Littelfuse Varistor Design Examples

Transcription

1 Lielfuse Varisor Design Examples Applicaion Noe January 998 AN9772 [ /Tile (AN97 72) /Subjec (Harris Varisor Design Examples) /Auho r () /Keywords (TVS, Transien Suppression, Proecion, ESD, IEC, EMC, Elecromagne ic Compaibiliy, Harris Sup- This noe is mean o be a guide for he user in selecing a varisor by describing common applicaion examples, and illusraing he soluion process o deermine he appropriae varisor. Also described are varisor fusing and series/parallel connecion rules. Applicaions Power Supply Proecion Agains Line Transien Damage I is desired o preven failure of he power supply shown in Figure B o be used on residenial 7V AC lines. A represenaive ransien generaor is o be used for esing, as shown in Figure A. If he ransien is applied o he exising circui, he recifier will receive high negaive volages, ransmied hrough he filer capacior. The LC nework is here o preven RFI from being ransmied ino he power line (as in a TV se), bu also serves o reduce he ransien volage. An analysis shows ha he ransien will be reduced approximaely by half, resuling in abou 2.5kV insead of 5kV a he recifier. This is sill oo high for any pracical recifier, so some suppression mus be added. I is desirable o use he buil-in impedance of he coil o drop he remaining volage, so he suppressor would bes be applied as shown. A selecion process for a Lielfuse Varisor is as follows: SOLUTION Seady-Sae Volage The 7V AC, % high line condiion is 29V. The closes volage raing available is 30V. 5kV V T = ± 5kV sin 5 π X e Ω Energy and Curren The µh inducor will appear o be abou 30Ω o he ransien. The 30Ω is derived from he inducive reacance a he ransien generaor source frequency of 5 π rad. Taking a firs esimae of peak varisor curren, 2500V/80Ω = 3A. (This firs esimae is high, since i assumes varisor clamping volage is zero.) Wih a enaive selecion of a 30V Lielfuse Varisor, we find ha a curren of 3A yields a volage of from 325V o 380V, depending on he model size, as shown in Figure 2A and Figure 2B. Revising he esimae, I (2500V - 325V)/80Ω = 27.2A. For model V30LA20A, 27.2A coincides closely wih a 320V clamping level. There is no need o furher refine he esimae of peak curren if model 20A remains he final selecion. To arrive a an energy figure, assume a sawooh curren waveform of 27A peak, dropping o zero in wo ime consans, or 20µs. Energy is hen roughly equal o (27A x 320V x 20µs)/2, he area under he power waveform. The resul is 0.086J, well wihin he capabiliy of he varisor (70J). Peak curren is also wihin he 6500A raing. Model Selecion The acual varisor selecion is a rade-off beween he clamping volage desired and he number of ransien curren pulses expeced in he life of he equipmen. A 70J raed varisor will clamp a 35V and be capable of handling over 6 such pulses. An J uni will clamp o approximaely 385V and be capable of handling over 5 such pulses. Furhermore, he clamping volage deermines he cos of he recifier by deermining he volage raing required. A smaller, lower cos varisor may resul in a more expensive higher volage recifier diode. µh D + 0.µF 50µF - FIGURE A. TRANSIENT GENERATOR FIGURE B. TYPICAL POWER SUPPLY CIRCUIT FIGURE. POWER SUPPLY PROTECTION or Copyrigh Lielfuse, Inc. 998

2 MAXIMUM PEAK (V) UL499 CORD CONNECTED AND DIRECT PLUG-IN CATEGORY V30LA2 V30LA5 V30LAA V30LA20A IMPULSE GENERATOR LOAD LINES (IMPLIED) UL449 PERMANENTLY CONNECTED CATEGORY, AND ANSI IEEE C6.4 (IEEE587) CATEGORY B PEAK AMPERES 8/20µs WAVESHAPE MAXIMUM PEAK (V) PEAK AMPERES 8/20µs WAVESHAPE FIGURE 2A. FIGURE 2B. FIGURE 2. V30LA VARISTOR V-I CHARACTERISTICS I V VARISTOR CURRENT 27 µs 20µs VARISTOR CURRENT 27 I V APPROXIMATION µs 20µs FIGURE 3A. FIGURE 3B. FIGURE 3. ENERGY APPROXIMATION SCR Moor Conrol The circui shown in Figure 4 experiences failures of he recifiers and SCR when he ransformer primary is swiched off. The manufacurer has ried 600V componens wih lile improvemen. 480V AC 60Hz 4: FIELD R2 5kΩ ARMATURE R 330kΩ SPEED CONTROL R3 250kΩ C I 0.2µF FIGURE 4. SCR MOTOR CONTROL SUS SCR SOLUTION Add a varisor o he ransformer secondary o clamp he ransformer inducive ransien volage spike. Selec he lowes volage Lielfuse Varisor ha is equal o or greaer han he maximum high line secondary AC volage. The V30LA ypes fulfills his requiremen. Deermine he peak suppressed ransien volage produced by he ransien energy source. This is based on he peak ransien curren o he suppressor, assuming he wors-case condiion of zero load curren. Zero load curren is normally a valid assumpion. Since he dynamic ransien impedance of he Lielfuse Varisor is generally quie low, he parallel higher impedance load pah can be negleced. Since ransien curren is he resul of sored energy in he core of he ransformer, he ransformer equivalen circui shown in Figure 5 will be helpful for analysis. The sored inducive energy is: 2 E LM = -- L 2 M Î M -28

