hz.


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Alternating Current

The emf produced by an ac ELECTRIC GENERATOR is

SINUSOIDAL.

The current produced in a closed circuit connected to the generator isalso sinusoidal.

The equations for the voltage and current are as follows:

Vo is referred to as the peak voltage

f = 60 Hz in is used in Malaysia and on the ship

Io = Vo/R is referred to as the peak current


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Alternating Current Since the current oscillates between positive and negative values, the

average current in an AC circuit is ZERO.

Electrons do move back and forth in the circuit so heat and power areproduced

The power delivered to a resistor at any instant is:

Since sin2 2ft varies between 0 and 1, its

average value is 1/2. Thus the averagepower developed is equal to the dotted

line in figure 18-15


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Electric Power Work is required to transfer

charge through an electric circuit.

The work required depends onthe amount of charge transferred

through the circuit and the

potential difference between the

terminals of the battery:

W = QV.

The rate at which work is done to

maintain an electric current in a

circuit is termed ELECTRIC

POWER


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Electric Power ELECTRIC POWER equals the product of the current I

and the potential difference V, i.e., P = IV.

The SI unit of power is the watt (W), where

1 W = 1 J/s.

The kilowatt is a commonly used unit where

1 kilowatt = 1000 watts.

The electric energy produced by the source of emf isdissipated in the circuit in the form ofheat.

The kilowatt hour (kWh) is commonly used to representelectric energy production and consumption where

1 kWh = 3.6 x 106 J.


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Electric Power

In a circuit of resistance R, the rate at

which electrical energy is converted to

heat energy is given by:P = IV but V = IR, then P = I(IR) =I2R

where I2R is known as JOULE HEATING.

An alternate formula for power can be

written, since I = V/R, then P = IV =

(V/R)V = V2/R

P=V2/R= I2R are power formulas which

apply only to resistors P = IV Applies to any device


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Electrical Safety


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What is a Faulted Circuit?In a faulted circuit or electrical fault, current follows the wrongpath and bypasses the normal load. This happens in one of twoways.

1. Short Circuit

Two HOT wires or a HOT wire and a NEUTRAL wire touch. The current then bypasses the tool.

Short circuits cause shocks and damage equipment. They make excess heat that can start fires. With a short circuit, a tool usually will not work.

2. Ground Fault The HOT wire touches an outlet or tool casing. The outlet or tool may keep working until something like a

person touches it creating multiple paths to GROUND.

Ground faults cause shocks.


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What are the Harmful Effects of Electricity?

Shock

Burns Fires

Explosions


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How Do We Work Safely with Electricity?

At a minimum, employers must follow

the Electrical Standards: These standards provide protection forusing temporary wiring in construction.

The regulations include threeprotective methods:electrical isolation

grounding, and

circuit interruption


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What is Grounding? Grounding is a separate, low resistance

pathway for electricity when it does notfollow normal flow from HOT to NEUTRAL.

Grounding wont work if your resistance isless than the GROUND path. For example: If youre holding a metal pipe that goes directly to

GROUND,

If youre standing in water, or

If your tool doesnt have a GROUND connection.


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What is Circuit Interruption? The last method of electrical protection

required is by circuit interruption.

For electricity to flow, it must complete aloop or a circuit.

Circuit interrupters break the loop, openingthe circuit, so the electricity does not flow.

Circuit breakers or fuses for wiring and

equipment protection and Ground Fault Protection for shock

protection.


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Circuit Measurement

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Electrotec - Introduction