Applications of Magnetic

Applications

 

1. Ferrite Cores

• The transformers are the devices which transform voltages and currents and thus impedances. Practically in most of the cases core is made up of ferromagnetic material like iron which is used to produce low reluctance path. In some applications, it is necessary to have a core with a complicated shape. It is found that it is very difficult to prepare a very complicated shaped core using solid material like iron. To overcome this limitation ferrite materials are used in place of iron.

• Basically ferrite is a mixture of powered ferromagnetic materials. Then the mixture is moulded into the desired shape and then it is intered. In general, ferrite can be obtained from any ferromagnetic material. The ferromagnetic materials are the most useful magnetic materials which have relative permeability greater than 1 and tend to thousands or even higher. Some of the important ferromagnetic materials and their relative permeabilities are as given below.

• Most of the ferrites are obtained from iron oxide Fe₃O₄ i.e. magnetite. Iron oxide like Fe₃O₄ is mixed with bivalent compound like Barium Oxide (BaO), Nickel Oxide (NiO), Manganese Oxide (MnO) etc. to obtain ferrite material. Depending upon materials used and processes used, the ferrites obtained by mixing iron oxide with bivalent compounds can be made to have any shape and size and these ferrites can have properties of solid material. The main advantage of sintering process used to bind these compounds is that, the ferrite become hard and brittle. They can be further worked with grinding process only.

• The important properties of ferrites are as follows.

i) The conductivity of ferrite is very low. It is generally below 10_ 5 S/m. Some of the ferrites show conductivity of the order of 1 S/m.

ii) Ferrite cores are used in the applications where frequency of operation is higher.

iii) The coercive field intensity of ferrite is relatively low and typically it is below 100 A/m.

iv) The ferrites have relative permeability between 10 and 10000.

v) The ferrites show square magnetization curve and the remnant flux density low and it is below 0.5 T.

 

2. Magnetic Recording

• The most useful application of the magnetic materials is the recording of signals. Inspite of variations in composition of different magnetic recording media, the basic principle of recording remain same in all the methods. In the process of magnetic recording, the magnetic particles on the substrate are oriented differently with the aid of external magnetic field. There are two methods to produce magnetic medium for recording such as tape and disk.

i) In first method, the recording medium is coated with a base material consisting ferromagnetic particles in binding material. In this method, each particle is suspended independently. The domains of these particles can be oriented with the help of external magnetic fields. This method is generally used in magnetic tapes.

ii) In second method, a ferromagnetic alloy is deposited on the non-ferromagnetic base  material in the form of thin layer. This method is mostly used for disks. Various alloys of iron and cobalt are used as materials for disks.

• In both the methods, the particles are diluted in a binder to the extent of 25 to 50 % of volume and then coated on a thin substrate film of polyethelene or aluminium disks. The binder is very important as it contains lubricating substance which prevent magnetic tape or recording head from damage. There is no limitation on the number of particles of ferromagnetic materials. Typical ferromagnetic materials used are cobalt, nickel, iron and iron oxides. But most of the tapes are made up of Fe₂O₃.

a. Principle of Tape Recorders

• The principle of the magnetic tape recording is as follows. When a magnetic tape is passed through a recording head, the signal to be recorded appears as some magnetic pattern on the tape. This magnetic pattern is in accordance with the variations of original recording current. The recorded signal can be reproduced back by passing the same tape through a reproducing head where the voltage is induced corresponding to the magnetic pattern on the tape.

• When the tape is passed through the reproducing head, the head detects the changes in the magnetic pattern i.e. magnetization. The change in magnetization of particles produces change in the reluctance of the magnetic circuit of the reproducing head, inducing a voltage in its winding. The induced voltage depends on the direction of magnetisation and its magnitude on the tape. The emf, thus induced is proportional to the rate of change of magnitude of magnetisation i.e. e ɑ  N (dϕ / dt)

where N = Number of turns of the winding on reproducing head

ϕ = Magnetic flux produced.

• Suppose the signal to be recorded is V_m sin ωt.

Thus, the current in the recording head and flux induced will be proportional to this voltage. It is given by,

ϕ = k₁ • V_m sin ωt, where

k₁ = constant

• Above pattern of flux is recorded on the tape. Now , when this tape is passed through the reproducing head, above pattern is regenerated by inducing voltage in the reproducing head winding. It is given by,

• Thus, the reproduced signal is equal to derivative of input signal and it is proportional to flux recorded and frequency of recorded signal.

b. Methods of Recording

• The methods used for magnetic tape recording used for instrumentation purposes are as follows :

i) Direct Recording

ii) Frequency Modulation Recording

iii) Pulse Duration Modulation Recording

• For instrumentation purposes mostly frequency modu lation recording is used. The pulse duration modulation recording is generally used in the systems for special applications where large number of slowly changing variables have to be recorded simultaneously.

i) Direct Recording :

• This method of recording is the simplest one. This method usually requires one tape track for each channel. The input signal to be recorded is amplified and mixed with high frequency bias. This signal is then fed to recording head as recording current.