3 V PRIMARY Z P MUTUAL INDUCTANCE REPRESENTED BY IRON CORE I M L M IDEAL TRANSFORMER FIGURE 5. SIMPLIFIED EQUIVALENT CIRCUIT OF A TRANSFORMER V SECONDARY The designer needs o know he oal energy sored and he peak curren ransformed in he secondary circui due o he muual inducance, L M. A no load, he magneizing curren, (I NL ), is essenially reacive and is equal o I M. This assumes ha he primary copper resisance, leakage reacance and equivalen core resisive loss componens are small compared o L M. This is a valid assumpion for all bu he smalles conrol ransformers. Since I NL is assumed purely reacive, hen: V pri X LM = I NL and i M = I NL I NL can be deermined from nameplae daa. Where nameplae is no available, Figure 6 and Figure 7 can guide he designer. Assuming a 3.5% value of magneizing curren from Figure 7 for a 20kVA ransformer wih 480V AC primary, and 20V AC secondary: i M = ( 0.035) 20kVA V =.46A î M = 2i M X LM = 480V/.46A = 329Ω Wih his informaion one can selec he needed semiconducor volage raings and required varisor energy raing. Peak varisor curren is equal o ransformed secondary magneizing curren, i.e., î M (N), or 8.24A. From Figure 2, he peak suppressed ransien volage is 3V wih he V30LAA selecion, 295V wih he V30LA20B. This allows he use of 300V raed semiconducors. Safey margins exis in he above approach as a resul of he following assumpions:. All of he energy available in he muual inducance is ransferred o he varisor. Because of core hyseresis and secondary winding capaciance, only a fracion less han wo-hirds is available. 2. The exciing curren is no purely reacive. There is a % o 20% safey margin in he peak curren assumpion. N Z S L M = X LM /ω = 0.872H 0.872( ) E LM = =.85J Afer deermining volage and peak curren, energy and power dissipaion requiremens mus be checked. For he given example, he single pulse energy is well below he V30LA20B varisor raing of 70J a 85 o C maximum ambien emperaure. Average power dissipaion requiremens over idling power are no needed because of he non-repeiive naure of he expeced ransien. Should he ransien be repeiive, hen he average power is calculaed from he produc of he repeiion rae imes he energy of he ransien. If his value exceeds he V30LA20B capabiliy of.0w, power varisors of he HA, DA, or DB Series may be required. Should he ambien emperaure exceed 85 o C or he surface emperaure exceed 85 o C, he single pulse energy raings and he average power raings mus be deraed by he appropriae deraing facors supplied on he daa shee. PERCENT MAGNETIZING CURRENT PERCENT MAGNETIZING CURRENT Conac Arcing Due o Inducive Load f = Hz TRANSFORMER RATING (kva) FIGURE 6. MAGNETIZING CURRENT OF TRANSFORMERS WITH LOW SILICON STEEL CORE f = Hz TRANSFORMER RATING (kva) FIGURE 7. MAGNETIZING CURRENT OF TRANSFORMERS WITH HIGH SILICON STEEL CORE OR SQUARE LOOP CORE To exend he life of he relay conacs shown in Figure 8 and reduce radiaed noise, i is desired o eliminae he conac arcing. -29

4 + C C 28V DC L R C RELAY C C = STRAY CAPACITANCE L = RELAY COIL INDUCTANCE R C = RELAY COIL RESISTANCE subsequen welding. To avoid his inrush curren, i is cusomary o add a series resisor o limi he capaciive discharge curren. However, his addiional componen reduces he nework effeciveness and adds addiional cos o he soluion. When relays or mechanical swiches are used o conrol inducive loads, i is necessary o use he conacs a only abou 50% of heir resisive load curren raing o reduce he wear caused by arcing of he conacs. The energy in he arcing is proporional o he inducance and o he square of he curren. Each ime he curren in he inducive load is inerruped by he mechanical conacs, he volage across he conacs builds up as -L di/d. When he conacs arc, he volage across he arc decreases and he curren in he coil can increase somewha. The exinguishing of he arc causes an addiional volage ransien which can again cause he conacs o arc. I is no unusual for he resriking o occur several imes wih he oal energy in he arc several imes ha which was originally sored in he inducive load. I is his repeiive arcing ha is so desrucive o he conacs. In he example, R C is 30Ω and he relay conacs are conducing nearly A. The conacs will draw an arc upon opening wih more han approximaely 0.4A or 2V. The arc coninues unil curren falls below 0.4A. SOLUTION FIGURE 8. RELAY CIRCUIT To preven iniiaion of he arc, i is necessary o reduce he curren and volage of he conacs below he arc hreshold levels a he ime of opening, and hen keep hem below breakdown hreshold of he conacs as hey open. Two obvious echniques come o mind o accomplish his: ) use of a large capacior across he conacs, and 2) a volage clamp (such as a varisor). The clamp echnique can be effecive only when he minimum arc volage exceeds he supply volage. In his example a clamping device operaing above he supply volage will no preven arcing. This is shown in Figure 9. The capacior echnique requires he capaciance o be sufficienly large o conduc he inducor curren wih a volage rae-of-rise racking he breakdown volage rae-of-rise of he conacs as hey mechanically move apar. This is shown in Figure A. The limiaions in using he capacior approach are size and cos. This is paricularly rue for hose cases involving large amouns of inducive sored energy. Furhermore, he use of a large capacior alone creaes large discharge currens upon conac reclosure during conac bouncing. As a resul, he conac maerial may mel a he poin of conac wih ARC VOLTAGE (V) 50 ARCING VOLTAGE CLAMP ABOVE ARC VOLTAGE VOLTAGE CLAMP BELOW ARC VOLTAGE BREAK TIME (µs) BREAKDOWN LEVEL FIGURE 9. VOLTAGE CLAMP USED AS ARC SUPPRESSOR A hird echnique, while no as obvious as he previous wo, is o use a combinaion approach. This echnique shown in Figure B parallels a volage clamp componen wih an R-C nework. This allows he R-C nework o preven he low volage iniial arcing and he clamp o preven he arcing ha would occur laer in ime as he capacior volage builds up. This approach is ofen more cos effecive and reliable hen using a large capacior. Also, wih AC power relays he impedance of a single large R-C suppressor migh be so low ha i would allow oo much curren o flow when he conacs are open. The combinaion echnique of a small R-C nework in conjuncion wih a varisor is of advanage here, oo. In his example a 0.22µF capacior and Ω resisor will suppress arcing compleely, bu by reducing he capaciance o 0.047µF, arcing will sar a 70V. Thus, o use a varisor as a clamp in conjuncion wih he R- C nework, i mus suppress he volage o below 70V a A and be capable of operaing a a seady-sae maximum DC volage of 28V + %, or 30.8V (assumes a ±% regulaed 28V DC supply). The hree candidaes ha come closes o meeing he above requiremen are he MA series V39MA2B model and he ZA series V39ZA and V39ZA05 models, all of which have maximum seady-sae DC volage raings of 3V. The V39MA2B and V39ZA05 V-I characerisics a A shows a maximum volage of 73V, while he V39ZA characerisic a A shows a maximum volage of 67V. Thus, he laer varisor is seleced. Use of a 0.068µF capacior in place of he 0.047µF previously chosen would allow use of he V39MA2B or V39ZA

