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







Related Discussions:- Magnetic permeability and the b/h curve, Assignment Help, Ask Question on Magnetic permeability and the b/h curve, Get Answer, Expert's Help, Magnetic permeability and the b/h curve Discussions

Write discussion on Magnetic permeability and the b/h curve
Your posts are moderated
Related Questions
Discuss the role of a bus arbiter in a multiprocessor configuration. Bus arbiter: It functions to resolve priority among bus masters and permits only one device at a time to

Q. What are the applications of JFET? · FET is used as a buffer in measuring instruments, receivers since it has high input impedance and low output impedance. · FET's are u

In second technique, a concentrator expander (CE) is used near cluster of users and another one at exchange end as demonstrated in figure. Only a few junction lines are run between

(a) A universal shift register can shift in both the left-to-right and right-to-left directions, and it has parallel-load capability. Draw a circuit for such a shift register.

Q. How does the value of Idss and Vp change with the change in Vgs The value of Vp and and Idss can be controlled by adjusting the value of the gate to sorce voltage ie Vgs. T

Q. A 10-kVA, 200:400-V, single-phase transformer gave these test results: • Open-circuit test (LVwinding supplied): 200V, 3.2 A, 450 W • Short-circuit test (HV winding suppli

Q. Is a depreciation of the dollar/euro exchange rate correlated with a decrease in the dollar return on U.S. deposits? Answer: No, suppose that the Interest Parity is maintai

precision

Q. Why FET is called a voltage-controlled device? Why its input resistance is high? In the case of a FET the output current ID is a function of the voltage VGS applied to the i

Q. Consider an elementary three-phase, four-pole alternator with a wye-connected armature winding, consisting of full-pitch concentrated coils.  Each phase coil has three turns,