|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|5. Magnetic Field|
|5.3. Electromagnetic Induction|
Phenomena of induction are utilised very frequently in engineering. The examples given below are a very limited selection of typical applications.
Fig. 5.20. Right-hand rule (generator rule)
· Magnetic head of magnetophone (induction of rest)
The magnetophone process is a high-grade sound storage method. The sound store is a thin (18 to 50 µm) polyester or acetate tape with a ferromagnetic film applied to it. The sound information is entered in the tape in the form of a more or less intense magnetisation in the running direction of the tape and thus stored. For the reproduction of the stored information, a magnetic replay head is required. This is a magnetic circuit with a very small air gap (1 to 100 am) which carries an induction coil. The magnetised tape is transported past the air gap. The flux caused by the individual permanent magnets of the tape in the ferromagnetic circuit induces a voltage proportional to the sound information in the induction coil surrounding the circuit (Fig. 5.21.). This voltage is amplified and fed to a loudspeaker.
Fig. 5.21. Reproducing head of a magnetic recorder
1 - to amplifier
2 - Iron core
3 - Magnetic tape
· Transformer (induction of rest)
The transformer is a stationary electrical machine and one of the most important components of electrical engineering. In accordance with Fig. 5.18., the transformer is provided with two coils which are galvanically separated but magnetically coupled. When a voltage is applied, to the primary coil which periodically changes as to magnitude and direction (such a voltage is called alternating voltage, see Section 7), a magnetic field is produced in both coils which also changes periodically its magnitude and direction. In an ideal case, a 100 per cent coupling is effected, i.e. the coefficient of coupling k = 1. Then the magnetic flux F1 completely penetrates the secondary coil as F2, hence, F1 = F2 = F
Fig. 5.22. Transformer
According to the law of induction, the induced voltage is directly proportional to the number of turns when the rate of flux variation is given.
The ratio of the primary voltage to the secondary voltage is called ratio of transformation trr and is written as
trr = U1/U2 = N1/N2
In a loss-free transformer, the ratio of the voltages is equal to the ratio of turns in the coils.
According to the law of conservation of energy, the primary power must be equal to the secondary power, hence, P1 = P2.
According to Section 4.1., equation (4.3), power is written as P = UI, that is to say,
U1I1 = U2I2
U1/U2 = I2/I1
Taking equation (5.19.) into account, we have
trr = U1/U2 = I2/I1 = N1/N2
In a loss-free transformer, the currents are in inverse ratio to the numbers of turns of the coils.
This shows that a given alternating voltage can be transformed into any desired, higher or lower alternating voltage by means of a transformer. Therefore, the transformer is an important connecting link between energy generator and the distribution network or between the distribution network and the consumers. In information electrical engineering, the transformer is frequently used for impedance matching. Since P1 = P2, we have inaccordance with Section 4.1.,
I12R1 = I22R2
R1 = (I2/I1)2 · R2 with trr = I2/I1 we read
R1 = trr2 R2
The load resistance R2 acts on the primary with the square of the transmission ratio.
· Generator (induction of motion)
In a homogeneous magnetic field, a conductor loop or a coil is arranged. If it is turned about its central axis which is perpendicular to the field direction, then the magnetic flux penetrating the coil area varies (see Fig. 5.23.).
Fig. 5.23. Generator (principle)
1 - North pole
2 - South pole
Since, according to the law of induction, any change of the magnetic flux causes an electromotive force, a voltage is induced in the rotating coil. Its direction can be deter-minded with the help of the right-hand rule. A generator is the inversion of the motor principle described in Section 5.2. Generalising, we can say that a pivoted coil in a magnetic field is the basic design of all rotating electrical machines (motors, generators).
· Eddy-current brake
In planar conductors voltages are induced by magnetic flux variations in the same manner as in wires and coils. The induction currents associated with these induced voltages are high because the current paths in a planar conductor are closed in themselves and act as short-circuits (see Fig. 5.25.). An experiment sketched in Fig. 5.24. shows the action of such induced currents. A metal plate of copper (or of another electrically conductive material such as aluminium) is suspended in such a way that it is allowed to swing through a magnetic field kie a pendulum. In this manner, currents are induced which, according to the Lenz rule, built up magnetic fields of opposite direction and thus damp the motion. The pendulum will come to rest very quickly.
Fig. 5.24. Electromagnetic induction in planar conductors
1 - Pendulum of non-ferromagnetic conductor material
2 - Total deflection of oscillation
3 - Oscillation in the magnetic field
4 - North pole
5 - South pole
Fig. 5.25. Eddy currents in planar conductors
Fig. 5.26. Reduction of the eddy currents by means of slots in planar conductors
Fig. 5.25. shows the closed current paths in the metal surface. Because of the apparently irregular course taken by the current, these currents are called eddy currents. Eddy currents can be avoided to a great extent when fine slots are made into the metal surface as shown in Fig. 5.26. The pendulum of such a slotted metal plate will hardly be damped; the braking action and thus the eddy-current formation are cancelled to a great extent.
Eddy-current brakes operate on the above described principle. They are used now and then for the braking of rail vehicles, for damping the deflection of electrical indicator operating mechanisms, and for braking electrical machines.
In most cases, eddy currents are not desired. They occur both by induction of rest in stationary electrical machines and by induction of motion in rotating electrical machines. Because of their short-circuit character, they heat the metal mass involved, thus, uselessly doing work. These eddy-current losses must be avoided as far as possible. This is achieved by avoiding compact metal masses. This is possible by composing metallic bodies of individual sheets insulated against each other and arranged in parallel to the direction of flux. Further eddy-current losses can be avoided when using ferromagnetic materials having a small electric conductivity, for example, sheet iron alloyed with silicon or certain iron-oxide compounds.