5 ARC VOLTAGE (V) 50 SMALL C WITH R (ARCING) CONTACT BREAKDOWN LEVEL LARGE C WITH R (NO ARCING) ARC VOLTAGE (V) 50 CONTACT BREAKDOWN LEVEL SMALL C WITH R AND VOLTAGE CLAMP COMBINATION BREAK TIME (µs) BREAK TIME (µs) FIGURE A. R-C ARC SUPPRESSION FIGURE B. R-C AND CLAMP ARC SUPPRESSION FIGURE. RELAY ARC VOLTAGE SUPPRESSION TECHNIQUES Placing only a Lielfuse Varisor raed for 3V DC across he conacs resuls in arcing up o he 66V level. By combining he wo, he capacior size and volage raing are reduced and suppression complee. Besides checking he varisor volage and arcing eliminaion, he designer should review energy and peak curren requiremens. Varisor energy is deermined from a measuremen of he coil inducance and he calculaion E = /2 Li 2. Peak curren, of course, is under A. Power dissipaion is negligible unless he coil is swiched ofen (several imes per minue). In hose cases where muliple arcs occur, he varisor energy will be a muliple of he above /2 Li 2 value. The peak curren is well wihin he raing of eiher he MA or ZA series of varisors, bu he number of conac operaions allowable for eiher varisor is a funcion of he impulse duraion. This can be esimaed by assuming a L/R C ime consan a he A or peak curren value. Since he volage across he varisor is 67V a A, he varisor saic resisance is 67Ω. The coil R C value is 28V/A, or 28Ω. The coil inducance was found o be 20mH. Thus, he approximae ime consan is: From he pulse raing curves of he V39ZA model, he number of allowable pulses exceeds million. Noise Suppression 20mH τ = L/R C = = 2µs 95 Swiching of a small imer moor a 20V, 60Hz, was causing serious malfuncions of an elecronic device operaing from he same power line. Aemps were made o observe he ransien noise on he line wih an oscilloscope as he firs sep in curing he problem. Observed waveforms were hash, i.e., no readily idenifiable. Noise in an elecromechanical sysem is a commonly experienced resul of inerruping curren by mechanical conacs. When he swich conacs open, a ho cahode arc may occur if he curren is high enough. On he oher hand, low curren will permi swich opening wihou an arc, bu wih ringing of circui resonances. As a consequence, volages can exceed he conac gap breakdown resuling in a replica of he old spark gap ransmier. I is he low curren case ha produces he mos serious noise disurbances which can resul in malfuncions or damage o elecrical equipmen. These pulses cause noise problems on adjacen lines, rigger SCRs and riacs, and damage semiconducors. In addiion, hey can disrup microprocessor operaion causing memory o be los and vial insrucions o be missed. SOLUTION A es circui (Figure ) was se up wih lumped elemens replacing he measured circui values. The moor impedance was simulaed by R, L, and C, and he AC line impedance by L 2 and C 2. A DC source allowed repeaable observaions over he full range of curren ha could flow hrough he swich in he normal AC operaion. A diode deecor was used o observe he RF volage developed across a 2 lengh of wire (50nH of inducance). V CC + L 2 5µH C 2 4 P F V 2 S 2" AWG #22 WIRE The supply is se a 25mA o represen he peak moor curren in normal 20V AC operaion. As swich S was opened, he waveform in Figure 2 was recorded. Noe he showering arc effec. The highes breakdown volage recorded here is 20V, and he highes RF deecor oupu (shown in he lower race) is 32V. V C FIGURE. TEST CIRCUIT 80 P F R L 6.8H 448Ω V RF -3

