عدد المساهمات : 15
النقاط : 29
تاريخ التسجيل : 08/12/2009
الاقامة : الدامر
|موضوع: Ealctrical A C Basic الأربعاء يناير 27, 2010 8:34 pm|| |
Voltage Wave Forms
We now know that there are two types of current and voltage, that is, direct current and voltage and alternating current and voltage. If a graph is constructed showing the amplitude of a dc voltage across the terminals of a battery with respect to time, it will appear in figure (1) view A. The dc voltage is shown to have a constant amplitude. Some voltages go through periodic changes in amplitude like those shown in figure (1) view B. The pattern which results when these changes in amplitude with respect to time are plotted on graph paper is known as a WAVEFORM. Figure (1) view B shows some of the common electrical waveforms. Of those illustrated, the sine wave will be dealt with most often.
Figure (1). - Voltage waveforms:
(A). Direct voltage; (B) Alternating voltage.
The sine wave illustrated in figure (1) view B is a plot of a current which changes amplitude and direction. Although there are several ways of producing this current, the method based on the principles of electromagnetic induction is by far the easiest and most common method in use.
The fundamental theories concerning simple magnets and magnetism were discussed in Module 1, but how magnetism can be used to produce electricity was only briefly mentioned. This module will give you a more in-depth study of magnetism. The main points that will be explained are how magnetism is affected by an electric current and, conversely, how electricity is affected by magnetism. This general subject area is most often referred to as Electromagnetism. To properly understand electricity you must first become familiar with the relationships between magnetism and electricity. For example, you must know that:
<LI class=MsoNormal style="MARGIN-LEFT: 36pt; DIRECTION: ltr; MARGIN-RIGHT: 0mm; unicode-bidi: embed; TEXT-ALIGN: left">An electric current always produces some form of magnetism.
<LI class=MsoNormal style="MARGIN-LEFT: 36pt; DIRECTION: ltr; MARGIN-RIGHT: 0mm; unicode-bidi: embed; TEXT-ALIGN: left">The most commonly used means for producing or using electricity involves magnetism.
- The peculiar behavior of electricity under certain conditions is caused by magnetic influences.
In 1819 Hans Christian Oersted, a Danish physicist, found that a definite relationship exists between magnetism and electricity. He discovered that an electric current is always accompanied by certain magnetic effects and that these effects obey definite laws.
MAGNETIC FIELD AROUND A CURRENT-CARRYING CONDUCTOR
If a compass is placed in the vicinity of a current-carrying conductor, the compass needle will align itself at right angles to the conductor, thus indicating the presence of a magnetic force. You can demonstrate the presence of this force by using the arrangement illustrated in figure (2). In both (A) and (B) of the figure, current flows in a vertical conductor through a horizontal piece of cardboard. You can determine the direction of the magnetic force produced by the current by placing a compass at various points on the cardboard and noting the compass needle deflection. The direction of the magnetic force is assumed to be the direction in which the north pole of the compass points.
Figure (2). - Magnetic field around a current-carrying conductor.
In figure (2-A), the needle deflections show that a magnetic field exists in circular form around the conductor. When the current flows upward (see figure (2-A)), the direction of the field is clockwise, as viewed from the top. However, if you reverse the polarity of the battery so that the current flows downward (see figure (2-B)), the direction of the field is counterclockwise.
The relation between the direction of the magnetic lines of force around a conductor and the direction of electron current flow in the conductor may be determined by means of the LEFT-HAND RULE FOR A CONDUCTOR:
if you grasp the conductor in your left hand with the thumb extended in the direction of the electron flow (current) (- to +), your fingers will point in the direction of the magnetic lines of force. Now apply this rule to figure (2). Note that your fingers point in the direction that the north pole of the compass points when it is placed in the magnetic field surrounding the wire.
