Applications

Applications

AU : Dec.-17

1. Transformers

• A transformer is an a.c. device which is used to transform voltages and currents and hence the impedance. It is based on the principle of Faraday's law. The transformer works on the principle of mutual induction. It transfers electrical energy from one circuit to other eventhough there is no connection between two circuits.

a. Principle of Working

• According to the principle of mutual induction, when two coils are inductively coupled and if current in one coil is changed uniformly, then e.m.f. gets induced in other coil. Such induced e.m.f. in coil 2 can drive current when a closed path is provided to it. The transformer works on the same principle. In its elementary form it consists of two inductive coils which are electrically separated but linked through a common magnetic path which is made up of ferromagnetic material such as iron which gives low reluctance high permeability magnetic path. The two coils usually have high mutual inductance. The basic transformer is as shown in the Fig. 9.10.1 (a) and its symbolic representation is as shown in the Fig. 9.10.1 (b).

• In general, the core of the transformer is laminated which helps in reducing induced currents in core and thus it reduces losses due to core current.

Ideal Transformer

A transformer is said to be ideal if

i) It has no losses.

ii) Its winding have zero resistance (ideal coils).

iii) The flux produced by primary coil completely links with secondary coil.

iv) The permeability of core is high such that only negligible current is required to establish flux in it. Ideally the permeability is ∞.

v) The magnetic path is closed.

• The flux of magnetic circuit is given by,

ϕ = ∑ N_iI_i / ∑ Ɽ_j

N_i = Number of turns of ith coil

I_i = Current in ith coil

Ɽ_j = Magnetic reluctance of jth segment of the path

• But the reluctance of the magnetic path is given by,

Ɽ = lm / µS

• where Zm is the length of magnetic path and S is the cross-sectional area of path. Assuming zero core losses or coil losses, the magnetic flux can be written as,

ϕ = N₁I₁ – N₂I₂ / Ɽ

• Negative sign of term N₂I₂ indicates that I₂ is the induced current and it opposes the flux produced I2. Assuming p to be as the core is made up of high permeability material like iron, then Ɽ → 0.

• Thus we can write,

N₁I₁ – N₂I₂ = ϕ Ɽ ≈ 0

• This equation is an approximation which holds good for many application and gives solution almost same as ideal solution.

• Thus we can write,

 N₁I₁ = N₂I₂

• That means the total flux in the core is same in both coils the e.m.fs. can be expressed as,

V₁ = - N₁ (d ϕ / dt)

and V₂ = - N₂ (d ϕ / dt)

• From above conditions obtained for voltages and currents, we can write,

V₁ / V₂ = I₂ / I₁ = N₁ / N₂ = a

• Where a is known as turns ratio or transformer ratio.

• By definition, the transformer transforms voltage or current from one circuit to other circuit. But the transformer can be used to transform impedance of the circuit. The impedance of primary winding is given by,

But V₂ / I₂  is nothing but the impedance of secondary

winding, i.e. Z₂

• Thus transformer can be used for impedance matching by properly adjusting number of turns.

2. Magnetic Brake

• The magnetic brake is very useful application of an electromagnetic induction.

• Consider an electromagnet which produces magnetic field   in the gap which is constant throughout. A piece of aluminium or any conducting material which is flat is placed on electromagnet with support such that the pendulum like flat piece move easily into the gap. When the current carried by the coil of electromagnet is zero (i.e. I = 0), the oscillations of flat piece are not affected. But when the current flows through the coil of an electromagnet, the magnetic field is  developed in the gap. As a flat piece of conducting material oscillates i.e. moves in a constant magnetic field  , the current is generated into flat plate. This current opposes the cause producing it i.e.   according to Lenz's law. The induced current continues to oppose the flux density   maintaining conditions as shown in the Fig. 9.10.3.

• The magnitude of the induced current depends on the velocity with which the plate penetrates into the gap of an electromagnet. The relative directions of velocity of plate, magnetic field and current are as shown in the Fig. 9.10.4.

