HOW TO MAKE LOW-PRESSURE TRANSFORMERS 'RICE 40 CENTS How to Make Low-Pressure Transformers THIRD EDITION WITH ADDITIONS , BY PROF. F. E. AUSTIN, HANOVER, N. H. Ik A AUTHOR OF "EXAMPLES IN ALTERNATING- CURRENTS" "EXAMPLES IN MAGNETISM" "DIRECTIONS FOR DESIGNING, MAKING AND OPERATING HIGH-PRESSURE TRANSFORMERS." Srond Edition Copyright 1916 by F. E. Austin. Third Edition Copyright 1916 by F. K Austin. HANOVER, N. H PREFACE TO SECOND EDITION. The hearty reception of the first edition of this small book has been very gratifying to the author who takes opportunity to answer, in the second edition, in the form of additional subject matter, a number of questions pertaining to fundamental principles, addressed to him by those interested in transformer construction; hoping by this method to aid many who desire to depart somewhat from the printed specifications and construct with similar material, transformers for a variety of application. Pertaining to design it is probable there is no form of core that will effect so high operative efficiency as the form of discs recommended. The numerous transformers made by amateurs according to the specifications and submitted to the writer for test, have all shown won- derfully high efficiencies for so small devices. PREFACE TO THE THIRD EDITION. The increasing popularity of "How to Make Low-Pressure Trans- formers", has rendered a third edition necessary. The wide spread sale of the book has been a pleasing feature. Copies have been sold in Alaska, the Panama Canal Zone, the Philippines, Norway, Italy, and throughout the United States, Canada and England. While in the third edition the book has been greatly enlarged, the price has not been increased. In this edition a new and very simple form of core construction is described on page 14. Due to the desire of many amateurs to make a small transformer without the use of discs for a core, a very simple type is described in this edition, beginning on page 16. A ve/y novel utilization of discarded "tin cans" as transformer cores is also explained on page 20; being a lesson in the conservation of resources;: scrA-the making of one transformer where none existed before. .' Introductory Being constantly in receipt of inquiries regarding the design, construc- tion, and operation of small transformers for exparimental purposes, the fol- lowing instructions have been put into the present form, to enable anyone to build a small transformer, at a small cost, without the use of .expensive tools or machinery, that may be connected with any house circuit where the pressure is 110 volts or less, and the frequency about 60 cycles. The particular shape adopted, while excellent as regards ease of con- struction, is also conducive to high operative efficiency; meaning that the meter bill will be a minimum. The high efficiency is due to the high permeability of the iron used and also to the fact there are no "magnetic joints" in the core of the transformer. Many of these transformers have been made by students in our lab- oratories, with a variety of windings on the same size and shape of core, to produce a variety of pressures and out-puts. The transformer here described, is a "step-down" transformer, to re- duce the pressure from 110 volts, to about 8 volts as a minimum, for ex- perimental purposes; such as operating low pressure tungsten lamps; ringing bells, operating small direct-current series motors, used with fans or small electric cars; operating sparking devices for gasolene engines; operating small arc-lights, and for a variety of other uses. While the primary winding of this transformer is the same, regardless of the secondary or step-down pressure when connected with the 110 volt mains at 60 cycles, the secondary pressure may be varied at will by vary- ing the number of turns in its secondary windings, when constructing the transformer. The amateur is advised not to make a "step-up" transformer of the material here described, 'since this type of transformer is not designed for high pressures and the result of applying high pressures might be disastrous for the transformer, if not resulting in injury to the house wiring and ex- rimenter. If it is desired to build a high-pressure transformer for wireless experiments, information may be obtained from the book entitled "Direc- tions for Making and Operating a High Pressure Transformer" by Prof F. E. Austin, mentioned in the last part of this book. While the' transformer to be described in the following outline, may be rated as a 100 watt transformer, when operating at its maximum effi- ciency, it will transform much more power without over heating. As may be seen from the efficiency curve, figure 6, page 12, which was drawn from 345211 floir to Mdl'f ficw-Preaffure Transformers. actual experiment :J dut-i obtained from .osting