An arrow is generally used in electrical diagrams to denote the direction of current in a length of wire (see figure (3-A)).Where a cross section of a wire is shown, an end view of the arrow is used. A cross-sectional view of a conductor that is carrying current toward the observer is illustrated in figure (3-B). Notice that the direction of current is indicated by a dot, representing the head of the arrow. A conductor that is carrying current away from the observer is illustrated in figure (3-C). Note that the direction of current is indicated by a cross, representing the tail of the arrow. Also note that the magnetic field around a current-carrying conductor is perpendicular to the conductor, and that the magnetic lines of force are equal along all parts of the conductor.
Figure (3). - Magnetic field around a current-carrying conductor, detailed view.
When two adjacent parallel conductors are carrying current in the same direction, the magnetic lines of force combine and increase the strength of the field around the conductors, as shown in figure (4)(A).Two parallel conductors carrying currents in opposite directions are shown in figure (4-B). Note that the field around one conductor is opposite in direction to the field around the other conductor. The resulting lines of force oppose each other in the space between the wires, thus deforming the field around each conductor. This means that if two parallel and adjacent conductors are carrying currents in the same direction, the fields about the two conductors aid each other. Conversely, if the two conductors are carrying currents in opposite directions, the fields about the conductors repel each other.
Figure (4). - Magnetic field around two parallel conductors.
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Magnetic Field Of A Coil
Figure (3-A) illustrates that the magnetic field around a current-carrying wire exists at all points along the wire.
Figure (5) illustrates that when a straight wire is wound around a core, it forms a coil and that the magnetic field about the core assumes a different shape. Figure (5-A) is actually a partial cutaway view showing the construction of a simple coil. Figure (5-B) shows a cross-sectional view of the same coil. Notice that the two ends of the coil are identified as X and Y.
Figure (5). - Magnetic field produced by a current-carrying coil.
When current is passed through the coil, the magnetic field about each turn of wire links with the fields of the adjacent turns. (See figure (4-A)). The combined influence of all the turns produces a two-pole field similar to that of a simple bar magnet. One end of the coil is a north pole and the other end is a south pole.
Polarity of an Electromagnetic Coil.
Figure (2) shows that the direction of the magnetic field around a straight wire depends on the direction of current in that wire. Thus, a reversal of current in a wire causes a reversal in the direction of the magnetic field that is produced. It follows that a reversal of the current in a coil also causes a reversal of the two-pole magnetic field about the coil.
When the direction of the current in a coil is known, you can determine the magnetic polarity of the coil by using the LEFT-HAND RULE FOR COILS. This rule, illustrated in figure 1-6, is stated as follows:
Figure (6). - Left-hand rule for coils.
Grasp the coil in your left hand, with your fingers "wrapped around" in the direction of the electron current flow. Your thumb will then point toward the north pole of the coil.
Strength of an Electromagnetic Field
The strength or intensity of a coil's magnetic field depends on a number of factors. The main ones are listed below and will be discussed again later.
<LI class=MsoNormal style="MARGIN-LEFT: 36pt; DIRECTION: ltr; MARGIN-RIGHT: 0mm; unicode-bidi: embed; TEXT-ALIGN: left">The number of turns of wire in the coil.
<LI class=MsoNormal style="MARGIN-LEFT: 36pt; DIRECTION: ltr; MARGIN-RIGHT: 0mm; unicode-bidi: embed; TEXT-ALIGN: left">The amount of current flowing in the coil.
<LI class=MsoNormal style="MARGIN-LEFT: 36pt; DIRECTION: ltr; MARGIN-RIGHT: 0mm; unicode-bidi: embed; TEXT-ALIGN: left">The ratio of the coil length to the coil width.
- The type of material in the core.
Losses in an Electromagnetic Field
When current flows in a conductor, the atoms in the conductor all line up in a definite direction, producing a magnetic field. When the direction of the current changes, the direction of the atoms' alignment also changes, causing the magnetic field to change direction. To reverse all the atoms requires that power be expended, and this power is lost. This loss of power (in the form of heat) is called HYSTERESIS LOSS. Hysteresis loss is common to all ac equipment; however, it causes few problems except in motors, generators, and transformers. When these devices are discussed later in this module, hysteresis loss will be covered in more detail.
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