Notes 2

http://cipco.apogee.net/foe/home.asp

What is Electricity? It's a lot easier to describe what it does than what it is. For example, electricity operates our lights, runs our refrigerators and powers our electric motors. The word "electric" comes from the Greek word "amber" and has been used to describe a wide range of related phenomena. We can't see electricity, but we can see its effects, such as light. Electricity can exist in a number of forms, but there are two types of commonly used electricity: 1. Direct Current, which is provided by batteries, 2. Alternating Current which is provided by electric utilities or other power generators in the form of electrons -- called current -- flowing through a wire -- called a conductor.

Electrons To gain an insight into how electricity flows through a material, we need to understand the structure of atoms -- nature's building blocks. All matter is made up of carbon, hydrogen, and other atoms. Each atom is comprised of protons, which are positively charged; neutrons, which have no charge; and electrons, which are negatively charged. The protons form the nucleus of the atom and the electrons travel in orbits around the nucleus much like the earth travels around the sun. Protons and electrons follow specific laws of attraction. Since they have opposite charges, they attract to one another. If an atom has the same number

of protons as electrons, then the atom is balanced, and stable. The orbiting electrons remain in their orbits as long as nothing upsets the balance. When something upsets this balance, then some of the electrons become "knocked" out of their orbits. The are called "free electrons". This unbalanced condition can be caused by rubbing cat's fur on amber, passing a wire through a magnetic field, or putting two chemicals together, as in a dry cell battery.

The free electrons are attracted to atoms where there is an electron missing and will fill the space just vacated by the first free electron. When this conditions occurs continuously, the movement of electrons becomes the basis for the flow of electrical energy we call "current".

Voltage

Voltage is the electrical force that causes free electrons to move

from one atom to another. Just as water needs some pressure to force it through a pipe, electrical current needs some force to make it flow. "Volts" is the measure of "electrical pressure" that causes current flow. Voltage is sometimes referred to as the measure of a potential difference between two points along a conductor.

Voltage is typically supplied by either a generator or battery. Generators are analogous to a water pump in a water piping system, and batteries are similar to water towers. Both systems have a potential difference between the source of the power and someplace downstream from the source. The scientific symbol for voltage is an "E", dating to early days of electricity when it was called the "Electromotive force". Scientists and engineers use the

"E" symbol for voltage, while electricians and wiring books use "V" as the voltage symbol. This can create some confusion, since either may be encountered. In this title, we'll use the practical symbol "V" for voltage.

Current Current is a measure of the rate of electron flow through a material. Electrical current is measured in units of amperes or "amps" for short. This flow of electrical current develops when electrons are forced from one atom to another.

One amp is defined as 6.28 x 10 18 electrons per second. When current flows in a conductor, heat is produced. This happens because every conductor offers some resistance to current flowing. That is why the amperage flow in a circuit is important, since the more amps flowing, the more heat is produced. Most people notice this heating effect when the cord of any appliance or electrical device heats up after the device has been running for an extended period. Recognizing this heat production is important in specifying wire sizes. When a wire carries more amps than it can handle without overheating, we say it is "overloaded". Overloaded wires can melt the insulation and create shocks or even fires.

The scientific symbol for amperage is an "I", dating back to the early days of electricity. It is still used by scientists and engineers. Electricians and wiring guides use "A" as the

amperage symbol. In this title, we'll use the practical symbol "A" for current flow in amps.

Direct Current (DC) Direct current is produced when electrons flow constantly in one direction. It's abbreviated as "DC". Since direct current flows in one direction only, its electrical pressure or voltage is always oriented in one direction, or "polarity".

Interestingly, the first commercial electrical systems set up by Thomas Edison and others were direct current systems. But, for economic reasons, these were later changed to alternating current or AC systems, and are described in the Alternating Current section of this program. Today, batteries, solar panels, fuel cells and special DC generators such as wind turbines produce direct current.

