  The developments in the field of electrical engineering which have rendered feasible the transmission of high pressure currents over long distances, together with the reliability and efficiency of modern generating units, have resulted in notable economies in the generation and distribution of electric current. This has been accomplished largely by the use of distant water power or the centralization of the generating plants of a large territory in a single power station. The transformer is one of the essential factors in effecting the economical distribution of electric energy, and may be defined as an apparatus used for changing the voltage and current of an alternating circuit. A transformer consists essentially of:
Basic Principles.—If a current be passed through a coil of wire encircling a bar of soft iron the iron will become a magnet; when the current is discontinued the bar loses its magnetization. Conversely: If a bar of iron carrying a coil of wire be magnetized in a direction at right angles to the plane of the coil a momentary electric pressure will be induced in the wire; if the current be reversed, another momentary pressure will be induced in the opposite direction in the coil. These actions are fully explained in chaps. X and XI, and as they are perfectly familiar phenomena, a detailed explanation of the principles upon which they depend is not necessary here. From the first two statements given above it is evident that if a bar of iron be provided with two coils of wire, one of which is supplied from a source of alternating current, as shown diagrammatically by fig. 1,916, at each impulse of the exciting current a pressure will be induced in the secondary coil, the direction of these impulses alternating like that of the exciting current. Ques. What name is given to the coil through which current from the source flows? Ans. The primary winding. Fig. 1,916.—Diagram of elementary transformer with non-continuous core and connection with single phase alternator. The three essential parts are: primary winding, secondary winding, and an iron core. Ques. What name is given to the coil in which a current is induced? Ans. The secondary winding. Similarly, the current from the source (alternator) is called the primary current and the induced current, the secondary current. Ques. What is the objection to the elementary transformer shown in fig. 1,916? Ans. The non-continuous core. With this type core, the flux emanating from the north pole of the bar has to return to Ques. How is this overcome? Ans. By the use of a continuous core as shown in fig. 1,917. Ques. Is this the best arrangement, and why? Ans. No. If the windings were put on as in fig. 1,917, the leakage of magnetic lines of force would be excessive, as indicated by the dotted lines. In such a case the lines which leak through air have no effect upon the secondary winding, and are therefore wasted. Fig. 1,917.—Diagram of elementary transformer with continuous core and connections with alternator. The dotted lines show the leakage of magnetic lines. To remedy this the arrangement shown in fig. 1,918 is used. Ques. How is the magnetic leakage reduced to a minimum in commercial transformers? Ans. In these, and even in ordinary induction coils (the operating principle of which is the same as that of transformers) the magnetic leakage is reduced to the lowest possible amount by arranging the coils one within the other, as shown in cross section in fig. 1,918. The Induced Voltage.—The pressure induced in the secondary winding will depend on the ratio between the number of turns in the two windings. For example, a transformer with 500 turns of wire in its primary winding and 50 turns in its secondary winding would have a transformation ratio of 10 to 1, and if it were supplied with primary current at 1,000 volts, the secondary pressure at no load would be 100 volts. Fig. 1,918.—Cross section showing commercial arrangement of primary and secondary windings on core. One is superposed on the other. This arrangement compels practically all of the magnetic lines created by the primary winding to pass through the secondary winding. EXAMPLE.—If ten amperes flow in the primary winding and the transformation ratio be 10, then 10×10=100 amperes will flow through the secondary winding. Thus, a direct proportion exists between the pressures and turns in the two windings and an inverse proportion between the amperes and turns, that is:
From the above equations it is seen that the watts of the primary circuit equal the watts of the secondary circuit. Ques. Are the above relations strictly true, and why? Ans. No, they are only approximate, because of transformer losses. In the above example, the total wattage in the primary circuit is 1,000×10=10 kw., and that in the secondary circuit is 100×100 = 10 kw. Hence, while both volts and amperes are widely different in the two circuits, the watts for each are the same in the ideal case, that is, assuming perfect transformer action or 100% efficiency. Now, the usual loss in commercial transformers is about 10%, so that the actual watts delivered in the secondary circuit is (100×100)×90%=9 kw. Fig. 1,919.—Wagner transformer coil formed, ready for taping. These are known as "pan cake" coils. They are wound with flat cotton covered copper strip. In heavy coils, several strips in parallel are used per turn in order to facilitate the winding and produce a more compact coil. The No Load Current.—When the secondary winding of a transformer is open or disconnected from the secondary circuit no current will flow in the winding, but a very small current called the no load current will flow in the primary circuit. The reason for this is as follows: The current flowing in the primary winding causes repeated reversals of magnetic flux through the iron core. These variations of flux induce pressures in both coils; that induced in the primary called the reverse pressure is opposite in direction and very nearly equal to the impressed pressure, that is, to the pressure applied to the primary winding. Accordingly the only force available to cause current to flow through the primary winding is the difference between the impressed pressure and reverse pressure, the effective pressure. Fig. 1,920.—Wagner coils with insulation ready for core assembly. The flat coils, sometimes called pancake coils are wound of flat, cotton covered, copper strip with ample insulation between layers. In heavy coils several flat strips in multiple are used per turn in order to facilitate the winding and produce a more compact coil. In many cases normal current flow per high tension coil is very low and could be carried with a very small cross sectional area of copper; however, flat strip is almost always used on account of the increased mechanical stability thus obtained. The Magnetizing Current.—The magnetizing current of a transformer is sometimes spoken of as that current which the primary winding takes from the mains when working at normal pressure. The true magnetizing current is only that component of this total no load current which is in quadrature with the supply pressure. The remaining component has to overcome the various iron losses, and is therefore an "in phase" component. The relation between these two components determines the power factor of the so called "magnetizing current." Figs. 1,921 and 1,922.—Assembled coils of Westinghouse 10 and 15 kva. transformers; views showing ventilating ducts. The true magnetizing component is small if the transformer be well designed, and be worked at low flux density. Action of Transformer with Load.—If the secondary winding of a transformer be connected to the secondary circuit by closing a switch so that current flows through the secondary winding, the transformer is said to be loaded. The action of this secondary current is to oppose the magnetizing action of the slight current already flowing in the primary winding, thus decreasing the maximum value reached by the alternating magnetic flux in the core, thereby decreasing the induced pressure in each winding. The amount of this decrease, however, is very small, inasmuch as a very small decrease of the induced pressure in the primary coil greatly increases the difference between the pressure applied to the primary coil and the opposing pressure induced in the primary coil, so that the primary current is greatly increased. In fact, the increase of primary current due to the loading of the transformer is just great enough (or very nearly) to exactly balance the magnetizing action of the current in the secondary coil; that is, the flux in the core must be maintained approximately constant by the primary current whatever value the secondary current may have. When the load on a transformer is increased, the primary of the transformer automatically takes additional current and power from the supply mains in direct proportion to the load on the secondary. When the load on the secondary is reduced, for example by turning off lamps, the power taken from the supply mains by the primary coil is automatically reduced in proportion to the decrease in the load. This automatic action of the transformer is due to the balanced magnetizing action of the primary and secondary currents. Fig. 1,923.—Rear view of Fort Wayne distributing transformer, showing hanger irons for attaching to pole cross arm. Classification of Transformers.