one of the transformers made by students in our laboratories, the output may be 400 watts; and this without serious heating, if not too long continued. However the efficiency at this output is only slightly over 70%; while at 100 watts output the efficiency is over 90 per cent; a high efficiency for a small device. The power input to the transformer is always greater than the power output because of the various losses within it. The power efficiency of the transformer at any output is always the ratio of the output, in watts, to the input, in watts, when the "load" connected with the secondary is "non-inductive"; that is not consisting of coils or motors, but of lamps, (incandescent) or a liquid rheostat. The core of the transformer consists of 50 annular rings or washers 9 inches diameter over all with a 6 inch diameter central hole, punched from thin plates of so called "electrical-steel" about .015 of an inch in thickness. The total thickness of the 50 rings, unvarnished, when tightly clamped together is about .744 inch. Since the width of the annular portion is 1 5 inch, the cross sectional area of the iron in the core is 744 = 1500 x 744 1.5 X 7.44 = 1.11 inch 1000 1000 1000 10 (approx.) The total weight of the 50 rings is about t> pounds and 11 ounces. It should be noted that the useful power output of this transformer is over 5 horse-power. Directions 1. Carefully remove all sharp edges and burrs from the edges of the rings by means of a fine half-round file or sand paper. 2. Carefully count the rings, weigh them and measure their thickness and width. 3. Coat both sides and edges of each punching or disc with shellac varnish or some other good insulating varnish. Shellac varnish seems to give as good results as any varnish and can be easily made by any one; be- ing simply gum-shellac dissolved in sufficient denatured alcohol to give the proper consistency to spread easily, with a brush. If too thick, it may be thinned by adding more alcohol. Do not use the powdered shellac if the orange flake shellac can be obtained. 4. As soon as each disc has been carefully varnished, hang it on a hor- izontal stick, to become thoroughly dried. This may require five to ten How to Make Low- Pressure Transformers. 5 hours in a warm room. The supporting stick may be long enough and of sufficient strength to support all 50 discs. Do not allow the varnished discs to come into contact with one another before they are dry. See figure 1, a. page 5. 5. Cut carefully from thin brown or white wrapping paper, 50 annular rings having the same size as the steel punchings, to be used to insulate each punching from its neighbor when assembled. A large sheet of the wrap- ping paper may be folded into a square somewhat larger than a punching then with a punching as a pattern, using a small sharp pen knife, a number of paper rings may be cut at one operation. These may be cut while the varnished discs are drying. 6. After the varnished discs are well dried, they should be assembled by placing one disc on a level board between three or more wooden pegs which are inserted in holes in x the board so the pegs are perpendicular to the surface of the board, and at such distance from each other as to allow the disc to just touch them when placed between them. See figure l,b. Ap- ply a thin coat of varnish to the upper surface of the first disc and immed- iately place a paper ring on the newly varnished disc. Place another steel disc on top of the paper ring and apply a thin coating of varnish to its upper surface, upon which place another paper ring. Continue this process until all the steel discs have been used. 7. After the assembled pile of discs have been allowed to dry for several hours carefully remove the pile from the board and clamp together in a vice, and in two or more places by wooden hand screws. The vise is not necessary but serves to hold the assembled discs firmly in position for the next operation. FIG. 1. How to Make Low- Pressure Transformers. . I H mBi . Ss I ' :...:...-. FIG. 2. FIG. 3. 8. Begin to wind ordinary electricans' sticky tape, about f inch wide, around the assembled discs between two of the attached clamps, which should be placed about two inches apart. Lap the edges of tape about inch. After the tape has been wound tightly over this space, remove the clamp up to which the tape has been wound, and place it so as to again clamp the discs at a distance of about two inches from the point where the winding of the tape was stopped. Continue to tape over this space of two inches, up to the clamp. Proceed in this manner until the whole transfor- mer core has been tightly taped. See figure 1, page 5. If the adhesive tape is lapped | inch on the outer circumference of the core, it will of course lap more in passing around the inner circumference. Two layers of tape will be advisable. 