• The volumetric force density is given by,

• As all vectors are perpendicular to each other, over volume V the total force is given by,

F = σvB²V  ... (iv)

• The direction of this force is opposite to This force damps the oscillations of plate into the gap. With the finite conductivity |σ of the plate material, the power due to the induced current gets dissipated in plate itself and thus effectively the plate deaccelerates. This process continues till the plate attains a static state of equilibrium into the gap and stops oscillating completely.

• The principle of magnetic brake is used in vehicles such as trucks. A practical magnetic brake used in vehicle is as shown in the Fig. 9.10.5.

• A conducting circular disk is mounted on the axial at vehicle. An electromagnet is placed near disk such that the disk can move in the gap of electromagnet.

• During application of a mechanical brake, current is applied to the electromagnet also. Because of this alongwith mechanical brake effect, the effect of magnetic brake is also observed. But the braking effect assumes velocity  Thus a vehicle cannot be completely stopped using electromagnetic brake. Thus it is also called magnetic damper or magnetic retarder. 

3. Induction Heating

• The heating method is based on the principle of electromagnetic induction. When alternating current flows in a conductor it produces alternating flux. If any other conducting material is placed in this magnetic flux which is alternating, the e.m.f. gets induced in the conducting material. This induced e.m.f. drives eddy currents in that piece. The power loss due to such eddy currents appears as heat. The action of inducing e.m.f. in other material due to alternating flux produced by a current carrying conductor is a transformer action.

Key Point : The only difference between transformer and induction heating is a transformer electrical energy available in secondary is utilised outside secondary while in induction it is used to heat the charge itself which acts as a short circuited secondary.

• The Fig. 9.10.6 shows the principle of induction heating.

• The heating effect due to such principle depends on following factors :

1. It is proportional to relative permeability. So heating produced in magnetic material is more than non magnetic material. In addition to the eddy currents, there is loss due to hysteresis in magnetic material which contributes to generate more heat. But hysteresis loss becomes negligible at high frequencies.

2. For a given material and supply frequency, heating is proportional to magnetomotive force (m.m.f.). The force can be varied either by changing current or by varying number of turns of coil as m.m.f. is NI.

3. For a given material and magnetising force, heating effect can be increased by employing high frequency supply.

Induction heating can be also classified as,

i) Direct induction heating :

In this, currents are induced in the charge itself. This is usually used in furnaces for smelting (extraction of metal from ore), melting of metals etc. This requires very high frequency supply. These are further classified as core type and coreless type direct induction furnaces.

ii) Indirect induction heating :

In this method, eddy currents are induced in the heating element. Thus heat produced by heating element is then transferred to the charge by radiation or convection.

4. Magnetic Levitation

• A magnetic levitation is a method by which an object is suspended without any support other than magnetic fields. It is thus also known as magnetic suspension or simply maglev.

• The basic principle behind a magnetic levitation is that the opposite poles of magnets attract each other while like poles repel each other. The object is said to be under magnetic levitation when the object floats because of the repelling quality of magnets. So when the force generated by the electromagnetic repulsion is sufficiently strong such that the weight of the object is balanced, then that object is under magnetic levitation. The effects of gravitation acceleration and all other accelerations are counter-acted by mangetic pressure.

• The magnetic materials and the system are capable of attracting or pressing each other (towards each other or away from each other) because of a force which depends on the magnetic field and area of magnets. As discussed earlier, the magnetic pressure counteracts the gravitational acceleration, we can define magnetic pressure mathematically as follows. 

P_mag = B² /  2 µ₀

where          P_mag = Magnetic pressure which is magnetic force per unit area and it is measured in pascles.

B = Magnetic field measured in tesla

µ₀ = Permeability of vacuum

= 4 π × 10^-7 N/A²

a. Methods for Obtaining Magnetic Levitation

• In general there are various methods for obtaining magnetic levitation. It is proved that to levitate stably against gravity only static magnetism is not sufficient. But it should be accomplished with use of diamagnetic materials, servomechanisms, superconductors and systems with eddy currents. The diamagnetic substances repel the magnetic field. This effect can be used for levitation of light objects. More perticularly, superconducting substances are perfect diamagnetic substances which can lift heavier weight objects. Some of the important methods to obtain magnetic levitation are as follows.