—As in the case of motors, the great variety of transformer makes it necessary that a classification, to be comprehensive, must be made from several points of view, as: 1. With respect to the transformation, as Step Up Transformers.—This form of transformer is used to transform a low voltage current into a high voltage current. Such transformers are employed at the generating end of a transmission line to raise the voltage of the alternators to such value as will enable the electric power to be economically transmitted to a distant point. Fig. 1,924.—Diagram of elementary step up transformer. As shown the primary winding has two turns and secondary 10 turns, giving a ratio of voltage transformation of 10÷2=5. Since only ? as much current flows in the secondary winding as in the primary, the latter requires heavier wire than the former. Copper Economy with Step Up Transformers.—To comprehend fully the bearing of the matter, it must be remembered that the energy supplied per second is the product of two factors, the current and the pressure at which that current is supplied; the magnitudes of the two factors may vary, but the value of the power supplied depends only on the product of the two; for example, the energy furnished per second by a current of 10 amperes supplied at a pressure of 2,000 volts is Now the loss of energy that occurs in transmission through a well insulated wire depends also on two factors, the current and the resistance of the wire, and in a given wire is proportional to the square of the current. In the above example the current of 400 amperes, if transmitted through the same wire as the 10 amperes current, would, because it is forty times as great, waste sixteen hundred times as much energy in heating the wire. It follows that, for the same loss of energy, the 10 ampere current at 2,000 volts may be carried by a wire having only 1/1,600th of the sectional area of the wire used for the 400 ampere current at 50 volts. The cost of copper conductors for the distributing lines is therefore very greatly economized by employing high pressures for distribution of small currents. Fig. 1,925.—Diagram of elementary step down transformer. As shown the primary winding has 10 turns and the secondary 2, giving a ratio of voltage transformation of 2÷10=.2. The current in the secondary being 5 times greater than in the primary will require a proportionately heavier wire. Step Down Transformers.—When current is supplied to consumers for lighting purposes, and for the operation of motors, etc., considerations of safety as well as those of suitability, This involves that the high pressure current in the transmission lines must be transformed to low pressure current at the receiving or distributing points by step down transformers, an elementary transformer being shown in fig. 1,925. Figs. 1,926 and 1,927.—Core type transformer. It consists of a central core of laminated iron, around which the coils are wound. A usual form of core type transformer consists of a rectangular core, around the two long limbs of which the primary and secondary coils are wound, the low tension coil being placed next the core. Transformers of this type have a large number of turns in the primary winding and a small number in the secondary, in ratio depending on the amount of pressure reduction required. Core Transformers.—This type of transformer may be defined as one having an iron core, upon which the wire is wound in such a manner that the iron is enveloped within the coils, the outer surface of the coils being exposed to the air as shown in figs. 1,926 and 1,927. Shell Transformers.—In the shell type of transformer, as shown in fig. 1,928, the core is in the form of a shell, being built around and through the coils. A shell transformer has, as a rule, fewer turns and a higher voltage per turn than the core type. Ques. What is the comparison between core and shell transformers? Ans. The relative advantages of the two types has been the subject of considerable discussion among manufacturers; the companies who formerly built only shell type transformers, now build core types, while with other builders the opposite practice obtains. Fig. 1,928.—Shell type transformer. In construction the laminated core is built around and through the coils as shown. For large ratings this type has some advantages with respect to insulation, while for small ratings the core type is to be preferred in this respect. The shell arrangement of the core gives better cooling; with this arrangement minimum magnetic leakage is easily obtained. Ques. Upon what does the choice between the two types chiefly depend? Ans. Upon manufacturing convenience rather than operating characteristics. The major insulation in a core type transformer consists of several large pieces of great mechanical strength, while in the shell type, there are required an extremely large number of relatively small pieces of insulating material, which necessitates careful workmanship to prevent defects in the finished transformer, when thin or fragile material is used. Both core and shell transformers are built for all ratings; for small ratings the core type possesses certain advantages with reference to insulation, while for large ratings, the shell type possesses better cooling properties, and has less magnetic leakage than the core type. Fig. 1,929.—View illustrating the construction of cores and coils of Maloney transformers. Fig. 1,930.—Maloney mica shield between primary and secondary coils, showing lapping feature which prevents the wrinkling and cracking of the mica. Combined Core and Shell Transformers.—An improved type of transformer has been introduced which can be considered either as two superposed shell transformers with coils in common, or as a single core type transformer with divided magnetic circuit and having coils on only one leg. It is best considered however, as a combined core and shell transformer, Figs. 1,931 and 1,932.—The Berry combined core and shell transformer. It consists of a number of inner and outer vertical and radial laminated iron blocks built up of the usual thin sheet iron, with the coils between. The magnetic circuit is completed at the top and bottom by other laminated blocks placed horizontally, and the whole is held together between top and bottom cast iron frame plates by a bolt passing right down the center. Fig. 1,931 gives a general view, W being the winding, and B, B, B, etc., the outer laminated blocks. The construction will be better understood from fig. 1,932, where it may be supposed that the top cap and laminated cross pieces have been removed. Here I, I, I and O, O, O are respectively the inner and outer radial vertical blocks, P the primary, and S, S the secondary; the latter being in two sections with the primary sandwiched between, as an extra precaution against shock. It will be evident that this form of transformer possesses excellent ventilation; and this is still further enhanced by opening out the winding at intervals to leave ventilating apertures, as at A, A, A. Fig. 1,932 shows only 6 sets of radial blocks, but the usual plan is to provide 24 or 36, according to the size of the transformer. Fig. 1,932 shows a cross section of the first transformer of this type to be developed commercially, and known as an "iron clad" transformer; this construction has been used in England for some time. Fig. 1,933 shows the American practice. Fig. 1,933.—Plan of core of General Electric combined core and shell transformer. The core used contains four magnetic circuits of equal reluctance, in multiple; each circuit consisting of a separate core. In this construction one leg of each circuit is built up of two different widths of punchings forming such a cross section that when the four circuits are assembled together they interlock to form a central leg, upon which the winding is placed. The four remaining legs consist of punchings of equal width. These occupy a position surrounding the coil at equal distances from the center, on the four sides; forming a channel between each leg and coil, thereby presenting large surfaces to the oil and allowing its free access to all parts of the winding. The punchings of each size transformer are all of the same length, assembled alternately, and forming two lap joints equally distributed in the four corners of the core, thereby giving a magnetic circuit of low reluctance. Ques. How is economy of construction obtained in designing combined core and shell transformers? Ans. The cross section of iron in the central leg of the core is made somewhat less than that external to the coils, in order to reduce the amount of copper used in the coils. Single and Polyphase Transformers.—A single phase transformer may be defined as one having only one set of primary and secondary terminals, and in which the fluxes in the one or more magnetic circuits are all in phase, as distinguished from a polyphase transformer, or combination in one unit of several one phase transformers with separate electric circuits but having certain magnetic circuits in common. In polyphase transformers Ques. Is it necessary to use a polyphase transformer to transform a polyphase current? Ans. No, a separate single phase transformer may be used for each phase. Figs. 1,934 and 1,935.