File Name: ac current and voltage .zip
In Direct Current DC , the flow of electric charge is unidirectional. In DC, the voltage and current maintain a constant polarity and direction. The source of direct current is the battery.
See the Design Science License for more details. Available in its entirety as part of the Open Book Project collection at:openbookproject. Illustrations reworked in standard graphic eps and jpeg format. Source files translated to Texinfo format for easy online and printed publication. Source files translated to SubML format. New sections added, and error corrections made, since the fourth edition. To explain the details of why this is so, a bit of background knowledge about AC is necessary.
If a machine is constructed to rotate a magnetic field around a set of stationary wire coils with the turning of a shaft, AC voltage will be produced across the wire coils as that shaft is rotated, in accordance with Faraday's Law of electromagnetic induction. Connected to a load, this reversing voltage polarity will create a reversing current direction in the circuit. The faster the alternator's shaft is turned, the faster the magnet will spin, resulting in an alternating voltage and current that switches directions more often in a given amount of time.
While DC generators work on the same general principle of electromagnetic induction, their construction is not as simple as their AC counterparts.
With a DC generator, the coil of wire is mounted in the shaft where the magnet is on the AC alternator, and electrical connections are made to this spinning coil via stationary carbon "brushes" contacting copper strips on the rotating shaft. All this is necessary to switch the coil's changing output polarity to the external circuit so the external circuit sees a constant polarity: Figure 1 Step 3Step 4 The generator shown above will produce two pulses of voltage per revolution of the shaft, both pulses in the same direction polarity.
The diagram shown above is a bit more simplified than what you would see in real life. The problems involved with making and breaking electrical contact with a moving coil should be obvious sparking and heat , especially if the shaft of the generator is revolving at high speed. If the atmosphere surrounding the machine contains flammable or explosive vapors, the practical problems of spark-producing brush contacts are even greater.
An AC generator alternator does not require brushes and commutators to work, and so is immune to these problems experienced by DC generators. The benefits of AC over DC with regard to generator design is also reflected in electric motors. While DC motors require the use of brushes to make electrical contact with moving coils of wire, AC motors do not. This relative simplicity translates into greater reliability and lower cost of manufacture.
But what else is AC good for? Surely there must be more to it than design details of generators and motors! Indeed there is. There is an effect of electromagnetism known as mutual induction, whereby two or more coils of wire placed so that the changing magnetic field created by one induces a voltage in the other. If we have two mutually inductive coils and we energize one coil with AC, we will create an AC voltage in the other coil. When used as such, this device is known as a transformer: Figure 1 The fundamental significance of a transformer is its ability to step voltage up or down from the powered coil to the unpowered coil.
The AC voltage induced in the unpowered "secondary" coil is equal to the AC voltage across the powered "primary" coil multiplied by the ratio of secondary coil turns to primary coil turns. If the secondary coil is powering a load, the current through the secondary coil is just the opposite: primary coil current multiplied by the ratio of primary to secondary turns.
This relationship has a very close mechanical analogy, using torque and speed to represent voltage and current, respectively: Figure 1. When transmitting electrical power over long distances, it is far more efficient to do so with stepped-up voltages and steppeddown currents smaller-diameter wire with less resistive power losses , then step the voltage back down and the current back up for industry, business, or consumer use.
Transformer technology has made long-range electric power distribution practical. Without Step-up transformer steps voltage up and current down. Step-upStep-down the ability to efficiently step voltage up and down, it would be cost-prohibitive to construct power systems for anything but close-range within a few miles at most use.
As useful as transformers are, they only work with AC, not DC. Because the phenomenon of mutual inductance relies on changing magnetic fields, and direct current DC can only produce steady magnetic fields, transformers simply will not work with direct current. Of course, direct current may be interrupted pulsed through the primary winding of a transformer to create a changing magnetic field as is done in automotive ignition systems to produce high-voltage spark plug power from a low-voltage DC battery , but pulsed DC is not that different from AC.
Perhaps more than any other reason, this is why AC finds such widespread application in power systems. Often, the number of turns in each coil is set to create a voltage increase or decrease from the powered primary coil to the unpowered secondary coil.
When graphed over time, the "wave" traced by this voltage of alternating polarity from an alternator takes on a distinct shape, known as a sine wave: Figure 1. If we were to graph the trigonometric function of "sine" over a horizontal range of 0 to degrees, we would find the exact same pattern as in Table 1. The reason why an electromechanical alternator outputs sine-wave AC is due to the physics of its operation. The voltage produced by the stationary coils by the motion of the rotating magnet is proportional to the rate at which the magnetic flux is changing perpendicular to the coils Faraday's Law of Electromagnetic Induction.
That rate is greatest when the magnet poles are closest to the coils, and least when the magnet poles are furthest away from the coils. Mathematically, the rate of magnetic flux change due to a rotating magnet follows that of a sine function, so the voltage produced by the coils follows that same function. If we were to follow the changing voltage produced by a coil in an alternator from any point on the sine wave graph to that point when the wave shape begins to repeat itself, we would have marked exactly one cycle of that wave.
This is most easily shown by spanning the distance between identical peaks, but may be measured between any corresponding points on the graph.
The degree marks on the horizontal axis of the graph represent the domain of the trigonometric sine function, and also the angular position of our simple two-pole alternator shaft as it rotates: Figure 1 Since the horizontal axis of this graph can mark the passage of time as well as shaft position in degrees, the dimension marked for one cycle is often measured in a unit of time, most often seconds or fractions of a second.
