Frequency dependence of loss-improvement of grain oriented silicon steels by laser scribing

ELSEVIER

Journal of Magnetism and Magnetic Materials 133 (1994) 177-179

journalof magnetism ~ l ~ and magnetic ~ l ~ materials

Frequency dependence of loss-improvement of grain oriented silicon steels by laser scribing B. Weidenfeller *, W. Riehemann Institut fiir Werkstoffkunde und Werkstofftechnik Agricolastr. 6, D-38678 Clausthal-Zellerfeld, Germany

Abstract Dynamical hysteresis loops of unscribed and laser scribed grain oriented silicon steels were measured in the frequency range from 0.05 to 500 Hz for polarizations from 1.4 to 1.7 T to determine frequency dependent loss improvement. For low frequencies the plot of fractional loss change show increased power losses due to laser scribing. However by increasing the frequency the fractional loss change is decreased until an optimum in relative loss reduction is reached, dependent on laser treatment. The loss change due to laser scribing depending on frequency can be explained by the assumption that the number of movable domains is increased by the same amount for all frequencies.

1. Introduction

2. Experimental

Several techniques to reduce power losses by introducing surface defects into silicon steel sheets have been developed (e.g. [1-5]). One of these techniques is the well known laser scribing to refine magnetic domains. The induced surface defects lead to an increase of hysteresis loss while the dynamic losses are decreased. For optimum improvement of the power losses one has to find, for a certain kind of electrical steel, a certain amplitude of polarization and a certain frequency, a special kind of surface defect as well as its arrangement and density. The measurement and description of the relationship between power loss improvement and these variables should lead to a progress in understanding of the mechanisms responsible for the variation of the power losses and to further improvement in loss reduction. In this contribution the dependence of frequency on loss improvement is discussed.

Dynamic hysteresis loops of unscribed and scribed grain oriented 3.2 wt% silicon electrical steel sheets with a thickness of 230 Ixm and coated with forsterite (B 8 = 1.945 + 0.003 T, ORSI H, EBG, Gelsenkirchen, Germany) have been measured in the range of 0.05 Hz to 500 Hz for nineteen different frequencies and the polarizations 1.4 T, 1.5 T, 1.6 T and 1.7 T using Epstein frames and a computer controlled device [6]. From the dynamic hysteresis loops the dynamic and hysteresis losses, the dynamic coercive field, anomality factors and the fractional loss improvement and number of domains have been evaluated. The static domain configuration was observed using a decoration technique [7]. Scribing has been done with a N d : Y A G laser (JK-700, Lumonics) of 300 W average power for various pulse intensities and distances of laser points, while the diameter of the focal point of laser light on the sample surface was kept constant at 50 Ixm.

3. Results

* Corresponding author.

The effect of laser treatment on the electrical steel sheets was observed optically and with SEM. Loss

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178

B. Weidenfeller, W. Riehemann / Journal of Magnetism and Magnetic Materials 133 (1994) 177-179 020

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Fig. l. Domain pattern in the vicinity of laser lines in static case observed with decoration technique.

improvements were obtained for light intensities of about 104 W / c m 2 when no influence of laser scribing could be detected in the forsterite coating or the coating was partly destroyed but the FeSi matrix material was still coated. A typical domain pattern of scribed and unscribed areas of a sheet can be seen in Fig. 1. The fractional loss change A p / p = p ~ / p 1 dependent on frequency is shown by Fig. 2. P and Psc~ are the total losses without and with scribing respectively. 2~P/P

Ph.scr + r/scr Pcl

1.

Ph + nP~,

(1)

Ph is the hysteresis loss which has been estimated by interpolation of losses P ( f - - , 0). rl is the anomaly

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Fig. 2. Frequency dependent fractional loss improvement at 1.4 T amplitude of polarization for various scribing conditions. Symbols represent measured values, solid and broken lines are calculated by Eq. (1).

.

