Created it, 05/10/15
Update it, 05/11/24
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AC CURRENT “1st PART”
As specified at the end of the preceding lesson entitled “electromagnetic Induction”, we now will analyze a new type of current completely different from that considered up to now. However, you know certainly all this current, at least by the name, since it is about the AC current.
We always examined circuits traversed by the current provided by one or more piles, running called D.C. current because it always has the same direction of circulation. The current circulating in the electric circuit of the figure 1-a is a D.C. current.
The current traversing this circuit is always directed, according to the conventional direction, of the positive pole to the negative pole of the pile : it enters resistance by the end located using letter A and leaves by that located using letter B. In such a circuit, the tension generating the D.C. current is called continuous tension. There exists on the other hand of other types of generators, which deliver a current, which, by its characteristics AC current is called.
To include/understand the difference between these two types of current, it is necessary first of all to refer to the figure 1-b. This one represents the same electric circuit as the figure 1-a, with the difference close which it is fed by a generator of AC current of which we can note with the passage the graphic symbol.
In this figure 1-b, polarities of the generator of AC current, indicated by the signs “+” and “-” are identical to the polarities appearing in the circuit of the figure 1-a. Consequently, in these two cases of figure, the current circulates in the same direction crossing the resistance of A towards B.
However, in the case of an alternate generator, the current circulates in a direction only during a very short time, with the end of which it is reversed. We are then in the presence of the figure 1-c where the polarities of the generator are reversed and where the current crosses the resistance of B towards A. Même in this new direction of circulation, the current persists only during a very short time for then returning in the case of the figure 1-b and so on.
We can say that the current changes its direction of circulation periodically, in other words whom it traverses resistance alternatively A towards B and of B towards A lasting of the very short periods of time. of this explanation, we include/understand the origin of the name of the AC current.
The intensity of a AC current varies constantly, in the case of the figure 1-b it increases by zero up to a maximum value determined by the generator and resistance, then decreases to total zero. At the moment when the intensity is null, the generator reverses its polarities, we are in the case of the figure 1-c, the intensity increases again until the same maximum that previously and goes down again then to zero. At this moment, it reproduces a change of polarities and the cycle starts again.
Since resistance is fixed, the variations of intensity of current I can be due only to similar variations of the power provided by the generator. This tension has the same characteristics as the current that it provides and is called alternating voltage.
There are thus well two fundamental types of electrical current which are :
The D.C. current symbolized by initials D.C. and the AC current symbolized by the initials A.C.
It is wise to remember that the AC current is much more widespread than the D.C. current, since he is used in industry and the dwellings. The AC current is produced by means of generator called alternators and installed in the power stations.
1. 1. - PRODUCTION OF THE AC CURRENT
To include/understand how the AC current can periodically change its direction of circulation and vary its intensity, we should consider the principle of operation of a generator of AC current.
The operation of such a generator is founded on the phenomenon of electromagnetic induction analyzed in the preceding theoretical lesson. Indeed, this generator comprises a primary circuit fed in D.C. current to produce the flow of induction necessary, and an induced circuit in which is precisely induced the desired AC current.
Figure 2 are represented in a very simplified way these two circuits.
The primary circuit is made of two rollings up connected in series and supplied with a pile. Between these two rollings up is laid out the induced circuit represented figure 2 by a simple whorl. The ends of this whorl constitute the poles of the generator and are connected to a resistance which represents the circuit external with the generator. With this provision, the whorl of the induced circuit is crossed by the lines of induction of the flow produced by the primary circuit.
The variation of inductive flow necessary to the creation of a current induced in the whorl is obtained in our case by a rotation of the complete primary circuit around the whorl. In figure 2, the arrows represent the direction of rotation while item 0 materializes the center of the movement. It should be noted that rotation takes place at constant speed.
Figure 3 are shown eight positions various catches by the flow of induction during a full rotation of the primary circuit (circuit which is not represented any more with an aim of not overloading the figures).
The primary circuit is supposed to spend eight seconds to achieve a full rotation and thus spends a second to pass from one position to the following one.
While following the examples of figure 3, we see immediately why the generator inverts at a certain time its polarities and consequently reverses the direction of circulation of its current.