—Top view showing core and coils in place, and view of coils of Westinghouse distributing transformer. The coils are wound from round wire in the smaller sizes of transformers and from strap copper in the larger sizes. Strap wound coils allow a greater current carrying conductor section than coils wound from large round wire, as there is little waste space between the different turns of the conductor. The coils are arranged concentrically with the high tension winding between the two low tension coils, this arrangement giving the fine regulation found in these transformers. The low tension coils are wound in layers which extend across the whole length of the coil opening in the iron, while the high tension coils are wound in two parts and placed end to end. This construction reduces the normal voltage strains to a value which will not give trouble under any condition of service. The magnetic circuit is built up of laminated, alloy steel punchings, each layer of laminÆ being reversed with reference to the preceding layer and all joints butted. This gives a continuous magnetic circuit of low reluctance, low iron loss and low exciting current. When assembled, the magnetic circuit consists of four separate parallel circuits encircling the coils and protecting the windings from mechanical injury. Separate high and low tension terminal blocks of glazed porcelain are mounted upon extensions of the upper end frames. All danger of confusing the leads or inadvertently making an electrical connection between the high and low tension sides of the transformer is thus averted. The high tension winding has four leads brought to the studs in the terminal block. Adjustable brass connectors or links between the studs provide for series or multiple connections between two points of the high tension winding. The position of the studs and the length of the links are so proportioned that wrong connections on the block are impossible. Barriers on the porcelain block separate the studs and prevent danger of arcing. Leads with means of preventing creeping of oil by capillary action are attached to these studs and brought out of the core through porcelain bushings. Ques. Is there any choice between a polyphase transformer and separate single phase transformers for transforming a polyphase current? Ans. Yes, the polyphase transformer is preferable, because less iron is required than would be with the several single phase transformers. The polyphase transformer therefore is somewhat lighter and also more efficient. Figs. 1,936 and 1,937.—Core and shell types of three phase transformer. In the core type, fig. 1,936, there are three cores A, B, and C, joined by the yokes D and D'. This forms a three phase magnetic circuit, since the instantaneous sum of the fluxes is zero. Each core is wound with a primary coil P, and a secondary coil S. As shown, the primary winding of each phase is divided into three coils to ensure better insulation. The primaries and secondaries may be connected star or mesh. The core B has a shorter return path than A and C, which causes the magnetizing current in that phase to be less than in the A and C phases. This has sometimes been obviated by placing the three cores at the corners of an equilateral triangle (as in figs. 1,939 and 1,940), but the extra trouble involved is not justified, as the unbalancing is a no load condition, and practically disappears when the transformer is loaded. The shell type, fig. 1,937, consists practically of three separate transformers in one unit. The flux paths are here separate, each pair of coils being threaded by its own flux, which does not, as in the core type, return through the other coils. This gives the shell type an advantage over the core type, for should one phase burn out, the other two may still be used, especially if the faulty coils be short circuited. The effect of such short circuiting is to prevent all but a very small flux from threading the faulty coil. Ques. Name two varieties of polyphase transformer? Ans. The core, and the shell types as shown in figs. 1,936 and 1,937. Ques. How should a three phase transformer be operated with one phase damaged? Ans. The damaged windings should be separated electrically from the other coils. The pressure winding of the damaged phase should be short circuited upon itself and the corresponding low pressure winding should also be short circuited upon itself. The winding thus short circuited will choke down the flux passing through the portion of the core surrounded by them without producing in any portion of the winding a current greater than a small fraction of the current which would normally exist in such portion at full load. Transformer Losses.—As previously mentioned, the ratio between the applied primary voltage and the secondary terminal voltage of a transformer is not always equal to the ratio of primary to secondary turns of wire around the core. The commercial transformer is not a perfect converter of energy, that is, the input, or watts applied to the primary circuit is always more than the output or watts delivered from the secondary winding. Fig. 1,938.—Interior of General Electric oil cooled 500 kva. 33,000 volt outdoor transformer showing lifting arrangement. This is due to the various losses which take place, and the
The iron or core losses are due to
Figs. 1,939 and 1,940.—Triangular arrangements of cores of three phase transformer. Fig. 1,939, form with three cornered yokes at bottom and top of cores; fig. 1,940, form with circular yokes. While these designs give perfect symmetry for the three phases, there is some trouble in the mechanical arrangement of the yokes. If these be stamped out triangularly and inserted horizontally between the three cores, it is necessary to interpose a layer of insulation, otherwise there would be objectionable eddy currents formed in the stampings. Those which are classed as copper losses are due to
Hysteresis.—In the operation of a transformer the alternating current causes the core to undergo rapid reversals of magnetism. This requires an expenditure of energy which is converted into heat. Fig. 1,941.—View showing mechanical construction of coil and core of Moloney pole type ½ to 50 kw. transformer. Moloney standard transformers of these sizes are regularly wound for 1,100 to 2,200 primary volts. For 1,100 volts the primary coils are connected in parallel by means of connecting links; for 2,200 volts, they are connected in series. The porcelain primary terminal board is provided with two connecting links so that connections can be made for either 1,100 or 2,200 volts. This loss of energy as before explained is due to the work required to change the position of the molecules of the iron, in reversing the magnetization. Extra power then must be taken from the line to make up for this loss, thus reducing the efficiency of the transformer. Ques. Upon what does the hysteresis loss depend? Ans. Upon the quality of the iron in the core, the magnetic density at which it is worked and the frequency. Fig. 1,942.—Fort Wayne transformer coils and core complete. Ques. With a given quality of iron how does the hysteresis loss vary? Ans. It varies as the 1.6 power of the voltage with constant frequency. Ques. In construction, what is done to obtain minimum hysteresis loss? Ans. The softest iron obtainable is used for the core, and a low degree of magnetization is employed. Fig. 1,943.—Top view of Fort Wayne (type A) transformer cover removed, showing assembly of coils and core and disposition of leads. Eddy Currents.—The iron core of a transformer acts as a closed conductor in which small pressures of different values are induced in different parts by the alternating field, giving rise to eddy currents. Energy is thus consumed by these currents which is wasted in heating the iron, thus reducing the efficiency of the transformer. Ques. How is the loss reduced to a minimum? Ans. By the usual method of laminating the core. The iron core is built up of very thin sheet iron or steel stampings, and these are insulated from each other by varnish and are laid face to face at right angles to the path that the eddy currents tend to follow, so that the currents would have to pass from sheet to sheet, through the insulation. Ques. In practice, upon what does the thickness of the laminÆ or stampings depend? Ans. Upon the frequency. The laminÆ vary in thickness from about .014 to .025 inch, according as the frequency is respectively high or low. Fig. 1,944.—General Electric 10 kva., (type H) transformer removed from tank. That part of the steel core composing the magnetic circuit outside of the winding is divided into four equal sections. Each section is located a sufficient distance from the winding so that all portions of the winding and core are equally exposed to the cooling action of the oil. On all except the very smallest sizes the winding is divided by channels and ducts through which a continual circulation of oil is maintained. The result is uniform temperature throughout the transformer, thus eliminating the detrimental effects of unequal expansion in the coils with consequent rubbing and abrasion of the insulation. Ques. Does a transformer take any current when the secondary circuit is open? Ans. Yes, a "no load" current passes through the primary. Ques. Why? Ans. The energy thus supplied balances the core losses. Fig. 1,945.