1. Mechanical constraint (pseudo levitation)

• In mechanical constraint levitation, the main lifting force is provided by the magnetic levitation but the stability is provided by a mechanical bearing which bears little load. This is called pseudo levitation.

• In this type of levitation two magnets are mechanically constrained along a single axis and arranged such that they strongly repel each other. This effectively levitates one of the magnets above other. If in case the two magnets are attracted towards each other, then the magnets are constrained using a tensile member like string or cable from touching each other.

2. Direct diamagnetic levitation

• A diamagnetic substance repel magnetic field. Actually all materials have diamagnetic properties but the effect is very weak. Also these properties are overcome by the paramagnetic or ferromagnetic properties of the object. Thus any material, in which diamagnetic component is strongest, will be repeled by a magnet eventhough the repulsive force is not strong enough.

The minimum criterion for the diamagnetic levitation is given by,

B (dB / dz) = µ₀ρ g/ χ

where          B = Magnetic field

dB / dz = Rate of change of magnetic field along vertical i.e. z-axis.

µ₀ = Permeability of vacuum or free

space = 4 π × 10^-7 N/A²

ρ = Density of material

g = Gravitational force

χ = Magnetic susceptibility

• Practically the diamagnatic levitation is used to levitate light weight pieces of graphite or bismuth above strong permanent magnet. Water being diamagnetic in nature, the diamagnetic levitation can be used very effectively to levitate water droplets. The main drawback of direct diamagnetic levitation is that very high magnetic fields are required. The typical range of required magnetic field is 12 to 16 tesla. This creats serious problems in ferromagnetic materials present nearby.

• Under ideal conditions, along vertical axis, the water levitates at 1400 T²/m while graphite levitates at 375 T² / m

3. Superconductors

• Basically superconductors are perfect diamagnets with µr = 0. The superconductors have unique property of completely expelling magnetic fields due to Meissner effect during formation of superconductivity in initial stages. In the levitation system using superconductors, the magnet is stabilized because of the flux pinning within the superconductor itself. This stops the superconductor from leaving the magnetic field. This principle is used in Electrodynamic Suspension (EDS) systems, superconducting bearings, flywheels. 

4. Servomechanisms

• The force of magnet decreases with increasing distances and increases for closer distances. But this is unstable condition. For stable operation, the object should be placed at stable position always. In case there is variation from a stable position, then it should be pushed back to the target position. For this certain corrective action is necessary.

• To achieve stable magnetic levitation, the position and the speed of an object being levited is measured. By using feedback loop, the corrective action adjusts one or more electromagnets to locate object at a target position by correcting speed of the object. This forms a servomechanism.

• In most of the magnetic levitation systems, magnetic attraction is used for pulling object upwards against gravity. But some of the systems, use combination of magnetic attraction and magnetic repulsion to push object upwards. The systems with such combinations are called Electromagnetic Suspension (EMS) Systems.

• The basic principle is that the electromagnet placed above the object being levited is turned off when the object gets closer while it is turned on when the object falls away from the target position. The example of an application based on EMS system is magnetic levitation trains (commonly called Maglevs). The train wraps around the track and the train is pulled upwards against gravity using very strong magnetic fields. Using effective servo controls, the train is safely kept at a constant distance from the track. It is illustrated in the Fig. 9.10.7.

5. Electromagnetic Propulsion of Ships and Submarines

• An electromagnetic propulsion is a process of accelerating any object using utilization of flowing electrical current and magnetic fields. The magnetic field of opposite type is created by such electrical current. It can also be utilized for charging of a fluid. When a current flows through conductor placed in a magnetic field, Lorentz force i.e. electromagnetic force is produced. This force pushes conductor in a direction perpendicular to direction of current through conductor and direction of magnetic field. Such an electromangetic force causes propulsion which is commonly known as Electromagnetic Propulsion (EMP). Thus propelling of an object using basic concept of electromagnetism is nothing but electromagnetic propulsion system.

