In this article, we will define and discuss the following topics; direct current, alternating current, opposition to current flow, magnetism, and power. However, before we begin to discuss these topics, we need to look at some of the units we will be using, and what they mean.
In the power generation industry, we use certain prefixes to express large numbers easily. Please use the chart below as a reference. An example would be a 500,000 watt generator set, which is the same as saying a 500 kilowatt generator set.
Prefix : Multiplier
One : 1
Kilo : 1000
Mega : 1000000
Giga : 1000000000
Electric current is the flow of electrons within a closed circuit, very much like the flow of water. Initially, electric current was believed to flow from positive charges to negative charges. However, it was later discovered that the opposite is true, electron flow is from negative to positive. The unit we use to measure the flow of electrical current is called the ampere. The ampere (amp) was named after the esteemed French scientist, Andre-Marie Ampere, in the year 1881. 1 ampere is equal to 6.24 x 10^18 electrons moving past a given point in 1 second. In order to accurately measure the amount of current, or number of amperes, in a given circuit, it is recommended to use an ammeter. This hand held device is must be inserted in series within the circuit, so that all the current flows through it.
The volt is another important measurement within an electrical circuit. It is the measurement of electromotive force (EMF), and was named after Alessandro Volta, an Italian scientist. 1 volt causes 1 ampere to flow in a circuit with a resistance of 1 ohm (we will define an ohm in the following paragraph). Voltage is also referred to as potential difference across a circuit. Voltage can be measured using a voltmeter across any 2 points in an electrical circuit. This is different that the ammeter, which must be measured in series so all the current (amperes) can flow through it and be measured.
Resistance is defined as the hindrance to current flow in an electrical circuit. Resistance is measured using the unit ohm, which was named after the famed German scientist Georg Ohm. It can be helpful to think of ohms as the opposition to current flow in an electrical circuit. 1 ohm of resistance will cause a 1 volt drop when 1 ampere is flowing. Low resistance value materials are used to fabricate conductors, such as copper or aluminum. High resistance materials such as rubber are used to fabricate insulators. Ohm’s law can be defined as V=I x R, where V=volts, I=amps, and R=ohms. Ohm’s law proves that the flow of current in an electrical circuit is directly proportional to the electromotive force across the circuit, and inversely proportional to the resistance of the circuit. This means that with a constant voltage across a circuit, increasing the resistance (R) will reduce the current flow (I). Resistivity can be tested by taking a sample wire of a given material that is about 1/1000th of an inch in diameter, and 1 foot long. The value of resistance measured on the test sample of each material is termed the resistivity of the material. Resistance is like friction, it creates heat when current flows through it. The amount of heat that is generated is a function of the amount of current running through the given circuit. These sort of measurements can be taken using a resistance temperature device or detector. A resistance temperature device is a wire wound resistor whose resistance is a known linear function of its temperature.
There are 2 basic types of electrical current; direct current (DC) and alternating current (DC). Direct current flows in only one direction, while alternating current is bidirectional. However, DC currents can be reversed by reversing the polarity (or the direction) of the current flow in the circuit. This can be done by reversing the positive and negative leads feeding a DC circuit. Current flow will rise quickly to a specific level, and remain constant at that level for a period of time. Although DC current flows in one direction, it can still rise to a certain level, and then fall back down to zero. This is known as pulsating DC. As this cycle repeats, DC current flows from zero to peak and back to zero. Keep in mind that DC current does not drop below the zero threshold. Periods of current flow will typically be equal.
AC current has alternately positive and negative values. It is an electric current that reverses its direction in regularly recurring intervals of time. Unlike DC current, AC current has no polarity, as the current flow reverses every half cycle regardless if the negative or positive feeds are changed. The AC current flow forms the shape of a sine wave. During a cycle of AC current, the current will rise from zero to its peak positive, then drop back to zero, and continue to fall until it hits the peak negative, then rise back to zero. There are 2 ways to measure an AC voltage sine wave; using the root mean square, or the average voltage.
The amount of time for a cycle of a sine wave is called the frequency. 1 full cycle of an AC sine wave is the amount of time it takes for the wave to reach a peak positive value, drop down and reach a peak negative value, and then rise back to zero. Frequency is measured in Hertz, and the instrument that is used to measure frequency is called a frequency meter. One Hertz is equal to 1 cycle per second, so if a sine wave creates 60 cycles per second, that sine wave would have a frequency of 60 Hertz. This means that a sine wave with a frequency of 60 Hertz completes one cycle in 1/60th of a second, or .0166 seconds. In North America, we run at a frequency of 60 Hertz, while other parts of the world such as Europe and South America run at a frequency of 50 Hertz.