—Cover construction of Wagner 350 kva., oil filled 1,100-2,200 volt transformer. In transformers with corrugated cases, the base and top ring are cast to the corrugated iron sheets. Ques. Are the iron or copper losses the more important, and why? Ans. The iron losses, because these are going on as long as the primary pressure is maintained, and the copper losses take place only while energy is being delivered from the secondary. Strictly speaking, on no load (that is when the secondary circuit is open) a slight copper loss takes place in the primary coil but because of its smallness is not mentioned. It is, to be exact, included in the expression "iron losses," as the precise meaning of this term signifies not only the hysteresis and eddy current losses but the copper loss in the primary coil when the secondary is open. The importance of the iron losses is apparent in noting that in electric lighting the lights are in use only a small fraction of the 24 hours, but the iron losses continue all the time, thus the greater part of each day energy must be supplied to each transformer by the power company to meet the losses, during which time no money is received from the customers. Some companies make a minimum charge per month whether any current is used or not to offset the no load transformer losses and rent of meter. Figs. 1,946 to 1,948.—Methods of connecting the low tension sides of Westinghouse transformers using the connectors illustrated in figs. 1,949 to 1,953. Ques. How may the iron losses be reduced to a minimum? Ans. By having short magnetic paths of large area and using iron or steel of high permeability. The design and construction must keep the eddy currents as low as possible. As before stated the iron losses take place continually, and since most transformers are loaded only a small fraction of a day it is very important that the iron losses should be reduced to a minimum. With a large number of transformers on a line, the magnetizing current that is wasted, is considerable. During May, 1910, the U. S. Bureau of Standards issued a circular showing that each watt saving in core losses was a saving of 88 cents, which is evident economy in the use of high grade transformers. Copper Losses.—Since the primary and secondary windings of a transformer have resistance, some of the energy supplied will be lost by heating the copper. The amount of this loss is proportional to square of the current, and is usually spoken of as the I2R loss. Figs. 1,949 to 1,953.—Westinghouse low tension transformer connectors for connecting the low tension leads to the feeder wires. The transformers of the smaller capacities have knuckle joint connectors and those of the larger sizes have interleaved connectors. These connectors form a mechanically strong joint of high current carrying capacity. Since the high tension leads are connected directly to the cut out or fuse blocks, connectors are not required on these leads. The use of these connectors allows a transformer to be removed and another of the same or a different capacity substituted usually without soldering or unsoldering a joint. The connectors also facilitate changes in the low tension connections. Ques. Define the copper losses. Ans. The copper losses are the sum of the I2R losses of both the primary and secondary windings, and the eddy current loss in the conductors. Ques. Is the eddy current loss in the conductors large? Ans. No, it is very small and may be disregarded, so that the sum of the I2R losses of primary and secondary can be taken as the total copper loss for practical purposes. Ques. What effect has the power factor on the copper losses? Ans. Since the copper loss depends upon the current in the primary and secondary windings, it requires a larger current when the power factor is low than when high, hence the copper losses increase with a lowering of the power factor. Fig. 1,954.—Method of bringing out the secondary leads in Wagner central station transformers. Each primary lead is brought into the case through a similar bushing. Observe the elimination of all possibility of grounding the cable on the case or core. Ques. What effect other than heating has resistance in the windings? Ans. It causes poor regulation. This is objectionable, especially when incandescent lights are in use, because the voltage fluctuates inversely with load changes, that is, it drops as lamps are turned on and rises as they are turned off, producing disagreeable changes in the brilliancy of the lamps. Cooling of Transformers.—Owing to the fact that a transformer is a stationary piece of apparatus, not receiving ventilation from moving parts, its efficient cooling becomes a very strong feature of the design, especially in the case of large high pressure transformers. The effective cooling is rendered more difficult because transformers are invariably enclosed in more or less air tight cases, except in very dry situations, where a perforated metal covering may be permitted. Figs. 1,955 and 1,956.—Westinghouse transformer terminal blocks for high and low tension conductors. The final degree to which the temperature rises after continuous working for some hours, depends on the total losses in iron and copper, on the total radiating surface, and on the facilities afforded for cooling. There are various methods of cooling transformers, the cooling mediums employed being
The means adopted for getting rid of the heat which is inevitably developed in a transformer by the waste energy is one of the important considerations with respect to its design. Ques. What is the behaviour of a transformer with respect to heating when operated continuously at full load? Ans. The temperature gradually rises until at the end of some hours it becomes constant. The difference between the constant temperature and that of the secondary atmosphere is called the temperature rise at full load. Its amount constitutes a most important feature in the commercial value of the transformer. Figs. 1,957 to 1,960.—Porcelain bushing for Westinghouse transformers. Ques. Why is a high rise of temperature objectionable? Ans. It causes rapid deterioration of the insulation, increased hysteresis losses, and greater fire risk. Dry Transformers.—This classification is used to distinguish transformers using air as a cooling medium from those which employ a liquid such as water or oil to effect the cooling. Air Cooled Transformers.—This name is given to all transformers which are cooled by currents of air without regard to the manner in which the air is circulated. There are two methods of circulating the air, as by
Ques. Describe a natural draught air cooled transformer. Ans. In this type, the case containing the windings is open at the top and bottom. The column of air in the case expands as its temperature rises, becoming lighter than the cold air on the outside and is consequently displaced by the latter, resulting in a circulation of air through the case. The process is identical with furnace draught. Figs. 1,961 to 1,963.—Fuse blocks for Westinghouse transformers. The fuses furnished with the transformers are mounted in a weather proof porcelain fuse box of special design. The stationary contacts are deeply recessed in the porcelain and are well separated from each other. The contacts are so constructed that the plug is held securely in place by giving it a partial turn after inserting it. When the plug is in position, the fuse is in sight and its condition can be noted which eliminates all danger of pulling the fuse while same is still intact and the transformer is under load. Ques. Describe a forced draught or air blast transformer. Ans. The case is closed at the bottom and open at the top. A current of air is forced through from bottom to top as shown in fig. 1,964 by a fan. Ques. How are the coils best adapted to air cooling? Ans. They are built up high and thin, and assembled with spaces between them, for the circulation of the air. Ques. What are the requirements with respect to the air supply in forced draught transformers? Ans. Air blast transformers require a large volume of air at a comparatively low pressure. This varies from one-half to one ounce per square inch. The larger transformers require greater pressure to overcome the resistance of longer air ducts. Fig. 1,964.—Forced draught or "air blast" transformer. As is indicated by the classification, this type of transformer is cooled by forcing a current of air through ducts, provided between the coils and between sectionalized portions of the core. The cold air is forced through the interior of the core containing the coils by a blower, the air passing vertically through the coils and out through the top. Part of the air is sometimes diverted horizontally through the ventilating ducts provided in the core, passing off at one side of the transformer. The amount of air going through the coils, or through the core, may be controlled independently by providing dampers in the passages. Ques. How much air is used ordinarily for cooling per kw. of load? Ans. About 150 cu. ft. of air per minute. In forced draught transformers, the air pressure maintained by the blower varies from ½ to 1½ oz. per square inch. Forced draught or air blast transformers are seldom built in small sizes or for voltages higher than about 35,000 volts. Oil Cooled Transformers.—In this type of transformer the coils and core are immersed in oil and provided with ducts to allow the oil to circulate by convection and thus serve as a medium to transmit the heat to the case, from which it passes by radiation. Fig. 1965.—Looking down into a Wagner central station transformer, showing the connection board, which provides facility for varying the ratio of transformation and also for interchanging the primaries. Ques. Explain in detail the circulation of the oil. Ans. The oil, heated by contact with the exposed surfaces of the core and coils, rises to the surface, flows outward and Ques. How may the efficiency of this method of cooling be increased? Ans. By providing the case with external ribs or fins, or by "fluting" so as to increase the external cooling surface. Fig. 1966.