• The concept of electromagnetic propulsion is very much similar to that of magnetic launcher commonly called rail gun. For propulsion of ships and submarines, the rails are submerged into sea water. Now being a conductive in nature, the sea water makes the space between rails completely conductive in nature. When the current passes through the sea water, a force is developed on the sea water. If the magnetic field also exists between the rails submerged, the sea water is pushed out of the space between rails. The application can be made more useful by covering rails with non-conductive surface, the water is expelled as a jet. Similar to the water jet boat, the reaction force moves water craft forward. This device is useful for submarines as a propulsion mechanism and it is called magnetohydrodynamic pump.

• The main advantage of an electromagnetic propulsion is that reduction in the noise. But the limitations of this process is the requirement of large currents and reduction in the overall efficiency because of low conductivity of sea water. The efficiency can be increased by producing very large magnetic fields with the help of superconducting magnet along with associated cooling equipment.

• Using this technique, pumping of molten metals is very easy. In typical nuclear power plant, pumping of molten sodium is required. This method is found to be effective as it can be used as pump without moving parts and also all the metals are highly conductive in nature.

6. Electromagnetic Launcher (EML)

• An electromagnetic launcher (EML) is the practical application of force exerted on currents. The electromagnetic launchers are commonly called electromagnetic rail launchers or rail guns.

• A typical electromagnetic launcher consists of two parallel rails made up of a electrically conductive material and are attached with a narrow space between them to form a barrel. The projectile that is to be propelled from the electromagnetic launcher is attached to the back of barrel. The projectile is made up of conducting material so that the rails get started to each other as shown in the Fig. 9.10.8.

• Due to the current in rails, magnetic flux density is produced between the rails as shown in the Fig. 9.10.8 (b). The interaction of current in the projectile and the field forces the projectile out of rails. The electromagnetic launchers have capability to a achieve velocities greater than that attained in thermodynamic guns and explosives for military applications. With high enough velocities achieved, the electromagnetic launchers are used in important commercial, military and scientific applications. The EML can be used to fire satellite into orbit. Using EML much more damage is possible used in military applications as escape velocities are possible with it. The EML can be used is scientific applications to accelerate very small masses in area of particle research. The electromagnetic accelerators designed to launch projectiles at hypervelocities must be evacuated in order to avoid the unwanted atmospheric drag effects.

7. Electromagnetic Forming

• An electromagnetic forming is a process which includes high velocity, cold forming of electrically conductive metals, like copper and aluminium. This process is also known as EM forming or Magneforming. The piece of metal can be reshaped by high intensity pulsed magnetic field which induces a current in the piece of metal. The repulsive magnetic field is also produced which rapidly reshapes the portion of the piece of metal. This technique is also known as high velocity forming.

Principle of electromagnetic forming

• The rapidly changing magnetic field around the coil induces a current in a conductor placed nearby coil according to the principle of electromagnetic induction. The current flowing through the conductor sets up a corresponding magnetic field around it. Then according to Lenz's law the magnetic fields within coil and conductor repel each other strongly.

Operation

• The piece of metal to be fabricated is placed in the proximity of heavily constructed coil of wire (which is called work wire) known as forming coil. A large pulse of current is forced through the forming coil by rapidly discharging high voltage capacitor bank using switch. In practical process, an ignition or spark gap is used as switch. This high pulse of current thus in turn produces a fast oscillating, ultrastrong magnetic field around the forming coil.

• When the switch is closed, the capacitor bank discharges electrical energy stored through the forming coil which produces a rapidly changing magnetic field. This changing magnetic field induces a current in the piece of metal placed within  forming coil placed in supporting coil casing. The current flowing in the piece of metal develops a opposite magnetic field. Correspondingly which rapidly repels the piece of metal from the forming coil, thus the reshaping of metal piece is done. The effect of reciprocal forces acting against the forming coil is eliminated by placing a forming coil inside a supportive coil casing.

