A paper presented at the 22d Annual Conven­tion of the American Institute of Electrical Engineers, Ashevilli, N. C.,fune 19-23, 1905.

Copyright 19C5. By Α. I. Ε. E.

N O T E S ON T H E P O W E R - F A C T O R O F T H E A L T E R N A T « I N G - C U R R E N T A R C .

BY G E O . D. SHEPARDSON.

The alternating-current arc presents a wide field for research because of the many variable elements. During the last year the effect of the wave-form of electromotive force applied t o the terminals of a standard enclosed arc-lamp, and the ele­ments affecting the power-factor, were studied in connection with the graduating thesis work of Messrs. L. S. Billau, C. B . Gibson, Ε. H. LeTourneau, and R . Morris at the University of Minnesota. Some of the results obtained seem of sufficient interest for presentation before the I N S T I T U T E .

Electromotive forces having sine, flat-topped, and peaked wave-forms, Fig. 1, were obtained from a 7.5-kw., 60-cycle,. General Electric double-current machine provided with three interchangeable sets of pole-pieces.- Another wave-form, shown in the oscillographic curves in Figs. 5 to 8, was obtained from the circuit of the local lighting company . Current from these sources was sent through an enclosed constant-pressure lamp of standard manufacture, or through a hand-feed lamp having in series the reactance and feeding coils of the standard lamp. With the latter, all the conditions of a standard lamp are re­produced, except that the length of the arc is controllable.

For part of the experiments, wave-forms were recorded b y a modified Duddell oscillograph; unfortunately this broke several times during the progress of the work, and many of the tests had to be made without oscillographic records.

Much difficulty was experienced from lack of uniformity in the carbons, although these came from a well-known maker o f high-grade carbons and were supposed to be of the best quali ty.

651

052 SHEPARDSON: POWER-FACTOR. [June 22

In fact, the irregularities in the carbons entirely masked some of the variables which were being studied, and led to results so erratic that only a small part of the investigation gave re­sults worthy of confidence. Variations in the performance of the carbons affected the candle-power of the light more th?n did changes in wave-form.

ELECTROMOTIVE FORCE WAVE-FORMS

FIG. 1.

Study of the power-factor of the arc under varying condi­tions shows that it is practically independent of the form of the electromotive force applied to the lamp terminals. The power-factor of the enclosed arc with soft cored carbons is found to be practically constant for current strengths between 4.5 and 8.5 amperes with 7/16-in. carbons, varying between 95 and 99 per cent. See Fig. 2. The power-factor of the

1905.] SHEPARDSON: POWER-FACTOR. 653

whole lamp-circuit, including reactive coil and feeding coil, is practically independent of the wave-form of impressed electro­motive force, being higher for small currents and lower for large currents. The change in the power-factor of the lamp-circuit, from 76 to 43 per cent, in this case, is evidently due to the smaller resistance of the arc with larger current, the reactance of the circuit being sensibly constant within the range con­sidered. With the same grade of soft cored carbons and with open arc, the power-factor of the arc increases from 94 per cent, with 5.5 amperes to 99 per cent, with 8.5 amperes, Fig. 3, being independent of wave-form of impressed electromotive force, at

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least within the limits of accuracy available. With open arc and with copper-plated hard carbons, such as commonly used with open arcs, the power-factor of the arc varies from 80 to 90 per cent. See Fig. 4.

The power-factor of the arc is found to remain constant when the arc pressure is constant, whatever the range of current and whatever the wave-form of impressed electromotive force. Likewise when the length of the arc (measured b y the image thrown by a lens) is maintained constant, the power-factor of the arc is independent of changes in current and in wave-form of electromotive force applied to the lamp terminals. The quality of the carbons and the exposure of the arc to the air affect the

6 5 4 SHEPARDSON: POWER-FACTOR. [June 22

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1 9 0 5 . ] SHEPARDSON: POWER-FACTOR. 6 5 5

power-factor of the arc considerably. The distortion of the cur­rent wave-form doubtless affects the wave-form of electromotive force developed b y the transformer or b y a small generator.

FIG. 5 .

The effect of the lamp coils doubtless tends to make all impressed electromotive force waves alike when they reach the arc. Future investigations are expected to determine the wave-form of

F I G . 6.

electromotive force actually applied to the arc under varying circumstances, the effect of changing it, and the reactions of the arc upon the source of electromotive force.

656 SHEPARDSON: POWER-FACTOR. [June 22

F I G . 7.

parture of the arc power-factor from unity is evidently due to distortion of the current and electromouve-force curves, tl.ere being little if any phase difference. In some of the recoups

F I G . 8.

there is a slight unsymmetrical displacement of zero values. Duddell and Marchant show similar displacement of zeros in their paper before the British Institution of Electrical Engineers

B y means of a modification of the Duddell oscillograph, curves were obtained showing the variations in the instantaneous values of line pressure, arc pressure, and current. The de-

1 9 0 5 . ] SHEPARDSON: POWER-FACTOR. 657

(Jour. Inst. Elec. Eng., Vo l . 28, pp . 60 to 86, 1899). This phe­nomenon seems not to have been explained satisfactorily, and suggests a field for fruitful investigation.

The hard carbons show a much more sudden rise in the differ­ence of potential around the arc than do the soft cored carbons or soft solid ones. Characteristic of the soft cored carbons is the secondary peak on the arc potential curve, occurring nearly simultaneously with the max imum current value and thereby raising the power-factor. Differences between the positive and negative loops are probably due to slight differences in the composit ion of the carbons. The more gradual growth of the current wave with the long arc will be noted, probably being due t o the more rapid cooling of the arc gas during the extinction. Wi th long arcs there is a tendency toward a double peak on the front of the potential wave. This portion of the wave changes greatly with the condition of burning and with irregularities in the carbons. Wi th the short arc the potential wave rises to a marked peak at the rear. The current waves are concave to the axis when rising and convex when falling, due to the resistance being less in the hot vapor of the decreasing arc than in the cooler vapors of the increasing arc. The effect of the inductive action of the reactive coils is plainly seen in the lag of the current (marked " I " ) and arc potential (marked " P. D . Α . " ) waves behind that of the impressed electromotive force (marked " P. D . L . " ) in Figs. 5 to 8. Wi th the hard carbons and open arc, the arc potential waves vary considerably with the length; with short arc the potential rises almost vertically to a high peak, is nearly horizontal on top , and*rises to a peak at the rear; with the long arc, the peaks are not so prominent'and^the potential wave does not rise so rapidly.