Alternating Current (AC)

Alternating current, or "AC" as it's often called, is the kind of power we are all familiar with. We rely on this kind of power in our homes, businesses, and industries. That's because AC power is much more economical to produce and use than DC power. The first commercial AC power was set up by George Westinghouse in 1886. At that time, Edison was still providing DC current to homes, but the range of power transmission was about one mile from his plant in New Jersey. Because AC power was found to be much cheaper to distribute, it became the obvious preference. The primary characteristic of AC power that makes it so economical is the ability to change the voltage levels by using transformers. The voltage can be

stepped up or down as the need arises. This allows the power to be distributed as widely as needed. Unlike DC voltage and current, which remain steady, AC voltage and current changes -- or cycles -- 60 times per second in North America. AC power in Europe cycles 50 times per second.

Sine Wave Characteristics AC power is represented graphically by a sinusoidal or sine waveform. -- called sine wave for short. As you look at this sine wave, remember that this apparently stable picture changes 60 times every second. In doing so, we think in terms of averages of current, voltage and any changes in frequency. There are five characteristics of AC power; Amplitude, Cycles, Frequency, Peak to Peak, and RMS.

Conductors

Materials that are made up of atoms whose electrons are easily freed are called conductive materials or "conductors". Platinum, gold, and silver are examples of the very best conductors of electricity. Gold is used extensively in small quantities for high-value products like microelectronics, high quality audio components, computer chips and telecommunications satellites. Copper and aluminum are also quite excellent conductors of electricity and much less expensive. Almost all electrical wiring is aluminum or copper.

Resistance

Electrical resistance is defined as the resistance to flow of electricity through a material. Even the best conductors, such as gold, have some resistance.

Resistance elements essentially fall somewhere between a conductor and an insulator. Resistance can also be considered a measurement of how tightly a material holds onto its electrons. For example, common resistance elements in a circuit are lights, motors, and electrical resistance heaters. The electrical resistance of a material is measured in units called "ohms". The lower the resistance of a material, the better the material acts as a conductor. For example, copper has a lower electrical resistance than aluminum; copper is a better conductor. The resistance value for most materials is listed in physics or science books. We can use a water piping system as an analogy. The resistance in the water pipe to the flow of water comes mainly from the size of the pipe. Rust and corrosion inside the pipe, objects stuck inside the pipe, and the number of bends and fittings all add up to increase the resistance to the flow of water. The same is true of current flow in an electric circuit. A number of factors determine the resistance to current flow such as wire diameter, wire length and any impurities in the wire's makeup. For example, smaller wires have more resistance than larger diameter wires and longer wires have more resistance than shorter wires.

When electricity flows through any resistance, energy is dissipated in the form of heat. If the heat becomes intense enough, the conductor resistor may actually glow. This is exactly how an incandescent light bulb works. The filament is made of a material that will resist the current enough to heat up and glow. The scientific symbol for electrical resistance, which is measured in ohms, is the Greek letter Omega. Electricians and practical wiring books typically use an

"R" to represent resistance. So in this title, we will use the practical symbol "R" to represent resistance in ohms.

Insulators Insulators are materials that have structural properties exactly opposite of conductors. These materials are made up of atoms whose electrons are not easily "freed". These electrons are said to be tightly bound to the nucleus, and are very stable. Insulators are used to prevent the flow of electrical current. The rubberized power cord and plastic coverings on appliances are typical examples of insulators. Glass, rubber, porcelain, and most plastics are good insulators.

Power Power is a measure of the amount of work an electric current can accomplish in a specified period of time. The most common unit of electrical power measurement is the watt, or kilowatt, which is 1,000 watts. Power is the rate at which electrical energy is converted into some other form of energy such as light, heat or mechanical work or horsepower. For any electrical device, the higher its power rating in watts, the greater its consumption of electrical energy, not necessarily the amount of work it produces. For example, consider a 100-watt incandescent light bulb. The 100 watts does not represent how much light it produces, but how much electrical power it uses. A 17-watt fluorescent lamp may produce much or even more light, while using only 17% of the power. Appliance manufacturers normally indicate how much electrical power an appliance uses in units of watts. Electric utilities measure the power consumption of their customers in kilowatts, thousands of watts, and measure the power produced by a generator or power plant in units of megawatts, or millions of watts. U.S. motor manufacturers still rate motors in units of horsepower where one horsepower equals 746 watts.

