Transformers may be divided into two main groups, the classification being made according to the relation between the magnetic circuit of the transformer and the primary and secondary windings. When the two windings surround the magnetic circuit of a transformer, as indicated in Fig. 9, the transformer is said to be of core type. If the magnetic circuit surrounds the windings, as indicated in Fig. 10, the transformer is said to be of the shell type. The following instructions are for a shell-type transformer.
Fig. 9—Core-Type Transformer
Any mass of magnetic material, such as a piece of soft iron, when placed in a magnetic field that is produced by an alternating current, will be rapidly magnetized and demagnetized, the rapidity of the change depending upon the frequency of the current producing the field. When a piece of iron is magnetized and demagnetized, as just stated, there will be a certain amount of heat generated in it and this heat represents energy that must come from the electrical circuit producing the magnetic field in which the iron is placed.
Fig. 10—Shell-Type Transformer
The heat that is generated in the iron is due to two causes: First, the hysteresis loss which is due to a property of the iron that causes the magnetism in the iron to lag behind the magnetizing influence, or the changes that are constantly taking place in the field strength due to the alternating current. This loss cannot be entirely eliminated, but it may be reduced to a very low value by using a soft grade of iron, or one having what is called a low hysteretic constant. Second, the eddy-current loss which is due to the circulation of currents through the mass of metal. These currents are due to unequal electromotive forces set up in the different parts of the piece of metal when there is a change in the strength of the field in which the metal is placed. This loss cannot be entirely eliminated, but it can be greatly reduced by breaking the mass of metal up into parts and insulating these parts from each other, which results in the paths in which the eddy currents originally circulated being destroyed to a certain extent.
The breaking up of the metal is usually made in such a way that the joints between the various parts are parallel to the direction of the magnetic field. When the joints are made in this way, they offer less opposition to the magnetizing force. This is one of the principal reasons why induction-coil cores are made up of a bundle of wires instead of a solid piece. These wires are annealed or softened to reduce the hysteresis loss that would occur. The combined hysteresis and eddy-current losses, which are spoken of as the iron losses, will of course be very small in the transformer you are going to construct, but the above discussion is given to show why the magnetic circuits of transformers are built up from sheets of soft iron, called laminations. The core is said to be laminated.
The dimensions of the complete magnetic circuit, of the transformer you are going to construct, are given in Fig. 11. The primary and secondary windings are both to be placed about the center portion C, and it is apparent that the winding of these coils would be very tedious if the wire had to be passed back and forth through the openings A and B. This procedure in winding can be prevented by first forming the part of the magnetic circuit upon which the windings are placed; then wind on the coils and, after they are completed, finish building up the magnetic circuit with pieces cut to the proper size and shape.
