Converter for measurement of non-sinusoidal current peak value

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Converter for measurement of non-sinusoidai current peak value P. Butvin ~.*, B. Butvinovfi u O.V. Nielsen h p. Brauer h "* btsti/me el Plo'~ics SASc, Dt~hrcrskCti t',~ta 9, 842 28 Broil.elate. Sloi~ukia h btstttutC o/Azltamad.~z, The Tcdmkal/.h,dersiO" o] !Octtlttark,DK.2?¢O0Lrngbv, Demmlrk

Abstract A linear-response toroid with core wound of rapidly quenched ,.,oft magnetic metallic ribbon and titted with two windings is used to enable correct measurement of mean peak value of non->inusoidal and not noise-free alternating current, The time derivative of the current fed through the primary winding is obtained nit the ~condmy of the sensor (or peak ,~ahte converter. PVC), Evaluating the a','erag¢ value of this fi~,nal recovers the peak value of the primary current, A good quality average responding voltmeter is then used instead of peak-capture circuits which are often more susceptible to interferences and errors due In the superimposed noise. Certain advantages of using such sensor-converter as smoothing, amplification etc, us well as some drawbacks as possible distortion or peak intensification of the signal ~re discussed in some detail. Cores with characteristics linearized by different methods (e,g. field or stress anneal) are compared as to their PVC performan,:e, :O 1997 Elsevier Science S,A. Keyword~:Current peak-value measurement; Linear response magnetic cores: Rapidly quenched soft-magneticmaterials

1. Introduction The variation of the amplitude permeability with the intensity of the excitation field is frequently used to characterize soft-magnetic materials. Except 'linear' materials, the request of sinusotdal flux condition inevitably leans to the need of measuring the actual peak value of the intensity {i,e. of non-sinusoidal exciting current). Exploiting a core with linear characteristic simplifies the job significantly if compared to the use of a boxcar averager or a standard peak capturing circulhy. Such a core with windings connected as seen in Fig. I serves as a peak value converter, PVC. The background is really simple (also exploited earlier, using rather huge air-core toroidal windings). The arerag¢ value of the induced voltage U~ is a measure of the peak value of the exciting current I since Bp = yHp =/IN~Ip and recalling that U , ( t ) = - N : S d B / d t we get I/' T ~l~U, t t ) d t <_2JNzSB p _,, (it is the constant permeability of PVC, N~,, and S are its primary, secondary tvrns and cross section respectively, 1 is the frequency) as argued in many basic texts [1]. The integral is a standard device function for full-wave rectifying average responding voltmeters. It equals l.L,~g= 4fN,.SBp where* Corresponding author. [1924-4247;97;S17.flO~) 1997 Elsevier ScienceS.A. All rights reserved. PII S0924-4247(97J014'~a.4

from

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for the common antisymmetric waveforms, It is instructive to use the possibility to express also more complicated (periodic) induced voltage wavcforms by Fourier trigonometric series and the above concept can he checked by performing the integration of the series. The use of a boxcar averager checking the voltage drop on a series resistor necessitates an auxiliary sense/delay circuit for the sample to be correctly positioned on the peak. It is because the peak of the current shifts signifi-

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Fig. I Siinplilie6eii"euttdiagram of the seiap tbr amplitude penneability measurement.TUT is the +toroidunder test++w h o s e I+,,vs, H variation is to be determined. The excitation branclt comprises: fur.:.:tiongenerator F(;. low output imp'dance power amplilier AMP, series resistor ar,d the PVC. Measuring brunches:a'.'g-valuevoltmeter aVB, selective voltmeter spV and Ih¢ avg-value voltmeter aVH to measure the output of PVC. Optim|ally, an elc~troni,zhaegnalw and a digital st,,)rag~oscilloscopeare also connected.

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Table I The hlrokls tested f,r [he fiinclion of PVC ton~id Tornid #

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tautly with respect to the zero crossings of the readily available signals (used to trigger the sampling circuitry) as the peak excitation level changes and the current becomes non-sinusoidal. The standard risk of using simple peak capturing circuitry (diode blocked capacitor charging) is the superposition of noise peaks of the currently checked polarity.

2. Experimental and results The most distinctive test can he perlormed by peaked sigmds quickly oh:raging with the excitation level. Such conditions are obtained by inserting a high permeability, rather square loop toroid as TUT. All the presented results are obtained using nanncrystallinc Ferc~,~Nb4.~CuiBxSi,, in this position. The low output impedance power amplifier AMP is dri~:n by sinusoidal voltage with continuously variable level. Each measuring run is preceded by a.c. neutralisation (continuous decline) followed by 5 min pause. Low, reliably sinusoidal currents are me',tsurcd by selective pV-meter, above 0,4 mA a boxcar averager measures both current maxima to have the reference level of I~. A frequency of 217 Hz has been used to obtain all the results presented. Nevertheless the variation of frequency was tested too, showing no unpredictable results• We tested the PVC performance of four rather different cores wound or" rapidly quenched ribbons. The size of the toroids differs significantly as it tbl!,ws rather the genuine width of the ribbons used amJ the demand of having wide spread of inserted ~aductivity with comparable windings. Cutting the width by simple means has been lbund a risky operation (unrecoverable phtstic strain is possible), The "gain' is the ~,~g/I e ratio according to Eq. 11). Useful range has ,,he lower limit controlled by noise and lincarity whereas the rpper limil depends essentially on approaching the saturation of PVC's core. The measured response of PVC progressively diverges from the correct value by more than 5% off the limits, The nonlinearity of PVC rules the error within the liqlits although for toroid #4 only it controls the very limits (Table I). This toroid is the most interesting one from the material viewpoint: to our knowledge, no other rapidly