The symbol for "power" is a capital "P". The kilowatt is shown as "kW" with a little k and capital W. The megawatt is shown as "mW" with a little m and capital W.

Energy Electrical energy is the average amount of power used over a given time period and is commonly measured in "kilowatt-hours." Electric utility electric meters accurately measure the kilowatt-hour energy use by the customer, and may also measure peak power use during a specified time interval. Let's calculate the energy use for a blow dryer. Say the blow dryer is rated at 1,500 watts by the manufacturer. This is how much electric power it uses when it operates. If the blow dryer is operated for a total of 2 hours each month, the blow dryer consumes 1,500 watts x 2 hours = 3000 watt-hours. Since utility rates are based on kilowatt-hours, divide by 1,000 to get 3 kilowatt-hours. This shows how power consumption and operating time are important in determining energy use.

Magnetism The generation of electric power depends on magnetism or the principles of magnets. Most of us have seen a magnets' ability to attract certain metals, such as iron. Any material that can attract metals is called a "magnet". The attractive ability of these materials is called "magnetic force". Certain specimens of iron ore possess this attracting property when they are taken from the earth. One name for this material is magnetite or lodestones.

Magnets The basic atomic structure of a magnet seems to align most of the molecules in the same direction. It's possible to see this force through a simple experiment: Put a bar magnet under a sheet of glass and sprinkle iron filings on the glass. The lines of force from the magnet show up clearly as the filings form a pattern. Notice that the attractive forces are greatest at the two ends of the magnet, where the majority of filings gather. We call these ends "poles".

The density of the pattern represents the strength of the field, which is the magnitude of the force exerted upon a magnetic material placed at the point in the field. These lines are called lines of magnetic flux. If we suspend a magnet by a string from its center so that it is free to turn, it will turn until there axis lines up with its poles, lying along the earth's magnetic north and south poles. The pole which points north is called the north pole and the other is called the south pole. These are usually designated by an N and S marked on the magnets. Let's add another magnet to our experiment and we will notice another key property of magnets. The like poles will repel one another, while the unlike poles will attract one another. This is a very important principle since the generation of electric power depends on these laws of attraction. Almost all commercially available magnets are artificial. They were manufactured to be magnets by using other magnets to create the correct molecular alignment. There are two types of magnets: temporary and permanent. Temporary magnets are those which will hold their magnetism only as long as the magnetizing force is maintained. These are usually found inside motors. Permanent magnets are those which will hold their magnetism after the magnetizing force has been removed and will continue to be magnets for as long as they are not disturbed by being jarred or heated.

Electromagnetic Fields The flow of electricity through a conductor produces both an electric and magnetic field around the conductor. Collectively, these two fields are referred to as an electromagnetic field or EMF. The strength of the electric field is measured in volts per meter and varies with the amount of the source

voltage. The higher the source voltage, the higher the strength of the field. Electric field strength decreases rapidly with distance

from the source.

Electric fields are produced both naturally and by any conductor carrying electricity. The strength of the earth's natural electric field varies, but on average is about one-thousandth of a volt per meter. Electric field strength typically varies from 10 to 150 volts per meter under electric distribution lines and 5 to 100 volts per meter inside homes and workplaces. The strength of a magnetic field is typically measured in units of gauss or milligauss and varies with the amount of current moving through a conductor. Lines or devices requiring high levels of current flow produce stronger magnetic fields than those with low current flow. For example, the measure of a magnetic field directly under a high voltage transmission line is somewhere between 20 to 650 milligauss. The magnetic field measured underneath a lower power distribution line is .5 to 30 milligauss. Magnetic fields produced by electrical circuits drop off rapidly with distance from the source. The magnetic field produced by a microwave at 1 foot is 70 to 100 milligauss while at five feet away, the magnetic field strength drops to five milligauss. Electric fields are blocked by shielding such as walls, houses, trees, other vegetation, soil, and other large dense objects. Magnetic fields, on the other hand, pass easily through most objects and are only blocked by structures containing large amounts of iron or iron alloy metals.

