Hysteresis and Eddy-current losses

The two main magnetic losses are Hysteresis and Eddy-current losses....

1. Jamhuriya University Of Science & Technology Faculty Of Engineering Department of Electronics and Communication Engineering Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

2. Hysteresis and Eddy current losses Referenced from:- Hysteresis and Eddy current losses journal by prof P. Parthasaradhy & S.V. Ranganayakulu. Magnetic circuits and core losses Module 6 by kharagbur. Principles and Applications fourth edition by Allan R. Hambley Electrical Circuit Theory & Technology 4 th Edition By John Bird. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

3. Losses in Magnetic Circuit There are two types of magnetic losses Hysteresis losses Eddy Current Losses Total iron loss is the sum of these two losses Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

4. Hysteresis loss Hysteresis is the ‘lagging’ effect of flux density B whenever there are changes in the magnetic field strength H. When an initially unmagnetized ferromagnetic material is subjected to a varying magnetic field strength H, the flux density B produced in the material varies. Hysteresis loss is associated with the phenomena of hysteresis and is an expression of the fact when ferromagnetic material is involved, not all the energy of the magnetic field is returned to the circuit when mmf is removed. It is known as hysteresis loss. The core of a transformer is subjected to an alternating magnetizing force and for each cycle of emf a hysteresis loop is traced out. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

5. The lagging phenomenon of B behind H is called hysteresis. The tip of hysteresis loop can be joined to obtain the magnetization characteristics. Hysteresis results in a dissipation of energy which appears as a heating of the magnetic material. The energy loss associated with hysteresis is proportional to the area of the hysteresis loop. For AC-excited devices the hysteresis loop is repeated every cycle of alternating current. Thus a hysteresis loop with a large area is often unsuitable since the energy loss would be considerable . Silicon steel has a narrow hysteresis loop, and thus small hysteresis loss, and is suitable for transformer cores and rotating machine armatures. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

6. Core Loss B-H curves exhibits “hysteresis” Power loss due to hysteresis is proportional to frequency, assuming constant peak flux. The relationship between B and H is not linear for the types of iron used in motors and transformers. Hysteresis & Eddy Current losses ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

7. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

8. 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. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

9. The area of a hysteresis loop varies with the type of material. The area, and thus the energy loss, is much greater for hard materials than for soft materials. hard material, which has a high remanence and a large coercivity. soft steel, which has a large remanence and small coercivity. ferrite, this being a ceramic-like magnetic substance made from oxides of iron, nickel, cobalt, magnesium, aluminium and mangenese; the hysteresis of ferrite is very small. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

10. The area of a hysteresis loop varies with the type of material. The area, and thus the energy loss, is much greater for hard materials than for soft materials. A disturbance in the alignment of the domains of a ferromagnetic material causes energy to be expended in taking it through a cycle of magnetization. This energy appears as heat in the specimen and is called the hysteresis loss Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

11. Separation of core loss The sum of hysteresis and eddy current losses is called core loss as both the losses occur within the core (magnetic material). For a given magnetic circuit with a core of ferromagnetic material, volume and thickness of the plates are constant and the total core loss can be expressed as follows. Core loss = Hysteresis loss + Eddy current loss It is rather easier to measure the core loss with the help of a wattmeter (W) by energizing the N turn coil from a sinusoidal voltage of known frequency. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

12. Eddy current loss An eddy current is an electric current set up by an alternating magnetic field . These losses arise from the fact that the core itself is composed of conducting material, so that the voltage induced in it by the varying flux produces circulating currents in the material. Eddy current loss depends upon the rate of change of flux as well as the resistance of the path, it is reasonable to expect this loss to vary as the square of both the maximum flux density and frequency Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

13. Eddy Current Eddy current: As we saw, a flux induces a voltage on a coil. Q: Why not inducing a voltage on the core itself? A: It actually does. The result is eddy current. That is why the transformers core are laminated. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015

14. Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad Eddy current loss flux Current flux Because of time variation of flux flowing through the magnetic material as shown, current is induced in the magnetic material, following Faraday’s law. This current is called eddy current. This current can be reduced by using laminated (thin sheet) iron structure, with Insulation between the laminations. Laminations 11/11/2015

15. Eddy-Current Loss As the magnetic field changes in a material, it causes “eddy currents” to flow. Power loss due to eddy currents is proportional to the square of frequency, assuming constant peak flux . Eddy currents result in a loss of power, with consequent heating of the material . Hysteresis & Eddy Current ©Associated to you by AbdulAziz Ahmed Siyad 11/11/2015