B H Curve Experiment Pdf
The Hysteresis Loop and Magnetic Properties
Experiments for First Year Electrical Engg Lab 2014-15 Dept of Electrical Engineering, Shri Ramdeobaba College of Engineering and Management, Nagpur 440013 INDIA Page 8 On LHS by Hand with Pencil EXPERIMENT NO. Aim: To study R-L-C series circuits (AC) Apparatus: Range/Rating Make 1.
A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop. An example hysteresis loop is shown below.
The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point 'a' almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When H is reduced to zero, the curve will move from point 'a' to point 'b.' At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost their alignment.) As the magnetizing force is reversed, the curve moves to point 'c', where the flux has been reduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material.
As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point 'd'). Reducing H to zero brings the curve to point 'e.' It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point 'f' back to the saturation point where it with complete the loop.
From the hysteresis loop, a number of primary magnetic properties of a material can be determined.
- Retentivity - A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, it is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point b on the hysteresis curve.)
- Residual Magnetism or Residual Flux - the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.
- Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero. (The value of H at point c on the hysteresis curve.)
- Permeability, m - A property of a material that describes the ease with which a magnetic flux is established in the component.
- Reluctance - Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.
This example shows calculation and confirmation of a nonlinear transformer core magnetization characteristic. Starting with fundamental parameter values, the core characteristic is derived. This is then used in a Simscape™ model of an example test circuit which can be used to plot the core magnetization characteristic on an oscilloscope. Model outputs are then compared to the known values.
Open Model
Specification of Parameters
Fundamental parameter values used as the basis for subsequent calculations:
Permeability of free space,
Relative permeability of core,
Number of primary turns,
Number of secondary turns,
Effective magnetic core length,
Effective magnetic core cross-sectional area,
Core saturation begins,
Core fully saturated,
Calculate Magnetic Flux Density Versus Magnetic Field Strength Characteristic
Where:
Magnetic flux density,
Magnetic field strength,
Linear representation:
Nonlinear representation (including coefficient, a):
Use Parameters in Simscape Model
The parameters calculated can now be used in a Simscape model. Once simulated, the model is set to output a Simscape logging variable, simlog_ee_custom_transformer, and some signals using output ports, yout. Circuit parameters are:
Voltage source magnitude,
Voltage source frequency,
Voltage source resistance,
Operational amplifier input resistance,
Operational amplifier feedback resistance,
Operational amplifier feedback capacitance,
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Calculations on Logging and Output Data
The data needs to be processed to provide the Magnetic field strength and Magnetic flux density data for comparison. Where:
Magnetomotive force,
Magnetic flux,
Operational amplifier input voltage,
Operational amplifier output voltage,
Equations to be used are as follows:
Conclusion
The three characteristics can now be overlaid:
Defined characteristic: calculated from fundamental parameters
Characteristic from logging: calculated from internal Simscape logging data
Characteristic from measurement: obtained by measurement and calculation using electronic test circuit
Due to leakage and parasitic parameters, the characteristic obtained from the electronic test circuit differs to the defined characteristic. However, the test circuit and its parameterization is shown to find the characteristic for the given transformer within suitable tolerances.