Electromagnets Electromagnets play an essential role in the operation of generators, motors, transformers, and relays. Electromagnets are constructed by wrapping an insulated conductor wire around an iron object, like a large nail, and then passing an electrical current through the wire. The strength of the electromagnet depends on the number of wraps, the size of the wire, and the amount of current flowing through the wire.

Magnetic Induction Principles Michael Faraday discovered in 1831 that if a coil of copper wire is rotated in a magnetic field in such a way as to cut across the lines of magnetic force, an electric charge is created or induced in the wires. This is the basic principle by which practically all our present day electric current is generated. Generators use magnetic induction to produce electrical energy. Electrical current is generated by moving wires through a magnetic field. The wire loop inside the generator is mechanically driven by some source of rotary motion. The source of power for the rotation might be fossil fuels, falling water or nuclear energy. As the wire loop spins inside the magnetic field, an electric current is produced in the wire. This current becomes the basis for commercially available electrical energy.

DC Generator A single loop of wire in a magnetic field can be used as a DC generator. When the loop is stationery, it is not cutting any magnetic lines of force and the current and voltage are zero. As the loop of wire is rotated through the magnetic field, it starts to break the magnetic lines of force, and current and voltage are induced in the wire loop. The magnetic lines of force induce current into the wire loop in the same direction of flow as the loop moves in a circle, so the electricity produced is DC since current flow is always the same direction.

AC Generator One of the easiest ways to think about AC or electric power generation, is to think about it as the opposite of electric power use -- kind of like a motor running backwards. Motors convert electricity into power and motion. Generators convert motion and power into electricity.

A typical generator has a large electromagnet spinning inside a stationary coil of wire. As the magnetic field produced by the ends of the magnet moves across the turns of wire in the stationary coil, an electric current is set up in the wire. Increasing the number of turns of wire in a ring or doughnut configuration increases the additive current in the wire. There are two types of alternating current commonly in use today: 

Single-Phase



Three Phase

Single-Phase Single-phase alternating current is most often used in homes, small businesses and on farms. In large commercial buildings and industrial locations where larger motors are used, single phase power is not usually adequate. The production of single-phase alternating current is best described by thinking of the generator as a simple bar magnet rotating inside a single coil shaped loop of wire. When the magnet rotates, the magnetic lines of force cut through the coiled wires. The strength of the field created depends on the number of these lines that are cut each second. At a constant speed, more coils of wire will be cut per second as the loop approaches the one-fourth revolution point and the generated voltage reaches a maximum at this point. As the north pole moves from the one-fourth revolution point to the one-half revolution point, fewer wire coils are being cut per second. The voltage decreases and goes to zero at the one-half revolution point where the magnetic field is parallel to the coils of wire. As the magnet continues to rotate, the South pole's magnetic field cuts the coiled wires in the opposite direction, producing an opposing voltage which again builds up to a maximum at the three-fourths revolution point. As the north pole moves from the three-fourths turn to one full revolution, the voltage then decreases to zero. One complete revolution of the magnetic field is called a cycle. If there was only one coil of wire in the outer portion of the generator this would be a single phase device. By adding two additional coils of wire to the generator, we could then generate current in three individual coils or phases, or three phase power.

Three Phase Three-phase power is designed especially for large electrical loads where the total electrical load is divided among the three separate phases. As a result, the wire and transformers will be less expensive than if these large loads were carried on a single phase system.

Three-phase generators usually have three separate windings, each producing its own separate single-phase voltage. Since these windings are staggered around the generator circumference, each of the single-phase voltages is "out of phase" with one another. That is, each of the three reaches the maximum and minimum points in the AC cycle at different times. Electricity is generated at power companies in these three phases. But, if three phase power is better than single phase, why not four, five or six phase? Theoretically, these would be even better, but equipment manufacturers would have to build motors to use it, and that just wouldn't be cost effective given the installed base of three phase equipment that must continue to be powered. The word "phase" is often abbreviated using the Greek letter "phi" and is written as a zero with a slash mark through it.