—Section through Westinghouse ½ kilovolt ampere type S transformer. Fig. 1967.—Section through Westinghouse 50 kilovolt ampere type S transformer showing large oil ducts. Ques. In what types of transformer is this mode of oil cooling used? Ans. Lighting transformers. In such transformers, the large volume of oil absorbs considerable heat, so that the rise of temperature is retarded. Hence, for moderate periods Ques. In what other capacities except that of cooling agent, does the oil act? Ans. It is a good insulator, preserves the insulation from oxidation, increasing the breakdown resistance of the insulation, and generally restores the insulation in case of puncture. Fig. 1,968.—Wagner 300 kva, 4,400 volt three phase oil cooled transformer. In this type of transformer the case is filled with oil and fluted so as to increase the cooling surface, an oil drain valve is screwed to a wrought iron nipple cast into the base, the duct to which is in such a position as to make it possible not only to drain all of the oil from the transformer, but when desirable, to draw off a small quantity from the bottom. Should any moisture be in the oil it is therefore drawn off first. Ques. What is the special objection to oil? Ans. Danger of fire. Ques. What kind of oil is used in transformers? Ans. Mineral oil. Ques. What are the requirements of a good grade of transformer oil? Ans. It should show very little evaporation at 212° Fahr., and should not give off gases at such a rate as to produce an explosive mixture with the air at a temperate below 356°. It should not contain moisture, acid, alkali or sulphur compounds. Fig. 1,969.—Section through Fort Wayne (type A) transformer showing interior of case, core conductors, and insulation, also division of laminÆ. The presence of moisture can be detected by thrusting a red hot nail in the oil; if the oil "crackle," water is present. Moisture may be removed by raising the temperature slightly above the boiling point, 212° Fahr., but the time consumed (several days) is excessive. Water Cooled Transformers.—A water cooled transformer is one in which water is the cooling agent, and, in most cases, oil is the medium by which heat is transferred from the coils to the water. In construction, pipes or a jacketed casing is provided through which the cooling water is passed by forced circulation, as shown in figs. 1,970 and 1,971. Fig. 1,970.—Water cooled transformer with internal cooling coil, that is, with cooling coil within the transformer case. In this type, the cooling coil, through which the circulating water passes, is placed in the top of the case or tank, the latter is filled with oil so that the coil is submerged. The oil acts simply as a medium to transfer the heat generated by the transformer to the water circulating through the cooling coil. In operation a continual circulation of the oil takes place, as indicated by the arrows, due to the alternate heating and cooling it receives as it flows past the transformer coils and cooling coil respectively. In some cases tubular conductors are provided for the circulation of the water. Water cooled transformers may be divided into two classes, as those having:
Ques. Describe the first named type. Ans. Inside the transformer case near the top is placed a coil of wrought iron pipe, through which the cooling water is pumped. The case is filled with oil, which by thermo-circulation flows upward through the coils, transferring the heat absorbed from the coils to the water; on cooling it becomes more dense (heavier) and descends along the inside surface of the casing. Fig. 1,971.—Water cooled transformer with external cooling coil. In this arrangement the cooling coil is placed in a separate tank as shown. Here forced circulation is employed for both the heat transfer medium (oil) and the cooling agent (water), two pumps being necessary. The cool oil enters the transformer case at the lowest point and absorbing heat from the transformer coils it passes off through the top connection leading to the cooling coil and expansion tank. Since the transformer tank is closed, an expansion tank is provided to allow for expansion of the oil due to heating. The water circulation is arranged as illustrated. Ques. How much circulating water is required? Ans. It depends upon the difference between the initial and discharge temperatures of the circulating water. Fig. 1,972.—Interior of General Electric water cooled 140,000 volt transformer showing cooling coil. Ques. In water cooled transformers how much cooling surface is required for an internal cooling coil? Ans. The surface of the cooling coil should be from .5 to 1.3 sq. in. per watt of total transformer loss, depending upon the amount of heat which the external surface of the transformer case will dissipate. For a water temperature rise of 43° Fahr., 1.32 lbs. of water per minute is required per kw. of load. Transformer Insulation.—This subject has not, until the last few years, been given the same special attention that many other electrical problems have received, although the development In transformer construction it is obviously very important that the insulation be of the best quality to prevent burn outs and interruptions of service. Ques. What is the "major" insulation? Ans. The insulation placed between the core and secondary (low pressure) coils, and between the primary and secondary coils. Fig. 1,973.—Assembled coils of General Electric water cooled 500 kva., 66,000 volt transformer. It consists usually of mica tubes, sometimes applied as sheets held in place by the windings, when no ventilating ducts are provided, or moulded to correct form and held between sheets of tough insulating material where ducts are provided for air or oil circulation. Ques. Describe the "minor" insulation. Ans. It is the insulation placed between adjacent turns of the coils. Since the difference of pressure is small between the adjacent turns the insulation need not be very thick. It usually consists of a double Ques. What is the most efficient insulating material for transformers? Ans. Mica. It has a high dielective strength, is fireproof, and is the most desirable insulator where there are no sharp corners. Fig. 1,974.—Three Westinghouse 20 kva, outdoor transformers, for irrigation service. These are mounted on a drag so that they may be readily transported from place to place. 33,000 volts high tension; 2,200 and 440 volts low tension, 50 cycles. These outdoor transformers are of the oil immersed, self-cooling type and have been developed to meet the requirements for transformers of capacities greater or of voltages higher than are usually found in distribution work. They are in reality distributing transformers for high voltage, outdoor installations, single or three phase service, for voltages up to 110,000. Where the magnitude of the load does not warrant an expensive installation, transformers of the outdoor type are particularly applicable. The cost of a building and outlet bushings which is often the item of greatest expense is eliminated where outdoor type transformers are installed. Oil Insulated Transformers.—High voltage transformers are insulated with oil, as it is very important to maintain careful insulation not only between the coils, but also between the coils and the core. In the case of high voltage transformers, any accidental static discharge, such as that due to lightning, which might destroy one of the air insulated type, might be successfully withstood by one insulated with oil, for if the oil insulation be damaged it will mend itself at once. By providing good circulation for the oil, the transformer can get rid of the heat produced in it readily and operate at a low temperature, which not only increases its life but cuts down the electric resistance of the copper conductors and therefore the I2R loss. Efficiency of Transformers.—The efficiency of transformers is the ratio of the electric power delivered at the secondary terminals to the electric power absorbed at the primary terminals. Accordingly, the output must equal the input minus the losses. If the iron and copper losses at a given load be known, their values and consequently the efficiency at other loads may be readily calculated. EXAMPLE.—If a 10 kilowatt constant pressure transformer at full load and temperature have a copper loss of .16 kilowatt, or 1.6 per cent., and the iron loss be the same, then its
At three-quarters load the output will be 7.5 kilowatts; and as the iron loss is practically constant at all loads and the copper loss is proportional to the square of the load, the
