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Product Details
| ISBN-13: | 9781504965439 |
|---|---|
| Publisher: | AuthorHouse |
| Publication date: | 01/03/2016 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 148 |
| File size: | 9 MB |
Read an Excerpt
Electrical Power Simplified
By Prashobh Karunakaran
AuthorHouse
Copyright © 2016 Dr. Prashobh KarunakaranAll rights reserved.
ISBN: 978-1-5049-6542-2
CHAPTER 1
Introduction
Electricity has provided huge benefits for mankind. The small village homes depend on it for lighting and irrigation of crop lands. At the other end of the spectrum, the prime mover of the largest machines in the world have over the past few decades moved away from combustion engines and hydraulics and to electric induction motors. Extremely hot areas of the earth like the Arabian Peninsula have been made habitable with electricity powered air conditioning and extremely cold places like Alaska can attract human populations with electricity powered heating. Thus electricity has become a fine compatriot of humanity. But, if we touch an electricity carrying wire, we will be burnt. Thus it is imperative that as many people as possible have proper knowledge of the limits, dangers of electricity and respect it. This book hopes to disseminate the knowledge of the electric power system to as many people as possible. Calculations are avoided as far as possible. The most useful formulae in electrical power are Ohm's law (1) and it's derivation, the power law (2)
V = IR (1)
P=V (2)
Where V = Voltage or EMF (Electromotive Force) in volt units, I= current in ampere units and R = resistance in ohm units and P = power in watt units.
The simple principle of electricity is that generation of power should always equal to the customer demand for it. If this is not balanced, there will be effects on voltage and frequency. The control of this balance is the biggest complication in an AC (alternating current) electrical power system. Nowadays intermittent renewal energy is fast replacing conventional hydrocarbon energy which suddenly made the system even more complicated.
Propagation speed of electricity is affected by insulation. In an unshielded copper conductor, it is about 96% of the speed of light, while in a typical insulated coaxial cable it is about 66% of the speed of light (3 X 108 m/s). But actual speed of electrons is near 0 m/s in AC and comparable to putty flow down a wall in DC (direct current). As an analogy to explain this is if a pipe with ends named A and B are filled with table tennis balls. And if the first ball at point A is given a little push, immediately the ball at end B will move. In a similar way, a push of the first slow moving electron at point A of an electrical wire will cause the electron at point B of the wire to move immediately; this immediate action is termed current (I). This action happens at about 96% the speed of light in a bare electrical wire (conductor). If the conductor is covered with in insulation, current flow can drop up to 66% the speed of light.
For current carrying wire, the analogy of the finger which pushed the first ball in the pipe is replaced by a term called voltage or EMF. Thereby voltage is the force which pushed the first electron giving a force to each and every ball all along the pipe. If the finger pushed the first ball with a great force at end A, the tennis ball at point B will jump out and fly quite a distance. The combination of the finger force plus how rough or smooth the pipe is (resistance = R) for all the tennis balls to flow is termed the power (so P=VI, whose units is watts). If the pipe is rough (high R), even a forceful push (high V) of the ball at point A will not result in the ball at point B moving much. But if the pipe is made as smooth as humanly possible (low as possible R) even a slight finger push (small V) will cause the ball at point B to move with close to exactly the finger pushing force exerted at point A.
Another analogy is a powerful BMW next to a tiny Daihatsu on a straight road. The BMW has high power (high EMF or V) and the tiny Daihatsu has low power (low EMF or V). The speed of the car is the current (I); sometimes the tiny Daihatsu can actually beat the powerful BMW if the driver is good enough, that is, the current of the Daihatsu can be greater than the BMW even though it has a low V. When the two cars meet a traffic jam, they are experiencing R.
The 'roughness in the pipe' is termed resistance (R). There are exactly three ways electron flow can be slowed:
1. Resistance: roughness of the pipe which restricts the table tennis balls from flowing is equivalent to resistance.
2. Inductive reactance: assuming the table tennis balls are replaced with ball bearings and a magnet it placed over the pipe, that will slow electron movement. Note that current carrying wire forming a coil is a magnet by itself; it is called a solenoid. Andre-Marie Ampere discovered the fact that electrons moving in a circle parallel to each other (as in a solenoid) forms a magnet. While in a permanent magnet, unpaired electrons spins line up to each other in regions called domains causes magnetism. This slowing of electron flow (current flow) as it moves into a solenoid is called inductive reactance.
3. Capacitive reactance: assuming the negatively charged table tennis balls have built up on the first plate and the second plate is neutral (having an equal number negatively charged tennis balls and stationary positive charged balls). If more and more of negatively charge balls accumulate on the first plate, a critical charge will be reached to enable these balls to have enough energy to jump to the nearby positively charged plate. This accumulation of electrons on one place before jumping onto a nearby plate is called capacitive reactance.
Therefore all three, resistance, inductive reactance or capacitive reactance slows down current flow.
