Eddy Current & Hysteresis Loss

 

22.1 Lesson goals

In this lesson we shall show that (i) a time varying field will cause eddy currents to be induced in the core causing power loss and (ii) hysteresis effect of the material also causes additional power loss called hysteresis loss. The effect of both the losses will make the core hotter. We must see that these two losses, (together called core loss) are kept to a minimum in order to increase efficiency of the apparatus such as transformers & rotating machines, where the core of the magnetic circuit is subjected to time varying field. If we want to minimize something we must know the origin and factors on which that something depends. In the following sections we first discuss eddy current phenomenon and then the phenomenon of hysteresis.

Finally expressions for (i) inductance, (ii) stored energy density in a magnetic field and (iii) force between parallel faces across the air gap of a magnetic circuit are derived.

Key Words: Hysteresis loss; hysteresis loop; eddy current loss; Faraday's laws;

After going through this section students will be able to answer the following questions. After going through this lesson, students are expected to have clear ideas of the following:

 

1. Reasons for core losses.

 

2. That core loss is sum of hysteresis and eddy current losses.

 

3. Factors on which hysteresis loss depends.

 

4. Factors on which eddy current loss depends.

 

5. Effects of these losses on the performance of magnetic circuit.

 

6. How to reduce these losses?

 

7. Energy storing capability in a magnetic circuit.

 

8. Force acting between the parallel faces of iron separated by air gap.

 

9. Iron cored inductance and the factors on which its value depends.

 

22.2 Introduction

While discussing magnetic circuit in the previous lesson (no. 21) we assumed the exciting current to be constant d.c. We also came to know how to calculate flux (φ) or flux density (B) in the core for a constant exciting current. When the exciting current is a function of time, it is expected that flux (φ) or flux density (B) will be functions of time too, since φ produced depends on i. In addition if the current is also alternating in nature then both the

Version 2 EE IIT, Kharagpur

magnitude of the flux and its direction will change in time. The magnetic material is now therefore subjected to a time varying field instead of steady constant field with d.c excitation. Let:

The exciting current i(t) =Imax sin ωt
Assuming linearity, flux density B(t)	=		μ0 μr H(t)
=		μ0 μr Nil
=		sin max0rN Iωtμ μl
∴ B(t)	=		Bmax sin ωt