basic electrotechnical training

7/30/2019 Basic Electrotechnical Training

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Basic Electrical Technology

Training Material

For Entrepreneurs

September 2011

ADAMA, ETHIOPIA


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Content page

Preface

PART I

CHAPTER I: WHAT IS ELECTRICITY? 4

CHAPTER II: ELECTRICITY WHERE DO THEY COME FROM? 7

Example 1: Static & Dynamic Electricity

Example 2: Electrochemical cells

Example 3: Solar Cells or Photovoltaic Energy

Water Analogue

CHAPTER III: Electrical Quantities 11

Voltage, current and ResistanceOhms Law

Power

Energy

PART II 14

Experiment1: Introduction to Multimeters

Digital Multimeter

Analogue Multimeter

Sensitivity of Analogue mulimeter

Measuring Current & voltage

Measuring Resistance

Experiment2: Ohms Law

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Preface

The organizer of this training material does not claim to have produced

a well organized course material. Rather, the aim has been to provide

entry level introduction to Electrical Technology to Entrepreneurs in

Solar photovoltaic business that have not been passed through

Electrical Courses at any level.

This training material has got two Parts. Part one deals basic theories

and definition of terms. Part II: deals with practical activities.

Tafesse Aserat

Peter Adelmann

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PART I

CHAPTER I: WHAT IS ELECTRICITY?

Electricity figures every where in our lives. Electricity lights up our homes, cooks our food, powers ourcomputers, television sets, and other electronic devices. Electricity from batteries keeps our cars running

and makes our flashlights shine in the dark.

But what is electricity? Where does it come from? How does it work? Before we understand all that, we

need to know a little bit about atoms and their structure.

All matter is made up of atoms, and atoms are made up of smaller particles. The three main particles

making up an atom are the proton, the neutron and the electron.

Electrons spin around the center, or nucleus, of atoms, in the same way the moon spins around the

earth. The nucleus is made up of neutrons and protons.

Fig 1. Structure of an Atom

Electrons contain a negative charge, protons a positive charge. Neutrons are neutral they have neither

a positive nor a negative charge.

There are many different kinds of atoms, one for each type of element. An atom is a single part that

makes up an element. There are 118 different known elements that make up every thing! (Elements in a

periodic table) Some elements like oxygen we breathe are essential to life.

Each atom has a specific number of electrons, protons and neutrons. But no matter how many particles

an atom has, the number of electrons usually needs to be the same as the number of protons. If the

numbers are the same, the atom is called balanced, and it is very stable.

So, if an atom had six protons, it should also have six electrons. The element with six protons and six

electrons is called carbon. Carbon is found in abundance in the sun, stars, comets, atmospheres of most

planets, and the food we eat. Coal is made of carbon; so are diamonds.

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Some kinds of atoms have loosely attached electrons. An atom that loses electrons has more protons

than electrons and is positively charged. An atom that gains electrons has more negative particles and is

negatively charge. A "charged" atom is called an "ion."

Electrons can be made to move from one atom to another. When those electrons move between the

atoms, a current of electricity is created. The electrons move from one atom to another in a "flow." One

electron is attached and another electron is lost. Scientists and engineers have learned many ways to

move electrons off of atoms. (Friction, cell, photocell, generators etc)

Since all atoms want to be balanced, the atom that has been "unbalanced" will look for a free electron to

fill the place of the missing one. We say that this unbalanced atom has a "positive charge" (+) because it

has too many protons.

Since it got kicked off, the free electron moves around waiting for an unbalanced atom to give it a home.

The free electron charge is negative, and has no proton to balance it out, so we say that it has a"negative charge" (-).

So what do positive and negative charges have to do with electricity?

Scientists and engineers have found several ways to create large numbers of positive atoms and free

negative electrons. Since positive atoms want negative electrons so they can be balanced, they have a

strong attraction for the electrons. The electrons also want to be part of a balanced atom, so they have a

strong attraction to the positive atoms. So, the positive attracts the negative to balance out.

The more positive atoms or negative electrons you have, the stronger the attraction for the other. Since

we have both positive and negative charged groups attracted to each other, we call the total attraction

"charge."

When electrons move among the atoms of matter, a current of electricity is created. This is what

happens in a piece of wire. The electrons are passed from atom to atom, creating an electrical current

from one end to other.