6 0 A 200V/cm 26V V+ 50Ω 200mH V C V C V+ 26V I C 470Ω B 0 200µs/cm 20V/cm PERIOD OF HIGH SOA REQUIREMENT FIGURE 4A. BASIC SOLENOID CIRCUIT UPPER V : 200V/cm LOWER V RF : 20V/cm : 0.2ms/cm FIGURE 2. UNPROTECTED CONTACTS Obviously, some correcive acion should be aken and he mos effecive one is ha which prevens he repeaed breakdown of he gap. Figure 3 shows he waveform of V (upper race) and V RF (lower race) for he same es condiions wih a Lielfuse Varisor, ype V30LAA, conneced direcly across he swich erminals. The varisor compleely eliminaes he relaxaion oscillaions by holding he volage below he gap breakdown volage (abou 300V) while dissipaing he sored energy in he sysem V/cm V/cm 200µs/cm UPPER V : 200V/cm LOWER V RF : 20V/cm : 0.2ms/cm FIGURE 3. VARISTOR PROTECTED CONTACTS Proecion of Transisors Swiching Inducive Loads The ransisor in Figure 4 is o operae a solenoid. I may operae as frequenly as once per second. The circui (wihou any suppression) consisenly damages he ransisor. The inducor drives he collecor volage up when he ransisor base is grounded (urning off ). The inducor forces curren o flow unil he energy sored in is field is dissipaed. This energy is dissipaed in he reverse bias condiion of he ransisor and is sufficien o cause breakdown (indicaed by a sudden collapse of collecor volage during he pulse). I C 26V V+ I V V C = COLLECTOR EMITTER VOLTAGE SOLUTION This condiion can be eliminaed eiher by shuning he ransisor wih a suppressor or by urning i on wih a varisor conneced collecor-o-base. The firs mehod will considerably reduce he demands upon he safe operaing area (SOA) of he ransisor. If he volage is kep below is breakdown level, all energy will be dissipaed in he suppressor. The laer mehod will cause he ransisor o once again dissipae he sored energy, bu in he forward-bias sae in which he ransisor can safely dissipae limied amouns of energy. The choice is deermined by economics and reliabiliy. A suppressor conneced collecor-emier (C-E) will be more expensive han one conneced C-B, since i is required o absorb more energy, bu will allow he use of a ransisor wih reduced SOA. If a collecor-emier varisor is used in he above example, i is required o wihsand 28.6V DC wors-case (26 + % regulaion). The sored energy is /2 Li 2 or /2 (0.20) (0.572) 2 = J. The energy conribued by he power supply is roughly equal o his (coil volage supply volage, since varisor clipping volage 2 x supply volage). Ignoring coil resisance losses for a conservaive esimae, varisor energy dissipaion is 0.065J per pulse. The peak curren will be 0.572A, he same as he coil curren when he ransisor is swiched off. If he ransisor operaes once per second, he average power dissipaion in he varisor will be 0.065W. This is less han he 0.20W raing of a small 3V DC varisor (V39ZA). From he daa shee i can be seen ha if he device emperaure exceeds 85 o C, deraing is required. The V C V+ 26V I C FIGURE 4B. SOLENOID CIRCUIT WITH VARISTOR PROTECTION FIGURE 4. TRANSISTOR SWITCHING OF AN INDUCTIVE LOAD I V -32

7 nonrecurren joule raing is.5j, well in excess of he recurren value. To deermine he repeiive joule capabiliy, he curren pulse raing curves for he ZA series mus be consuled. Two are shown in Figure 5. To use Figure 27, he impulse duraion (o he 50% poin) is esimaed from he circui ime consans and is found o be 240µs. From Figure 27A, for his example, he 7mm V39ZA would no be limied o a cumulaive number of pulses. In cases where he peak curren is greaer and inersecs wih he recommended pulse life curves, he designer mus deermine he maximum number of operaions expeced over he life of he circui and confirm ha he pulse life curves are no exceeded. Figure 5B shows he curves for he larger, 4mm V39ZA6 device and, illusraes he resulan higher capabiliy in erms of number of ransiens for a given peak pulse curren and duraion. Also, i may be necessary o exrapolae he pulse raing curves. This has been done in Figure 6 where he daa from Figure 5B is ransposed. A low currens he exrapolaion is a sraigh line. RATED PEAK PULSE CURRENT (A) INDEFINITE MODEL SIZE 7mm V8ZA - V68ZA ,000,000 IMPULSE DURATION (µs) FIGURE 5A. ZA SERIES V8ZA TO V68ZA2 (MODEL SIZE 7mm) RATED PEAK PULSE CURRENT (A), INDEFINITE 20,000,000 IMPULSE DURATION (µs) FIGURE 5B. ZA SERIES V8ZA3 TO V68ZA (MODEL SIZE 4mm) MODEL SIZE 4mm V8ZA3 - V68ZA Finally, he V-I characerisics curves mus be consuled o deermine he varisor maximum clamping volage in order o selec he minimum ransisor breakdown volage. In his example, a 0.572A he V39ZA6 (if chosen) provides a maximum of 6V requiring ha he ransisor have abou a 65V or 70V capabiliy. PEAK PULSE CURRENT (A) ZA SERIES V8ZA3 TO V68ZA NOTE: PULSE RATING CURVE FOR,240µs PULSE WIDTH NUMBER OF PULSES FIGURE 6. EXTRAPOLATED PULSE RATING CURVES Moor Proecion Frequenly, he cause of moor failures can be raced o insulaion breakdown of he moor windings. The source of he ransiens causing he breakdown may be from eiher inernal magneic sored energy or from exernal sources. This secion deals wih he self-generaed moor ransiens due o moor saring and circui breaker operaion. Exernally generaed ransiens and heir conrol are covered in AN9768. In he case of DC moors he equivalen circui consiss of a single branch. The magneic sored energy can be easily calculaed in he armaure or field circuis using he nameplae moor consans. Wih AC inducion moors he equivalen magneic moor circui is more complex and he circui consans are no always given on he moor nameplae. To provide a guide for moor proecion, Figures 7, 8, 9 were drawn from ypical inducion moor daa. While he acual sored energy will vary according o moor frame size and consrucion echniques, hese curves provide guidance when specific moor daa is lacking. The daa is conservaive as i assumes maximum moor orque, a condiion ha is no he ypical running condiion. Sored energy decreases considerably as he moor loading is reduced. Experience wih he suppression of magneic energy sored in ransformers indicaes ha Lielfuse Varisors may be used a heir maximum energy raings, even when muliple operaions are required. This is because of he conservaism in he applicaion requiremens, as indicaed above, and in he varisor raings. Thus, no aemp is made o derae he varisor for muliple operaion because of he random naure of he ransien energy experienced. -33