• The magnetic pattern recorded on the magnetic tape is directly proportional to the magnetic flux density produced at the air gap. The input current i.e. recording current is sinusoidal. But the magnetisation pattern developed during recording is non-sinusoidal. Thus the current in the winding and flux density in the air gap are having non-linear relationship between them. The relationship between winding current and flux density in air gap is as shown in Fig. 8.13.1.

• The distortion can be avoided by applying a high frequency bias of constant frequency with the signal input. The amplitude and frequency of this high frequency bias is greater than maximum amplitude and highest frequency of the signal to be recorded. The bias frequency is generally 4 times greater than the highest frequency while the amplitude is about 5 to 30 times greater than the input signal current. The exact value of bias amplitude depends on tape and head characteristics.

• The mixing of bias and the input signal is accomplished by using a linear mixing process. The peak value of the combined signal is limited in such a way that it lies on the linear portion of B-H curve. This signal is passed through recording signal as shown in Fig. 8.13.2.

 • The reproducing head exactly reproduces same waveform. This reproduced waveform is then passed through filter block which removes unwanted high frequency components and gives original waveform. This signal may be amplified to get suitable higher magnitude.

• The output voltage of the reproducing head is directly proportional to the frequency of input signal. Hence direct recording procedure can not be used to record d.c. signal (having zero frequency) as voltage developed in reproducing head is zero. As frequency of input signal decreases, the output voltage in reproducing head decreases. Thus, there is a limitation on lower operating frequency. Below certain limit of frequency, voltage in reproducing head will be equal to noise in the system. If the signal with frequency less than this frequency is used, signal will be completely overcome by the noise.

• Similar to lower frequency, there is a limitation on higher frequency. The higher frequency is limited by gap of length of reproducing head. When the tape passes the air gap of the reproducing head, the wavelength of the tape is given by,

λ = (V / f) m

where V = Speed of tape in m/sec

f = Frequency of recorded signal in Hz.

•  From above equation it is clear that when speed of tape increases, the wavelength A also increases at any given frequency. If the air gap in the reproducing head is significant in relation with wavelength A then the small changes in the signal can not be reproduced. Thus the air gap must be small compared to the wavelength of the highest frequency to be reproduced.

• The output voltage in the reproducing head increases with frequency. This continues till the length of the gap equals half value of the recorded wavelength. After this, output voltage decreases rapidly and then becomes zero. This is called as gap effect which restricts high frequency response of the tape recorder.

Advantages of Direct Recording :

i) This has a wide frequency response from 50 Hz to 2 MHz for tape speed of 3.05 m/s. It has a very high bandwidth.

ii) It requires simple and cheap electronic circuits only. 

iii) It has a good dynamic ratio without increase in distortion.

iv) It is used to record signals such as spectrum analysis of noise where the information is in relation with frequency and amplitude.

v) It can be used to record voice signals.

Disadvantages of Direct Recording :

i) Due to certain random surface inhomogeneities in the tape, there may be amplitude instability in the recorded signal.

ii) Due to poor manufacturing and dirt on the tape, some portion may not be perfectly recorded.

iii) It can be used only when maximum bandwidth is needed.

iv) It can not record dc signals.

 

ii) Frequency Modulation (FM) Recording :

The major disadvantage of direct recording is that it is difficult to record dc signals. This difficulty is overcome by using frequency modulation recording in which accurate dc response is obtained.

Principle of Operation :

• In the FM recording, the carrier frequency 1^- is modulated by the input signal. FM recording uses the variation of frequency to carry the required information instead of varying the amplitude. The modulated signal is then recorded using the recording head in normal way. The reproducing head reproduces the signal in normal way. This reproduced signal is passed through FM demodulator, low pass filter to get original signal.

Operation :

• The basic FM recording system is as shown in Fig. 8.13.3.

• In this system, the carrier frequency is called as center frequency f_C. This frequency is modulated by the level of the input signal. When the input signal is zero, the modulation contains only the center frequency oscillation. The positive input voltage deviates the carrier frequency by specified percentage in one direction. The negative voltage deviates the carrier frequency    by specified percentage in other direction. For dc inputs the modulated output is a signal of constant frequency and for ac inputs the modulated output is a signal of variable frequency. The frequency variation is directly proportional to the amplitude of input signal. During the playback, the output of the reproducing head is passed through FM demodulator. The demodulated signal is passed through the filter which removes the carrier frequency f_C and the unwanted signals. The FM demodulator converts the difference between center frequency and frequency on the tape, to a voltage proportional to frequency difference. Thus the FM recording enables to record signals from dc to several thousand Hz.