9. Next wind on the core, over the tape, as many turns as possible in one layer, of No. 22 double cotton covered copper magnet wire, so as to cover one-half of the core. To facilitate the process of winding, this wire should first be wound onto a spool that is small enough to be easily passed through the circular hole of the transformer core. Fasten one end of the wire by clamping it, or by any method that w r ill not injure it or the tape on the core, and passing the spool of wire through the central hole, wind the wire so the successive turns as they pass through the central hole, lie close to each other. The proper action of the completed transformer depends largely on the care observed in winding the coils. The turns of wire as they pass around the outer circumference will be slightly separated from each other and this distance should be the same be- tween each turn if possible. Each terminal of the primary should be properly tagged to show the number of turns and whether the terminal is an inside one or an outside one. See figure 2, page 6. How to Make Low- Pressure Transformers. 7 The four terminals of the two primary sections are shown at P, P'; figure 2, page 6 the other eight terminals being those of the secondary coils. The second half of the core should be wound with the same kind of wire, observing the same precautions as with the first half, and should be wound in the same direction. The two coils should consist of about 484 turns, total, and constitute the ''primary" of the transformer. See figure 2, P. P', page 6. There will be four free ends of wire, and when two adjacent ends are connected together and the two remaining free ends connected with a cricuit, the two coils will act together', or not in "opposition." 10. Apply two coats of varnish to the wire of the primary coils, allowing the first coat to dry during several hours before applying the >'<< >nd coat. 11. Carefully count the number of turns of wire on the primary, and record the number. 12. Cut two annular rings from thick card board, each ring having an over all diameter about inch greater than the overall diameter of the transformer as already constructed; that is, inclusive of the primary windings. The diameter of the central hole in each cardboard disc should be about inch less than the diameter of the central hole in the transformer a.< wound. 13. These cardboard discs should be carefully varnished with at least two coats of shellac varnish and very thoroughly dried. Certain kinds of cardboard may need three or more coats. FIG. 4. > Hoic to Make Loiv- Pressure Tram/onm 14. Cut strips from the same card-board material from which the cardboard discs are made, which have a width equal to the thickness of the transformer inclusive of the primary winding, and long enough to encircle the inner circumference of the transformer and also the outer circumference. Two strips may be used for the outer circumference if one strip cannot be obtained of sufficient length. These strips should be varnished like the card board discs. 15. Carefully wind each terminal P. P', figure 2, page G, of the pri- mary coil with insulating tape to insulate the terminals as they pass out through the cardboard insulation that is placed between the pri: and the secondary. Considerable care regarding this feature should be taken, that no short-circuit or ground can take place between the primary and secondary windings. 16. Place one of the cardboard discs on one side of the transf against the primary wires, and the other disc on the opposite side. Fi varnished cardboard strips around the edges of the transformer bei the circumferences of the two cardboard discs and hold all in po- temporarily by means of string. The primary is now completely co 1 with varnished cardboard which serves to insulate the prii secondary windings. 17. Next wind carefully over the cardboard covering, N double cotton covered copper magnet wire in four sections. The first two sections occupy \ of the transformer per section; or \ of the transformer for both sections. Another section should occupy f of the unwound half or $ of the whole transformer, while the last section occupies the reni i 5 of , or 5 of the whole transformer. The secondary is wound in one layer, with the turns on circumference lying close together and evenly spaced on cumference, and is to be wound always continuously in th< as was the primary. That is in such manner that if all th free ends were connected together it would form one continuous wincing. Figure 4, page 7, will illustrate. 18. The secondary winding should be given two or three coats of varnish; allowing each coat to dry thoroughly before applying the succeed- ing coat . 