The matter of efficiency is important, especially in the case of large transformers, as a low efficiency not only means a large waste of power in the form of heat, but also a great increase in the difficulties encountered in keeping the apparatus cool. The efficiency curve shown in fig. 1,975, serves to indicate, however, how slight a margin actually remains for improvement in this particular in the design and construction of large transformers. Fig. 1,975.—Efficiency curve of Westinghouse 375 kw., transformer. Pressure 500 to 15,000 volts; frequency 60. Efficiencies at different loads: full load efficiency, 98%; ¾ full load efficiency, 98%; ½ full load efficiency, 97.6%; ¼ full load efficiency, 96.1%; regulation non-inductive load, 1.4%; load having .9 power factor, 3.3%. The efficiency of transformers is, in general, higher than that of other electrical machines; even in quite small sizes it reaches over 90 per cent., and in the largest, is frequently as high as 98.5 per cent. To measure the efficiency of a transformer directly, by measuring input and output, does not constitute a satisfactory method when the efficiency is so high. A very accurate result can be obtained, however, by measuring separately, by wattmeter, the core and copper losses. The core loss is measured by placing a wattmeter in circuit when the transformer is on circuit at no load and normal frequency. The copper loss is measured by placing a wattmeter in circuit with the primary when the secondary is short circuited, and when enough pressure is applied to cause full load current to flow. If it be desired to separate the load losses from the true I2R loss, the resistances can be measured, and the I2R loss calculated and subtracted from the wattmeter reading. The losses being known, the efficiency at any load is readily found by taking the core loss as constant and the copper loss as varying proportionally to the square of the load. Thus,
All Day Efficiency of Transformers.—This denotes the ratio of the total watt hour output of a transformer to the total watt hour input taken over a working day. To compute this efficiency it is necessary to know the load curve of the transformer over a day. Suppose that this is equivalent to 5 hours at full load, and 19 hours at no load. Then, if W1 be the core loss in watts, W2 the copper loss at rated load, and W the rated output,
and the all day efficiency is equal to
Commercial or all day efficiency is a most important point in a good transformer. The principal factor in securing a high all day efficiency is to keep the core loss as low as possible. The core loss is constant—it continues while current is supplied to the primary, while copper loss takes place only when the secondary is delivering energy. In general, if a transformer is to be operated at light loads the greater part of the day, it is much more economical to use one designed for a small iron loss than for a small full load copper loss. Figs. 1,976 and 1,977.—Westinghouse double pole fuse box; views showing box open with tubes in place, and with tubes removed. Transformer Fuse Blocks.—These may be of either the single pole or double pole type. Fig. 1,976 shows a double pole fuse box opened, and fig. 1,977, the fuse box opened and the tubes removed. Of the four wires, W, W, W, W, entering the box from beneath, two are from the primary mains, and two lead to the primary coil of the transformer. These wires terminate in metallic receptacles R, R, R, R, in the porcelain plate P, fig. 1,977, which are bridged over in pairs by fuse wires placed inside porcelain tubes T, T, as shown in fig. 1,976. These tubes are air tight except for a small outlet O in each, which fit into the receptacles B, B, in the porcelain plate and open out at the back of the block, as shown in fig. 1,977. The fuse wires are connected between metallic spring tubes S, S, S, S, which fit into the receptacles R, R, R, R. If a sudden load or a short circuit occur in the transformer, the intense heat, By this arrangement, sustained arcing is prevented, as the action of the tube causes the arc to extinguish itself automatically when the current is interrupted. The porcelain tubes are held in position by the spring K, and the primary of the transformer becomes entirely disconnected from the circuit when the tubes are lifted out. This form of construction enables the lineman to detach the tubes from the fuse box, and insert the fuse at his convenience. Furthermore, when inserting a fuse in a short circuited line, he does not run the risk of being hurt, as the heated vapor of the exploding fuse can escape through the outlet provided for that purpose, and in a predetermined direction. The method of attaching the lid not only permits of quick access to the interior of the box, but enables the lineman to tighten the joints by means of the thumb screws L, L, so as to keep the box waterproof. Fig. 1,978.—Diagram illustrating connections and principles of auto-transformers as explained in the accompanying text. Auto-transformers.—In this class of transformer, there is only one winding which serves for both primary and secondary. On account of its simplicity it is made cheaply. Auto-transformers are used where the ratio of transformation is small, as a considerable saving in copper and iron can be effected, and the whole transformer reduced in size as compared with one having separate windings. Fig. 1,978 illustrates the electrical connections and the relations between the volts and number of turns. By using the end wire and tapping in on turn No. 20 a current at 20 volts pressure is readily obtained which may be used for starting up motors requiring a large starting current and yet not draw heavily on the line. Since the primary is connected directly to the secondary it would be dangerous to use an auto-transformer on high pressure circuits. This type of transformer has only a limited use, usually as compensator for motor starting boxes. Figs. 1,979 and 1,980.—Two winding transformer and single winding or auto-transformer. Fig. 1,979 shows a 200:100 volt transformer having a 10 amp. primary and a 20 amp. secondary, the currents being in opposite directions. If these currents be superposed by using one winding only, the auto-transformer shown in fig. 1,980 is obtained where the winding carries 10 amp. only and requires only one-half the copper (assuming the same mean length of turn). If R be the ratio of an auto-transformer, the relative size of it compared with a transformer of the same ratio and output is ((R - 1)/R):1. For example, a 10 kw. transformer of 400 volts primary and 300 volts secondary could be replaced by an auto-transformer of 10×(1.33-1)/1.33=2.5 kw.; or, in other words, the amount of material used in a 2½ kw. transformer could be used to wind an auto-transformer of 400:300 ratio and 10 kw. output. Constant Current Transformers for Series Arc Lighting.—The principle of the constant current transformer as used for series arc lighting is readily understood by reference Fig. 1,981.—Elementary diagram illustrating the principles of constant current transformer as used for series arc lighting. Since the induced currents in the secondary are repelled by the primary there is a tendency for the secondary coil to jump out of the primary field, and in case of a very large current due to a short circuit in the lamp circuit, the secondary current is quickly reduced to normal by the rapid movement of the coil upward. By adjusting the counterweight for a given number of amperes required by the arc, the current will be maintained constant by the movement of the secondary coil. The magnetic field produced by the primary must be kept the same by a constant current from the alternator, therefore, when the lamp load is increased the primary voltage increases similar to that of an ordinary series wound direct current dynamo. In other words the alternator and regulating transformer supply a constant current and variable voltage. Fig. 1,982.—Mechanism of General Electric air cooled constant current transformer. It operates on the principle explained in the accompanying text and is built to supply 25 to 100 arc lamps at 6.6 to 7.5 amperes. The transformers are interchangeable and will operate on 60 or 125 cycles. The relative positions of the two coils may be changed in order to regulate the strength of the current more closely, by shifting the position of the arc carrying the counterbalance by means of the adjusting screw on it. A dash pot filled with special oil prevents sudden movements of the secondary coil and keeps the current through the lamps nearly constant, when they are being cut in or out of the circuit. In starting up a constant current transformer, it is necessary to separate the two coils as far as possible and then close the primary circuit switch and allow the two coils to come together. If the primary circuit be thrown directly on the generator the heavy rush of current which will follow due to the two coils being too close together might injure the lamps. Constant current incandescent lighting systems for use in small towns also use this method for automatically regulating the current. Regulation.—This term applies to the means adopted either to obtain constancy of pressure or current. In the transformer, regulation is inherent, that is, the apparatus automatically effects its own regulation. The regulation of a transformer means, the change of voltage due to change of load on the secondary; it may be defined more precisely as: the percentage increase in the secondary voltage as the load is decreased from its normal value to zero. Thus, observation should be made of the secondary voltage, at full load and at no load, the primary pressure being held constant at the normal value. Fig. 1,983.—General Electric air cooled constant current transformer. View showing external appearance with case on. The regulation is said to be "good" or "close," when this change is small. In the design of a transformer, good regulation and low iron losses are in opposition to one another when the best results are desired in Transformer Connections.