8 STORED ENERGY PER PHASE (J) V50PA20 V5HA32 4 POLE 2 POLE V320PA40 460V RMS LINE - LINE 230V RMS LINE V32HA32 4 POLE Y CONNECTED 2 POLE STORED ENERGY PER PHASE (J) MOTOR STORED ENERGY AT START 230V RMS LINE - LINE Y CONNECTED V27HA V32HA32 V5PA80 V5HA32 Y CONNECTED CONNECTED 460V RMS LINE - LINE CONNECTED MOTOR (hp) NOTES:. Y conneced 60Hz. 2. Energy a Max orque slip speed. 3. See Figure 20 for varisor circui placemen. FIGURE 7. STORED ENERGY CURVES FOR TYPICAL WYE-CONNECTED INDUCTION MOTOR STORED ENERGY PER PHASE (J) DELTA CONNECTED 230V RMS LINE - LINE 2 POLE 4 POLE V27HA32/V275PA40 460V RMS LINE - LINE 4 POLE V5PA80 2 POLE V5HA MOTOR (hp) NOTES: 4. Dela conneced a 60Hz. 5. Energy a maximum orque slip speed. 6. See Figure 20 for varisor circui placemen. FIGURE 8. STORED ENERGY CURVES FOR TYPICAL DELTA-CONNECTION INDUCTION MOTOR MOTOR (hp) NOTES: 7. 60Hz, see Figure 20 for varisor circui placemen. 8. Energy a sar, i.e., SLIP =. 9. Inducion moor.. 2, and 4 pole moors. FIGURE 9. STORED ENERGY CURVES FOR A TYPICAL MOTOR WITH STALLED ROTOR As an aid in selecing he proper operaing volage for Lielfuse Varisors, Table gives guidelines for wyeconneced and dela-conneced moor circuis a differen line-o-line applied volages. Figure 20 provides guidance in proper placemen of he varisor. Inerrupion of moor saring currens presens special problems o he user as shown in Figure 9. Since he sored magneic energy values are approximaely imes he running values, proecion is difficul a he higher horsepower levels. Ofen he moor is sared by use of a reduced volage which will subsanially reduce he sored energy. A reducion in saring curren of a facor of wo resuls in a fourfold reducion in sored energy. If a reduced volage sarer is no used, hen a decision mus be made beween proecion for he run condiion only, and he condiion of locked roor moor curren. For mos applicaions, he saring condiion can be ignored in favor of selecing he varisor for he wors-case run condiion. TABLE. PREFERRED VARISTOR VOLTAGE RATINGS FOR DELTA- AND WYE-CONNECTED MOTORS RMS Line Volage (Line-Line) Dela Conneced Applied V. Varisor Raings / / / / Y Conneced Applied V. Varisor Raings /

9 To proec a wo-pole, 75hp, 3φ, 460V RMS line-o-line wye-conneced moor from inerrupion of running ransiens. Specific Moor Daa Is No Available SOLUTION Consul Figure 7 along wih Table. Sandard varisors having he required volage raings are he 320V RMS raed models. This allows a 20% high-line volage condiion on he nominal 460V line-o-line volage, or 266V line-neural volage. Figure 7 shows a wo-pole 75hp, wye-conneced inducion moor, a he running condiion, has 52J of sored magneic energy per phase. Eiher a V320PA40 series or a V32HA32 series varisor will mee his requiremen. The HA series Lielfuse Varisor provides a greaer margin of safey, alhough he PA series Lielfuse Varisor fully mees he applicaion requiremens. Three varisors are required, conneced direcly across he moor erminals as shown in Figure 20. provide he riggering o he SCR, a high-volage deecor is needed. High volage avalanche diodes are effecive bu expensive. An axial leaded Lielfuse Varisor provides an effecive, inexpensive subsiue. In he circui of Figure 2, he volage, wihou proecion, can exceed wice he normal 240V peaks, damaging componens downsream. A simple arrangemen o crowbar he supply is shown. + V - V A C.B. FULL WAVE (RECTIFIED) C6D NORMAL VOLTAGE < 240V PEAK ABNORMAL VOLTAGE > 400V PEAK FIGURE 2. CROWBAR CIRCUIT V L-L V VARISTOR = V 3 L L FIGURE 20A. WYE CONNECTED V L-L Power Supply Crowbar Occasionally i is possible for a power supply o generae excessively high volage. An accidenal removal of load can cause damage o he res of he circui. A simple safeguard is o crowbar or shor circui he supply wih an SCR. To M V VARISTOR = V L-L FIGURE 20B. DELTA CONNECTED FIGURE 20. VARISTOR - 3φ INDUCTION MOTOR CIRCUIT PLACEMENT The supply shown can provide 2A RMS of shor-circui curren and has a A circui breaker. A C6D SCR having a 4A RMS capabiliy is chosen. Triggering will require a leas 0.4V gae-ocahode, and no more han 0.8V a 200µA a 25 o C ambien. SOLUTION Check he MA series Lielfuse Varisor specificaions for a device capable of supporing 240V peak. The V270MA4B can handle 2 (7V RMS ) = 242V. According o is specificaion of 270V ±%, he V270MA4B will conduc ma DC a no less han 243V. The gae-cahode resisor can be chosen o provide 0.4V (he minimum rigger volage) a ma, and he SCR will no rigger below 243V. Therefore, R GK should be less han 400Ω. The highes value 5% olerance resisor falling below 400Ω is a 360Ω resisor, which is seleced. Thus, R GK is 378Ω maximum and 342Ω minimum. Minimum SCR rigger volage of 0.4V requires a varisor of 0.4V/378Ω, or.06ma for a minimum varisor volage of 245V. The maximum volage o rigger he circui is dependen upon he maximum curren he varisor is required o pass o rigger he SCR. For he C6 a 25 o C, his is deermined by calculaing he maximum curren required o provide 0.8V across a parallel resisor comprised of he 360Ω R GK seleced and he equivalen gae-cahode SCR resisor of 0.8V/200µA, since he C6 requires a maximum of 200µA rigger curren. The SCR gae inpu resisance is 4kΩ and he minimum equivalen gae-cahode resisance is he parallel combinaion of 4kΩ and R GK(MIN), or 360Ω -5%, 342Ω. The parallel combinaion is 35Ω. Thus, I VARISTOR for maximum volage-o-rigger he C6 is 0.8V/35Ω, or 2.54mA. According o he specificaion shee for he V270MA4B, he varisor will no exceed 330V wih his curren. The circui will, herefore, rigger a beween 245 and 330V peak, and a 400V raed C6 can be used. The reader is cauioned ha SCR -35

