THE SKIN EFFECT

The resistance of a conductor is not the same for alternating current as it is for direct current. The reason is this. A straight conductor carrying current has a magnetic field in the

form of concentric circles. This field is produced both around and inside the conductor.

The current through the conductor can be pictured as several parallel streams. Those closer

to the center of the conductor link a greater magnetic flux. The inductance of a current flow and the inductive reactance are directly proportional to the magnetic flux linked with it. Therefore,

the inductance of the central parts of the conductor is greater than that of the outer skin, as is

the inductive reactance, and the current flows mainly at and near the surface of the conductor where the reactance is least. The useful cross-section of the conductor is less than the actual area, and the effective resistance is consequently higher. This is called the skin effect. The skin effect is negligible at a standard frequently of 50 c/s, in conductors with small cross-sections and

in copper wires, but is considerable at high frequencies, in conductors of large cross-section and in iron or steel wires.

F. Pick out the electric terms from the text, you have read, translate them, refer to a

glossary or a dictionary if necessary.

G. Read and translate the text, make up some questions and answers and present them in

the form of a dialogue.

BASIC LAW OF ELECTROMAGNETIC INDUCTION

The basic law of electromagnetic induction states that any change in the magnetic flux piercing the surface embraced by a conducting loop induces a current in the loop. However,

by varying in exactly the same way the magnetic flux through loops made of different materials but similar in all other respects, we shall see that different currents are induced in them. Let us make, for example, two coils of the same shape and size and having the same number of turns, but one made of a copper wire and the other of a nichrome wire of the same length and cross-section, place them in the same magnetic field (into a long solenoid) and orient them identically relative to the direction of the field. Switching off the magnetic field, we shall observe induced currents in the coils but the current in the copper coil will be 70 times as strong as the current in the nichrome coil. Carrying out various experiments of this kind, we shall see that other conditions being equal, the induced current is the stronger the lower the electric resistance of

the coil. This circumstance leads us to the conclusion that under given experimental conditions

a certain EMF is induced in the coil, and the current induced as a result is determined by Ohm’s law and therefore turns out to be inversely proportional to the electric resistance of the circuit.

Indeed, a simple experiment can be made to show that Ohm’s law is valid for induced currents. Let us connect the ends of a coil in which a current is induced to a circuit whose resistance can be varied and perform appropriate measurements.

The concept of EMF was encountered when we considered the condition of emergence and maintaining of electric current in a circuit. For a galvanic cell, accumulator or thermocouple,

it could be established that an EMF emerges in a certain part of the circuit ( i.e. at the interface between a metal and an electrolyte or at the contact between two different metals) .In the case of electromagnetic induction, the EMF is not concentrated in a certain part of the circuit but acts in

the entire induction circuit, i.e. at each point of the circuit where the magnetic flux changes.

In the case of a loop embracing the magnetic field lines, an EMF is induced at all points of the loop and can be calculated for the loop as a whole. If we have several loops (turns), the same takes place in each of them: the EMF of a coil is the sum of EMF’s of individual turns.

H. What is the Russian for?

poly-phase system displacement

symmetrical value

non-interlinked armature

interlinked cross-section

phase winding rotate

wires wave

transmit prime rectifier

revolve combustion

I. Read and translate the texts, pick out the ideas, which you think are most informative

and most interesting from these texts.

Poly-phase systems (three- phase systems)

A poly-phase system is an alternating current circuit or network to which are applied

two or more EMFs of the same frequency but displaced in time phase. Each EMF may act independently of the others in the system. Such systems are called non-interlinked. Each EMF of a poly-phase system is referred to as a phase. In a non-interlinked system, the separate phases are not interconnected either electrically or magnetically, and they can be calculated from formulas for single-phase alternating current. A serious drawback of non-interlinked systems is that they use a large number of wires. In the case of a three-phase system, six wires will be required. Poly-phase systems in which the individual phases are electrically connected are called interlinked and are widely used. A poly-phase system has many advantages. The same power can be transmitted over smaller wires than in the case of a single-phase alternating-current system. It can produce a revolving magnetic field in stationary coils or windings, which is a decisive advantage in many types of electrical apparatus.

Of all poly-phase systems, the three-phase system is used most widely. The EMFs in

a three-phase system are generated as follows. If three turns are placed between the poles N and S of a uniform magnetic field so that they are spaced 120* apart and are rotating at a constant angular velocity, the EMFs induced in them will also be spaced 120* apart.

Date: 2014-12-28; view: 1082