LESSON 4. Hysteresis and eddy current losses

Hysteresis

As a varying voltage is applied across some windings, a time varying current will flow as starting from 0 to complete its sinusoidal cycle (Fig. 2.5). The magnetization that it will form is known as hysteresis loop (Fig. 2.6).

Fig 2.5

Fig. 2.5 Magnetic circuit applied with a varying voltage

Fig 2.6

Fig. 2.6 Formation of hysteresis loop

• Initially unmagnetized – o

• i and H increased slowly – oa

• i and H removed : br residual flux density

• H reversed to –hc (coercivity): flux density vanishes

• During first period B-H curve will follow the bath oacdefga’ - the loop does not close

Fig 2.7

Fig. 2.7 Locus of the B-H curve

• The locus of the tip of the hysteresis loop is called the magnetization curve (Fig. 2.7)

• After few periods the loop almost closes the hysteresis loop (Fig. 2.8)

• B-H relation is nonlinear and multi- valued

• The B lags behind H

Fig 2.8

Fig. 2.8 Complete hysteresis loop

Hysteresis losses occur when the flux changes continuously both in value and direction. The magnetic material absorbs energy each cycle and dissipates it as heat.   To reduce hysteresis losses, magnetic materials are selected that have a narrow hysteresis loop. The loss of power in the core due to the hysteresis effect is called hysteresis loss

As electric circuits have losses due to its resistance and flow of current, the magnetic circuits also have losses generated, by the varying action of flux and induced voltages, in cores due to magnetic field, namely hysteresis and Eddy current losses.

Eddy current loss

The other type of losses, that occurs in magnetic circuits, is the eddy current loss.

Time varying magnetic field induces eddy currents in conducting material Eddy currents occur when AC voltages are induced in a conductor by a changing magnetic field.

Eddy currents dissipate power as resistive losses in the conductor

To reduce eddy current losses, magnetic materials are laminated (for a given core size, eddy current losses decrease in proportion to the square of the number of laminations).

Fig 2.9

Fig. 2.9 Eddy current loss

A power loss proportional to Ri2 will be caused.

Eddy current loss can be reduced by:

– increasing the resistivity of the core material

– using laminated cores

Adapted from EE360 course on Electric Energy Engineering, KFUPM Open Courseware

Last modified: Tuesday, 1 October 2013, 7:05 AM