By A. E. Andrews

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.

9   Core Type Transformer

Ill: Fig. 9 - Core-Type Transformer

10   Shell Type Transformer

Ill: Fig. 10 - Shell-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.

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.

Procure a small quantity of soft, thin sheet iron and cut out a sufficient number of rectangular pieces, 3 in. by 4 1/4 in., to make a pile 3/4 in. in height when firmly pressed together. Now cut a rectangular notch in each of these pieces, 2 in. wide and 3% in. long. The sides of this notch can be cut with a pair of tinner's shears, and the end can be cut with a sharp cold-chisel. Be careful not to bend either piece any more than you can help. The outside piece, or the one in which the notch is cut, should have dimensions corresponding to those given in Fig. 12. When all of these pieces have been cut, as indicated above, the rectangular pieces, 2 in. by 3% in., that were cut out to form the notch in the larger pieces, should have two of their corners cut away, so as to form pieces whose dimensions correspond to those given in Fig. 13. These last pieces are to form the core and part of the end of the transformer. Now make sure that all the edges of the pieces are perfectly smooth and that they are all of the same size; then give each one a coat of very thin shellac.

Construction Of A Small Bell Ringing Transformer P 835

Now cut from a piece of insulating fiber, that is about 1/16 in. thick, two pieces whose dimensions correspond to those given in Fig. 14. When these pieces are completed, the core of the transformer can be assembled as follows : Place the T-shaped pieces, whose dimensions correspond to those given in Fig. 13, through the openings in the pieces of insulation, alternate pieces being put through the openings from opposite sides. The distance from outside to outside of the pieces of insulation should be exactly the same as the length of the vertical portion of the T-shaped pieces forming the core, or 3 in.

Cut from some soft wood four pieces having cross sections whose dimensions correspond to those given in Fig. 15, and of such a length that they will just slip down between the two pieces of insulation. These pieces should now be placed on the four sides of the iron core and covered with several layers of heavy insulating cloth. Each layer of the cloth should be shellacked as it is put on, which will increase the insulation and at the same time help in holding the wooden pieces in place. You are now ready to start winding the transformer.

The secondary, which is the low-voltage side in this case, as you are using the transformer to reduce or step down the voltage, will have the smaller number of turns, and larger wire should be used in winding it than in the primary, as it will carry a larger current. On account of the secondary being of larger wire, it will be placed on the core first. For this winding you will need a small quantity of No. 26 B. & S. gauge, single cotton-covered wire.

Drill a small hole through one of the insulating washers, down close to the cloth covering the core, being careful at the same time to keep the hole as far from the metal part of the core as possible. Pass the end of a short piece of No. 18 or 20 B. & S. gauge, double cotton-covered wire through this opening and solder it to the end of the No. 26 wire. Insulate the joint with a piece of paraffin paper or cloth, and bind the piece of heavy wire to the core of the transformer with a piece of linen thread.

14   Insulating 'Washer Fig. 15   Wood Filler

Ill: Fig. 14 - Insulating 'Washer Fig. 15 - Wood Filler

Now wind the No. 26 wire on the core as evenly as possible, to within about 1/8in. of the end of the spool. Place over the first layer two layers of paraffin paper and wind on a second layer of wire. Three layers should give you the required number of turns in the secondary winding and a resistance of approximately 3 1/2 ohms. The end of the secondary winding should be terminated in the same way as the winding was started. Outside of the completed secondary winding place at least six layers of paraffin paper, or several layers of insulating cloth. The paraffin paper used should be approximately five mills in thickness. You can make your own paraffin paper by taking a good quality of writing paper about two mills thick and dipping it into some hot paraffin, then hanging it up by one edge to drain.

The primary winding is to be made from No. 34 B. & S. gauge, single silk-covered copper wire. The inside end of this winding should be started in the same way as the secondary, but at the end opposite to the one where the secondary terminated. Wind about 240 turns on each layer and place one layer of paraffin paper between each layer of wire. The primary winding should have at least 12 layers, and the outside end should be terminated as the inside end. Outside of the completed windings, place several layers of insulating cloth to serve as an insulation, and at the same time provide a mechanical protection for the windings.

The outside part of the magnetic circuit can now be put in place. When the U-shaped pieces are all in place, the magnetic circuit will have the form and dimensions shown in Fig. 11. A clamp should now be made for each end of the transformer, to hold the pieces forming the magnetic circuit together, and at the same time give an easy means of mounting the transformer. Cut from a piece of sheet iron, about 1 /16 in. in thickness, two pieces whose dimensions correspond to those given in Fig. 16, and two pieces whose dimensions correspond to those given in Fig. 17. Drill the holes in these pieces as indicated, and bend the larger ones into the form shown in Fig. 18. These pieces can now be clamped across the ends of the transformer with small bolts, as shown in Fig. 19.

A box should now be made from sheet iron to hold the transformer. The box should be of such dimensions that it will be at least 1/8 in. from the transformer at all points. This box should be provided with a cover that can be easily removed.

Now mount the transformer in the box by means of small bolts, that pass through the holes in the supports and holes in the bottom of the box. Two binding-posts can now be mounted on one end of the box, and insulated from it, to serve as terminals for the secondary winding. Two pieces of stranded No. 14 B. & S. gauge, rubber-covered copper wire should now be soldered to the terminals of the primary circuit and passed out through insulating bushings mounted in holes cut in the end of the box opposite to the one upon which the binding-posts were mounted. These heavy wires should be firmly fastened to the iron part of the transformer inside the box, so that any outside strain placed upon them will not, in time, break them loose from the smaller wires. Be sure to insulate all joints and wires well inside the box.

16   Upper Clamping

Ill: Fig. 16 - Upper Clamping

Fig. 17 - Lower Clamping Pieces

Fig. 18 - Shape

Fig. 19 - Method of Clamping Pieces and Mounting Supports of Support Transformer Together

A circuit can now be run from a 110-volt lighting or power circuit, observing the same rules as though you were wiring for lights, and connected to the heavy wires, or primary circuit. The binding-posts, or secondary winding should be connected to the bell circuit and the transformer is complete and ready to operate. You may have to change the adjustment of the bells, but after a little adjustment they will operate quite satisfactorily.