Document o93qwnZZKGm6rrx1m3DMQ40mR

'/a | motors: characteristics With new insulations, steels, designers obtain more motor With 85% of Industry's machines powered by motors, changos in practice have wide significance. One is a swing to lull-voltage starting, even for motors up to 3000 hp. This means simple, toss-costly control, less maintenance. But before adopting it, check your generating equipment or utility supply to be sure it can handle the starting currents. Another trend: Growing use of ad{usiable-speod equipment and dc motors meets the precision production-volume demands ol modern machim Perhaps most important: Motor manufocti pack, more horsepower into each pound of This has been going on for years. But, as ot J< I9S4, you'll see the first trickle of now NEMAt**, motors coming into industry. Some will fit tvtoe'l much horsepower into the same sizes. Today's frames pack more hp Tor the time being, this NEMA rorating ol motor* 1b conlined to Integral alias to 30 hp. How did It come about? Remembot. many motor Irame iliei used today were established 23 years ago. During that time there hove been two significant advances: better Insulations and belter lamination materials. Motor insulations, bosed on 1929 standards. Included cottoncovered wire, paper and cotton slot and phase coverings. Impregnating varnishes were made of natural resins, oils and solvents. So to get sultoble dielectric strength, a considerable mass ol Insulation was needed. Insulations today incorporate synthetic materials. For ex ample. today's synthetic-resin coating on wire is thinner but olfors higher resistance to leakage than Ihe old doublo-colton covering. Synthetic materials in stole, between Ihe phases and in lead protection, are doing an all-around better Job. Synthetic vamlehet more thoroughly impregnate, bind and protect the windings. Thus synthetics mean less Insulation mass Is required. Less mass ol Insulation means more copper can be put into a stator slot. Right off, this gives a stronger magnetic circuit. But a big secondary advantage is In heal transfer. Less mass el insulation means greater heat transler from copper through to the iron and oil to the cooling medium. Better lamination materials give reduced Iron lose. These rerated motors will operate at higher flux densHies. meaning lees iron required lot a given horsepower. But while frame sixoe come down, shafting sizes hold the same. Why? There have been no comparable advancos In shotting design and materials, so more output from a given frame site means a proportionately larger shall. This NEMA rorating is a rocommondatlon only, so all manu facturers don't have to adopt it. But they will. To you, rorating will mean need to slock additional tenewal parts and some additional motors during the transition period. Will you get tho same degroo of liberality In design? Yes. More honepower Is being packed into smaller frames, but each motor will hold to oJI existing NEMA performance standards cover ing torque, starling current, temperature rise. otc. For most jobs, oc motors suit To gal the proper perspective on today's motor picture, remember that Ihe ac polyphase induction motor is used in 9 out ol 10 Industrial applications. This group includes the ver-populor squirrel-cog*, the wound-rotor, and the adjustable-speed commutator motor. The squirrel cage Is the granddaddy and the widest used. In one form or another. It drives more machines In Industry than any other motor type. It is simplest in eonsj'ucilon, has lowest wearing parts. This boils down to low maintenance and low first cost. The usual squlrrel-cago motor rang at constant speed. The "muftf-epeed" type has additional windings, permitting ft to operate at two or more fixed speeds. Whatever the speed. It it held reasonably steady rogardless el load changes. And because most Industrial machines do run at a constant the on* or more fixed rates available troen a squirt make it sellable lor many applications. The second polyphaso induction machine is the rotor, often coiled Ihe sllp-rtng type. Speed con be adjS but will vary with load when tar below rated speed * wound-rotor type fits on a conveyor drivo, lor example; k can be increased or decreased to meat changes in the |, Keep it in mind also on power systoms with limited capodj It provides high starting ability with lew inrush current. <, Third in Ihe polyphase Induction group is the odjustafalj speed commutator motor. Although little used compared L. its brothers In the induction group, this motor oilers a tpesd] variation of about 4:1 (or conilnous operation and over lor Intermittent operation. This type olten tokos ihe'pJaeeoT a squirrel-cage motor combined with an odJuttabU-speed' mechanical transmission. * Whsn it comas to synchronous motors, w* find most of thso' driving heavy machine*. They can start and oceelerm* sts ihe heaviest loads, pull ihem at constant speed. About SCO rpm it dividing line belwoon so called low and high speed Many plants today are spedliylng synchronous motors with' 0,9 loading power factor lor all btg drives. In addition to constant speed and power lector improvement, synchronous motors are also used because: (1| they olten cost lass than ether motors when property applied 12) they oiler a power- cost saving. This results from high eftidoncy " up In ihs JJ to 67% range. But when examining the high-oMctency angle keep in mind the somewhat greater maintenance resulting from dc exdtation, brushes, slip rings and rotating field For speed control, dc gets nod Where an ac motor won't do the fob satlslactorily. consider a dc motor. They are premium priced products, espedally to larger sizes. But they oiler topnotch adjustable speed, eifedivs and simple control oi torque, acceleration and decelsroltoa Even with limitations ol a commutator they eon and .do handle tough duty cycles. Yes. the dc motor Is growing rather than slipping. Need tv critical speed adjustment in continuous processes olten finds dc motors operated in tandem with speed ot each unit chang ing proportionally over a wide range. Speed can be adjusted in the dc motor in two ways. Arena- lute voltago can be varied, ot you can vary Held current In some cases, both are varied. Raising armature voltage spoeds up the motor, raising field current slows It. A lew words about dc motor size: Rule ot thumb says lh* maximum economical size (s not exceeded as tong as product ol horsepower and speed ts tees than 1.5 million. Vollogo-wlxe. think in terms ol 250 volts (or units up to 210 hp, 690 v lor 600 to 1000 hp, 700 or 900 v above 1000 Hp. Whero do you get the dc voltage? A close lolly today shows that 95% ot all dc motors operate tram ac systems, throvg conversion equipment (see pp 120,12)). HO TODAY'S EUCTIICAI MACTICt fOWfl Synchronous or induction}' ' One of today's big motor questions is whon to uso a syn chronous unit and when an induction. Here's the thinking: Synchronous motors eon be used io? any application a NEMA d*ign-B squitrel-cage will handle. But whether a synchronous should be used it another matter. Cost-wise, here* a handy rule: Synchronous motors are fees expensive than squlrrel-cago if rating exceeds i hp per rpm. But this doesn't toke Into account the higher power factor and etilcieney ol tho synchronous motor. These two (actors alone are mighty important in low-speed motors USO rpm ot lets), where pi and efficiency of squirrel-cage motors are poor. For especially low starting kvo, when controllable torque, kva or variable speed is wanted, either a wound rotor or a constant-speed motor and slip coupling may be used. Power lector and efficiency of today's 3600-rpm squirrel-cage motors are good. And this holds too for 1600 rpm. So unless pi Im provement Is especially important, squirrel-cage motors are logical first choice. But rarely will you find induction motors doing a good Job below 500 rpm because ol low pi and effi ciency; synchronous motors, with leading or unity pf. show good efficiencies down to 72 rpm. What about teading-power-facior suitors? They're generally lees efficient than unity-pf. But they, do double duty. Uso them in your plant when: (I) you need' pbwer-loelor correction (2) you want motors to handle exceptionally high starting, pull-in Or pull-out torque. summary of motor characteristics and applications dfc arrdf tlnglo-phase motors I 2- and 3-phtne motors ten . 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