10 gae characerisics are sensiive o juncion emperaures, and a value of 25 o C for he SCR emperaure was merely chosen as a convenien value for demonsraing design procedures. The maximum energy per pulse wih his waveform is deermined as approximaely /2 x K x I PK x V PK x τ (duraion of /2 wave pulse), or 0.52mJ for his example. Since he volage does no drop o zero in his case, he SCR remains on, and he varisor sees only one pulse; hus, no seady-sae power consideraion exiss. General Proecion of Solid Sae Circuiry, Agains Transiens On 7V AC Lines Modern elecronic equipmen and home appliances conain solid sae circuiry ha is suscepible o malfuncion or damage caused by ransien volage spikes. The equipmen is used in residenial, commercial, and indusrial buildings. Some es sandards have been adoped by various agencies (see applicaion noes AN9769 and AN9773) and furher definiion of he environmen is underway by he IEEE and oher organizaions. The ransiens which may occur on residenial and commercial AC lines are of many waveshapes and of varying severiy in erms of peak volage, curren, or energy. For suppressor applicaion purposes, hese may be reduced o hree caegories. Firs, he mos frequen ransien migh be he one represened by a 30kHz or khz ring wave. This es surge is defined by an oscillaory exponenially decaying volage wave wih a peak open circui volage of 6kV. This wave is considered represenaive of ransiens observed and repored by sudies in Europe and Norh America. These ransiens can be caused by disan lighning srikes or disribuion line swiching. Due o he relaively high impedance and shor duraion of hese ransiens, peak curren and surge energy are lower han he second and hird caegories. The second caegory is ha of surges produced by nearby lighning srokes. The severiy of a lighning sroke is characerized in erms of is peak curren. The probabiliy of a direc sroke of a given severiy can be deermined. However, since he lighning curren divides in many pahs, he peak curren available a an AC oule wihin a building is much less han he oal curren of he sroke. The sandard impulse used o represen lighning and o es surge proecive devices is an 8/20µs curren waveshape as defined by ANSI Sandard C68.2, and also described in ANSI/IEEE Sandard C and IEC 664- (992). A hird caegory of surges are hose produced by he discharge of energy sored in inducive elemens such as moors and ransformers. A es curren of /0µs waveshape is an acceped indusry es impulse and can be considered represenaive of hese surges. Alhough no hard-and-fas rules can be drawn as o he caegory and severiy of surges which will occur, a helpful guideline can be given o sugges varisors suiable in ypical applicaions. The guideline of Table 2 recognizes consideraions such as equipmen cos, equipmen duy cycle, effec equipmen downime, and balances he economics of equipmen damage risk agains surge proecion cos. Failure Modes and Varisor Proecion Varisors are inherenly rugged and are conservaively raed and exhibi a low failure rae. The designer may wish o plan for poenial failure modes and he resulan effecs should he varisor be subjeced o surge currens or energy levels above is raing. Failure Modes Varisors iniially fail in a shor-circui mode when subjeced o surges beyond heir peak curren/energy raings. They also shor-circui when operaed a seady-sae volages well beyond heir volage raings. This laer mode of sress may resul in he evenual open-circuiing of he device due o meling of he lead solder join. When he device fails in he shored mode he curren hrough he varisor becomes limied mainly by he source impedance. Consequenly, a large amoun of energy can be inroduced, causing mechanical rupure of he package accompanied by expulsion of package maerial in boh solid and gaseous forms. Seps may be aken o minimize his poenial hazard by he following echniques: ) fusing he varisor o limi high faul currens, and, 2) proecing he surrounding circuiry by physical shielding, or by locaing he varisor away from oher componens. TABLE 2. LITTELFUSE VARISTOR SELECTION GUIDELINE FOR 7V AC APPLICATIONS APPLICATION TYPE DUTY CYCLE LOCATION EXAMPLE SUGGESTED MODEL Ligh Consumer Very Low A Mixer/Blender V07E30 or VE30 Consumer Low A Porable TV/Elecronics V4E30 Consumer Medium A Home Theaer, PC V4E30, V20E30 Ligh Indusrial/Office Medium B Copier, Server V20E30, V20E40 Indusrial Medium B Moors, Solenoid, Relay V20E40, V3HA32 Indusrial High B Large Compuer Moor Conrol V3DA40 or DB40 Indusrial High B Elevaor Conrol Heavy Moors V5DA40 or DB40-36