CHAPTER 2Electrical Safety
We cannot say current or voltage is more dangerous. The combination of both, the power whose equation is below
P = VI cos θ (3)
is what causes electric shocks. Where θ or cos θ is an indication of the degree to which the current waveform have shifted with respect to the voltage waveform. This variation in the travelling speed of the current waveform with respect to the voltage waveform is call phase shift. The phase shift only occurs in AC moving through a capacitor or inductor and doesn't happen if the AC is moving through a resistor. Phase shift also do not happen in DC.
It takes about 40 volts of force for the negative charge from a current carrying conductor to jump into a human skin. This 40 volts plus or minus will form a sigmoid curve if human resistivity data is collected. People with moist hands will conduct electricity slightly better than those with dry hands. An equipment can have very high power but insufficient voltage for that current to jump into your skin. For example a very big and high powered speaker can have 12V leads but high current (amps) to deliver such huge power to the speakers (as in some dance places). A human can touch this 12V lead even when the speaker is live.
Definition of danger in electrical installation: any source of voltage which is high enough to cause sufficient current flow to the muscles or hair.
Electric currents are always finding a pathway to go to Ground (Earth). If there is an easier pathway for it to reach Ground (Earth) via a copper wire, it will avoid choosing human bodies to reach ground. The ground resistance (earth resistance) must be <100Ω for homes or offices, it must be <10Ω for a steel electric pole and <1Ω for substations and power stations. To put into perspective why electricity loves to go to the ground; if two 10Ω resistors are connected in parallel, the resultant resistance is 5Ω. If four 10Ω resistors are connected in parallel the resistance drops to 2.5 Ω. If eight 10 Ω resistors are connected in parallel the resistance drops to 1.25Ω. Now 1cm3 of silicon has a resistance of 200kΩ (sand is silicon dioxide SiO2). If billions of these sand particles are touching a Ground (Earth) rod, they are effectively billions of 200kΩ resistors in parallel and the Ground (Earth) resistance can easily reach less than 100Ω which is the requirement for Ground (Earth) rods of every home, office or factory.
Below is an EXCEL calculation of resistor in parallel following the equation:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)
This was done till column KQ which is adding 303 resistors in parallel of 200,000 Ω each. The final resistance of 303 resistors in parallel is 662Ω. Thus finer grain size means even more 'resistors' in parallel. A highly compacted fine sand around the Ground (Earth) rod will add even more 'resistors' in parallel. A wet ground (mineral filled water conducts electricity but not distilled water) will lower the resistance of each sand particle. In this author's experience one location which is even famous for flooding has a relatively high Ground (Earth) resistance because the water is from a river which has less minerals.
A 240V shock will be more serious than a 120V shock because the force (EMF) for the current to jump into the skin is double. A person dealing with electricity should not be allowed to wear any gold, silver or other conductive ornaments. There are even cases of 6V sending high current through a person's gold ring sometimes even taking the finger off. When this author graduated with an electrical engineering degree and going through the ceremony at South Dakota State University, USA, an oath ceremony had to be gone through where oaths like not designing anything which will endanger humans and down the list is one oath whose statement was, "I will never wear gold on my body."
The weakest point in the human body with regards to ability to withstand electric shock is the heart and brain. So these two portions of the body should always be as far as possible from any electric conductor while performing electrical installation. The CPR (Cardio Pulmonary Resuscitation) is performed on a victim of electric shock. The human body is basically an electric machine so when electricity goes into the body from an external source, the body system is disrupted; electrical signals that instruct the heart to pump are disrupted. The heart 'forgets' how to pump. CPR is done to reteach the heart how to pump. 30 pushes to a point one inch above the bottom of the sternum (meeting point of the ribs), two mouth to mouth blows and then another 30 push. This cycle of 30 plus 2 must be repeated five times before performing a test of the victim's blood circulation. The recommended method to do this test is by placing the pointing and index fingers at the soft region of the victim's neck in-between the harder throat and the hard muscles at the side of the neck. If he still has no circulation, the process must be repeated till the ambulance comes. If there is circulation the victim must be paced in the recovery position; body lying on the side, bottom arm stretched out and other arm on this arm, top leg touching floor and over the bottom leg.
It has to be noted that all metal work used in electrical systems must have a Ground (Earth) wire joined to it. The reason for this is that as mentioned before, electricity loves the ground and if there is a Ground (Earth) wire joined to the metal body, electricity will use it to go to ground rather than a via the human body touching the switchgear (any device used to control electricity flow is called switchgear). The resistance of a human body to electricity flow is too high to be a choice for electricity to use to go to Ground (Earth) if there is a very low resistance (low O) pathway like a Ground (Earth) wire. So the human will not experience any shock touching an electricity leaking switchgear which has a ground wire as shown in Fig. 21. One of the main reasons electricity leaks to the body of a switchgear is when plastic insulated wires goes into the switchgear via sharp holes drilled into it's metal body. Cable glands must be used whenever a cable is running through a metal hole.
CHAPTER 3History of Electricity
In 1600 William Gilbert coined the word 'electric' which comes from the Greek word 'elektron' meaning amber. He wrote a book named, 'On Magnetism' which was published in 1600.