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_2

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Fig. 3. Frequency dependent fractional loss improvement at various amplitudes of polarization for a fixed distance of laser lines. Symbols represent measured values, solid and broken lines are calculated by Eq. (1).

factor and Pd is the so-called classical loss:

Pc, = ( 2 k d J o f ) Z / ( 3 P ) •

(2)

k is the form factor of the induced signal and was calculated numerically using the form of the induced signal. The index scr in Eq. (1) is used for the corresponding values of the scribed sheets. The measured values of losses were corrected to k = 1 by dividing Pdyn by the third root of k. This has been found empirically for the used sheets varying k in the frequency region from 10 to 500 Hz by the use of sinusoidal, triangular and square shaped field and induction signals. For low frequencies the curves of the fractional loss indicate increasing power losses due to higher hysteresis losses in the scribed sheets. With increasing frequency the fractional loss change runs through a minimum, where the largest improvement of losses appears. Though domain refining is relatively small a loss improvement of 8% can be observed at a frequency of 60 Hz. The position of the minimum in the frequency range between 10 and 100 Hz depends not only on the scribing parameters but also on the amplitude of polarization which can be seen in Fig. 3. For higher amplitudes of polarization the minimum is moved to lower frequencies and becomes deeper. Herewith the fractional change of hysteresis loss which equals the fractional loss change at low frequencies also decreases. An cquivalent result we observed for stronger defects. Due to the higher hysteresis losses the optimum distance of laser lines increases in that case.

B. Weidenfeller, W. Riehemann /Journal of Magnetism and Magnetic Materials 133 (1994) 177-179 4. Discussion

As can be seen in Fig. 4 the frequency dependence of "O can be described by

(no,

f
"q = { ' q o ( f / f o ) - * ,

f ~ f <~f(rl = 1.63),

I

~ 1.63,

(3)

f ( ' o = 163) < f .

f0 is evaluated by linear extrapolation of the two first frequency regions. Using the points in the range 1-100 Hz for linear regression % and x have been determined. Assuming that the behaviour of the nucleation of new domain walls with increasing frequency remains unchanged and only their number is increased, % is decreased by the scribing process to r/0,scr and x is decreased to Xscr in that way that both straight lines In r/0n f ) and In ~Tscr(lnf ) meet at the lowest anomaly factor that is possible. This should be near a = d and can be estimated to be 1.63 according to [9]. For higher frequencies -q is constant. This leads to x r/~ . . . . 1.63 Xscr - x "O0,scr"

(4)

~70,scr has been estimated by averaging the values of rt in the frequency region up to 1 Hz where "O is nearly constant [8]. Xs~~ has been evaluated by Eq. (4). The agreement between the experimentally determined and the calculated (Eq. (3)) anomaly factors is good for lower frequencies up to 100 Hz (Fig. 4). For higher frequencies a systematically increasing deviation occurs with frequency. The experimentally found values are lower than the calculated ones. Nevertheless, if the constants "00, "00,s~r, x and f0 determined by the fit in Fig. 4 and Xscr determined by Eq. (4) are used to describe the loss change in Eq. (1) the agreement between measured and calculated values is perfect within the scatter of experimental points for various 10

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19

179

laser parameters (Fig. 2) as well as for various polarizations (Fig. 3). This is due to the difference between the anomaly factors in Eq. (1) playing a crucial role, more than the absolute values. The anomaly factors determined by the mean domain wall distance of Fig. 1 ( % = 4.45; r/o,scr = 3.70) are lower than the anomaly factors for the static case ( % = 9.60; rt0,scr = 6.64) because not all visible domain walls are movable at low frequencies. With Ph and Ph,scr only five parameters are needed to describe the fractional loss improvement with Eq. (1). Ph, % and x are needed to characterize the behaviour of the unscribed electrical sheets. So only Ph,scr and %,scr are used to describe the frequency dependent loss change due to laser scribing.

5. Conclusions

The loss change by laser scribing of coated grain oriented electrical steel sheets depending on frequency can be quantitatively described by simple assumptions: The domain refinement leads to an increase of the number of domain walls and therefore to a decrease of the anomaly factor for low frequencies. The increase of the number of moving domain walls with increasing frequency can be described by straight lines in a logarithmic scaled plot of the anomaly factor versus frequency for unscribed as well as for laser scribed electrical steel sheets. These straight lines meet at high frequencies at "O = 1.63. Knowing the properties of the unscribed material the frequency dependent loss change caused by laser scribing can be calculated by the increase of hysteresis loss and the decrease of the anomaly factor.

Acknowledgements. This work was supported by the Deutsche Forschungsgemeinschaft. The authors wish to thank Dr. M. Hastenrath, EBG, Gelsenkirchen, for providing us with grain oriented silicon steel sheets.

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"x_ . . . . . .

scribed not ........

treated i 1.00

........

i 10.00

........

i 100.00

........

i 1000.0

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FREQUENCY (Hz)

Fig. 4. Frequency dependence of anomaly factor at 1.4 T amplitude of polarization for a scribed and unscribed silicon steel sheet.

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