Mark Krinker, City College of Technology, Department of Electrical Engineering and Telecommunication Technology, CUNY, New York

Transcription

1 Mark Krinkr, ity ollg of Tchnology, Dpartmnt of Elctrical Enginring and Tlcommunication Tchnology, UNY, Nw York Nw On-Prob Mthod and Instrumnts for Instant In-ircuit Masuring Quality of apacitors. Infinitly Rmot Point oncption - Physical Bas of th Mthod. Unlik wll-known In-ircuit mthods, basd on injction th probing currnt through th following pass: Mtr-first prob-first trminal of capacitor-plat-dilctric-plat-scond trminal of capacitor-scond prob of th mtr-th mtr, th nw Mthod is basd on th principally nw approach. According to it, th probing currnt flows through th following circuit: Mtr-th prob-nvlop of capacitor- intrnal contnt-any trminal-conducting nvironmnt of th trminal. Physical bas and appropriat quations wr st forth in USA Patnt 6, 98,90, March 00. Thr is no scond prob. Th probing currnt is formd du to polarization of conducting nvironmnt associatd with trminal of th capacitor as wll as displacmnt currnts. Th dvlopmnt of physical bas of th Mthod brought to origination of th Mthod of Infinitly Rmot Point. Th Mthod and instrumnts wr succssfully tstd at NASA tsting facility in summr of 00. In lctronic quipmnt, up to 60% of malfunctions ar dirctly or indirctly rlatd to capacitors. Exploitation of th Mthod and its instrumnts mad tsting capacitor almost instant and drastically dcrasd tim of rpair of lctronic quipmnt. Th Mthod is capabl to tst capacitors in powrd circuits. Th papr considrs physical bas of th Mthods in dtails.. Gnral oncption of th Mthod Thr ar two vrsions of th mthod. Thy hav on common fatur: a probing currnt is bing injctd through th jackt of th capacitor and thn passs through th stuffing of th capacitor, collcting th information about its quality [,].. Two-Trminal Vrsion of th Mthod Th nw mthod to tst capacitors in a circuit is basd on rplacmnt of th convntional way of flowing altrnating masurmnt currnt through th circuit lik mtr-trminal of capacitor-plat-dilctric-anothr plat-trminal-mtr for a nw approach: mtr trminal of capacitor plat dilctric nvlop of capacitor

2 masurmnt prob mtr. This allows avoiding influnc of any circuit connctd to capacitor in paralll and maks dirct masurmnt in a circuit possibl. This vrsion mploys two-trminal masurmnt instrumnt. Fig. illustrats this approach. W hav to not that shown in Fig. implis capacitors with a mtal jackt. This can b both lctrolytic and non-lctrolytic ons. For this purposs, a tip-lik prob can b mployd. Mtal surfac of th jackt plays a rol of a masurmnt lctrod. For capacitors with an insulating jackt, th prob has a dvlopd conductiv surfac as this was shown in th Patnt. Said surfac initiats a displacmnt currnt, which probs th mdia insid th capacitor. Fig. Employing Two-Trminal vrsion of th Mthod for in-circuit tsting capacitors. Masurmnt currnt is injctd through th mtal jackt and thn passs through th body of th capacitor. For th insulting jackt capacitors, th uppr prob has a dvlopd conductiv surfac to initiat th displacmnt currnt.. On-Trminal Vrsion of th Mthod Anothr vrsion of th mthod dos not us apparnt closd circuit of passing masurmnt currnt at all. Instad of that, masurmnt currnt passs th following way: mtr masurmnt lctrod nvlop of capacitor dilctric plat trminal of capacitor total lctric capacitanc of all conductiv nvironmnt, coupld to th trminal. This nw approach nabld to crat principally nw, onlctrod masurmnt mthod and instrumnts. Th natur of formation of masurmnt probing currnt is a displacmnt currnt causd by polarization procsss both in th intr-plat dilctric and said total conductiv nvironmnt. This vrsion mploys on-trminal masurmnt instrumnt. Fig. illustrats On-Trminal vrsion of th Mthod.