19. For convenience the transformer may be mounted on a board base about 12^ or 13 inches square and about 1 inch thick, and the var- ious terminals brought out to binding posts as indicated in figure 3. pane 6, which shows a finished transformer, having 12 binding posts; one for each primary and each secondary terminal. In the figure the four posts below the white line are the four primary connections and the other eight posts are the secondary connections. The following tabulated data applies to a transformer made in our laboratory according to directions given in this book. How to Make Low-Pressure Transformers. " 9 Core : Primary : Secondary TRANSFORMER. 50 annular discs of "electrical steel". Total weight of 50 discs, 107 ozs. (6 Ibg. 11 ozs. ) Weight of each disc, 2.14 ozs. Total thickness of 50 discs not varnished, 0.744 inch. Average thickness of each disc, - inch . . 1000 Outside diameter of disc, 9 inci Diameter of central hole, 6 inches. Width of annular portion, 1 \ inch. Cross sectional area 1.11 square inch. 485 turns, of No. 22 D. C. C. copper magnet wire. Wound in two sections. Total resistance of primary wire, at 70 F (or 2lC) 3.8 ohm. Total length of primary wire, 204 feet. Total weight of primary wire, 7 ozs. 2.S7 turns, of No. 16 D. C. C. copper magnet wire. Wound in four sections. Total resistance of secondary at 70F (or 21C) is 0.63 ohm. Total length of secondary 137 feet. Tot al weight of secondary 24 ozs. or 1 Ib. 8 oz. FIG. 5. 10 How to Make Low-Pressure Transformers. The reason for winding the secondary in unequal sections, is to obtain a wide variation in pressure. Without direct reference to any particular transformer that, has been built, suppose diagram figure 4, page 7 denotes a transformer with a secondary having four sections, two of which, 1 and 2 are equal to each other, and the pressure of each section is | the total secondary pressure. Let it be assumed the total no-load secondary pressure is 64 volts; that is with all the sections connected together in series so that their pres- sures are all added together. The sum of the pressures of the two sections having the same number of turns will be 32 volts when they are connected together in series; SP<> between a and d, figure 4, page 7. Now suppose sections 1 and 2 are connected together in opposition, by changing their terminal connections; that is terminal d be connected with terminal b; then the pressure between a and c will be zero. Two equal pressures, numerically, being opposed to each other. Next suppose terminals of coils 3 and 4 are connected as at f and g; then the pressure between e and h will be 8 + 24 =32 volts. There- fore the two sections formed by coils 1 and 2 in series, and coils 3 and 4 in series, could be connected together in parallel, for heavier current out- put. Both sections thus formed could supply twice as much current to a load as could one section alone. Coil 4 having three times as many turns as coil 3. Figure 4 indicates the pressure obtainable from each of the four coils; namely 8, 16 and 24 volts, together with two other possible pressures. .32 and 64, resulting from properly connecting the coils together. A large number of different pressures may be obtained by connecting certain coils in opposition. Instead of connecting 3 and 4 properly in series to produce 32 volts, suppose the two coils are connected together in opposition, by reversing their terminal connections. Then the pressure between the free ends will be 24-8 or 16 volts. The following pressures as tabulated, show some possibilities. Connections of Coils Resulting Pressure 3 or 2-3 or 1-3 1 or 2 or 4 - 3 4 or 2 + 3 or 1 + 3 1 + 2 or 3 + 4 4 + 1 or 4 + 2 4 + 3 + 2 or 4 + 3 + 1 4 + 1+2 16 - 8 or 8 volts 24 - 8 or 16 " 16 + 8 or 24 " -..SiJfe- " 24 + 16 or 40 " 24 + 8 + 16 or 48 " 24 + 16 + 16 or 56 " How to Make Low- Pressure Transformers. - 11 The negative or - sign denotes that the coils are connected together in opposition; while the positive or + sign denotes that the coils are so con- nected that their pressures are added together. Figure 5, page 9, gives some idea of the application of the transformer as a "booster", or auto- transformer, if part of the primary is employed. The arrangement indicated in figure 5 is such that the pressure applied may be boosted, in steps, from 110 to 174 volts. In order not to over heat the transformer, no greater current should be taken from the device when used as booster, than the primary winding is designed for. Other pres- sures than those indicated, may be obtained; as between A D, is 48 volts; between A E, 64 volts. Also if secondary coil 4, figure 5, is connected in opposition to the primary pressure, then about 110-24 = 68 volts may be obtained, the transformer acting as a "crusher", or negative booster. The primary pressure, less any of the other coil pressures, will give other possible values. When wound according to the instructions given here, the input to the transformer at no load secondary, when primary is connected across ser- vice mains having a pressure of 110 volts at 60 cycles, is about 11 watts. If the cost of electric power is 15 cts per kilo-watt hour, (1000 watt- hours) the cost per day of 24 hours of having the unloaded transformer connected with the mains, would be 11 X 24 X 15 = 3.9 cents; say four cents. 