Magnetic permeability and the b/h curve, Electrical Engineering

Magnetic permeability and the B/H curve

To measure the relationship between field strength H and flux density B for a material, a sample is made into a closed ring and a magnetising coil wound onto it. Current flowing in this coil creates a m.m.f. equal to NI which in turn produces a magnetic field

H= NI/L

in the ring. So  H is proportional to the current in the energising coil and may be obtained from direct measurement of I. If a change in the current occurs, the flux density changes and this will induce a transient 'back e.m.f' in a second coil that is also wound onto the ring.

 

1372_Magnetic permeability and the B-H curve.png

 

The integral of the back emf is proportional to B. Hence the relationship between B and H may be determined from measurement of I (to give H) and the induced back e.m.f in the coils (to give B).

The result of such measurements on various materials produces two types of curves - straight lines of low

B/H =µr

values and curves for  which the value of B/H =µr   varies. This first type are sub divided into paramagnetic
varies. This first type are sub divided into paramagnetic materials if B/H =µr >1 and diamagnetic if B/H =µr <1. The second type are ferromagnetic and have a more complicated dependence of B on H.  At low H these have B/H =µ>>1 (typically 100 - 100000)

85_Magnetic permeability and  B-H curve.png

 

 

It is also found that ferromagnetic materials lose these high µr properties above a well-defined temperature (different for different materials) known as the Curie temperature. This typically  r ranges from 200 - 750 °C and limits the maximum temperature that many electromagnetic devices can operate.


The temperature dependence of the ferromagnetic properties give a clue to the physical reason for the different

permeabilities of the materials. Diamagnetic and paramagnetic properties are attributable to the orbiting electrons within the atoms of the material. Ferromagnetic properties arise due to the orientation of atoms on a macroscopic scale so that the magnetic fields created by the orbiting electrons within each atom reinforce each other. These 'domains' of high magnetic strength may contain ~10-12   atoms each. The domains are initially randomly directed but under the influence of an external magnetic field, re-orientate to point in the same direction. Initially this is along the direction of the nearest crystal axis to the direction of the magnetic field.

 

Steadily increasing the external field brings them directly into line with the external field direction.If the external field is now removed, the domains relax back to point in the nearest crystal direction, leaving a residual magnetism. Materials that have a strong residual magnetism are used for permanent magnets. Cycling the current over one complete cycle produces a hysteresis B/H curve

 

 

 

1726_hysteresis B-H curve.png

Posted Date: 8/22/2012 5:56:39 AM | Location : United States






Your posts are moderated
Related Questions
Q. Define Transconductance? The control that the gate voltage has over the drain current is measured by the transconductance gfs and is similar to the transconductance  gm of t


The potential drop between the terminals of a battery is equal to the battery's EMF when: a) No current is drawn from the battery b) A very large current is drawn from th

Sketch the Fermi-Dirac distribution function, F(E), alongside the energy band diagram for an n-type semiconductor, indicating the position of the Fermi level, EF, and the donor lev

Q. Biasing the BJT? A simple method of biasing the BJT is shown in Figure. While no general biasing procedure that will work in all cases can be outlined, a reasonable approach

For the circuit of Figure, given that V CC = 5V, R C = 1k, β = 100, and the high range is 4 to 5 V, choose R B such that any high input will saturate the transistor with the ba

Explain the NAND Gates - Microprocessor? The NAND GATE is a AND gate with the output inverted. Consequently the outputs of a NAND gate would be the opposites of the outputs of a A

Convolution :   (i) Express this function in terms of kernel coefficients and convolve with image class.png, (x,y are coordinates of pixels) Io(x,y)=I(x+1,y)-2*I(x,y)+I(x-1,

Q. Explain the drain characteristics of JFET with external bias? It gives relation between Id and Vds for different values of Vgs.(which is called the running variable). The ab

Example of Registers Example : Write  the instruction to copy  data byte 24H  stored  in register C to register B  Solution : The instruction to copy  data byte  is  a