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full load rediirrs thr lifr of the insulat,ion. Principal Effects of High Voltage on Induction Motors. The most, significant efferts of too high voltage are inrreased tnr(lue, inr,rrasrd starting rurrent, and decreased porer factor. The increased torque may muse rouplings to shear off or damage t o driven equipment. Increased starting curretit raiiscs greater voltage drop in the power system, henre increases light, flirker. Uecreased po~vzr factor is particularly disadvantageous where power-fartor peualty rlanses Effect on Induction Motors. Principal Effects of l ow Voltage on Induction Motors. 206 VOLTAGE-STANDARD RATiNGS, VARIATIONS, CALCULATION OF DROPS TABLE 4.6 General Effect of Voltage Variation on Induction-motor Characteristics Starting and maximum running torque... ................. Synchronous speed.. .......... Per cent dip. . ............... Full-load speed. .............. Efficiency: Full load.. ................ 9% load. .................. )i load . . ................. Full land.. ................ I( load.. ................. 36 load. .................. Full-load ~urrent . ............. Starting wrrenl . . ............. Temperature rise, full load. ..... Maximum torque capocity.. .... Magnetic n0ire.m load in parlicu- lor.. ..................... Power faclor; I Voltage Variotion Decrease 19% No change Increase 23% Decreore 136% Decrease 2 points Proclicolly no change Increase 1 to 2 point$ Increase 1 point Increase 2 lo 3 point! Incrcoie 4 lo 5 points Increase I1 Yo Decrease 10 to 12% Increose 6 to 7 C Decrease 19% Decrease slightly 90% voltage Functionof voltage 110% voltage (Voltage)’ Cons1.nt 1 (voltagel~ ISyn. speod--.llpl InCreOle 21 No change Decrease 17% Increase 1 % .............. .............. .............. .............. .............. .............. .............. Voitoge IV0l togeJ~ .............. .............. Small increo*e Decrease 1 to 2 points Procticdiy no change Decrease 3 points Decrease 4 points Decrease 5 lo 6 points Decrease7% Inc,eo.e 10 to 12% Decrease I lo 2 C Increa3e 21 % Increase slightly This table s h o w gencral effcets, which will vary somewhat for specific ratings. are applied by the utilities. The higher the motor voltage rises, the lower the power fartor mill become. This may result in a greater penalty and hence a higher power bill. While the temperature rise at full load on standard motors decreases slightly for moderate overvoltages, the temperature rise may increase on certain types of sperial motors a t even very small overvoltages. Over- voltages of 10 to 1.5 per cent have caused numerous burnouts on special four-speed grinder motors. Motors rated for intermittent load are also materially affected by overvoltagcs. While marry drive applications are not seriously affected by voltage deviations as much as plus or minus 10 per cent from rated voltage, there are import,ant applications that are. Effect on Synchronous Motors. The effect of voltage variation on the performance of synchronous motors is similar to that on induction motors. However, while t,he starting torque varies as the square of the voltage, the maximum or pull-out torque varies directly with the voltage. From the above discussions it will be noted that, in general, voltages slightly in excess of motor name-plate rating have less detrimental effect VOLTAGkSTANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 207 on motor performance than voltage helow the name-plate rating. This is one of the bases on which the voltage spreads in Table 4.9 mere deter- mined. A s an example, the figures show a recommended spread of 420 to 180 volts for the 480-volt nominal system voltage, which is approxi- mately 4 per cent below and 9 per cent above the 440-volt motor rating. The light output and life of incan- descent filament lamps are critically affected by the impressed voltage. In Table 4.7 is shown the relationship of lamp life arid output to voltage for a vokage range from 80 to 120 per cent of rated voltage. In general i t may be said that for incandescent filament lamps a 1 per cent deviation from rated voltage causes a change of 3 to 335 per cent in light output. It can be seen from Table 4.7 that a 10 per cent reduction in lamp voltage results in a 30 per cent reduction in light output. In other words, when the voltage is 10 per cent low, the investment in the lighting system is working at only 70 per cent efficiency-thus, 30 per Effect on Incandescent lamps. !i! 2 3 a 0 c PER CENT NORMAL VOLTS 3 9 a FIG. 4.10 average of many lampr. Characteristics of large gar-filled incandescent type C lampr. There are the 208 VOLTAGbSTANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS cent of the investment is lost. With an overvoltage of 10 per cent the lamp-life is reduced to less than oue-third~-t,hus lamp-replacement costs are three times as great as a t normal voltage. Other dat,a arc shown in Fig. 4.10, from which it, should be noted that the lumens per watt., or lamp efficieilcy, rises sharply at voltages above 100 per cent. In some cases, operating eronnmies result from hurriing lamps at higher efficiency and short life, or vice versa. TABLE 4.7 Effect of Voltage Variations on Gar-filled Incandescent-lamp Choracteristics 80 85 90 92 94 96 98 100 102 104 106 108 110 115 I20 Socket voltage ~ 96.0 102.0 108.0 110.4 112.8 115.2 117.6 120.0 122.4 124.8 127.2 129.6 132.0 138.0 144.0 ~- 47 58 70 75 81 87 93 I00 105 115 I20 I30 I40 I60 185 Per cent Per cent rated rated voltage lighl output Per <en1 heoretical* life 1900 850 400 300 225 170 130 100 75 60 45 35 30 15 10 * Throrrticnl l i fv in thc nhsrrrcc of any mcrhanicnl hrcakagc.. In onlinary sprvire, mcchanird hrrakage r cdn r r s t h p l i ip expectanrc a t tlrr lo\ver roltagrs. The changes in lamp characteristics iI-ith rariatioii in cirruit, voltage arc given in Fig. 4.1 1. III general, 1 per cent variatiim i n line voltage n-ill changc t,he lumeir oudput only about 1 per cent. Toltage is a factor in starting reliahility, and voltages l o w r than recommeiided may result in unsatisfactory starting. It will be noted that the ores-all efficiency (if the fluoresrerrt, lamp decreases if the line volt,age is raised above normal. The increased line volt,age causes the choke t,o pass more current to the lamp. This loivers the resistance of the arc. column, rcsulting in a lower voltage drop i n the lamp itself. The input, Ti-atts t o the lamp are slightly increased, and t,herefore the lumen output increases over a cert,aiii range. In this condition, however, the higher currcnt density priiduces the short ultraviolet radiation less effirieutly; wilserpently t,he luminous efficiency of the lamp decreases. Effect on Fluorescent lamps. VOLTAGGSTANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 209 Fluorcsreiit lamps are fa r less af'ectrd hy circuit voltage variatioli tllan filament lamps, from t,he standpoint of life. The life of preheat-type lamps should he quite satisfartmy throrlghollt, the range of published voltage fur the various I ial lasts; tlicsr volt:tg(. ranges, iii general, are 110 ti] 125, 100 t o 2l(i, 220 to 250. :ind 240 t o 280. There may be some derrease i n life performalire \\-3ir11 opwitcd a t maxi- mum vokage as compared with that, a t miiiimiim vdt:igr. I I ~ i ~ e v t ~ r , there are a numher of other fartors, SWIM: of whidi arc i ~ i r p r ~ ~ d i r ~ t a l ~ l ~ ~ , tlr;lt,
