Electric power in a steel plant
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Figure 6. Load circuit of a resonant-circuit constant-current regulator is suddenly short-circuited
Figure 7 (r ight). A resonant-circuit constant-current regulator is suddenly switched on a ful ly loaded Mazda-lamp circuit
provided the input voltage is held constant. If an input and an output transformer are used, the constancy of the output current is decreased as a result of the input-trans-former-voltage regulation and the change in current transformation ratio caused by the magnetizing current of the output trans-former. The magnetizing current increases with increasing output load. Thus the out-put current is somewhat less at full load than a t no load (load circuit short-cir-cuited) . For these reasons the Air Forces permitted a current variation of plus or minus two per cent.
The highest constancy of the output cur-rent occurs for noninductive or slightly capacitive loads. Heavy inductive con-stant-power-factor loads decrease the ratio of full-load current to the short-circuited current (no load). But constant power-factor capacitive loads increase said ratio, and it may reach undesirable values for ex-tremely capacitive loads. Figure 3, A and B, explain said relations.
An inductive or capacitive output load not only effects the constancy of the output current but also effects the voltages on the capacitors and reactors of the monocyclic square. For inductive loads the capacitor voltages are higher and the reactor voltages are lower than for noninductive loads of the same kilovolt amperes. For capacitive loads, the capacitor voltages decrease, and the reactor voltages increase.
Calculations based on the analytic methods developed by Steinmetz are very accurate. However, in practice a quicker inside view of the operating characteristics is obtained by vector diagrams and particu-larly by so-called circle diagrams. Figure 4 shows a circle diagram for an L-connected circuit. This diagram together with dia-grams for a monocyclic square and other special cases appeared in a paper by H. Weichsel.1
Mr. Kevern touched in his paper on the effect of frequency changes on the output current. He cites a test on a regulator oper-ating at 50 and 60 cycles.
The theoretical relation between the out-put current and impressed frequency for a monocyclic square without input and output transformers is given by Figure 5. With input voltage held constant, the output cur-rent varies only slightly over a very large range of frequencies. I t is a simple matter to consider the effect of input and output transformers if used.
Figure 6 shows the change in useful out-put current and voltage when the useful load circuit is suddenly short-circuited.
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Figure 8. A ful ly loaded resonant-circuit constant-current regulator is suddenly discon-
nected from its supply circuit
The effective value of the output current changes only slightly during the very short transient period. The output voltage dropped to zero at the instant the short cir-cuit in the load circuit occurred.
Figure 7 shows the output current and voltage as well as input current to the regu-lator when regulator is suddenly switched on a fully loaded circuit. Only a small change in the output current occurs during the very short transient period. The out-put voltage increases gradually due to the temperature increase of the Mazda lamps' filaments. The latter results in increased watt output. Consequently, the input current to the regulator increases nearly proportional with the output voltage.
Figure 8 represents, in the two upper pic-tures, the output current in the useful load circuit when the unit is suddenly discon-nected from its supply circuit. The lower picture shows the corresponding input cur-rent to the regulator. The complete ab-sence of oscillations or transients is note-worthy.
The opening of the input circuit of a fully loaded monocyclic square results in only a small arc, contrary to the experience when opening the input circuit of moving-coil transformers. This difference is partly brought about by the high power factor of the input current to a monocyclic square, whereas the input circuit to a moving-coil transformer is highly inductive.
The foregoing gives some of the funda-mental laws which must be considered in the design as well as the operation of constant-current resonant regulators. There also exist certain problems in the manufacturing of these units. For instance, it is readily realized tha t neither the required capacitors nor the reactors can be manufactured to ex-actly the specified values. Theoretical study carried out on this very subject has shown that , in spite of said manufacturing variations, it is possible by suitable means to obtain a surprisingly high degree of con-stancy of the output current.
Mr. Kevern's paper forms a very impor-tant contribution to the problem of constant-current a-c circuits and doubtlessly will re-sult to a large extent in a better understand-ing of the great field in which constant-cur-rent resonant circuits can be used very ad-vantageously.