Electricity is conducted through some substances/things better than others do. Its resistance measures

how well something conducts electricity. Some things hold their electrons very tightly. Electrons do not

move through them very well. These things are called insulators. Rubber, plastic, cloth, glass and dry air

are good insulators and have very high resistance.

Other materials have some loosely held electrons, which move through them very easily. These are

called conductors. Most metals like copper, aluminum or steel are good conductors.

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Where Does the Word 'Electricity' Come From?

Electrons, electricity, electronic and other words that begin with "electr..." all originate from the Greek

word "elektor," meaning "beaming sun." In Greek, "elektron" is the word for amber.

Amber is a very pretty goldish brown "stone" that sparkles orange and yellow in sunlight. Amber is

actually fossilized tree sap! It's the stuff used in the movie "Jurassic Park." Millions of years ago insects

got stuck in the tree sap. Small insects which had bitten the dinosaurs, had blood with DNA from the

dinosaurs in the insect's bodies, which were now fossilized in the amber.

Ancient Greeks discovered that amber behaved oddly - like attracting feathers - when rubbed by fur or

other objects. They didn't know what it was that caused this phenomenon. But the Greeks had

discovered one of the first examples of static electricity.The Latin word, electricus, means to "produce from amber by friction."

So, we get our English word electricity from Greek and Latin words that were about amber.

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CHAPTER II: ELECTRICITY WHERE DO THEY COME FROM?

Example 1: Static & Dynamic Electricity

If we take the examples of wax and wool which have been rubbed together, we find that the surplus of

electrons in the wax (negative charge) and the deficit of electrons in the wool (positive charge) create an

imbalance of charge between them. This imbalance manifests itself as an attractive force between the

two objects:

Fig2. Static electricity

If a conductive wire is placed between the charged wax and wool, electrons will flow through it, as some

of the excess electrons in the wax rush through the wire to get back to the wool, filling the deficiency of

electrons there:

Fig3. Dynamic electricity.

Example 2: Electrochemical cellsAn electrochemical cell is a device that produces an electric current from energy released by a chemical

reaction.

Electrochemical cells have two conductive electrodes (the anode and the cathode). Theanodeis defined

as the electrode where releases electrons (Oxidation occurs) and the cathode is the electrode where the

addition of electrons (reduction) takes place. Electrodes can be made from any sufficiently conductive

materials, such as metals, semiconductors, graphite, and evenconductive polymers. In between these

electrodes is the electrolyte, which contains ions that can freely move.

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Fig4. Voltaic Cell

Principle of Operations

Two or more electrochemical cell connected together is called Battery. It is a device that converts

chemical energy directly to electrical energy. It consists of a number of voltaic cells; each voltaic cell

consists of two half cells connected in series by a conductive electrolyte containing anions and cations.

One half-cell includes electrolyte and the electrode to which anions (negatively charged ions) migrate,

i.e., the anodeor negative electrode; the other half-cell includes electrolyte and the electrode to which

cations (positively charged ions) migrate, i.e., the cathode or positive electrode. In the redoxreaction

that powers the battery, cations are reduced (electrons are added) at the cathode, while anions are

oxidized (electrons are removed) at the anode.The electrodes do not touch each other but are

electrically connected by theelectrolyte. Some cells use two half-cells with different electrolytes. A

separator between half cells allows ions to flow, but prevents mixing of the electrolytes.

Each half cell has an electromotive force (or emf), determined by its ability to drive electric current from

the interior to the exterior of the cell. The net emf of the cell is the difference between the emfs of its

half-cells, as first recognized by Volta. Therefore, if the electrodes have emfs and , then the net emf

is ; in other words, the net emf is the difference between the reduction potentialsof the half-

reactions.

Example 3: Solar Cells or Photovoltaic Energy

It is known today, that the sun is simply our nearest star. Without it, life would not exist on our planet.

We use the sun's energy every day in many different ways.

When we hang laundry outside to dry in the sun, we are using the sun's heat to do work drying our

clothes.

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Plants use the sun's light to make food. Animals eat plants for food. Decaying plants hundreds of millions

of years ago produced the coal, oil and natural gas that we use today. So, fossil fuels is actually sunlight

stored millions and millions of years ago.

Indirectly, the sun or other stars are responsible for ALL our energy.