Let us leave the figure 3-a in which the whorl is completely crossed by inductive flow and consider what occurs during the second during which flow moves to reach the position of the figure 3-b. During this second, flow moved of an angle of 45° is a eighth of turn in the direction indicated by the arrow. The consequence of this rotation is that, as shown in the figure 3-b, the whorl is not crossed any more by the totality of the flow of induction since some of the lines of this flow are external with the whorl.
Because of the reduction in embraced flow, it is induced in the whorl a current I whose direction of circulation is such, that it produces in its turn a flow of induction directed in the same direction as that of inductive flow and this according to the Lenz's law. We thus know the direction of the lines of induction of the flow induced in the whorl and by observing the rule of the corkscrew, we deduce the direction from flow of current I in the whorl. In the circuit external with the generator, in other words in resistance, the current circulates of A towards B. Since according to its conventional direction, the current is directed positive pole of the generator to its negative pole, we can indicate the polarities appearing at the boundaries of the whorl.
The end of the whorl connected to point A is of positive polarity while that connected to the point B is of negative polarity.
Continuing its rotation, the flow embraced by the whorl decreases and is cancelled completely at the end of 2 seconds when it reaches the position of the figure 3-c. During this second second, inductive flow turned of a new eighth of turn what makes in a whole quarter of turn compared to the figure 3-a is an angle of 90°. At the moment when flow crossing the whorl is cancelled, the current induces I still circulates in the same direction as previously and this always to produce a flow directed of the left towards the line with an aim of thwarting the cancellation of the flow embraced by the whorl. From the position of the figure 3-c, the flow embraced by the whorl starts again to increase taking into account rotation. When this flow still achieved a eighth of turn during the third second, it describes since its starting position an angle of 135°. A certain number of the lines of induction of this flow crosses the whorl again. Because of the rotation of flow, its lines of induction are now directed line towards the left. Inductive flow increases : the induced current generated in the whorl for thwarted this increase must produce a flow of opposed direction, therefore directed left towards the line.
This orientation is the same one as in the case of the figures 3-b and 3-c, consequently, the current induces I always circulates in the same direction.
The induced current continuous to circulate in this direction until 1 second later, inductive flow reaches the position of the figure 3rd having achieved a half-turn (flow is completely embraced by the whorl). On this figure, it is represented any more of current induced for the reason only we will see later. One second after, flow turned still of a eighth of turn and is in the position of the figure 3-f (swing angle of 225°). Flow crossing the whorl again decreased and the current induces I, for thwarted this reduction, must create a flow of induction directed in the same direction which inductive flow, is line towards the left, and this, always according to the Lenz's law.
(To facilitate the comprehension of figure 3 of it above, we defer it the same figure).
Thus knowing the direction of the lines of induction of the flow induced in the whorl and by observing the rule of the corkscrew, we deduce the direction from flow of current I in the whorl.
In the circuit external with the generator, in other words in resistance, the current circulates of B towards A. Since according to its conventional direction, the current is directed positive pole of the generator to its negative pole, we can indicate figure 3-f the new polarities appearing at the boundaries of the whorl.
The end of the whorl connected to point A is of negative polarity while that connected to the point B is of positive polarity.
We thus see that in correspondence of the inversion of the direction of flow of the current, the polarities of the generator are also reversed.
The direction of the induced current is reversed as soon as flow with exceeded the position of the figure 3rd and continuous to circulate in this new direction until flow, after having passed the positions of the figures 3-g (angle of 170°) and 3-h (angle of 325°) returns to its initial position which is that of the figure 3-a having thus achieved a full rotation (360°).
When flow reaches this position, the direction of the induced current is reversed again and the cycle starts again if, of course, the rotary movement applied to the primary circuit is maintained. In conclusion, we can say that the current circulates in a direction during a half-turn of flow of induction and in contrary direction during the following half-turn, this inversion of direction is carried out when the lines of induction of inductive flow are horizontal, case of the figures 3-a and 3-e.
Since any current results from a displacement of electrons, its inversion thus materializes by an inversion of the direction of displacement of the electrons which constitute it. So that this occurs, the electrons must initially stop their movement in a direction before setting out again in the other. There is thus a moment during which the electrons are motionless.
This immobility of the electrons results in the absence of current I in the figures 3-a and 3-e. In these two figures, the intensity of current I beam resistance is null.
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