11 Fusing he Varisor Varisor fusing should be coordinaed o selec a fuse ha limis curren below he level where varisor package damage could occur. The locaion of he fuse may be in he disribuion line o he circui or i may be in series wih he varisor as shown in Figure 22. Generally, fuse raher han breaker proecion is preferred. Breaker ripping may be oo slow o preven excessive faul energy in some applicaions. seen ha he V30LA20 single pulse wihsand capabiliy a ms impulse duraion is slighly in excess of A. This is adequae for applicaion in areas where lighning aciviy is medium o ligh. For heavy lighning aciviy areas, eiher a DA or DB series varisor migh be desirable o allow a capabiliy of wihsanding over 70 ransiens. In making he choice beween he LA series and higher energy series, he designer mus decide on he likelihood of a wors-case lighning sroke and resulan fuse replacemen should he varisor fail. V LINE F L F V PROTECTED CIRCUIT Assuming a low lighning aciviy area, he V30LA20A series is a reasonable choice. To coordinae he fuse wih he varisor, he single pulse surge raing curve is redrawn as I 2 vs impulse duraion as shown in Figure 23. The I 2 of he composie /0µs impulse is found from: []. FIGURE 22. FUSE PLACEMENT FOR VARISTOR PROTECTION In high power indusrial circuis he line currens are generally so high as o rule ou he use of a line fuse for varisor proecion. The fuse may no clear under a varisor faul condiion and would allow varisor failure. In low power (5-20A) applicaions i may be feasible o use he line fuse, F L, only. Use of a line fuse, F L, raher han F V, does no presen he problem of having he fuse arc volage being applied across he circui. Conversely, wih F V alone, he fuse arc volage adds o he varisor volage, increasing he V C, he ransien clamp volage. Since some fuses can have peak arc volages in excess of wice peak working volage, fuse clearing can have a significan effec on proecion levels. Anoher facor in he choice of locaion is he consequence of sysem inerrupion. Fuse locaion F L will cause a shudown of he circui while locaion F V will no. While he circui can coninue o operae when F V clears, proecion no longer is presen. For his reason i is desirable o be able o monior he condiion of F V. Fusing Example (Ligh Indusrial Applicaion) A process conrol minicompuer is o be proeced from ransiens on a 5V nominal line. The minicompuer draws 7.5A from he line, which is guaraneed o be regulaed o ±% of nominal line volage. A V30LA20A varisor is chosen on he basis ha he wors-case surge curren would be a /0µs pulse of A peak ampliude. The raionale for his surge requiremen is ha he incoming plan disribuion sysem is proeced wih lighning arresors having a maximum arresor volage of 5kV. Assuming a ypical 50Ω characerisic line impedance, he wors-case ransien curren hrough he varisor is A. The ms impulse duraion is aken as a wors-case composie wave esimae. While lighning sroke discharges are ypically less han µs, hey can recur in rapid fire order during a s duraion. From he pulse raing curves of he LA series size 20mm models, i is I 2 = -- Î 2 ( µs) Î 2 ( τ 3 ( 0.5) µs) When: τ ( 0.5) 200µs( ime for impulse curren o decay by 0.5) I Î 2 τ ( 0.5) Where: he firs erm represens he impulse I 2 conribued by he µs rise porion of he waveform and he second erm is he I 2 conribued by he exponenial decay porion. Figure 23 shows a cross-hached area which represens he locus of possible failure of he varisor. This area is equal o an I 2 value of from wo o four imes ha derived from he daa shee peak curren pulse life curves. The curve exending beyond he cross-hached area and parallel o i is where package rupure will ake place. The crieria for fuse selecion is given below: A) Fuse mels; i.e., opens, only if wors-case ransien is exceeded and/or varisor fails. B) If varisor fails, fuse clearing limis I 2 applied o varisor values below ha required for package rupure. C) Fuse is raed a 30V RMS. D) Fuse provides curren limiing for solid-sae devices. Based on he above, a Carbone-Ferraz 2A RMS, 30V RMS, Class FA fuse is enaively seleced. The minimum meling I 2 and maximum clearing I 2 curves for he 2A fuse are shown superimposed on he varisor characerisics. This fuse is guaraneed o mel a an I 2 of 40% above he esimaed wors-case ransien. Upon meling, clearing I 2 and clearing ime will depend upon available faul curren from he 30V RMS line. Table 3 liss clearing imes for he seleced fuse versus available prospecive circui curren. -37

12 ,000 I 2 - AMPERES SQUARED SECONDS,000 PT.2 2A FUSE PT. ASSUMED WORST CASE TRANSIENT I 2 µs µs ms ms IMPULSE DURATION (s) MAX. CLEARING I 2 2A FUSE MIN. MELTING I 2 PACKAGE RUPTURE POSSIBLE PACKAGE RUPTURE DEVICE FAILURE POSSIBLE DEVICE FAILURE (SHORT CIRCUIT) SINGLE PULSE LIFETIME RATING, I 2 (VI30LA20) FIGURE 23. LITTELFUSE VARISTOR - FUSE COORDINATION CHART TABLE 3. 2A FUSE - PROSPECTIVE CURRENT vs CLEARING TIME PROSPECTIVE CURRENT (A RMS ) CLEARING TIME (ms) I is desirable o indicae he saus of he varisor fuse if one is used in addiion o he line fuse. The circui shown in Figure 24 senses he presence of volage across he varisor by use of a phoocoupler. When he fuse inerrups he varisor circui, he LED of he coupler becomes deenergized, and he coupler oupu signal can be used o annunciae an unproeced condiion. Some fuse manufacurers provide indicaing means upon fuse operaion ha may also be used o rip an alarm As Figure 23 shows, a clearing ime of less han.5ms is desirable. For faul currens in excess of.2ka, he fuse will clear a less han 24A 2 s and.3ms. This will preven varisor package rupuring. However, he disribuion line may be sof, i.e., have a high source impedance a he 60Hz power frequency ha limis he faul curren o values below.2ka. Then, i is possible ha he fuse would no proec he varisor package from rupuring, hough i would serve o isolae he varisor in any case. 30V AC HIIAA2 AC OPTO COUPLER 2K 2W TO STATUS ANNUNCIATOR LIGHT/ALARM TO PROTECTED CIRCUIT Upon furher examinaion of his example, i is clear ha he varisor will be proeced from package rupuring even if he ransien pulse curren is 50% greaer han ha of he assumed value, resuling in an I 2 of 6A 2 S (Poin 2 on Figure 23). Placemen of he fuse for his example applicaion could be in he line or in series wih he varisor. If in series wih he varisor, he line fuse should be a medium o slow speed, such as a slow blow ype 5A fuse. Tha would assure a faul in he varisor would be isolaed by he varisor fuse wihou inerruping he line fuse. FIGURE 24. VARISTOR FUSE STATUS SENSING CIRCUIT In selecing a fuse, he reader is advised o avoid daa based on average values or daa aken a operaing condiions ha are grossly differen from he acual applicaion. For example, DC daa does no apply when he fuse will be used on an AC circui. Also, es daa aken in a resisive circui wih uniy power facor does no hold for low power facor operaion. -38