We can also change the sunlight directly to electricity using solar cells.

Solar cells are also called photovoltaic cells or PV cells for short and can be found on many small

appliances, like calculators, and even on spacecraft. They were first developed in the 1950s for use on

U.S. space satellites. They are made of silicon, a special type of melted sand.

When sunlight strikes the solar cell, electrons (red circles) are knocked loose.

Fig5. PV Cell operation

They move toward the treated front surface (dark blue color). An electron imbalance is created between

the front and back. When the two surfaces are joined by a connector, like a wire, a current of electricity

occurs between the negative and positive sides.

These individual solar cells are arranged together in a PV module and the modules are grouped together

in an array. Some of the arrays are set on special tracking devices to follow sunlight all day long.

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Fig6. PV cell, module, and Array

The electrical energy from solar cells can then be used directly. It can be used in a home for lights and

appliances. It can be used in a business. Solar energy can be stored in batteries to light a roadside

billboard at night. Or the energy can be stored in a battery for an emergency roadside cellular telephone

when no telephone wires are around.

Water Analogue

To understand how voltage and amperage are related, it is sometimes useful to make an analogy with

water. Look at the picture here of water flowing in a garden hose. Think of electricity flowing in a wire in

the same way as the water flowing in the hose. The voltage causing the electrical current to flow in the

wire can be considered the water pressure at the faucet, which causes the water to flow. If we were to

increase the pressure at the hydrant, more water would flow in the hose. Similarly, if we increase

electrical pressure or voltage, more electrons would flow in the wire.

Does it also make sense that if we were to remove the pressure from the hydrant by turning it off, the

water would stop flowing? The same is true with an electrical circuit. If we remove the voltage source, or

EMF, no current will flow in the wires.

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Fig7. Water flowing in a garden hose

Another way of saying this is: without EMF, there will be no current. Also, we could say that the free

electrons of the atoms move in random directions unless they are pushed or pulled in one direction by

an outside force, which we call electromotive force, or EMF.

CHAPTER III: Electrical Quantities

(Voltage, current and Resistance)Current: The movement of electric charge is known as an electric current, the intensity of which is

usually measured inamperes. The term amps are often used for short. An amp is the amount of

electrical current that exists when a number of electrons, having one coulomb of charge, move past a

given point in one second. A coulomb is the charge carried by 6.25 x 10^18 electrons. This quantity of

Electricity is measured by using an instrument called Ammeter.

Electromotive force (Voltage): the force that causes the electrons to move in an electrical circuit. This

force is called electromotive force, or EMF. Sometimes it is convenient to think of EMF as electrical

pressure. In other words, it is the force that makes electrons move in a certain direction within a

conductor. It is measured in volts.

This quantity of Electricity is measured by using an instrument called Voltmeter.

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Resistance: Resistance is a term that describes the forces that oppose the flow of electron current in a

conductor. All materials naturally contain some resistance to the flow of electron current. It is measured

in units called ohms.

This quantity of Electricity is measured by using an instrument called Ohmmeter.

Table 1: Summary

No Quantity Symbol Unit Measured by

1 Electromotive force

Potential Difference

Voltage

emf

Pd

V

Volt(v)

Voltmeter

2 Current I Amps (A) Ammeter

3 Resistance R Ohm () Ohmmeter

HOW VOLTAGE, CURRENT, AND RESISTANCE RELATE

The first, and perhaps most important, relationship between current, voltage, and resistance is

called Ohms Law. Ohms principal discovery was that the amount of electric current through a

metal conductor in a circuit is directly proportional to the voltage impressed across it, for any

given temperature. Ohm expressed his discovery in the form of a simple equation, describing

how voltage, current, and resistance interrelate:

E = I R

In this algebraic expression, voltage (E) is equal to current (I) multiplied by resistance (R).Using algebra techniques, we can manipulate this equation into two variations, solving for I

and for R, respectively:

I = E/R R = E/I

Electrical Power (P)

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Whenever current flows, work is done. A conductor may be heated, a motor may be spun, a bulb might

give off light, or some other form of energy may be released. There is a simple law that tells exactly how

much work may be done by a flowing current.

The amount of work done is equal to the voltage of the supply times the current flowing through the

wire.