—The alternating current has the advantage over direct current, in the ease with which the pressure and current can be changed by different connections of transformers. On single phase circuits the transformer connections can be varied to change current and pressure, and in addition on polyphase circuits the phases can also be changed to almost any form. Single Phase Connections.—The method of connecting ordinary distributing transformers to constant pressure mains is shown by the elementary diagram, fig. 1,984, where a transformer of 10 to 1 ratio is indicated with its primary winding connected to a 1,000 volt main, and a secondary winding to deliver 100 volts. Fig. 1,984.—Single phase transformer connection with constant pressure main. Fig. 1,985.—Usual method of single phase transformer connections for residence lighting with three wire secondaries. A balancing transformer is connected to the three wire circuit near the center of distribution as shown. Fig. 1,986 shows a transformer with each winding divided into two sections. Each primary section is wound for 1,000 volts, and each secondary section for 50 volts. By connecting the entire primary winding in series, the transformer may be supplied from a 2,000 volt main, as indicated, and if the secondary winding be also connected all in series, as shown, the no load voltage will be 100 between the secondary terminals. Fig. 1,986.—Diagram of single phase transformer having primary and secondary windings in two sections, showing voltages per section with series connections. The sections of the primary winding may be connected in parallel to a 1,000 volt main, and 100 volts obtained from the secondary, or the primary and secondary windings may be connected each with its two sections in parallel, and transformations made from 1,000 to 50 volts as represented in fig. 1,987. This is a very common method of construction for small transformers, which are provided with convenient terminal blocks for combining the sections of each winding to suit the requirements of the case. When the two sections of either winding are connected in parallel as shown in fig. 1,987, care must be taken to connect corresponding ends of the two sections together. Combining Transformers.—Two or more transformers built to operate at the same pressure and frequency may be connected together in a variety of ways; in fact, the primary and secondary terminals may each be considered exactly as the terminals of direct current dynamos, with certain restrictions. Fig. 1,987.—Diagram of single phase transformer with primary and secondary windings of two sections each, showing voltages per section with parallel connection. Ques. What are the two principal precautions which must be observed in combining transformer terminals? Ans. The terminals must have the same polarity at a given instant, and the transformers should have practically identical characteristics. The latter condition is not absolutely essential, but it is emphatically preferable. For example, if a transformer, which has 2 per cent. regulation, be connected in parallel, as indicated in fig. 1,988, with one which has 3 per cent. regulation, at no load the transformers will give exactly the same voltage at the secondary terminals, but at full load one will have a secondary pressure of, say, 98 volts, while the other has 97 volts. The result is that the transformer giving only 97 volts will be subject In case the transformers have practically the same characteristics it is necessary, as stated above, to make sure that the secondary terminals connected together have the same polarity at a given instant; it is not necessary to find out definitely what the polarity is, merely that it is the same for both terminals. This can be easily done as shown in fig. 1,989. Fig. 1,988.—Diagram showing unlike single phase transformers in parallel. Ques. What may be said with respect to operating transformer secondaries in parallel? Ans. It is seldom advantageous. Occasionally it may be necessary as a temporary expedient, but where the load is such as to require a greater capacity than that of a transformer already installed, it is much better to replace it by a large transformer than to supplement it by an additional transformer of its own size. Ques. How are the secondaries arranged in modern transformers and why? Ans. The secondary windings are divided into at least two sections so that they may be connected either in series or parallel. Ques. Explain how secondary connections are made for different voltages. Ans. If, for instance, the secondary pressure of a transformer having two sections be 100 volts with the terminals in parallel, as in fig. 1,990, then connecting them in series will give 200 volts at the free secondary terminals, as indicated in fig. 1,991. Ques. What precaution should be taken in connecting secondary sections in parallel in core type if the two sections be wound on different limbs of the cores? Fig. 1,989.—Method of comparing instantaneous polarity. Two of the terminals are connected as shown by a small strip of fuse wire, and then touching the other two terminals together. If the fuse blows, then the connections must be reversed; if it does not, then they may be made permanent. Ans. It will be advisable to make the connections ample and permanent, so that there will not be any liability to a difference between the current flowing in one secondary winding and that flowing through the other. Two Phase Connections.—In the case of two phase distribution each circuit may be treated as entirely independent of the other so far as the transformers are concerned. Two Figs. 1,990 and 1,991.—Methods of altering the secondary connections of a transformer having two sections in the secondary to obtain a different voltage. Fig. 1,990 shows the two sections in parallel giving say 100 volts; fig. 1,991 shows the two sections in series giving 200 volts. Ques. Is the above method usually employed? Ans. No, the method shown in fig. 1,997 is generally used. Three Phase Connections.—There is not so much freedom in making three phase transformer connections, as with single or two phase, because the three phases are inseparably interlinked. However, the system gives rise to several methods of transformer connection, which are known as:
Figs. 1,992 to 1,995.—Three phase transformer connections. Fig. 1,992 delta connection; fig. 1,993 star connection; fig. 1,994 delta star connection; fig. 1,995 star-delta connection. Delta Connection.—In the delta connection both primaries and secondaries are connected in delta grouping, as in fig. 1,992. Star Connection.—This method consists in connecting both the primaries and secondaries in star grouping, as in fig. 1,993. Delta-star Connection.—In this method the primaries are connected in delta grouping and the secondaries in star grouping, as in fig. 1,994. Star-delta Connection.—This consists in connecting the primaries in star grouping, and the secondaries in delta grouping, as in fig. 1,995. Fig. 1,996.—Two phase transformer connections. Two single phase transformers are used and connections made just as though each phase were an ordinary single phase system. Ques. What advantage has the star connection over the delta connection? Ans. Each star transformer is wound for only 58% of the line voltage. In high voltage transmission, this admits of much smaller transformers being built for high pressure than possible with the delta connection. Ques. What advantages are obtained with the delta connection? Ans. When three transformers are delta connected, one may be removed without interrupting the performance of the circuit, the two remaining transformers in a manner acting in series to carry the load of the missing transformer. Fig. 1,997.—Two phase transformer connections, with secondaries arranged for three wire distribution, the primaries being independently connected to the two phases. In the three wire circuit, the middle or neutral wire is made about one-half larger than each of the two outer wires. In fig. 1,996 it makes no difference which secondary terminal of a transformer is connected to a given secondary wire, so long as no transformers are used in parallel. For example, referring to the diagram, the left hand secondary terminal of transformer, A, could just as well be connected to the lower wire of the secondary phase, A, and its right hand terminal connected to the upper wire, the only requirement being that the two pairs of mains shall not be "mixed"; that is, transformer, A, must not be connected with one secondary terminal to phase, A and the other to phase, B. In the case shown by fig. 1,997, there is not quite so much freedom in making connections. One secondary terminal of each transformer must be connected to one of the outer wires and the other two terminals must be both connected to the larger middle wire of the secondary system. It makes no difference, however, which two secondary terminals are joined and connected to the middle wire so long as the other terminal of each transformer is connected to an outer wire of the secondary system. The desire to guard against a shut down due to the disabling of one transformer has led to the extensive use of the delta connection, especially for the secondaries or low pressure side. It should be noted that if one transformer be disabled, the efficiency of the other two will be greatly reduced. To operate a damaged three phase transformer, the damaged windings must be separated electrically from the other coils, the damaged primary and secondary being respectively short circuited upon themselves. Ques. What kinds of transformers are used for three phase current? Ans. Either a three phase transformer, or a separate single phase transformer for each phase. Fig. 1,998.