3 Fig.. On-Trminal vrsion of th Mthod. Lik Fig., this illustration implis th conductiv jackt capacitors. For th insulatd jackt capacitors, th prob has a dvlopd conductiv surfac..3.adquacy of th Mthod To b succssful, nw mthod has to b adquat with th commonly accptd. This is furnishd by th following: Both th losing and saving nrgy in capacitors dpnd on physical procsss in a mdia insid th body of capacitor bing tstd. onsquntly, disrgarding th way of passag probing currnt through th capacitor, a lost-to-savd nrgy ratio (i.. tangnt of dilctric loss angl, tgδ/ δ/q) rmains th sam. δ/ Antion: Th following lowrcas indxs will b accptd hr and furthr for th capacitanc and th tangnt of dilctric loss angl tgδtanδ: - trminal-to-trminal; t- nvlop-to-trminal; - nvironmntal, that is an lctric charactristic of th two-trminal whr th probing currnt is formd by a polarization of th associatd circuitry; ff- ffctiv valu, which rprsnts th total paramtr of a combind circuitry. Th first mntiond approach is ralizd by mans of applying A voltag across nvlop of capacitor and its ithr trminal. This crats a probing currnt, whos charactristics dpnd on physical procsss insid th capacitor bing tstd. Physical proprtis of mar don t dpnd on mthod and instrumnt with which w masur thm. Following from th statd abov, tgδ tgδ t () whr mans classical trminal-to-trminal mthod, whil t mans th nw,

4 nvlop-to-trminal approach. Following from th quity of dilctric losss ratios, with rgard to tgδ ωr, w hav R R (a) In particular, th masurmnt instrumnt, shown in Fig. can masurs a phas shift α btwn currnt and voltag across capacitor. In this cas, taking into considration o that α + δ 90, a scal of instrumnt can b calibratd ithr in tg δ or in its invrt Q/tgδ, which rprsnts th quality of capacitor..4 Employing th Mthod for ombind Tsting Inductors and apacitors. Th approach, ralizd in th shown masurmnt instrumnt, is also applicabl to incircuit tsting inductors. Prsnc of short turns in inductor causs incras of tgδ. Both in-circuit capacitors and inductors tsting wr mbodid in L-Mat, th pilot instrumnt, which can dtct both dfctiv capacitors and inductors with no switching functions..5 Elctric Equivalnt of On-Prob Vrsion of th Mthod. Infinitly Rmot Point. In On-Prob concption, th displacmnt currnt to prob capacitors is formd du to involvmnt of total conductiv surfac of circuitry nvironmnt. Fig.3 illustrats this. t t Fig.3 Gnral Elctric Equivalnt of On-Prob Mthod for tsting capacitors mploying a total conductiv surfac for forming th displacmnt probing currnt. Said conductiv surfac can b formd by a chassis and/or total surfac of intrconnctd circuitry. Fig.3 shows a gnral lctric prsntation of th unit diagram of th Fig.3. Th masurmnt instrumnt compriss A voltag gnrator and a mtr. Th mtr is a gnralization of any unit, which masurs ithr of A charactristics.

5 Th masurmnt instrumnt forms A lctric fild insid th capacitor. Du to a dvlopd conductiv surfac, coupld to th capacitor, said A lctric fild forms th displacmnt currnt, which altrnativly chargs and thn dischargs th conductiv surfac. This currnt flows through th capacitor, rprsntd by connctd in sris and R, prforming a probing function. Th instrumnt analyzs charactristics of th currnt and convrts thm into an output rading. Injcting probing currnt insid th capacitor can b don by two ways. First of thm is a touching mtal nvlop of th capacitor with a conductiv tip connctd to th masurmnt instrumnt, lik it s shown in Fig.3. This way works goodly for lctrolytic and som typs of non-lctrolytic capacitors. For capacitors with a non-conductiv nvlop, th prob has to hav a dvlopd conductiv surfac as this was shown in th patnt []. Th total conductiv surfac of th Fig.3 consists of two surfacs: total zro-potntial surfac of th masurmnt instrumnt and th total conductiv surfac of th tstd unit. As follows from said abov, said vrsion is an On-Prob Mthod. Equations of lctric circuits rquir a closd contour for calculations. In th smingly opn circuit of Fig.3 currnt nvrthlss flows du to th polarization procss. It looks lik th circuit joints in som fictional point, having som lctric potntial. Rmmbring a dfinition of lctric potntial at a givn point as a work, ndd to transfr a unit of a positiv charg from th infinity to th givn point, w call this point as an Infinitly Rmot Point. Fig.4 shows an lctric quivalnt of th Mthod, including th Infinitly Rmot Point. Fig.4. Elctric Equivalnt of th On-Prob, Infinitly Rmot Point Mthod of masuring quality of capacitors. apacitors and rprsnt conductiv surfacs of Fig.3, connctd at th Infinitly Rmot Point Th circuit consists of A voltag gnrator, which is coupld to R circuit of a masurmnt instrumnt and a mtal surfac, a trminal and a capacitor bing tstd, which is coupld to a chassis with its othr trminal. As applid to On-Trminal Mthod, th capacitor consists of intrnal rsistor Rt, markd as R in Fig.4, connctd in

