Magnetic Circuits

MAGNETIC CIRCUITS AND ELECTRICAL INSTALLATIONS

MAGNETIC CIRCUITS

The closed path followed by magnetic flux is called a magnetic circuit. In a magnetic circuit, the magnetic lines of force leave the north pole pass through the entire circuit and return to the starting point. A magnetic circuit usually consists of materials having high permeability such iron, soft steel etc. These materials offer very small opposition to the flow of magnetic flux. Consider a coil of N turns wound on an Iron core as shown in Fig. 2.1. When current is passed through the coil, magnetic flux (6) is set up in the core. The flux follows the closed path ABCDA and hence ABCDA is a magnetic circuit.

Basic Definition of Magnetic Circuit

(a) Magnet

A substance that attracts the iron pieces and pieces of some other metals is called magnet. A magnet can be classified into a permanent and temporary magnet.

 (b) Permanent magnet

This is made up of cobalt, steel or tungsten steel. It is used in moving coil instruments, energy meters, loud speakers, microphones etc.U

(c) Temporary magnet 2TIUAI

It is also called an electromagnet. The material used here is soft iron or silicon steel. A soft iron piece with a coil acts as a magnet as long as currents flows through the coil. It is used in electrical machines such as motor and generator.

(d) Magnetic lines of force

The imaginary magnetic lines which travels from north pole to south pole outside the magnet, and south pole to north pole, inside the magnet are called magnetic lines of force

(e) Magnetic Field

The region around a permanent magnet or the space around a current carrying conductor occupied by the lines of force is called the magnetic field of force or magnetic field.

(f) Magnetic flux

The amount of magnetic lines of force in a magnetic field known as magnetic flux. These flux lines from a closed path and the path followed by the flux lines is known as magnetic circuit. The unit of magnetic flux is weber (wb) and is represented by the Letter (4).

(g) Magnetic flux density

The flux density is the magnetic flux per unit area taken perpendicular to the direction of the magnetic flux. It is usually denoted by the Letter of 'B' and its unit is tesla (w/sqm). If a flux of (Φ) weber is passing through an area of A square metre, then the flux density B is given by B= Φ/A

(h) Permeability

The phenomena of magnetism and electromagnetism are dependent upon a certain property of the medium is called its permeability. Every medium posses two permeabilities.

• Absolute permeability (μ₀)

• Relative permeability (μ_r)

(i) Absolute permeability (μ₀) 

The magnetic conductivity of iron as compared to that of air is called permeability. The absolute permeability (μ₀) of a medium is given below. Permeability of the medium

(ii) Relative permeability μ_r

The relative permeability μ_r of a medium is given by

μ_r  = B / B₀

where, B = flux density in the medium under consideration in wb/m2

B₀ = Flux density in vacuum

μ_r = 1 (air, free space, vaccum)

(k) Reluctance (S)

Reluctance is the property of a material which opposes the establishment of magnetic flux in it. It is defined as the ratio of magneto motive force to the flux.

(I) Permeance

It is the reciprocal of reluctance and is a measure of the case with which flux can pass through the material.

(m) Magneto Motive Force (MMF)

 It is the driving force required to drive the magnetic flux through a magnetic circuit. The product NI is called magneto motive force.

MMF = NI (AT)

where, N→ No. of turns in the conductor

I→ Current in Ampere

(n) Magnetic Field Strength (H)

It is given by the force experienced by unit north pole place at that point. It is denoted by 'H'.