quenched material has boca reported to show s~h a linearization of the B vs. H characteristic when no field and no :aress anneal are applied. The toreid is wound of I0 mm wide Fer~Nb~CuiBl~.~Si4.~ ribbon under less than 2 MPa tensile stress, annealed half an hour at 510°C {then partly nanocrystalline), Toroid # 3 makes u.~ of another interesting property which i~ encountered at planar-flow cast ribbons--the as-cast anelastic strain re:~ahing in pronounced hard ribbon axis magnetic anisotropy for this Co-rich alloy. Toroid # 2 is a member of fluxgate ~ n ~ r cores family [2]. It has b~en cho~n for this work ft:r its outstanding linearity, low noise :tad neghgible influence on the exciting branch of the measuring circuit. The tiny cross section limits the "gain" and so pulls up the lower limit of its useful range as a resuh of not suppre~ing the c:.nrent noi~ of the amplifier and not surpassing the noise which affects the measuring branch. Toroid # I is made of an encapsulated core commercially available ITem Vacuumsehmdz¢ Hanau GmbH. The characteristics of the tornid cores are shown in Fig. 2. the resulting characteristics of TUT core measured with different PVCs is shown in Fig. 3. The nonlinearity error comes from assuming uncorrected (constantl "gain', i.e. constant tt of the PVC core throughout the useful range. Fig. ,1.

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Fig 2,/,, ~s. II ~:~riation~of the PVC Lx~res,Note tha: PVC toroids ha~e different ~indln~x to ma~,c the I~sl use of the core A 8 swing. tolcnzbI¢ nonlillCdril)', tic, It mcar~s th;tt the restllting current ~a]t,~ of the IWC~ differ I~'0ra each other.

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Fig, 3. It,. ~s. II :ariation uf TIlT measuredusing different PVCs, There are no discernibletncviatiousfrom rdi:r,meeVUlllgSIbr h~r(~id'.i # 2 alld # I. thus {1111~,Ihe ' result ,,hlUJllc'tlv,ith # I is displa,,xl a,. In~:correct 11U~,]'h¢ current below 1h¢ a~ful range i~ measuredhy selecli'.c/~-voltmelerand Ihe cur','cs;ire ploned to show a standard. ul)l PVC-depcndcut irrcpn~ducibility If numeric lin~arizutiun r,ax u,led, not only lhe de;'ialious would ealleel bul the range of # 4 ~ould spread notably, exphfiliugits large ~B swiug,

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Discussion

] h e use of PVC greatly simplifies the meusurement of non-siuusoidal, noisy current peak value. Namely as It part of an automated tq vs. H measuring setup, it provides three more advantages: it is capable of suppressing high frequency noise in the exciting branch, it Call amplify tile signal of interest, and it can be exploited fo~ sensitive detection of many of exciting current unsymmetries. Possible nonlinearity of PVC is not critical -.it is almost ~dways possible to choose a core sa:isfying partieuhtr demands or to use a correction table h~ computer-controlled setups (this has delibcralely not been done in this work). The major drawback is the requirement of tolerating large crest factors which is put on the voltmeter measuring ~he output of PVC. Here a compromise bet~eeu damping the current steep edges by PVC's inserted inductivity and keeping the exciting current genuine can be ibund. No materi'-d-spe-

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demonslr,'~tes the infltnence of the inserted inductivity illustrating also the risk of ovedoading the aVH voltmeter by high peaks if the induc;ivity is not used to damp the sharp edges of exciting current.

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Fig...1. (a] Exeitiugcurrent aud PV("outpttt as n'¢et)rded~.',hcntt)roid # 3 1270trill i~, used a~; PV(' Tile current i:.,iullueu,:'ed line edges damped, but uo problemsarke tilr the voltmeleraVH, Ib) The same as abo'~ebut for toroid # 2 (2,8 ~.tH).The IWC output is horiztmtallv mirrt~red fur the Ihiu p:uks uot to be t~bscured,'111¢prououueed pc,d::- [i:niz Ihe asd'ul range el' vohmder ~JVll.

cific handicap (after.effects. additional noise ~,cueration etcj is observed, mainly as a merit of the dominant hard ribbon axis magnetic anisotropy. No differences which could be due to different materials and methods to achieve this anisotropy 'are detccled in the ~::'r~' pertbrmance, except nonlinearity.

Refereuces [I] J.K, Wal~;uu, ,Ippliuath,t: ,I ~fitvm'fi,~m. Wile)', No'.'. York, 19~O. p, ~IL [2] O,'¢, Nidseu. P, Brauer, F, I'ritadahL I. Ri~bo. J.L. Jor~nscn,

('. Boo. M. Deycrlcrand S. Iluuerci:a.'n..~high-pl'ecisitlntriashd Ihl~,~ate set!~or I'~)¢ slUiCe applic:di~m'~:layout and choice of materials. ,'h,u.~,r.~mul¢|th,zh.r,~. :f5')llt~)71 168 176,