1000 This is about \ cent per hour. . According to the curve figure 6, page 12, when operating a load of 90 watts the input is 100 watts; and the cost per hour will be: 100 X 1 X 15 : = 1.5 cent. 1000 Compared with the expense of batteries to furnish the same amount of power, the expense is small. The upper curve of figure 6, page 12, shows the efficiency of one of these transformers under various conditions of output. The cost per-watt-hour of output is greater when the transformer 'out- put is 400 watts than when it is 90 watts. The cost per-watt-hour is the same when the output is 400 watts as when it is only 46 watts, as indicated by the dotted line. At any output below 44 watts, the cost per-watt-hour increases very rapidly, since the efficiency decreases very rapidly. The cost per-watt-hour is a minimum, when the efficiency is a maxi- mum. 12 How to Make Low- Pressure Transformer*. The lower curve in figure 6, page 12, shows what is called the "regu- lation" of the transformer; which is the ratio of the difference between the no-load secondary terminal pressure and the full-load secondary terminial pressure, to the full load secondary terminal pressure, when the load connected with the secondary is non-inductive, such as incandescent lights. The primary applied pressure must be kept constant while finding the regulation . uj ajnssajj PUB '11193 j3 trips by using a file, and to carefully varnish both sides and edges' of each strip with shellac varnish, allowing the varnish to thoroughly dry before beginning the assembling of the core. The ends of the spools or bobbins are made of wood, figure 11, which are glued to the cardboard tube made of a single piece, and folded into shape as indicated in figure 12. By cutting partly through the cardboard with a sharp pen knife as indicated in figure 12, the cardboard may be more readily formed into the required shape. f/g.14 Fig. 11. Fig is How to Make Low-Pressure Transformers. The two forms or spools may be wound with tape as indicated in figure 13; either ordinary electricians' sticky tape may be used, or cloth tape may be used and carefully shellacked and allowed to dry before winding the wires on the spools. To facilitate winding the coils on the spools a device such as illustrated in figure 15, will be found very useful, and consists of a slightly tapering wooden mandrel shown in figure 14, page 17, having a j " diameter hole lengthwise through its center to allow it to be slipped onto a \" diameter shaft of iron or brass rod, bent into a crank at one end. The tapering wooden mandrel may be made in two parts as indicated in figures 14 and 15, and provided with screws, either ordinary wood screws or stove bolts, for clamping the mandrel to the shaft. The forms for the coils may be slipped over the wooden mandrel, placed in position as indicated in figure 15, and the coils readily wound. How to Make Low-Pressure Transformers. 19 The primary winding on each spool consists of 260 turns of No. 25 copper magnet wire, double cotten covered, wound in two layers. After giving the primary windings two or three coats of shellac varnish, the secondary may be wound directly over the primary. The secondary consists of two layers of No. 14, double cotton covered copper magnet wire; 50 turns per layer. The primary may be wound in two sections per spool, bringing out four terminals. The secondary should be wound in two sections, bringing out four terminals. For 110 volts 60 cycles primary, connect all the primary sections together in series. The pressure between the terminals of each section of the secondary will be about 5 volts; or all connected together in series about 20 volts. If a tap is brought out from the middle of a secondary section (or layer) the pressure of 1\ volts per half section may be obtained. If desired an extra layer of 25 turns may be wound over the secondary to obtain 2| volts. From 10 to 15 ampers may be obtained from the secondary of this transformer. Even 20 to 25 ampers may be taken for short intervals. Fig. 16. 20 Hotr t r > M. Van Nostrand Company, 25 Park Place, New York, N. Y. Putnams, 2 West 45th St., New York, N. Y. THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO 5O CENTS ON THE FOURTH DAY AND TO $1.OO ON THE SEVENTH DAY OVERDUE. MAY 151933 MAY 16 1933 23 1936 APR MAR 1940 MAR 271942 IUN 3 1942 38 Y B 15685 345211 UNIVERSITY OF CALIFORNIA LIBRARY