• The central frequency is selected with respect to the tape speed. The frequency deviation selected is ± 40 % about carrier frequency. When the tape speed is changed, there is proportional change in the carrier frequency. So for the dc signal the wave length λ  remains same as λ = V / f and as speed V changes, the frequency also changes in proportion.

• There are two factors related to FM recording

i) Percentage deviation  ii) Deviation ratio

i) Percentage Deviation

• It is defined as ratio of carrier deviation to center frequency. It is denoted by M.

Percentage Deviation = M = Δf / f_C × 100

• It is also called as Modulation Index.

ii) Deviation Ratio :

• It is the ratio of carrier deviation from center frequency to the signal frequency or modulating frequency. It is denoted by δ.

δ = Δf / f_m where f_m = Modulating frequency.

Advantages of FM Recording

i) FM recording is useful mainly to record dc components.

ii) FM recording has wide frequency range from Hz to several kHz.

iii) In FM recording, drop out effects due to inhomogeneities are not possible.

iv) Amplitude variation is neglected in FM recording and input signal is correctly recorded.

v) FM recording is extensively used for recording non electrical quantities such as force, pressure etc.

vi) FM recording is extensively used for multiplexing in the instrumentation and process system.

Disadvantages of FM Recording

i) The tape speed fluctuations affect the FM recording.

ii) The circuitry used for FM recording is complicated as compared to that of direct recording.

iii) FM systems have limited frequency response.

iv) For FM recording high tape speed are required.

v) Better recording requires high quality tape transport and speed control mechanisms.

vi) It is comparatively expensive.

 

iii) Pulse Duration Modulation Recording (PDM)

• The pulse duration modulation is also called as pulse width modulation. The principle of operation is that the amplitude and starting time of each pulse of a signal is kept constant while width of pulse is made proportional to amplitude of signal at that instant.

• In this recording system, the input signal is converted to a pulse at the sampling instant. The width of each pulse is dependent on the a mplitude of the signal at that instant. The sampled signal is recorded at various instants instead of recording instantaneous values continuously. On playback original signal can be obtained by passing re corded signal to appropriate filter.

Advantages of Pulse Duration        Modulation Recording

i) PDM recording is mainly useful when large number of information from various channels is to be recorded simultaneously.

ii) PDM recording has high accuracy.

iii) PDM recording has high signal to noise ratio.

Disadvantages of Pulse Duration Modulation Recording

i) It has limited frequency response.

ii) Because of complex circuitry, reliability of recording is low.

iii) Only useful to record several slowly varying signals simultaneously. 

 

3. Magnetic Shielding

• In many applications, it is necessary to protect instruments from the effects of external magnetic fields. Often it is required to measure very low magnetic fields which are less than geomagnetic fields. In some applications, we may require to calibrate magnetic devices. In such applications, we cannot get correct results if there are external magnetic fields around the measuring instruments. In many applications like Magnetic Resonance Imaging (MRI), the electromagnets produce very large magnetic field of the order of 1 to 3 T. So due to such high magnetic fields, the measuring instruments, testing equipments, computers and even a human operator exposed to the fields, get affected. So it is very much needed to protect all these elements against high magnetic fields. This is done by magnetic shielding.

• In practice there is no known insulator for the magnetic flux. Suppose we place a non-magnetic material in the magnetic field, then it is observed that there is no appreciable change in the magnetic flux. For example, if we use glass plate in between magnetic flux, then there is no considerable effect on the magnetic field although the glass is good insulator of electricity. Then to protect the equipments against the external magnetic fields, a low reluctance path is provided using a piece of soft iron having high permeability such that the magnetic field is directed away from the area that is to be shielded. This is analogous to a short branch in an electric circuit so that the large current flows through the short branch protecting other branch with circuit elements connected in parallel with it.

• Suppose we want to contain the field inside the structure and also shield the interior of the structure from external mangetic fields. Consider a coil inside a shielded structure placed at the center as shown in the Fig. 8.13.4.

The current through the coil at center produces a magnetic field. To avoid the effect of this field outside the structure, a box of iron is placed around coil. The iron has high permeability i. e. low reluctivity and hence most of the field passes through the box of iron. Thus the field outside the iron box is zero. Also the same structure is used to shield the interior of the box from the external magnetic fields.

• In an application of MRI set-up, being huge set-up, the magnetic shield is placed within the walls of the room where the equipment is placed. Along with the walls, floor as well as ceiling are also provided with iron of say 50 mm thick (serving as magnetic shield) for a room of 4 × 4 × 2.5 m. The total volume of iron is typically 3.5 m³. Using such magnetic shielding, the magnetic field is contained within the room. It also protects the area inside the room from the magnetic fields produced outside the room.

Review Questions

1. Explain principle of magnetic recording.

2. Write a note on ferrite cores.

3. Explain magnetic shielding in brief.