Impedance Coils

In telephony electromagnets frequently serve, as already stated, to perform other functions than the producing of motion by attracting or releasing their armatures. They are required to act as impedance coils to present a barrier to the pa age of alternating or other rapidly fluctuating currents, and at the same time to allow the comparatively free pa age of steady currents. Where it is desired that an electromagnet coil shall po e high impedance, it is usual to employ a laminated instead of a solid core. This is done by building up a core of suitable size by laying together thin sheets of soft iron, or by forming a bundle of soft iron wires. The use of laminated cores is for the purpose of preventing eddy currents, which, if allowed to flow, would not only be wasteful of energy but would also tend to defeat the desired high impedance. Sometimes in iron-clad impedance coils, the iron shell is slotted longitudinally to break up the flow of eddy currents in the shell. Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed impedance , or retardation , or choke coils , since they are employed to impede or to retard or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each. Number of Turns . It should be remembered that an impedance coil obstructs the pa age of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force. Other things being equal, the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil, and this latter factor depends also on the reluctance of the magnetic circuit. Therefore, to secure high impedance we need many turns or low reluctance, or both. Often, owing to requirements for direct-current carrying capacity and limitations of space, a very large number of turns is not permi ible, in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance, usually a completely closed circuit. Kind of Iron. An important factor in the design of impedance coils is the grade of iron used in the magnetic circuit. Obviously, it should be of the highest permeability and, furthermore, there should be ample cro -section of core to prevent even an approach to saturation. The iron should, if po ible, be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects. Types. Open-Circuit:—Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding. IMG:384091999325875926_fig101_t.gif.png:Illustration_ Fig. 101. Section of Open-Circuit Impedance Coil Fig. 101. Section of Open-Circuit Impedance Coil View full size illustration. IMG:384091999325875926_fig102_t.gif.png:Illustration_ Fig. 102. Open-Circuit Impedance Coil Fig. 102. Open-Circuit Impedance Coil View full size illustration. IMG:384091999325875926_fig103_t.gif.png:Illustration_ Fig. 103. Closed-Circuit Impedance Coil Fig. 103. Closed-Circuit Impedance Coil View full size illustration. Closed-Circuit:—A type of retardation coil which is largely used in systems of simultaneous telegraphy and telephony, known as composite systems , is shown in Fig. 103. In the construction of this coil the core is made of a bundle of fine iron wires first bent into U-shape, and then after the coils are in place, the free ends of the core are brought together to form a closed magnetic circuit. The coils have a large number of turns of rather coarse wire. The conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity, and fortunately the added requirement, that it shall be placed in a very small space, does not exist. Toroidal:—Another type of retardation coil, called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire, is shown in diagram in Fig. 104. The two windings of this coil may be connected in series to form in effect a single winding, or it may be used as a "split-winding" coil, the two windings being in series but having some other element, such as a battery, connected between them in the circuit. Evidently such a coil, however connected, is well adapted for high impedance, on account of the low reluctance of its core. IMG:384091999325875926_fig104_t.gif.png:Illustration_ Fig. 104. Symbol of Toroidal Impedance Coil Fig. 104. Symbol of Toroidal Impedance Coil View full size illustration. This coil is usually mounted on a base-board, the coil being enclosed in a protecting iron case, as shown in Fig. 105. The terminal wires of both windings of each coil are brought out to terminal punch ings on one end of the base-board to facilitate the making of the nece ary circuit connections. IMG:384091999325875926_fig105_t.gif.png:Illustration_ Fig. 105. Toroidal Impedance Coil Fig. 105. Toroidal Impedance Coil View full size illustration. The usual diagrammatic symbol for an impedance coil is shown in Fig. 106. This is the same as for an ordinary bar magnet, except that the parallel lines through the core may be taken as indicating that the core is laminated, thus conveying the idea of high impedance. The symbol of Fig. 104 is a good one for the toroidal type of impedance coil. IMG:384091999325875926_fig106_t.gif.png:Illustration_ Fig. 106. Symbol of Impedance Coil Fig. 106. Symbol of Impedance Coil View full size illustration.