A maior rede de estudos do Brasil

Grátis
987 pág.
Beeman

Pré-visualização | Página 47 de 50

tube drterio- 
ration. While t,he effect of voltage change is most, important on the tube 
cathode, it is also undesirable ill ot,her parts of the ririwit. Electrotiic 
circuits, as all other electric cirruits, lost power mparity rapidly if the 
voltage is decreased from rating. Although critiml circuits normally 
contain voltage-regulator tubes and other mealis to hold a constant 
reference vokage in spit,e of line-voltage variat,ions, economic reasons pre- 
vent voltage regulation on t,he majority of rirruits, and henre thcir funr- 
tion will naturally be impaired by excessive voltage variation. This is 
especially true when magnetic sat,uration is part of the roiitrol function. 
I n this group fall solenoids, 
brakes, valves, and rlutrhes. The pull of the a-c solenoid varies approxi- 
mately as t,he square of the voltage. There is some deviation from this 
law, depending upon which part of the brake-horsepower cnrve the sole- 
noid is working. The temperature rise, too, varies approximately a s th r 
square of the vokage. 
In general, solenoids are liberally designed and standard rommerrial 
solenoids are designed to operate satisfartorily on 10 per cent overvoltage 
Effect on Solenoid-operated Devices. 
214 V O L T A G F S T A N D A R D RATINGS, VARIATIONS, CALCULATION OF DROPS 
and 15 per cent undervoltage. Since an a-c solenoid has an inrush current 
of approximately ten times the sustained value when sealed, the branch 
circuit sJpplying it should be of ample capacity to prevent an excessive 
voltage drop. 
The corrective capacity of capacitors varies 
with the square of the impressed voltage. A drop of 10 per cent in the 
supply voltage, therefore, reduces the corrective capacity by almost 20 
per cent, and where the user has made a sizable investment in capacitors 
for power-factor correction, he loses the benefit of 20 per cent of this 
investment. 
Effect on Capacitors. 
Nominal Commonly "red 
Iyllem ulilizolion-device 
volt.ge "Oltage rating. 
RECOMMENDED VOLTAGE SPREAD AT UTILIZATION EQUIPMEN1 
Rased on the foregoing effects of voltage variation on utilization equip- 
ment and an extensive poll of industrial plant operating engineers, the 
AIEE Committee on Industrial Power Applications established the 
recommended voltage spreads at the terminals of devices in industrial 
plants. These are shown in Tables 4.8 and 4.9.* 
TABLE 4.8 Recommended Voltage Spread at the Terminals of Utilization 
Devices in Industrial Distribution Systems 600 Volts and Below 
Recommended limib 
of volloge at terminals 
of ulilizolion devices 
480 440,* 460 420-480 
A00 ! 550,* 575 525-600 ! 
Drsigriations for nominal system voltages are those commonly used in industrial 
* ThPse are standard polyphase-motor voltage ratings. 
t Polyphase power loads may not operate satisfactorily a t this l o m ~ r limit 
In designing industrial power distribution systems, the system design 
engineer should design for voltage spreads not in excess of those mentioned 
in Tables 4.8 and 4.9. If anything, it would be desirable to design for 
closer limits to allow for critical utilization apparatus that may be devel- 
oped and widely used in the future. The history of electricity in indus- 
trial plants has been to extend its use to more and more functions. As 
plants. 
* Thcse rwommcndstions are in iuhstantial agreement with thP recommmdations 
of the joint EM-SEMA Committce whirh puhlishrd their findings in a report, Prc- 
ferrpd Voltage Ratings of AC Systems and Equipmcnt. 
VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 21s 
TABLE 4.9 Recommended Voltoge Spreod at the Terminols of Motors 
Served ot Primory Voltoge 
Nominal syitem 
d t a g e 
Motor-nome-plote 
*oltoge rating 
2400 
2400 ~ 2300" 1 
4160 
2160 
2250 
3920 
4500 
6470 
4800 4600 I 
6900 1 6600 I 