In 1744, a Dutch scientist named Pieter van Musschenbroek invented the Leyden jar and named it after his town of Leiden (Leyden). This was the first capacitor invented. He was basically trying to capture whatever is electric in a wine bottle. He accidently found that it stored more charge if he held it with his hand. This jar stores static electricity and was used by early researchers to perform experiments in electricity. The jar was a glass bottle coated with a metal foil which doesn't reach the cork top. The jar is partially filled with impure water. It has to be noted that pure water is not conductive. A metal wire passes through the cork top which closes the jar. This wire is connected to an external terminal which is a sphere to avoid losses by corona discharge. Early researchers believed the charge was stored in the water but Benjamin Franklin investigated it and concluded that the charge was stored in the glass container.
In 1752, 46 year Benjamin Franklin (American) with the help of his son flew a kite with a key tied to the bottom of a silk string. They also tied a thin metal wire from the key to a Leyden jar. He then attached a silk ribbon to the key and held it from inside a barn away from the rain. It was a raining day and lightning struck the kite and went down the wet silk string to the key and into the Leyden jar. Ben was unaffected by the negative charge because he was holding a dry silk string (insulator). There was a U shape between the key and the point Ben was holding the string. This is critical in electrical engineering installations; water will follow along the string (or electric cable) from a long way up but will fall at the bottom of the U, thereby the portion of the cable at the next termination or holder of the cable has little water. Thus in all electrical installations, there must be a U just before the next termination point of the wire.
When Ben later moved his hand near the iron key he received a shock because the negative charges from the key went to Ground (Earth) via his body. An arc jumped from the key to his hand. The experiment proved that lightning was static electricity. Ben was lucky to have survived this experiment because a later scientist who replicated his experiment was killed by the lightning. From this experiment Ben came up with the idea of the lightning conductor which later saved his home from a direct lightning strike. One person managed to capture a picture of lightning striking the Eiffel Tower showing lightning choosing to strike only the copper lightning conductor at the highest point and not any of the metal works. The steel metal work is a conductor and is joined to ground via a pile. The pile has a mesh of steel (BRC) and goes deep into wet ground, thereby making it a good Ground (Earth). But lightning chose to strike only the lightning conductor because it can differentiate the resistance difference between the steel framework and the copper conductor which is the lightning rod. Today various shapes of lightning conductor tops have been devised to attract lightning better but the basic principle from Ben is that if a Grounded (Earthed) copper rod is placed at the highest point of a building, it will attract lightning because lightning only wants to go to ground with the lowest resistance pathway it can find. If lightning can find a human body as a pathway, that will be chosen. This happened when three children played football on a rainy day in Kuching, Malaysia which lies on the Equator and therefore has a high lightning rate. It was a big football field but the lightning chose to strike two of the boys who died instantly. The boys have blood in them which contains iron which is a conductor. The lightning can differentiate this resistance difference between four feet of air and a child's body and therefore chose the body as the easier path to enter Ground (Earth).
Ben made a mistake that electricity will move from the positive to the negative and we are still living with his mistake today. The direction of current he postulated is opposite of the actual electron flow which causes current. Books were expensive in those days so when later scientist discovered that Ben actually made a mistake they decided to leave it that way. When electrical calculations are made totally the wrong way round, the answer will still be the same. Ben coined many electrical terms like conductor, condenser (capacitor), battery, charge, positively, negatively, plus and minus.
In 1780 Luigi Galvani (Italian) put a frog leg in-between two different metals. But he believed the electricity came from the frog leg and called it, 'animal electricity.'
(Continues...)
Excerpted from Electrical Power Simplified by Prashobh Karunakaran. Copyright © 2016 Dr. Prashobh Karunakaran. Excerpted by permission of AuthorHouse.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
Contents
Chapter 1 Introduction, 1,Chapter 2 Electrical Safety, 3,
Chapter 3 History of Electricity, 6,
Chapter 4 DC and AC currents, 16,
Chapter 5 Generator Principle, 17,
Chapter 6 Understanding 1Φ and 3Φ power, 29,
Chapter 7 Line voltage and phase voltage, 37,
Chapter 8 Conductors and Protection, 39,
Chapter 9 Motors, 53,
Chapter 10 Inductors, 71,
Chapter 11 Capacitors, 75,
Chapter 12 Transformers, 77,
Chapter 13 Electrical formulas, 88,
Chapter 14 Calculations, 97,
Chapter 15 Magnetism, 102,
Chapter 16 Low Voltage wiring, 105,
Chapter 17 Electric Cars, 108,
Chapter 18 Superconductivity, 110,
Chapter 19 Anode and Cathode, 111,
Chapter 20 Rectifiers, 113,
Chapter 21 Power Generation, 117,
Chapter 22 The Air Conditioning system, 124,
Chapter 23 The Grid, 126,
Chapter 24 HVDC Transmission, 130,
Chapter 25 Grid Control, 134,
Chapter 26 About the Author, 141,