TIME-OPTIMAL MAGNETIZATION OF INDUCTORS WITH PERMANENT MAGNET CORES D. Nedeljkovic, R. Fišer, V. Ambrožič Univerza v Ljubljani, Fakulteta za elektrotehniko, Ljubljana, Slovenija Keywords: magnetization processes, pulse magnetizing devices, permanent magnets, ferrites Abstract: Time-optimal accurate magnetization process for small magnetic cores in mass-production is presented. The procedure consists of magnetization to the saturation level, followed by optimal partial demagnetization, which sets the stable operating point of a magnet within required inductance tolerance (< 3 %). The basic topology of a pulse magnetizer/demagnetizer is described and some improvements in algorithm to calculate optimal demagnetization voltage are suggested. Thus, proper magnetization of a core can be achieved in less than 4 s per piece. Additionally, the production waste is drastically reduced. Časovno-optimalno magnetenje dušilk z jedrom iz trajnega magneta Kjučne besede: magnetilni postopl — with co LC^' 6 = r= 2L' CO (1) (2) (3, 4) (5, 6) When the "magnetizing" capacitor voltage Is at its reference value UcMo, charging is stopped and the charger is disconnected. Thyristor Tm is triggered, allowing the current II to flow through the magnetizing inductor L, in which the permanent magnet is placed, and the diode D. The aperiodic transient is shown in Fig. 5. Load current reaches its maximal value at t = 7/4: T U J =i (t hit ^^ 1 C 4 (7) Charger unit Cm — > ucm z f K "v L R iL Fig. 4. Principal magnetization circuitry. 'i(A) 0,0 i 1,0 2,0 3,0 4,0 5,0 m /(ms) Fig. 5. Current pulse for magnetization Fig. 6. Principai demagnetization circuitry. 't(A) A \ A \ V " y ^ ((ms) is controlled through its voltage. Therefore the capacitor with lower capacitance can store the same amount of energy at higher voltage, thus enabling wider voltage range with better precision. Consequently, frequencies and time constants (3, 5, 6) are different for demagnetization, where capacitance Co has to be considered before applying their values In (8). Magnetizing inductor is nevertheless the same for both actions. 3. Time-optimal magnetization procedure From Fig. 8 it is evident, that magnetic properties of magnets, made of the same material and with the same required dimensions, can differ significantly. Demagnetization curves for several linearity correctors of the same type were measured through pulse demagnetization. Magnets were magnetized to the saturation level and then gradually demagnetized by increasing the applied capacitor voltage. As it can be seen, the reference inductance L can be achieved by applying very different demagnetization voltages. Obviously the capacitor voltage, that would properly demagnetize the particular permanent magnet, has to be determined for each single piece separately. 150 200 250 300 350 Demagnetization voitage uo{v} Fig. 7. Current pulse for demagnetization. Fig. 8. Demagnetization characteristics for several permanent magnets of the same type. For demagnetization, dumped periodic transient can be used and applied by circuitry from Fig. 6. After charging the "demagnetizing" capacitor Co to the desired value Ucdo, the charger is disconnected and thyrlstorsToi and jd2 are triggered simultaneously, resulting in a current transient, shown in Fig. 7: coZ, e ^' sin(o) t) (8) The same charger unit can be utilized for both magnetization and demagnetization. Due to the process requirement, that the magnetization must always reach the saturation level, while the demagnetization should be executed partially and more precisely, it is reasonable to use two separate capacitors. Namely, the energy, stored In a capacitor. I 5? 5 100 g 80 C <0 ^ 60-"D ^ ■•o S 20 Silifted sampie curve Actual (unknown) curve V' /1 Sample curve l' witti toleranco range tC) /i' \ t 1 t' ~ ■ AUo(n) © © © 0'- 50 \ UM 150/ / 200 Udi 250 um uo,{ri) uum1) Demagnetization voitage UoiV) Fig. 9. Determination of demagnetization voltage. ® Magnetization to tlie saturation level T n:=0 %(0):=0 i(0) measurement at current iyo(H+l):= UosinyAUoin) ® Uoin+l) adaptation (if assigned) n-.