6 sris with capacitanc t ( in Fig.4). In trms of lctrical quivalncy, both mtal surfacs can b shown as two capacitors and connctd in sris at an th infinitly rmot point P. As was said, th validity of this dfinition outcoms from dfinition of lctric potntial of a givn point: this is a work don by a systm to bring unit of a positiv charg from infinity to th givn point of fild. In th Mthod, nrgy of th gnrator movs lctric chargs back and forth through th capacitor from th prob to infinity, bcaus thr ar no a scond pol lik it taks plac in convntional circuits. This is why w also call th Mthod as an Infinitly Rmot Point Mthod. Our purpos is to stablish a corrlation btwn st of paramtrs of traditional mthods and that of th nw approach. Dilctric losss ratio of combind sris circuit, comprising an intrnal rsistor Rt and a total capacitanc of conductiv nvironmnt and that intrnal, can b shown as, tgδ ωr () bcaus << t. From hr, taking into considration (a), on can show that tg δ tgδ / (3), whr tgδ is a tangnt of a dilctric loss angl at on-trminal masurmnt, and is a total capacitanc of all conductiv nvironmnt to which th capacitor is coupld. Equation (3) is a vry important to th Mthod bcaus it bridgs btwn traditional approach and th nw on. Studying this mthod has shown that for majority of capacitors t~ uf, Tabl, whil capacitanc of mtal chassis rprsntd by vary within tns of pf. This allows us to us / as a known cofficint, which simplifis th problm. t Bcaus of what a masurmnt instrumnt rally rads is tgδ, its valu will b always lowr than tgδ so, a scal has to b calibratd with rgard to a ratio (3). Bcaus <<t, th major componnts which dfin th valu of tgδ ar R t and, tgδ ω R t Whil rmains invariabl within a tstd unit, dfind by a siz of capacitor at invariabl δ t t Rt dpnds on t t which, in turn, is vs. total surfac of tg. Tabl shows t capacitor S dpndnc. As sn from th tabl, for th majority of capacitors, variation of t rmains within th limitd rang of quantitis. Only for high capacitanc it diffrs as much as -4 tims, that can b compnsatd tchnically. Bcaus is dfind by a total capacitanc of th conductiv nvironmnt of th capacitor and capacitanc of th masurmnt instrumnt, thr is always a chanc to adjust th last on to compnsat.

7 Siz, r*h (radius by hight, mm) t,uf ompany Total S, mm^ t/s uf50v 6.5* Elit uF35V 6.5* Elna uF35V 6.5* GE uF50V 8* 0.86 Elna uF50V 0.5* Elna uF350V * JH uF00V.5* Elna uF00V 6* Elna uF50V 8* Elna Tabl. onvntional capacitanc of various capacitors and thir capacitancs btwn th nvlop and th trminal t, as wll as othr comparativ charactristics..6. ombind Sris-Paralll R-ircuit Formd by Rsidual and Extrnal Elmnts at Invariabl Frquncy Th instrumnts, working with fr oscillations can us xtrnal capacitanc as an lmnt of RL circuit. For practical purposs, w nd to calculat quivalnt (ffctiv) paramtrs of th combind circuit: ffctiv rsistanc Rff, capacitanc ff and tangnt of dilctric loss angl tgδff. In this Mthod, connction to th xtrnal circuitry incrass th total complx capacitanc of th systm. So, w hav to considr a paralll circuitry, analyzing its bhavior. Lts considr th circuit formd by rsidual capacitanc and its sris rsistanc R whn xtrnal R circuit is connctd, Fig. 5. Fig.5. Paralll circuitry in ralizing th Mthod. R rprsnts th instrumnt, whil R- th masurd capacitor.