When expressed as a measurement, this is often called the period of a wave. The period of a wave in degrees is always , but the amount of time one period occupies depends on the rate voltage oscillates back and forth. A more popular measure for describing the alternating rate of an AC voltage or current wave than period is the rate of that back-and-forth oscillation.
This is called frequency. The modern unit for frequency is the Hertz abbreviated Hz , which represents the number of wave cycles completed during one second of time. In the United States of America, the standard power-line frequency is 60 Hz, meaning that the AC voltage oscillates at a rate of 60 complete back-and-forth cycles every second. In Europe, where the power system frequency is 50 Hz, the AC voltage only completes 50 cycles every second.
A radio station transmitter broadcasting at a frequency of MHz generates an AC voltage oscillating at a rate of million cycles every second. Prior to the canonization of the Hertz unit, frequency was simply expressed as "cycles per second. Many people believe the change from self-explanatory units like CPS to Hertz constitutes a step backward in clarity.
A similar change occurred when the unit of "Celsius" replaced that of "Centigrade" for metric temperature measurement. The name Centigrade was based on a count "Centi-" scale "-grade" representing the melting and boiling points of H 2 O, respectively. The name Celsius, on the other hand, gives no hint as to the unit's origin or meaning. Period and frequency are mathematical reciprocals of one another. That is to say, if a wave has a period of 10 seconds, its frequency will be 0. You may be familiar with the appearance of an ECG or EKG electrocardiograph machine, used by physicians to graph the oscillations of a patient's heart over time.
The ECG is a special-purpose oscilloscope expressly designed for medical use. Generalpurpose oscilloscopes have the ability to display voltage from virtually any voltage source, plotted as a graph with time as the independent variable.
The relationship between period and frequency is very useful to know when displaying an AC voltage or current waveform on an oscilloscope screen. By measuring the period of the wave on the horizontal axis of the oscilloscope screen and reciprocating that time value in seconds , you can determine the frequency in Hertz. Voltage and current are by no means the only physical variables subject to variation over time. Much more common to our everyday experience is sound, which is nothing more than the alternating compression and decompression pressure waves of air molecules, interpreted by our ears as a physical sensation.
Because alternating current is a wave phenomenon, it shares many of the properties of other wave phenomena, like sound. For this reason, sound especially structured music provides an excellent analogy for relating AC concepts. In musical terms, frequency is equivalent to pitch. Low-pitch notes such as those produced by a tuba or bassoon consist of air molecule vibrations that are relatively slow low frequency. High-pitch notes such as those produced by a flute or whistle consist of the same type of vibrations in the air, only vibrating at a much faster rate higher frequency.
Figure 1. C middle NoteMusical designation C C sharp or D flat Astute observers will notice that all notes on the table bearing the same letter designation are related by a frequency ratio of For example, the first frequency shown designated with the letter "A" is Hz. The next highest "A" note has a frequency of Hz -exactly twice as many sound wave cycles per second. The same ratio holds true for the first A sharp This similarity in sound is musically recognized, the shortest span on a musical scale separating such note pairs being called an octave.
Following this rule, the next highest "A" note one octave above Hz will be Hz, the next lowest "A" one octave below Hz will be Hz. A view of a piano keyboard helps to put this scale into perspective: Figure 1. The familiar musical mnemonic doe-ray-mee-fah-so-lah-tee -yes, the same pattern immortalized in the whimsical Rodgers and Hammerstein song sung in The Sound of Musiccovers one octave from C to C.
While electromechanical alternators and many other physical phenomena naturally produce sine waves, this is not the only kind of alternating wave in existence. Other "waveforms" of AC are commonly produced within electronic circuitry. Here are but a few sample waveforms and their common designations in figure 1.
They're simply a few that are common enough to have been given distinct names. Some waveforms are so complex that they defy classification as a particular "type" including waveforms associated with many kinds of musical instruments. Generally speaking, any waveshape bearing close resemblance to a perfect sine wave is termed sinusoidal, anything different being labeled as non-sinusoidal.
Ammeters are always connected in series with the circuit to be tested. It is obtained by multiplying the current in amperes by the time in hours during which current flows. For example, a battery which provides 5 amperes for 20 hours is said to deliver ampere - hours. One ampere is equal to a current flow of one coulomb per second. Apparent power is the product of the rms voltage and the rms current.
Made available to the public under the provisions of the Bureau of Indian Standards Act of and the Right to Information Act of In order to promote public education and public safety, equal justice for all, a better informed citizenry, the rule of law, world trade and world peace, this legal document is hereby made available on a noncommercial basis, as it is the right of all humans to know and speak the laws that govern them. IS pdf IS txt. Alternating current disconnectors isolators and earthing switches for voltages above V, Part 2: Rating. Gas insulated metal enclosed switchgear for rated voltages High-voltage switchgear and controlgear, Part AC insulation-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
Instantaneous power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the direction of energy flow. The portion of power that, averaged over a complete cycle of the AC waveform , results in net transfer of energy in one direction is known as active power more commonly called real power to avoid ambiguity especially in discussions of loads with non-sinusoidal currents. The portion of power due to stored energy, which returns to the source in each cycle, is known as instantaneous reactive power , and its amplitude is the absolute value of reactive power. In a simple alternating current AC circuit consisting of a source and a linear load, both the current and voltage are sinusoidal.
Electrical Engineering pp Cite as. This chapter focuses on the most basic measurement methods for electrical quantities using electrical measuring instruments. Unable to display preview. Download preview PDF. Skip to main content.
Отправляйся домой, уже поздно. Она окинула его высокомерным взглядом и швырнула отчет на стол. - Я верю этим данным.
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