Advantages

The advantages of an electromagnetic forming compared with conventional mechanical forming are as follows.

i) The amount of stretch in a conductor without tearing is improved. This property is called formability of conductor.

ii) The wrinkling in a metal (or conductor) is supressed considerably.

iii) This method allows to achieve very close tolerances.

iv) This method can be carried out using single sided dies, thus tooling cost reduces.

v) The process can be carried out in a clean room conditions because lubricants are not required in this method.

vi) In this method, a mechanical contact with a metal to be fabricated is not required. Thus there is no surface contamination and tooling marks observed in the metal. Thus prior to forming process, finishing of surface can be done.

vii) The electromagnetic forming can be combined with joining and assembling with dissimilar components such as glass, plastic, composites, other metals etc.

Disadvantages

The disadvantages of electromagnetic forming are as follows :

i) The direct forming of non-conductive metal is not possible. For forming of non-conductive metals, a conductive drive plate is required.

ii) The voltages and currents involved are very high. Hence safety considerations required to be monitored continuously.

iii) Forming of large sheet metals is not possible as the current in the large coils is limited.

Applications

• Basically an electromagnetic forming is used mostly to shrink or expand the cylindrical tubing. It can also be used to form a sheet metal repeling it onto a shaped die at very high velocity. Using electromagnetic pulse welding with a metallurgical weld, a high quality joints can be formed using this method. The electromagnetic forming is most effective with good conductors such as copper, aluminium (which are good conductors of electricity) but the process can be used with some modifications for materials like steel which is a poor conductor of electricity.

8. Eddy Current Testing of Materials

• Any conducting material can be tested for any flaw in it using eddy current testing method. Basically eddy current testing method comes under the class of non-destructive testing (NDT) which means testing of metallic conductors without destroying or damaging them. Many times there are defects in a material which are not visible due to paint or coating covering them. Sometimes there can be defects in materials which are not visible with human eyes or any visual method of inspection. In such cases eddy current testing plays very important role in detecting any such defects.

a. Basic Principle of Eddy Current Testing

• An eddy current testing is based on the phenomenon of electromagnetic induction. A simple coil is connected to constant a.c. voltage source (or constant current source). When an a.c. current flows through the coil at a perticular frequency, a magnetic field of oscillating type is generated near coil as shown in the Fig. 9.10.10 (a). If the conditions are kept unchanged, the inductance of the coil remains constant. So the equivalent impedance of the circuit also remains constant say Z₁ When the coil carrying a.c. constant current and oscillating magnetic field around it is moved closer to the conducting material to be tested, because of induced e.m.f., the induced currents flow through conducting material as shown in the Fig. 9.10.10 (b). As more power is to be delivered by the source, now the impedance of circuit consisting coil decreases to new value Z₂. Thus the current in the coil changes for constant a.c. voltage source (or voltage across coil changes for constant a.c. current source). The value of this current flowing through coil (or voltage across coil) is used as a reference reading for current (or voltage) and inturn for impedance. Now suppose there is a flaw or defect in a conducting material as shown in the Fig. 9.10.10 (c). 

When the coil and its magnetic field are brought closer to the conducting material, the flow of electrons (to be specific circular flow of electrons) called eddy current begin to move through the conducting material like a flow of swirling water. The eddy current flowing through conducting material set ups its own magnetic field which interacts with the magnetic field of coil through mutual inductance. Thus e.m.f. is induced in conducting material. Now where there is a defect in material to be tested, the amplitude and also the pattern of eddy current gets altered. So effectively it alters the resulting magnetic field due to the eddy current. As a result, movement of electrons in the coil gets affected and this ends in varying the impedance of coil to new value Z₃ with is obviously different than the reference value of impedance i.e. Z₂.

• Thus by measuring current for constant voltage source (or voltage for constant current source), we can get the direct information about the condition of material. Thus the variation in coil impedance directly detects t he defect in the material.

The defects in the material to be tested can be

i) Changes in material conditions such as crack, inclusion, corrosion.

ii) Changes in material property such as change in conductivity, change in permeability etc. 

• Many times, it is necessary to inspect or test tubes of heat exchanger unit and air conditioning units and power plants. For such testing a special probe is used. It consists two coils connected in series which provide differential output as shown in the Fig. 9.10.12.