8 Taking into considration that R tgδ / ω, a rsidual admianc of th lft portion of th circuits is ( tgδ + j) ω Y (4) tg δ + onncting xtrnal circuit R brings a paralll branch with th admianc ( tgδ + j) ω Y (5) tg δ + In fr oscillations vrsion of th masuring instrumnt only, on has to tak into considration that frquncy of fr oscillations gts lowr whn conncting R. ω ω Gnrally, o. As follows from that, tgδ0 not quals tgδ. Howvr, for th rlaxation typ of polarization, widly xisting in capacitors, and its scard dilctric loss band, tgδ rmains almost unchangabl in vicinity of ω0. Now, th total admianc is Y Y + Y (6) ( tg δ + )( tgδ + j) ω + ( tg δ + )( tgδ + j) ( tg δ + )( tg δ + ) Y (7) ω Hr, for convninc, tg δ + A and tg δ + B Taking into considration that Z / Y R j / ω on gts an ffctiv rsistanc of th combind circuit AB( Btgδ + Atgδ ) ( Btgδ + Atgδ ) + ( B A ) [ ] R ff ω + (8) Its ffctiv capacitanc And ffctiv tgδ: ( Btgδ + Atgδ ) + ( B + A ) AB( B A ) ff + (9)

9 tg R Btgδ + Atgδ δ ff ω ff ff (0) B + A.7. Spcial as of Low Dilctric Loss of Extrnal ircuitry. For th mthod, th cas of tg δ << has a spcial intrst bcaus, as it was said abov, in th On-Elctrod Mthod tgδ is drastically lss than that of tgδ. Hr, tg δ is tgδ. Bsid that, tgδ << for a good masurmnt systm. Taking this into considration, tgδ + tgδ tg ff + δ () tgδ + tgδ R ff ω + ( ) () And ff + (3) For dvloping On-Trminal Mthod, bhavior of Rff has a spcial valu. Within this mthod, is dfind mostly by capacitanc of conductiv nvironmnt, which is invariabl at last for tsting in on unit, whil is a rsidual capacitanc of th masurmnt instrumnt. onsquntly, Rff is th only (at th invariabl frquncy), which is rsponsibl for rading tgδ.7. ritical (Invrs) Point As sn from () and (), in som cass of low tg δ rsulting tgδ ff and R ff can b vn lowr than tg δ and tgδ / ω rspctivly. It s asily xplainabl bcaus contribution of capacitanc is a saving nrgy whil tg δ charactrizs its loss. So, if th systm gains its capacitanc whil th losss rmain low, it savs mor nrgy, i.. tgδ dcrass. Thr is com critical valu of xtrnal tgδ cr at which dissipation of nrgy dos not chang. This can b usd to mak a diffrntial scal whn thr is a critical point of tgδ cr in vicinity of which a ndl of a masurmnt instrumnt changs a dirction of dflction. If th tangnt of dilctric loss angl of an xtrnal circuit with th capacitor bing tstd quals tgδ, th loss of nrgy causd by imprfctivs of th circuit quals th cr

10 additional nrgy gaind by conncting th capacitor. A ndl of th masurmnt instrumnt stays at th rst whn th instrumnt rads th circuit with tgδ cr. Taking into account (), th following taks plac at a critical (invrs) point: tg tgδ + tgδ δ (4) + As it follows from (4), this taks plac at tgδ tgδ cr tgδ Lt s go to instrumnt-rlatd notations, whr is a rsidual capacitanc masurmnt instrumnt, tg δ is its rsidual tgδ r whil r of a is dfind by th capacitanc of th conductiv nvironmnt of th xtrnal circuit with th tstd capacitor. Th qustd - tg tgδ /. Taking into considration that th ratio of ths t capacitancs is a constant k at last within on tstd unit, tgδ ktgδ. From hr tgδ cr tgδ r (5) k R tg cr rcr (6) kωr For instanc, if th critical tg δ of th tstd capacitor is chosn 0 (which is ral in 3 lctrolytic capacitors) at k 0 (that is, th total capacitanc of a chassis is 00pF, whil th cross-capacitanc of th tstd capacitor t0.uf, thn tgδ 0. That is, in this particular cas, any tstd capacitor with tgδ lss than 0 causs th instrumnt to show a dcras of total dilctric loss, whil xtrnal tgδ mor than 0 will rsult in incras of th loss rading for th instrumnt with a rsidual tgδ 0.. Employing RL ircuits in Masuring Instrumnts Supporting th Mthod.. Basic oncptions of Instrumntal Approach to Implmntation of th Mthod. Fig.6. shows a simplifid application of On-Prob Mthod, ralizd on a bas of puls driv of L-circuit. Hr, x is a capacitanc btwn nvlop and th conducting chassis. Th pulss driv shock-xcitd damping oscillations, which dpnds on a procss of loss of nrgy in th tstd capacitor. rcr rcr

