Document ByMoge3QxmxNXkZrpKXLRr0k

tourcet of dc ILL! Industrie! applications ol dc grow, nof only among past big users like steel and aluminum but also in general. Reason: To meet needs lor degrees oi speed variation to be had only In dc motors, Recent thinking favors a number ol dc conversion points close to load centers, with short dc second aries, instead ol central conver sion. And equipment totlows the modern "packaged" theme. For sizeable applications, the package is a lulJ-fJedged unit substation, with an ac circuit breaker, step- down transformer, dc conversion unit, dc main breaker and smaller dc feeder breakers. Static roctifiere, with high over-all elUdohcy and low maintenance, gel the oall for many of today's jobs. In sleol mills, for example, mercury-arc units are maior source of 2S0-v dc and also appear to be taking over on main drives of con tinuous mills. But where reversing and variable speed ate needed, as on blooming mills, mg sets still serve best. Four basic conversion units out- IW lined below have their spedoft-l tuos but there are areas oi "oYtr-ll tap" -- where more than do the job reasonably well. ` s? Selection. First consider load onsetry" and then its electr^ characteristics: voltage, cunejft etc. Need lor widely variable v$' tag might point to an mg heavily fluctuating load with quent severe overloads might e to consider ambient con- ,^/spcwe requirements, and *fat}??TM!-pfosent {actors -- lirf operating cost. Senecas, Jn reduction of ''K--1V, pcoowwser outages ol cF^reVtUriieodni a few seconds can't be While not heccssorjjy i -l(jsls/crif,caf' ther dc ser- nomiefi won't justify cost ol addi tional units? Then use sfngle-pole high-speed anode switching to isolate individual tanks or tubes ol mercury-arc unit. This keeps it on line in spite of an arebaek or failure in one lank. Magnetic amplifiers arecoming in to conversion picture, particularly above 600 v. A small unit con pro vide an output proportional to dc current, hence measure it without vot iecV\Uer*. So would Biicqt^y: periods of no load or light Ida rectifiers have relatively low ni^i toad and partial-load losses. taking high-voltage dc to the b Ptotected against board. Similar idea can be used :'*i5linipliwt. Possible answers: (11 lor volts, watts, watthours. How rectifiers work Look on today's roctillors as #e(etff/v* electric valves. They allow only the positive portion ol an oc wave to flow through to the positive sldo ol the de bus. The negative wave is ei ther completely blocked (hallwav* reclitieaiion) or led to the negative dc bus (lull-wave rectification). The lull-wave recritter thus uses both sides ot the incoming ac voltage wave. 31/n today's load-center-localed ac-dc conversion station, italic' ^ij^role ac teed. (2) duplicate $&\&tcfertsilfoon resqufiprmeeqnl.uBeotnh tallyso gel the nod, but mg sols are still important ,\Ssipv; during scheduled mainte- ^narics shutdown. But what if eco- Before 1937, oil commercial mercury-arc remitter* were ot multl-anode type, with 6, 12. or 16 anodes per tank. Coming ot the more-efficient single-anode type (first tho ignftton. then the excitron! broadened applications in tower voltage range -- 200 to 300 v. The putnplets lube pushed practical usefulness to still lower ralings. tgnltron's distinguishing lecture Is n small stationary elec trode Immersed In cathode mercury pool. This Ignitor starts the cathode spat. Once established at mercury surface, main anode can pick it up and conduct in one direction. Method ot creating cathode spot dillers In the excitron. While rectifier operates, a pilot arc tram the oxcilaslon anode continuously maintains spot on the mercury sutlace Basle elements ol any metallic rectifier are a good conductor . `b spite oi advances In the rectifier art. and growing applica and a poor one (sotni-conductor), with a barrier layer. Current tion of static units, mg sets still handle a good t>ulk oi the flows more readily from semi-conductor lo good one. .\*Y y.fead- Big reason; An mg set can deliver anywhere from taro Advantages are; ruggednass, no' filament excitation, no .S' Bj'te.aaxiiBum voltage. In general, voltage variation is obtained time delay, long Ulo, high eliictency. But melallics ore likely by changing lield ilrongth of generator. Any reasonable volt- lo cost more than tubes, show poorer voltage regulallon and -'*5 eg# can be had by taking advantage ol the characteristics lower efficiency at high voltages. They lack grid control, weigh > ? dshunt.over- or undor-compounded Connections. more, lake up mote space. \u |V-.. Ambient condition* -- temperature, duet, humidity, haxard- Today's metafiles include magnesium.capper sulfide, cop -yrH atmospheres -- are Important lor mg sets. Temporoture by per sullide, copper-oxide, selenium. Germanium, a newcomer, .:p| llisll is a limiting lactor in insulation life, but modern insula- ie now available In unlit rated to 23 kw at )2S v. Metallic .