The sine wave that is created by an AC current that is described above, is known as single phase. It is called single phase because it refers to one power wire, and one neutral wire. In the United States, 120V is the standard single phase voltage. This type of power delivery is typical in residential applications such as a home, or a small office. Three phase power involves adding 2 more wires, giving you a total of 4; 3 phase wires, and 1 neutral wire. 3 phase power is used more in commercial or industrial applications, because it is a more efficient power delivery method that can help save on electrical costs. Within a given cycle, 3 phase power gives you 1.732 times more the amount of power than single phase. This is due to the fact that 2 more sine waves have been added to a given cycle in a 3 phase system.
Capacitors are an important topic when discussing electrical circuits. The principal function of a capacitor is to store electrons, whose movement can create an electrical current. A capacitor can be as simple as 2 metallic plates with some sort of insulator between them. The metal plates would each form a terminal for the capacitor. Let’s take an example, you have a simple circuit with a dc battery as a power source, a capacitor, a switch to open and close the circuit, and a load. Let’s start with the switch being open, so there is no circuit. When the switch is then closed, you will see that there will be no apparent current flow. However, what has actually happened, is the capacitor is being charged by the current flow, but is not allowing the current to pass due to the insulator between the 2 metallic plates. In this sense the capacitor is acting like a battery, and holding the electrical charge that is being produced by the circuit. Keep in mind that even after the circuit is broken by the opening of the switch, the capacitor is able to continue to store that electrical charge for an extended period of time, and can be dangerous.
Magnets and magnetism are necessary to create an electric charge. A magnetic field is made up of a concentration of what are called flux lines. In order to have a magnetic field, there must be a minimum of 2 poles, a north and a south pole. A simple example of this would be a bar magnet that is used in elementary school experiments. Flux lines become more concentrated towards the ends of each pole. This higher concentration of flux lines increase the magnetic strength at each pole end. Unlike poles attract each other, like poles repel each other. This can be seen by simply taking 2 small magnets and putting each end together. This simple experiment, while most often seen by children, is a great way to visualize how magnetic force works. Two positive (or negative) ends will repel each other, while a positive and a negative end will attract each other. Metal materials such as steel or iron, when introduced to a magnetic field, will become magnetized themselves. This phenomenon is known at retentivity, or the ability of a material to retain magnetism. It was discovered by Professor Hans Orstead that when an electric current flows through a conductor such as a copper wire, a magnetic field is created around the conductor. In a DC current circuit, if the leads on each end of the power source are changed, such as the connections on a battery, the current will reverse, along with the magnetic field. In a simple DC current circuit with a small DC battery, a wire with connecting leads, and a paper with small iron filings, you can perform a simple experiment and see the change in the magnetism as the lead ends are switched. The iron filings will move in different directions as the magnetic field changes. DC current is necessary to maintain a constant polarity, as it only flows in a single direction. AC current is bi-directional, and when it is introduced into a circuit, the AC current will constantly reverse the polarity of the current. Another very important and interesting phenomenon, is the fact that when you wrap a conductive wire into coils, the magnetic force increases with each additional coil added. This is called magnetomotive force, and it states that this force is equal to the product of the number of turns of the conductor forming the coil, times the current flow in the coil. An example of this would be a 10 turn coil carrying 50 amperes has the same magnetomotive force as a 500 turn coil carrying 1 ampere of current.
Permeability of the flux lines within a magnetic field is also important when trying to build a stronger magnetic field. This permeability is determined by the ease in which the flux lines can pass through a given material. Interestingly enough, air is less permeable for a flux line than certain materials such as iron. It is easier for a flux line to pass through a piece of iron than through the air. That means that an iron cylinder wrapped in a conductor with current running through it such as a copper wire, will produce a much stronger magnetic field than if the wire simply had air between the coils.
All of these principals of electro magnetism and electromagnetic induction are critical to the understanding of how an electrical generator functions. Michael Faraday, in 1831, discovered that when a conductor is able to move through, or cut, lines of flux in a magnetic field, a voltage is created. Flux lines can be cut by either moving the conductor (wire) through the magnetic field, or moving the magnetic field past the conductor. This principal is the basis for all electric generator, motor, and transformer operations and design.