Electrical Engineering ⇒ Topic : Leakage Flux

LEAKAGE FLUX

The part of the magnetic flux that has its path within the magnetic circuit is known as the useful magnetic flux. It has been seen that the magnetic flux has its part partly in magnetic circuit and partly in air. The second part is called the leakage flux. Sum of the two parts is called the total flux produced. Leakage flux A is defined as the ratio of total flux produced to useful flux. Its value is always greater than unity (1.12 to 1.25).

Consider a ring specimen of a conductor carrying a winding of N turns with an air gap as shown in Fig. (a). When the winding carries a current, flux will be established in the specimen. A small portion of this flux may cross the air gap and link with the search coil dd_i .This flux is called the useful flux. Some of the flux may not cross the air gap as useful flux and returns by paths a, b and c, and is known as leakage flux. The flux in the air gap bulges outwards as

                                                          Figure (a)       Magnetic leakage and fringing       

shown in Fig. (a), because of the variation in reluctance. This is known as fringing. The affect of fringing is to reduce the flux density in the air gap. The effect is more pronounced for larger air gaps. The flux through the air gap is known as useful flux and can be used for practical purposes. From Fig.(A), it can be seen that the effect of the leakage flux is to increase the total flux through the existing winding, and

The value of leakage coefficient varies from 1.15 to 1.25 for electrical machines. Thus, in electrical machines, total flux being known, the useful flux can be determined by assuming a suitable value of the leakage coefficient.