(tons (p 42) have this licked; Close H lo I0OC. Various types rectifiers find use fn battery charging, resistance welding. ` y). -].of enclosures (pp 40. 4l) cope with prohloms ol watet, hax* They supply dc lor email motor loads In increasing number y dou atmospheres and explosive conditions. Mechanical rectification was Introduced in this country just oiler World War II. and Ihora ate still only a limited number of units in operation. The rectifying element i* e set ol metallic contacts driven by a synchronous motor. These close when ac sine wave Is In desired direction, break contact when ac voltage reverses, fn other words, it Is a motor-driven switch, connecting and disconnecting ac ond de systems in synchro nism with sinusoidal variation of ac. Major problem it to open and close contacts only when ae wave is passing through sera, to minimise wear on contacts. This is now done by using a saturable or "commutating" reactor Inserted between iranslarmer terminals and oc bus. This stretches the tero-curtertl time ol the ac wave. 4 <0 Sealed or pumple., mercury arc. gain wide acceptance far .mall, medium load. With top efficiency promised, development of mechanical rectifier, goet ahead tlVi*' Simple, reliable and easy to maintain, sealed or pumpiess mercury-arc units are gaining wide acceptance on small to medium loads (730-1000 kw). Typical unit In metal-enclosed cubicle comprise* a number ol sealed tubes or tanks, associated eicitatlon equipment and aceeseoriee. Tube* are evacuated. degassed and sealed at the ladory. This eliminoies vacuum connections, equipment lor maintaining and Indicat ing vacuum, cuts maintenance. Tubes are water cooled. Alter passing through tank lockets, waler goes lo a heal exchanger before recirculation. Raw water /lowing through exchanger to drain carries away the hoot, Direct cooling by raw-waler supply is also employed. Where water is scarce, waleMo-ajf exchanger* can be used. Performance. How long do tubes lasl? Roughly five to seven years if properly applied and operated. Manulaclurers generally guarantee lor two years on a prorata adjustment basis. In genetol. \ho sealed motcury-ate rectifier will now show about 7% better full-load efficiency than an mg set. This wilt vary to approximately 19% at quar ter load on 600-v service. For 230-v service, efficiency curves stiff show a strong advantage for mercury-are units compared to mg sale, But pumpfeas mereury units have in herently lagging pf. In contrast, an mg set boasts pi between unity and 60% leading. Watch this II your utility con tract has a pi penally clause. Ai fuff load and rated voltage, a pumple** rectifier will run about 87% lagging. A bank of eopadtors (230 kvar (or 10 kw) would raise this (o 95%. ' Obcleney U the big reason industry . vatebss development of meehonJeoJ rec tifiers, Values ol 94 to 97% (over-all. ac to dc bus) are recorded. This is a major tOArideratfon where dc loads ore heavy, In electrochemical plants. Mechanical units handle fluctuating kd% between Untllx but axe critical an .. overload. Normal voltage span o/ to day's unlls is 3- lo 400-v de. Current ranges between 1500 and 10,000 amp; units are paralleled for forger toads. Any fixed voltage drop penalises ree. Hfler efficiency increasingly as voltage is lowerad. Mechanical recllllore havo no constant voltago drop anywhere tram ac to dc bus. That's a reason lor slant ing lo applications under 400 v. Pine voltage centre! can bo had, with quick response. Two methods are usod. One regulates supply voltage with an induction regulator or transformer lap changer. Second method, known a phase shift, delays point in ac wave whoro commutation starts. Rotating the stator of the synchronous drive mow does the job. Usual practice changes supply voltage lor coarse regulation, with phase shill lor close adjustment. Two limitations are; 1 k> Unit can't stand a current te*nOI lo remitter Hsell. (2) II ae teed Is lost, unit will shut down and some of silver contacts will generally need replacement. Today's maintenance problems center about the main silver contacts. Avail able liguros peg maintenance al about lYtYe ot total roefjfler cost epreod over a 5-year period. II the drawback ol short contact til* can be ((eked, as now eeetne indicated, we'll have another major rec tifying device to serve Industry. rooAr* KfOftcu fiACiiCf eown l\ OCTOSES 1*M TODAY'S (IIOIICAV MACItCt Ill ia&t