13 Series and Parallel Operaion of Varisors In mos cases he designer can selec a varisor ha mees he desired volage raings from sandard caalog models. Occasionally he sandard caalog models do no fi he requiremens eiher due o volage raings or energy/curren raings. When his happens, wo opions are available: varisors can be arranged in series or parallel o make up he desired raings, or he facory can be asked o produce a special o mee he unique applicaion requiremen. Series Operaion of Varisors Varisors are applied in series for one of wo reasons: o provide volage raings in excess of hose available, or o provide a volage raing beween he sandard model volages. As a side benefi, higher energy raings can be achieved wih series conneced varisors over an equivalen single device. For insance, assume he applicaion calls for a lead mouned varisor wih an V RMS raing of 375V AC and having a I TM peak curren capabiliy of 6000A. The I TM requiremen fixes he varisor size. Examining he LA series volage raings near 375V AC, only 320V and 420V unis are available. The 320V is oo low and he 420V uni (V420LA40B) resuls in oo high a clamp volage (V C of 60V a A). For a V30LA20B and a V250LA40B in series, he maximum raed volage is now he sum of he volages, or 380V. The clamping volage, V C, is now he sum of he individual varisor clamping volages, or 945V a A. The peak curren capabiliy is sill 6500A bu he energy raing is now he sum of he individual energy raings, or 200J. In summary, varisors can be conneced in series providing hey have idenical peak curren raings (I TM ), i.e., same disc diameer. The composie V-I characerisic, energy raing, and maximum clamp volages are all deermined by summing he respecive characerisics and/or raings of he individual varisors. Parallel Operaion of Varisors Applicaion requiremens may necessiae higher peak currens and energy dissipaion han he high energy series of varisors can supply individually. When his occurs, he logical alernaive is o examine he possibiliy of paralleling varisors. Forunaely, all Lielfuse Varisors have a propery a high curren levels ha makes paralleling feasible. This propery is he varisor's series-resisance ha is prominen during he up-urn region of he V-I characerisic. This up-urn is due o he inheren linear resisance componen of he varisor characerisic (see Applicaion Noe AN9767). I acs as a series balancing, or ballasing, impedance o force a degree of sharing ha is no possible a lower curren levels. This is depiced in Figure 25. A a clamp volage of 600V, he difference in curren beween a maximum specified sample uni and a hypoheical 20% lower bound sample would be more han 20 o. Thus, here is almos no curren sharing and only a single varisor carries he curren. Of course, a low curren levels in he range of A -A, his may well be accepable. PEAK VOLTAGE (V) LIMIT SAMPLE LOWER BOUND (20%) SAMPLE UNIT T A = -40 o C TO 85 o MODEL V25BA60 C PEAK CURRENT (A) FIGURE 25. PARALLEL OPERATION OF VARISTORS BY GRAPHICAL TECHNIQUE A high curren levels exceeding 0A, he up-urn region is reached and curren sharing improves markedly. For insance, a a clamp volage of 900V, he respecive varisor currens (Figure 25) are 2500A and 6000A, respecively. While far from ideal sharing, his illusraion shows he feasibiliy of paralleling o achieve higher currens and energy han achievable wih a single model varisor. Pracically, varisors mus be mached by means of high curren pulse ess o make parallel operaion feasible. Pulse esing should be in he range of over ka, using an 8/20µs, or similar pulse. Peak volages mus he read and recorded. High curren characerisics could hen be exrapolaed in he range of A -,000A. This is done by using he measured daa poins o plo curves parallel o he daa shee curves. Wih his echnique curren sharing can be considerable improved from he near wors-case condiions of he hypoheical example given in Figure 25. In summary, varisors can be paralleled, bu good curren sharing is only possible if he devices are mached over he oal range of he volage-curren characerisic. In applicaions requiring paralleling, Lielfuse should be consuled. Some guidelines for series and parallel operaion of varisors are given in Table

14 TABLE 4. CHECKLIST FOR SERIES AND PARALLEL OPERATION OF VARISTORS SERIES PARALLEL Objecive Higher volage capabiliy. Higher energy capabiliy. Non-Sandard volage capabiliy. Higher Curren Capabiliy Higher Energy Capabiliy Selecion Required No Yes Models Applicable All, mus have same I TM raing. All models Applicaion Range All volages and currens. All volages - only high currens, i.e., >A. Precauions I TM raings mus be equal. Mus be idenical volage raed models. Mus es and selec unis for similar V-I characerisics. Effec on Raings Clamp volages addiive. Volage raings addiive. Curren raings ha of single device. Energy W TM, raings addiive. Curren raings funcion of curren sharing as deermined graphically. Energy raings as above in proporion o curren sharing. Clamp volages deermined by composie V-I characerisic of mached unis. Volage raings ha of single uni. Reference For Lielfuse documens available on he inerne, see web sie - hp:// [] Kaufman, R., The Magic of I 2, IEEE Trans. IGA-2, No. 5, Sep.-Oc