11 Fig.6. Th simplifid application of On-Prob Mthod, ralizd on a bas of th shock-xcitd damping oscillations. Hr, x is a capacitanc btwn nvlop and th conducting chassis. Th pulsd driv damping oscillations, which dpnds on a procss of loss of nrgy in th tstd capacitor Fig.7 shows a practical application of th On-Prob Mthod of tsting capacitors incircuit with -Miracl masurmnt instrumnt. Fig.7. Masuring quality of capacitors with -Miracl instrumnt. Th instrumnt masurs a quality Q/tgδ Antion! Whn applying th shock-xcitd damping oscillations for ralization of th Mthod, highr harmonics of a puls train hav not match a rsonant frquncy of L tsting circuit in vicinity of its band-pass. Or, th amplitud of th highr harmonics has to b small nough to not compt with th fr oscillations. This can b achivd by mans of maximal spctral distanc of major frquncy of th puls

12 train and th fr oscillations. At last, coupl hundrd tim ratio of th frquncis is rquird... Displacmnt urrnt Stimulators, DS, as a Man to Bypass High Rsistanc oupling Problms Th On-Prob Mthod is basd on formation of displacmnt currnt through th capacitor du to polarization procsss of associatd conducting surfac. In majority of cass, on of trminals is dirctly coupld to dvlopd surfac of mtal chassis or intgratd surfac of PB. But in a lss numbr of cass, both of th trminals of th capacitor hav no dirct connction with th dvlopd conducting surfac. Ths trminals ar connctd to high-impdanc points of circuitry. By this rason, mploying th Mthod will rsult in a fals rading, causd by rsistors connctd in sris. To bypass this problm, th following solutions ar offrd []. Eithr trminal of th capacitor can b touchd with an insulatd mtal surfac lik a tub, having an lastic contact. Th insulatd tub, calld a Displacmnt urrnt Stimulator, DS, ar hold by an oprator, that provids a sufficint total capacitanc to dvlop th probing currnts. Anothr DS ralization is a soft conductiv surfac laid undr an opposit sid of PB with soldrd trminals. Antion! Nvr us DS in powrd quipmnt!.3. Advantags of th Mthod. Th Mthod has th following advantags whn compard to xisting In-ircuit mthods.. No nd to connct probs of th instrumnts to trminals of th capacitor, frquntly locatd inconvnintly;. No nd to know capacitanc of th capacitor bcaus th xisting in-circuit mthods and instrumnts masur Equivalnt Sris Rsistanc, ESR, rathr than an ultimat charactristic dilctric loss. Th ESR is not th ultimat charactristic, and can b usd only togthr with known capacitanc; 3. Instanc rsults on touching a cap of th capacitor for s is nough to gt th rsult. 4. Possibility to masur th quality or tangnt dilctric loss angl in powrd quipmnt. Antion! Although -Miracl and othr spcial instrumnts for ralizing th Mthod can oprat in powrd quipmnt, w don t rcommnd doing that. In any cas, a high-voltag dcoupling capacitor has to b placd btwn th L circuit and th prob. Th Mthod and Instrumnts wr succssfully xploitd sinc 996 in th computr maintnanc company, Nw York. Application of th Mthods and Instrumnt rsultd in considrabl incras of troublshooting rat in th company [3].

13 onclusion. Th principally nw masurmnt mthod and instrumnts to tst capacitors in-circuit hav bn dvlopd. Unlik xisting mthods of injcting a probing currnt through two trminals of th capacitor, du to applid voltag, th nw on forms th probing currnt by mans of applying th voltag to th nvlop of capacitor and furthr dvloping th probing currnt du to polarization procsss on a conductiv surfac, associatd with th trminal of th capacitor. Nw approach rsults in numrous advantags including instant tsting quality of capacitors and possibility to do this in powrd quipmnt. Litratur and Links. M. Krinkr. Mthod to Dtct Dfctiv apacitors in ircuit and Mtrs for That, US Patnt No March 00.. M. Krinkr. In-ircuit Tsting apacitors. Nw Approachs and Instrumnts. Infinitly Rmot Point Mthod of Masurmnt. hp:// Masur-Quality-of-apacitors 3. Rfrnc Lr from Ky Systms, company xploiting th Mthod and Instrumnts sinc May 996, Nw York.