R E F E R E N C E
1. C O N S T A N T - C U R R ^ T REGULATORS, H. Weich-sel. Wagner Electric Corporation, St. Louts, Mo., 1941.
Electric Power in a Steel Plant
Discussion of paper 45-140 by R. . Graham, published in A IEE TRANSAC-T I O N S , 1945 , December section, pases 8 1 6 - 8 .
C. G. Archibald (Westinghouse Electric Corporation, Buffalo, N. Y. ) : This paper presents in a condensed form the problems involved and the methods pursued by Mr. Graham and his associates in solving a very difficult problem in continuity of service. This paper shows tha t very thorough studies have been made of the system load requirements, the method of relaying, switching, and other problems associated with large load in a very small area, handling processes that require the highest degree of continuity.
The magnitude of this load may call to your attention that this is no small load; being, on the order of the amount of power consumed by central station load at peak, by a city of 350,000 to 400,000 population. However, it does have two unusual features about it that makes it a more interesting load. These arefirst, the very high load factor from 24-hour operation, on the order of 70 to 75 per cent; and second, practically unity power factor at all times.
994 Discussions AIEE TRANSACTIONS
The solution of operating problems by using fast relaying (both phase and ground), and sectionalizing of the plant, with double feeds wherever possible, appears to be the most economical and practical method of Tapid sectionalizing and maintenance of continuity to critical loads.
If all industrial plants placed an equal amount of emphasis on their electrical engi-neering problems, and solving these as we1l as this property has, greatly improved service would be maintained.
Capacitors for High-Frequency Induction-Heating Circuits
Discussion and authors* closure of paper 45-157 by F. M. Clark and M. E. Scoville, pub-lished in AIEE TRANSACTIONS, 1945, November section, pages 791-6.
Philip Bliss (The New Britain Machine Company, New Britain, Conn.): The writers have presented the problems of timing capacitors for high-frequency indus-trial use very well. The new oil and new method of construction make a much smaller, neater capacitor than those in present use employing mineral oil dielectric. Incidentally, the 15-kw industrial heater shown appears to be of far superior con-struction than previous models.
L. W. Foster (General Electric Company, Schenectady, N. Y.) : There are two points that I should like to make in connec-tion with this paper. First, Lectronol is a synthetic dielectric liquid whose character-istics may be controlled very closely in the manufacturing process; whereas mineral oil and castor oil are natural materials which will have a considerable variation in elec-trical properties depending upon their source of supply.
Secondly, the difference in dielectric strength a t high frequency of solids and liquids can be attributed mainly to thermal effects. The dielectric liquid is a good thermal insulator but in the case of the parallel-plate capacitor free circulation is possible, therefore, hot spots will not be readily formed. In the case of solid insula-tion like styrene or mica the hot spot-temperature may be very high compared to the capacitor case temperature, there-fore, the solid dielectric will have lower breakdown strength than the equivalent liquid dielectric because heat cannot be transmitted rapidly enough through the solid dielectric material.
L. J. Berberich (Westinghouse Electric Corporation, East Pittsburgh, Pa . ) : Every manufacturer of high-frequency heating equipment, during the war especially, has felt the need for a capacitor suitable for application in this type of equipment. Messrs. Clark and Scoville have presented a very satisfactory solution to this problem in the liquid-filled capacitor which promises to hold its own even in the postwar period when an abundant mica supply is again ex-
pected. Our analysis of the problem led us to the same conclusion reached by Messrs. Clark and Scoville; namely, that a liquid-filled capacitor should be the solution. We want to emphasize, even more strongly than the authors of the paper have, tha t liquid impregnated paper, despite its enviable rec-ord as a dielectric in low-frequency capaci-tors, is completely unsuited for high-voltage operation in the range from 100 kilocycles to 1 megacycle. The power factor of impreg-nated paper1 in this frequency range is from 20 to 50 times the somewhat arbi-trarily chosen maximum of 0.05 per cent which appeared feasible for this type of capacitor.