• When the probe with two coils move along a material of good condition, the output is zero. But as probe moves over a defect in material like hole in material, then the impedance of the coil over a defect shows different impedance as compared with other coil which is over portion of good condition material and thus non-zero is obtained as shown in the Fig. 9.10.12.

b. Factors Affecting Eddy Current Testing

• There are number of factors affecting the operation of eddy current testing. The eddy currents in materials with higher conductivity are more sensitive to the surface defects but will be less penetrating into the material. Basically penetration also depends on the test frequency of the input signal. Higher frequency test signals increase surface resolution but limit the depth of penetration. On the other hand, signals with low frequencies penetrate at greater depth. The large coils are capable of inspecting greater volume of material. But smaller coils are most sensitive to the small defects near surface as compared with larger coils. With the variation in permeability of material, there is a possibility of generation of noise which limits resolution. The basic properties of material like permeability, conductivity are beyond control, the selection of test frequency, coil type and coil size is done on the basis of test requirements.

9. Magnetohydrodynamic (MHD) Generator

• The magnetohydrodynamic (MHD) generator is a device used to generate electricity or electrical energy. The MHD generators are similar to the conventional electric generators. But conventional electric generators use solid conductors to generate electric power while the MHD generators use electrically conducting fluid. A typical setup of a MHD generator is as shown in the Fig. 9.10.13.

• Under high pressure, an electrically conducting fluid is passed through the magnetic channel formed between the conducting electrodes with very high speed. A strong magnetic field is produced in a channel formed by electrodes using superconducting magnets. The magnetic field inside the channel is usually about 3 to 5 tesla. The two electrodes are placed right angles to each other and are isolated from the magnets. When the conducting fluid is passed through the channel, the fluid experiences an electromotive force. According to the Faraday's law of electromagnetic induction, e.m.f. is induced in a coil whenever there is a change in magnetic flux linked with the coil. Note that the electromagnets are stationary but the conducting fluid is moving. This causes generation of electricity. The relation between the velocity of conducting fluid and the magnetic fluid is as shown in the Fig. 9.10.14 (a). This clearly indicates that the right plate becomes negative while the left plate becomes positive as shown in the Fig. 9.10.14 (b).

• The MHD generator works on the principle of Lorentz force law given by,

• The direction of motion of fluid is dependent on the direction of the magnetic field.

• Generally, the conducting fluids used in MHD generator are plasma and sea or salt water. In most of the MHD generators use highly ionized gases which are produced by burning fossil fuels. The conductivity of the ionized gases can be increased by adding conducting ions such as alkali metal vapours. This is called seeding of ionized gases. In case of a coal fired MHD generator, using potassium carbonate as a seed, the conductivity of the gas can be increased considerably.

• The MHD generators produce d.c. directly hence they are also called magnetohydrodynamic d.c. generators. The important point is that these generators are stationary. In case of rotating generators a.c. is produced than d.c. Inspite of having very low efficiency (less than 15 %), these generators are popular because they operate in closed circuits and the system is fully contained. But practically, these generator are hard to handle as the system needs very high magnetic fields, high pressures, temperatures to be maintained in the channel.

The MHD generators can be constructed in various designs such as Faraday generator, Hall generator, disc generator etc. The Faraday generator was the first MHD generator made by Micheal Faraday in 1831. The Faraday generator uses copper discs and horse shoe magnets to generate electricity.

Review Questions

1. Explain principle induction heating.

2. Describe the function of a transformer starting from fundamental principles.

3. Explain electromagnetic propulsion of ships and submarines.

4. Write a note on electromagnetic launcher.

5. Explain briefly electromagnetic forming. Write its advantages, disadvantages and applications.

6. Explain basic principle of operation of eddy current testing with the help of suitable diagrams.

7. What are the factors affecting eddy current testing ?

8. With the help of suitable diagram explain MHD generator.

9. Describe the applications where circuit theory is used and applications, where field theory is used.

AU : Dec.-17, Marks 7