Electrical Engineering ⇒ Topic : Magnetic Circuit

The magnetic effect of electric current was first discovered by Oersted in 1820. When a conductor carries an electric current, a magnetic field is produced around that conductor.

       In the course of his experiments, Oersted happened to lead a wire carrying current over a compass needle, and also parallel to it, as shown in Fig. (a). The needle was observed to deflect from its original position.On the other hand, when the wire was placed under the compass needle, the needle deflected in the opposite direction. Another example to show the magnetic effect of electric current is shown in Fig. (b) The figure shows a current carrying conductor pierced through a sheet of paper held horizontal to the surface. If iron filings are sprinkled on the paper, the iron filings align themselves in concentric circles around the current carrying wire. From the

 Fig.(a) Deflection of a compass needle by electric current                                      Fig.(b) Magnetic field due to a straight long conductor 

above observations Oersted concluded that the magnetic field lines are circular around the wire.

Magnetic Circuit

The closed path followed by magnetic flux is called a magnetic circuit.

In a magnetic circuit, the magnetic flux leaves the N-pole, passes through the entire circuit, and returns to the starting point. A magnetic circuit usually consists of materials having high permeability e.g. iron, soft steel etc. It is because these materials offer very small opposition to the 'flow' of magnetic flux. The most usual way of producing magnetic flux is by passing electric current through a wire of number of turns wound over a magnetic material. This helps in exercising excellent control over the magnitude, density and direction of magnetic flux.

Consider a coil of N turns wound on an iron core as shown in Fig.(a). When current is passed through the coil, magnetic flux (Φ) is set up in the core. The flux follows the closed path ABCDA and hence ABCDA is the magnetic circuit. The following points may be noted carefully :

                                                                            figure (a)

(1) The amount of magnetic flux set up in the core depends upon current (I) and number of turns (N). If we increase the current or number of turns, the amount of magnetic flux also increases and vice-versa. The product *NI is called the magnetomotive force (m.m.f) and determines the amount of flux set up in the magnetic circuit.

                                            m.m.f. = NI ampere-turns

(2) It can just be compared to electromotive force (e.m.f.) which sends current in an electric circuit.The opposition that the magnetic circuit offers to the magnetic flux is called reluctance. It depends upon length of magnetic circuit (i. e. length ABCDA in this case), area of X-section of the circuit and the nature of material that makes up the magnetic circuit