Page:Popular Science Monthly Volume 90.djvu/166

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��Popular Science Monthly

��Estimating the Windings of the Electromagnet Coils

IT is frequently desirable to rewind electromagnetic coils to obtain more efficient windings, or to make and wind the coil complete. It is usually difficult to estimate coil-windings correctly, with- out dealing more or less with complicated technical formulae.

The successful operation of every electromagnet depends upon the number of ampere-turns produced within the winding when a certain current is flowing through it. The value of this current is, of course, determined from the resistance of the coil and any of the apparatus that might be in circuit with it. For example, a certain electro- magnet is connected alone in a 4-volt circuit, the coil being wound with 500 turns of wire, giving a resistance of 10 ohms. By Ohm's law, the current C equals .4 ampere.

500 X .4 = 200 ampere-turns, which rep- resents the strength or pulling power of the magnet.

It may happen that this pulling power is not sufficient to produce the result desired, in which case it can be increased by either the number of turns or the amount of current in the circuit. By using a larger size of wire, the number of turns may be increased and the resistance reduced, thereby allowing more current to flow, which means a decided increase in the effective ampere- turns. It is not always possible to do this, however, on account of the small winding space available.

In this article we will consider only spools having round cores, these being the ones most generally used. The calculations would be simplified if it could be assumed that the convolutions of the wire were lying together, as if the wire were of square cross-section. Ex- perience has shown, however, that this is not the case. The condition more nearly approached is where the second and each succeeding layer of wire lie in the grooves formed by the preceding layers. To simplify matters, we will take the square of the diameter of the wire as the actual space occupied by each turn, which will be approximately correct and will give good results.

The diameter of the insulated wire can be determined by taking the diameter of commercial bare wire and adding the

��thickness of insulation, which varies with the manufacture. No fixed values can be given to cover all cases. A number of the leading manufacturers furnish handbooks or cards showing the average diameter over the insulation. The following table of thicknesses for different insulations will be found to give good results:

��B.&S. Gage

�COTTON

�SILK

�Enamel

�Single

�Double

�Single

�Double

�O-IO 10-18 18-26 26-34

34 up

�.007 .005 .004 .004

�.014 .010 .008

�.0020 .0018 .0018

�.0040 .0036 .0036

�.0018 .0014 .0009

���Unknown quantities are designated by let- ters and are determined by the formula below

Let

L = Winding length.

3' = Diameter of spool head.

d = Diameter of core plus any insulated covering wound over it.

x = Depth of winding.

Z> = Over-all diameter of winding. Also let

a = Diameter pf wire over insulation.

n = Number of turns.

K = Maximum winding space.

Case I. Given a spool and the size of wire, to find the number of turns and resistance, it being understood that the coil is to be wound as snugly as practic- able. For convenience assume the following values:

L = 2'' d = 4" y=i" Wire = No. 34

enamel. a = diameter over insulation = ,007" for No. 34 enameled wire. .007^ = .000049 sq. ins. (space occupied

��by one turn). y—d I — .4

��= .^ = x or maximum winding depth.

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