This paper presents a brief discussion of the current transformer as used with measuring and controlling apparatus tvith. special reference to the degree of accuracy which can be attained in the ratio and phase angle.A new type of current transformer is then described, in which it is possible to secure much higher accuracy with a given amount of iron and copper in the transformer.In this new device the transformation is effected in two stages, the first yielding in the usual way a secondary current which is approximately correct in magnitude and phase, and the second yielding an auxiliary corrective current which, when combined with the first secondary current, gives a resultant current which very closely approximates to the secondary current which would be furnished by an ideal current transformer having no errors.The two currents may easily be combined by having two like windings in the devices operated, one for the main and one for the auxiliary secondary current. The mathematical theory of the two-stage current transformer is developed and applied.Experimental curves are given to compare the performance of the new transformer with that of an ordinary simple current transformer of good average performance.The effect of mutual inductance between the external secondary circuits is discussed, and some' of the special advantages of the new transformer are given. THE SIMPLE CURRENT TRANSFORMERT HE term "current transformer" as ordinarilyused refers to a transformer used to deliver to electrical measuring and controlling devices a definite fraction of the line current. It consists essentially of a core of magnetic material on which are wound two coils, one of which, usually of a few turns of large wire, is connected in series with the highvoltage circuit, while the other coil (usually of a greater number of turns of smaller wire) supplies a secondary induced current which operates the measuring and controlling devices in the secondary circuit. The impedance of the external secondary circuit is properly referred to as the secondary burden.In order that a secondary current may be induced, a certain component of the primary current must be used to produce the necessary magnetization, and to supply the core loss. The core being of iron it is readily appre ciated that this component of current varies with (1) secondary burden, (2) frequency, (3) magnitude of the secondary current. Because this component does not vary directly with the secondary current, the ratio of the two currents varies with any changes in the above three factors occurring either separately or jointly. Also, the electrical phase difference between the primary current and the secondary current, which would be exactly 180 deg. in an ideal transformer, departs from 180 deg. by a small angle, the "phase angle," which varies with each of the three causes mentioned as affecting the ratio of currents. For the accurate opera tion of electrical measuring apparatus, especially wattmeters and watt-hour meters, it is necessary that the ratio of primary current to...
After a brief discussion of the various uses of reactors and the general characteristics of formulas for the self-inductance of coils, a number of useful general relations concerning the self and mutual inductance of geometrically similar coils are given with illustrative derivations of some of these relations. Three applications of some of these relations are developed in the remainder of the paper. The first is a simple and straightforward procedure for the design of air-core coils to serve as standards of self-inductance and is believed to be the first such design procedure for this specific purpose to be published. The second application proposes that the self and mutual inductance of the large current-limiting reactors used in power systems may be advantageously predetermined, or their computed values checked, by laboratory tests made on small-scale models. The third application proposes to use models for the checking or predetermination of the properties of iron-core air-gap reactors. In developing the theory of this application it is shown that such reactors have their desirable optimum performance that is, minimum power factor-when the relative length of the air gap is such as to make the copper loss equal to the core loss for the desired maximum magnetic flux density in the core. CONTENTS Page * Maxwell, Electricity and Magnetism, 2, sec. 706. 8 The geometric mean distance of a rectangle of sides b and c may be taken to be equal to 0.2236 '(b+c).
Chief. Engineer, Sangamo Electric Co. This paper presents a brief discussion of the current transformer as used with measuring and controlling apparatus with special reference to the degree of accuracy which can be attained in the ratio and phase angle. A new type of current transformer is then described, in which it is possible to secure much higher accuracy with a given amount of iron and copper in the transformer. In this newt! device the transformation is effected in tIVo stages, the first yielding in the usual way a secondary current which is approximately correct in magnitude and phase, and the second yielding an auxiliary corrective current which, when combined with the first secondary current, gives a resultant current which very closely approximates to the secondary current which wvould be furnished by an ideal current transformer having no errors. The two currents may easily be combined by having two like windings in the devices operated, one for the main and one for the auxiliary secondary currenzt.The mathematical theory of the two-stage current transvformer is developed and applied. Experiinental curves are given to compare the performance of the new transformer with that of an ordinary simple current transformer of good average performance. The effect of mutual inductance between the external secondary circuits is discussed, and some of the special advantages of the new transformer are given.ditions the total ampere-turns in the windings of each 1. This construction was suggested by Dr. F. B. Silsbee.
An absolute electrometer of the attracted-disk type is described . It is suitable for the measurement of alternating voltages up to 275,000 volts effective value with an accuracy of a few hundredths of a percent. A set of equally spaced coaxial guard hoops, maintained at equally spaced potentials, serves to produce a uniform field at the disk in spite of the large separation (110 cm) required to avoid sparkover at the high operating voltage. Formulas are derived by which corrections can be applied for any deviation of the individually measured hoop potentials from the ideal equal spacing.The disk hangs from one arm of a delicate balance which serves to measure the force of attraction. Light reflected from a mirror carried by the balance beam serves to magnify its motion and to indicate to the operator at a safe distance when a condition of equilibrium is reached. It also serves to indicate the height of the disk relative to its surrounding guard ring. The scale reading corresponding to the ideal coplanar condition is obtained from auxiliary measurements made with a pair of special microscopes adapted for measuring distances in the line of sight by a calibrated focus adjustment.The change in the attractive force as the disk moves away from the coplanar position has been measured and compared with that calculated theoretically. The effects of this change on the sensitivity and on the stability of the balance are worked out in detail.Trials of this instrument under various conditions, both normal and with certain adverse influences exaggerated, over the range from 10,000 to 100,000 volts, indicate that the probable error of values obtained with it is about 0.01 percent, and that this error will not be greatly increased when the instrument is used at 275,000 volts.
Direct observation of rain areas by radar yields new information on thunderstorm characteristics and behavior. A statistical summary of characteristics of 300 showers observed by radar at Spring Lake, N. J., June–Aug. 1945, is given.
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