This law is expressed in the form

P=IV, whereP is the power in watts,

I is the current in amps, and

V is the voltage in volts.

For example, if we find that a light bulb draws half of an amp at 120 volts, we simply multiply the 120

volts by half an amp to find that the bulb draws 60 watts of power.

Fig8. Ohms law Relationship between voltage, current, resistance and power.Electrical Energy (E)

Energy is defined as the ability to do work. It is measured in Joule. Power and energy are two

terms often used interchangeably. It is common to see even engineers making the mistake of

using the terms power and energy interchangeably. Remember Energy is the ability to do work;

while Power is the rate at which work is done.

Example: In this picture a garden hose is being used to fill a wading pool. The rate at which the

water comes out of the hose determines how long it will take to fill the pool. In the first image

the water is coming out very slowly and it will take a long time for the pool to fill.

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Fig 9: Low flow rate

In this second image the water is coming out of the hose more quickly allowing the pool to be

filled in a shorter time period.

Fig 10: High flow rate

In this analogy the rate at which the water comes out of the hose is equivalent to the power

output of a generator. The water that has filled the wading pool is equivalent to the energy

generated by a generator. While both hoses have produced the same amount of water (energy),

the rates at which they did so (power) were different. The jet of water coming from the second

hose was stronger, or more powerful, meaning that at any given moment more water was passing

through the nozzle.

Energy is calculated using the following formula

Energy (E) in Joule = Power (in watt) * Time (in second)

PART II

Experiment1: Introduction to Multimeters

Digital multimeters

All digital meters contain a battery to power the display so they use virtually no power from the circuit

under test. This means that on their DC voltage ranges they have a very high resistance (usually called

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input impedance) of 1M or more, usually 10M , and they are very unlikely to affect the circuit under

test.

Typical ranges for digital multimeters like the one illustrated:

(the values given are the maximum reading on each range)

DC Voltage: 200mV, 2000mV, 20V, 200V, 600V.

AC Voltage: 200V, 600V.

DC Current: 200A, 2000A, 20mA, 200mA, 10A*.

*The 10A range is usually unfused and connected via a special socket.

AC Current: None. (You are unlikely to need to measure this).

Resistance: 200 , 2000 , 20k , 200k , 2000k , Diode Test.

Digital meters have a special diode test setting because their resistance ranges cannot be used to test

diodes and other semiconductors.

Fig11: Digital Multimeter

Analogue multimeters

Analogue meters take a little power from the circuit under test to operate their pointer. They must have

a high sensitivity of at least 20k /V or they may upset the circuit under test and give an incorrect

reading. See the section below onsensitivityfor more details.

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Batteries inside the meter provide power for the resistance ranges, they will last several years but you

should avoid leaving the meter set to a resistance range in case the leads touch accidentally and run the

battery flat.

Typical ranges for analogue multimeters like the one illustrated:

(the voltage and current values given are the maximum reading on each range)

DC Voltage: 0.5V, 2.5V, 10V, 50V, 250V, 1000V.

AC Voltage: 10V, 50V, 250V, 1000V.

DC Current: 50A, 2.5mA, 25mA, 250mA.

A high current range is often missing from this type of meter.

AC Current: None. (You are unlikely to need to measure this).

Resistance: 20 , 200 , 2k , 20k , 200k .

These resistance values are in the middle of the scale for each range.

It is a good idea to leave an analogue multimeter set to a DC voltage range such as 10V when not in use.

It is less likely to be damaged by careless use on this range, and there is a good chance that it will be the

range you need to use next anyway!

Fig12: Analogue Multimeter

Sensitivity of an analogue multimeter

Multimeters must have a high sensitivity of at least 20k /V otherwise their resistance on DC voltage

ranges may be too low to avoid upsetting the circuit under test and giving an incorrect reading. To obtain

valid readings the meter resistance should be at least 10 times the circuit resistance (take this to be the

highest resistor value near where the meter is connected). You can increase the meter resistance by

selecting a higher voltage range, but this may give a reading which is too small to read accurately!

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On any DC voltage range:

Analogue Meter Resistance = Sensitivity Max. reading of range

e.g.

a meter with 20k /V sensitivity on its 10V range has a resistance of 20k /V 10V = 200k .

By contrast, digital multimeters have a constant resistance of at least 1M (often 10M ) on all their DC

voltage ranges. This is more than enough for almost all circuits.