2380 
2480 
4320 
5000 
71 30 
Recommended limits af 
voltage at terminalr of 
high-voltage moiors 
* I'rmrnt standard rnot,or voltagc rating. 
well as driving the utilization equipment, it is alço used for a11 types of 
rritical proccss control systems; therefore, its role is hecorniiig exceedingly 
important, and to fulFiI1 this role effectively, good voltage must he rnain- 
taiiied iii industrial plants. 
L I G H T FLICKER V O L T A G E REQUIREMENTS 
Relatively slom chaiiges in voltage are associated mith voltage spreads 
as discussrd iii tlie foregoiiig. There are, however, maiiy types of voltage 
changes 1rhii.h are of a traiisient nature aiid last only a feiv cycles. Thcse 
are commiiiily referred to as voltage flicker, aiid its primary effect is to 
cause flicker iii th r light ciiitput of lamps. The arnount of voltage varia- 
tioii as a fiiiirtioii of frequency of variation which can be xvithstood on 
iiicaiidesrent larnps aiid not cause ohjei:tionahle psychological effects is 
shown iii Fig. 4.15. These curves were preseiited in the General Electric 
Review, hugust, 1925. 
Fluoresceiit lamps are less suhject to flicker over a range of voltage that 
is beloiv that whirh mil1 piit them out. Iii industrial plants, voltage 
flicker i s caiised primarily hy the followiiig types of load: repetitive motor 
starting, large rei,iprocatiiig cornpressors, punch presses, etc., which dram 
a fluctiiating load; resistarice wcldcrs; aiid arc furnaces. 
To elimiiiatc objcctionable light flicker, the design of the systcm should 
be siich that the lirnits of Fig. 4.15 are adhered to. Wider lirnits may be 
iiscd uiider certaiii coiiditioiis without cornplaiiit from the personnel 
orrupyiiig tlie affei,tcd arca. Ho!rcv&, this subject is so cornplicated 
aiid involved that general guides other than Fig. 4.15 would probably not 
be of much use. 
216 VOLTAGE-STANDARD RATINGS. VARIATIONS. CALCULATION OF DROPS 
FLICKER OF INCANDESCENT LAMPS 
CAUSED 81 RECURRENT VOLTAGE DIPS 
I 
5 
0 
Y 
w 3 
0 
5 
' t 
t- z 
Y 
0 
w 
,' 
a 
0 
D l l O PL" "0"I DlPI PLI1 SECOND 
I F C O Y D L 
10 82 6 J 2 I 30 12 L 
Y l U U l L I 
T IME BETWEEN DIPS 
FIG, 4.15 Relation of magnitude of voltage dips to frequency of dips for incandescent 
IWlPS. 
METHODS OF REDUCING VOLTAGE SPREAD AND FLICKER 
REDUCING VOLTAGE SPREAD (See Fig. 4.271 
\Vitlr recommended values of voltage spread established by the N E E 
Industrial Power Systems Committee a i d EEI-SEA\Z.%, it is possible to 
study specitiv syst,ems to see hon. they romparc with these rcquiremerits. 
Where voltage spreads arc found t,o be heyorid t,hose limits, there are four 
11-ays of reducing the voltage spread. 
1 . Carry the power further a t a higher voltage and a t a lesser dist,aim 
at 1o\vcr voltage, i.e., use the load-center power system. 
2. 1tediii.p the impedance of the systrm. 
3. Use regiilat,iiig equipment to rompelisate for volt,age drop. 
4. Use s \~i t i~I ied capacitors. 
llaintaiiiiiig the volt,age at an average desirable I e i d also requires the 
judicious use of traiisformer ratios and taps. Traiisformer taps (for 
changing a t no load oilly) do trot, reduce the spread but affect only t,he 
general voltage level arid particularly the light load voltage in the plallt. 
VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 217 
Load-center Distribution Systems. The load-ceiiter distribiitioii sys- 
tem is 11011- almost uiriversally used i n industry for, among othcr reasons, 
it provides Ion- voltage drop, henre small voltage spread, berausc the 
power is carried right to the load i.etiter at high Iwltage. Refer to Chap. 
11 for a one-liiie diagram of a typical load-retiter system. 
I t 
is obvious from this table that the tiig gaiii is made by going from voltages 
iii the (i00-volt class to voltages il l the 2.4- to 13.8-kv class for rarryitig 
poll-er from the source to the load ceuter. To illust,rate