=n+l ® Demagnetize applying up{n) [i.e. Uoin+i) from step 5] Yes L measurements at other currents Ic Fig. 10. Basic steps of magnetization procedure. It is possible to acliieve the reference inductance L through several consecutive demagnetizations, starting from saturated magnet, by increasing the capacitor voltage in small steps. But this would result in numerous demagnetization steps, which would require too much time. Ideally, there should be only one demagnetization step, since the speed is paramount. To provide an optimal number of demagnetization steps, it is reasonable to measure the demagnetization curve for a sample (oran average curve for several samples), which is selected randomly among the magnets from the same batch. The form of this sample curve is then used to determine suitable demagnetization voltages for all individual magnets from the batch. The recursive principle is explained in Fig. 9 and Fig. 10, as follows: After the magnet is beforehand magnetized to the saturation and then par- tially demagnetized by demagnetization voltage Uoin) (sign a in Fig. 9, step 7 in Fig. 10), its inductance L{n) is measured (sign b, step 8) and its approximate relation to the sample curve can be established accordingly. Unknown demagnetization curve can be treated like a shifted sample curve (dashed), with the shift being estimated from the measured inductance of a magnet. Namely, the sample demagnetization curve reaches the same measured inductance L{n) (sign c) at demagnetization voltage Uosin) (sign d), which is for AL/D(n) higherthan the voltage Uoin). The same voltage difference MJoin) can be assumed at the reference inductance (sign e), i.e., the voltage, that has to be applied to this magnet, is for AUoin) lower than the voltage Uds, which provided demagnetization of the sample in orderte reach the reference inductance L at reference control current lc_rei. The new demagnetization voltage, which can provide proper demagnetization of this magnet, is now Uoin+D (sign f in Fig. 9, step 4 in Fig. 10). Although the actual curve does not match the "shifted" sample curve entirely, the inductance after the demagnetization with voltage Uoin+I) would be set within required limits (sign g). The most important is the demagnetization voltage Uoin) = L/d(1 ), which has to be applied for first demagnetization step. In the best case, this demagnetization should result with an inductance within tolerances of its reference value. Therefore, the above-described principle could be used directly after the magnetization to the saturation level (step 1 in Fig. 10); in this case the demagnetization voltage L/D(n) = UoiO) that is used in further calculation, is zero (step 2), i.e., only inductance L(0) after magnetization is measured (step 3). This approach gives excellent performance on magnets whose characteristics are close enough to the measured sample curve, because only one demagnetization is needed. In practice this condition cannot be assured, so undesired excessive demagnetizations can appear, i.e. the inductance L can exceed its reference value L . Consequently, new magnetization is needed, but with some magnetizing devices, which require longer time to charge magnetizing capacitor, this has to be avoided. The solution towards is to apply 75 % of voltage Uds for the first demagnetization, (step 5 in Fig. 10) thus avoiding the excessive demagnetizations for the expected range of magnets. 4. Conclusion The magnetizing procedure, described in this paper, was applied in mass production of linearity correctors with very good results. The obtained total time for the magnetization to the reference point was bellow 4 s. Beside the improved accuracy of the magnet's operating point, the production waste was significantly reduced. References /1/ D. Nedeljkovic, V. Ambrozio, J. Nastran, "An Improved Calibration of Ferrite Permanent Magnets in Mass-production," IEEE MELECON 2002 Proceedings, pp. 244-248, Cairo, Egypt, May 2002. /2/ p. Campbell, Permanent Magnet Materials and Their Application, Cambridge University Press, 1996. /3/ E. Steingroever, Magnetizing, Demagnetizing and Calibration of Permanent Magnet Systems, Magnet-Physik Brochure, Mag-net-Physik Dr. Steingroever GmbH, Köln, Germany, 1988. /4/ E. Spahn, G. Buderer, J. Wey, V. Wegner, F. Jamet, "The Use of Thyristors as Main Switches in EML Applications", IEEE Trans-actionson Magnetics,Vol.29,No. 1,pp. 1060-1065, January 1993. doc. dr. Rastl