Measuring voltage and current with a multimeter

1. Select a range with a maximum greater than you expect the reading to be.

2. Connect the meter, making sure the leads are the correct way round.Digital meters can be safely connected in reverse, but an analogue meter may be

damaged.

3. If the reading goes off the scale: immediately disconnect and select a higher range.

Multimeters are easily damaged by careless use so please take these precautions:

Always disconnect the multimeter before adjusting the range switch.

Always check the setting of the range switch before you connect to a circuit.

Never leave a multimeter set to a current range (except when actually taking a reading).The greatest risk of damage is on the current ranges because the meter has a low

resistance

Measuring resistance with a multimeter

To measure the resistance of a component it must not be connected in a circuit. If you try to measure

resistance of components in a circuit you will obtain false readings (even if the supply is disconnected)

and you may damage the multimeter.

The techniques used for each type of meter are very different so they are treated separately:

Measuring resistance with a DIGITAL multimeter

1. Set the meter to a resistance range greater than you expect the resistance to be.

Notice that the meter display shows "off the scale" (usually blank except for a 1 on the left).

Don't worry, this is not a fault, it is correct - the resistance of air is very high!

2. Touch the meter probes together and check that the meter reads zero.

If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again.

3. Put the probes across the component.

Avoid touching more than one contact at a time or your resistance will upset the reading!

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Measuring resistance with an ANALOGUE multimeter

The resistance scale on an analogue meter is normally at the top, it is an unusual scale because it reads

backwards and is not linear (evenly spaced). This is unfortunate, but it is due to the way the meter

works.

1. Set the meter to a suitable resistance range.

Choose a range so that the resistance you expect will be near the middle of the scale.

Hold the meter probes together and adjust the control on the front of the meter which is usually

labeled "0 ADJ" until the pointer reads zero (on the RIGHT remember!).

If you can't adjust it to read zero, the battery inside the meter needs replacing.

2. Put the probes across the component.

Avoid touching more than one contact at a time or your resistance will upset the reading!

Experiment 2: OHM'S LAW

PURPOSE: The purpose of this lab is to investigate the relationship between the three

variables involved in Ohm's Law - Current, Voltage and Resistance.

MATERIALS:

Circuits Experiment Board

D cell (or Variable DC-power supply)

Wire

Digital Multimeter (DMM)

Resistors (different size)

PROCEDURE:

1. Choose one of the resistors that you have been given. Using resistor color code, decode

the resistance value and record that value in the first column of the Data Table.

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Fig 13: Meter connection

2. Construct the circuit above, leaving the wire between the resistor and the battery unconnected.

Have the instructor check your wiring before going on.

3. Set the DMM to the 20 MA range, noting that the red lead should be connected to the plug onthe DMM designated for "MA" or "A". The black lead should always be connected to the

"COMMON" plug. When your circuit has been approved, connect the remaining wire and read

the current that is flowing through the resistor. Record this value in the second column of the

Data Table. Note that the decimal point is for MA or milliamps - 0.001 amps.4. Now replace the original resistor with the remaining resistors, one at a time, each time

recording the resistance value and the current you measured.5. When you have completed measuring all of the currents, disconnect the meter and connectthe circuit shown below. Change the meter to the 2VDC scale (or 2000 mV) and move the red

lead to the plug on the meter that indicates "V". Measure and record the voltage across the

resistor for each of the resistors. (On the 2000 mV scale, the decimal point is for millivolts.)6. When finished making measurements, turn the meter off, secure all of the resistors by placing at leastone end of each in a spring, and disconnect the wire(s) from the battery. Process the data you've

obtained. The results of this experiment are important and will definitely be covered on any quizes, etc.

DATA TABLE:

No Resistance Current Voltage Ratio (Voltage/Resistance)

12

3

4

5

6

7

ANALYSIS:

1. For each of your sets of data, calculate the ratio of Voltage/Resistance. Compare the

values you calculate with the measured values of the current.

2. Construct a graph of Current (vertical axis) vs Resistance.

DISCUSSION:

1. From your graph, what is the mathematical relationship between Current and Resistance

(for a constant voltage)?

2. Ohm's Law state that current is equal to the ratio of voltage/resistance. Does your data

concur with this?

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