Classification Of Magnetic Circuits

CLASSIFICATION OF MAGNETIC CIRCUITS

Magnetic circuits can be classified into

• Simple magnetic circuit

• Composite magnetic circuit

• Parallel magnetic circuit

A simple magnetic circuit is made up of a single magnetic material. But a composite magnetic circuit will have minimum of two different materials offering different magnetic properties. Both of them may be magnetic or one may be non magnetic.

 

Simple Magnetic Circuits

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Consider a circular solenoid or a toriod iron using as shown in Fig. 2.2. A windings of N turns is provided in the ring. Let I be the current flowing in the winding. ‘a' is the a new of cross section in m² and 7 is the mean length in metre.

MMF produced = NI ampere turns

The magnetising force inside the ring is

H = NI/ 1 Ampere turns/metre

1. Flux density inside the ring

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2. Total flux produced

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Composite magnetic circuit

A series magnetic circuit that has parts of different dimensions and materials is called a composite magnetic circuit. Each part will have its own reluctance. The total reluctance is equal to the sum of reluctances of individual parts.

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* Find the mean Length / of magnetic path for each part of the circuits.

* Find the mean length / of magnetic path for each part circuit.

* Find AT required for each part of the magnetic circuit using the relation AT = H × l.

* The Total AT required for the entire series circuit is equal to the sum of AT for various parts.

 

Parallel Magnetic Circuit

A magnetic circuit which has more than one path for flux is called a parallel magnetic circuit. It is similar to a parallel electric circuit which has more than one path for electric current. The concept of parallel magnetic circuit is illustrated in Fig.2.4. Here a coil of N turns is wounded on limb AD carries a current of I amperes. The flux o, setup by the coil divides at B into two paths, namely.

* Flux Φ₂ passes along the path BE

* Flux Φ₃ follows the path BCFE

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It is obvious from the figure Φ₁ = Φ ₂ + Φ ₃

The magnetic paths BE and BCFE are in parallel and from a parallel magnetic circuits. The AT required for this parallel circuit is equal to AT required for any one of the paths,

Let S₁ = reluctance of path EDAB

S₂ = reluctance of path BE

S₃ = reluctance of path BCFE

Total m.m.f required = m.m.f for path EDAB+m.m.f for path BE (or) mmf for path

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The Reluctances S1, S2 and S3 must be determined from a calculation ofmthose paths of the magnetic circuit in which 1, 2 and 3 exist respectively.

 

Leakage flux

The flux that does not follow the desired path in a magnetic circuit is called a leakage flux. In practical magnetic circuits a large part of flux path is through a magnetic material and the remainder part of flux path is through air. The ir. The flux in the a in the air gap is known as useful flux. An iron ring wound with a coil and having a narrow air gap. The total flux produced by the coil does not pass through the air gap as some of it leaks through the air (paths at /) surrounding the iron. These flux lines are called leakage flux (Φ_l).

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It may be seen that the useful flux passing across the air gap tends to bulge outwards as shown in Fig. 2.5. Thereby increasing the effective area of the gap and reducing the flux density in the gap. The effect is known as fringing. The longer the air gap, the greater is the fringing and vice versa.

 

Comparison between magnetic and Electrical circuit

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Magnetic field intensity

Magnetic field intensity (H) at any point in the magnetic field is defined as the force experienced by the unit north pole at that point. In simple terms, it is a measure of how strong or weak any magnetic field is. The SI unit of magnetic field intensity is Ampere/ meter (A/m).

Magnetic Flux Density

Magnetic Flux Density is amount of magnetic flux through unit area taken perpendicular to direction of magnetic flux. Flux Density (B) is related to Magnetic Field uber square m (H) by B = μH. It is measured in Webers per meter equivalent to Teslas [7].

The total number of magnetic field lines passing through a given area normally is called magnetic flux. In magnetic flux formula u is the permeability of the medium (material)

where we are measuring the fields. The B field is a vector field, which means it has a magnitude and direction at each p point in space.

 

Magnetic Fringing

When the magnetic field lines pass through an air gap, they tend to bulge out (Fig.2.6). It is because the magnetic field lines repel each other when passing through the air (or non- magnetic materials). This effect is known as magnetic fringing.

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Due to magnetic fringing, the effective area of the air gap is increased and thus the magnetic flux density is decreased in the air gap. The longer the air gap, the higher is the fringing and vice-versa.