—Three wire connections for transformer having two secondary sections on different legs of the core. If the secondary terminals be connected up to a three wire distribution, as here shown diagrammatically, it is advisable to make the fuse, 2, in the middle wire, considerably smaller than necessary to pass the normal load in either side of the circuit, because, should the fuse, 1, be blown, the secondary circuit through the section, Sa, will be open, and the corresponding half of the primary winding, Pa, will have a much higher impedance than the half of the primary winding, Pb, the inductance of which is so nearly neutralized by the load on the secondary winding, Sb. The result will be that the voltage of the primary section, Pa, will be very much greater than that of the section, Pb, and as the sections are in series the current must be the same through both halves of the winding; the drop or difference of pressure, therefore, between the terminals of Pa will be much higher than that between the terminals of Pb, consequently, the secondary voltage of Sb will be greatly lowered and the service impaired. As the primary winding, Pa, is designed to take only one-half of the total voltage, the unbalancing referred to will subject it to a considerably higher pressure than the normal value; consequently, the magnetic density in that leg of the transformer core will be much higher than normal, and the transformer will heat disastrously. If the fuse, 2, in the middle wire be made, say, one-half the capacity of each of the other fuses, this condition will be relieved by the blowing of this fuse, and as the lamps in the live circuit would not be anywhere near candle power if the circuit remained intact, the blowing of the middle fuse will not be any disadvantage to the user of the lamps. Some makers avoid the contingency just described by dividing each secondary coil into two sections and connecting a section on one leg in series with a section on the other leg of the core, so that current applied to either pair of the secondary terminals will circulate about both legs of the core. Figs. 1,999 to 2,002.—Three phase delta, and star connections using three transformers. There are two ways of connecting up the primaries and secondaries, one known as the "delta" connection, and illustrated diagrammatically by fig. 1,999, and the other known as the "star" connection, and illustrated by fig. 2,001. In both diagrams the line wires are lettered, A, B and C. Fig. 2,000 shows the primaries and secondaries connected up delta fashion, corresponding to fig. 1,999, and fig. 2,002 shows them connected up star fashion, corresponding to fig. 2,001. In both of the latter sketches the secondary wires are lettered to correspond with the respective primary wires. When the primaries are connected up delta fashion, the voltage between the terminals of each primary winding is the same as the voltage between the corresponding two wires of the primary circuit, and the same is true of the secondary transformer terminals and circuit wires. The current, however, flowing through the transformer winding is less than the current in the line wire, for the reason that the current from any one line wire divides between the windings of two transformers. For example, in figs. 1,999 and 2,000, part of the current from the line wire, A, will flow from A to B through the left hand transformer, and part from A to C through the right hand transformer; if the current in the line wire, A, be 100 amperes, the current in each transformer winding will be 57.735 amperes. When transformers are connected up star fashion, as in figs. 2,001 and 2,002, the current in each transformer winding is the same as that in the line wire to which it is connected, but the voltage between the terminals of each transformer winding is 57.735 per cent. of the voltage from wire to wire on the circuit. For example, if the primary voltage from A to B is 1,000 volts, the voltage at the terminals of the left hand transformer (from A to J) will be only 577.35 volts, and the same is true of each of the other transformers if the system is balanced. These statements apply, of course, to both primary and secondary windings, from which it will become evident that if the three transformers of a three phase circuit be connected up star fashion at the primaries, and delta fashion at the secondaries, the secondary voltage will be lower than if both sides are connected up star fashion. For example, if the transformers be wound for a ratio of 10 to 1, and are connected up with both primaries and secondaries alike, no matter whether it be delta fashion or star fashion, the secondary voltage will be one-tenth of the primary voltage; but if the primaries be connected up star fashion on a 1,000 volt circuit, and the secondaries be connected up delta fashion, the secondary voltage will be only 57.735 volts, instead of 100 volts. The explanation of the difference between the voltage per coil in a delta system and that in a star system is that in the former each winding is connected directly across from wire to wire; whereas in the star system, two windings are in series between each pair of line wires. The voltage of each winding is not reduced to one-half, however, because the pressures are out of phase with each other, being 120°, or one-third of a cycle, apart; consequently, instead of having 500 volts at the terminals of each coil in fig. 2,001 the voltage is 577.35. The same explanation applies to the current values in a delta system. The current phase between A and B, in fig. 1,999, is 120° removed from that in the winding between A and C; consequently the sum of the two currents, in the wire, A, is 1.732 times the current in each wire; or, to state it the opposite way, the current in each winding is 57.735% of the current in the wire, A. It will be well for the reader to remember that in all cases pressures differing in phase when connected in series, combine according to the well-known law of the parallelogram of forces; currents differing in phase, and connected in parallel, combine according to the same law. Ques. What points are to be considered in choosing between three phase and single phase transformers for the three phase current transformation? Ans. No specific rule can be given regarding the selection of single phase or three phase transformers since both designs are equally reliable; local conditions will generally determine which type is preferable. The following general remarks may, however, be helpful: Single phase transformers are preferable where only one transformer group is installed and where the expense of a spare transformer would not be warranted. In such installations the burn out of one phase of a three phase unit would cause considerable inconvenience for the reason that the whole transformer would have to be disconnected from the circuit before repairs could be made. If single phase transformers be used and connected in delta on both primary and secondary, With a three phase shell type transformer, if both the primary and secondary be delta connected, trouble in one phase will not prevent the use of the other two phases in open delta. By short circuiting both primary and secondary of the defective phase, and cutting it out of circuit the magnetic flux in that section is entirely neutralized. This cannot be done, however, with any but delta connected shell type transformers. Fig. 2,003.—Diagram showing three wire secondary connections General Electric (type H) transformer. As will be seen, the method adopted consists of distributing equally, on each side of the primary coil, both halves of the secondary winding, so that each secondary throughout its length is closely adjacent to the entire primary winding. In order to insure the exact equality of resistance and reactance in the two secondary windings necessary to obtain perfect regulation of the two halves, the inside portion of the secondary winding on one side of the primary coil is connected in series with the outside portion of that on the other side. As a result, the drop of voltage in either side of the secondary under any ordinary conditions of unbalanced load, does not exceed the listed regulation drop. This particular arrangement is used because it is the simplest and best method for this construction. Where a large number of three phase transformers can be used, it is generally advisable to install three phase units, the following advantages being in their favor as compared with single phase units:
Ques. What is the character of the construction of three phase transformers? Ans. The three phase transformer is practically similar to that of the single phase, except that somewhat heavier and larger parts are required for the core structure.
Figs. 2,004 to 2,011.—Connections of standard transformers. All stock transformers are wound for some standard transformation ratio, such as 10 to 1, but various leads are brought out by means of which ratios of 5, 10 and 20 to 1 may be obtained for one transformer. The figures show the voltage combinations possible with a standard transformer. Ques. How are transformers connected for four wire three phase distribution? Ans. When the secondaries of three transformers are star Fig. 2,012.—Method of determining core loss. Connect voltmeter and wattmeter as shown in the illustration to the low tension side of the transformer. By means of a variable voltage transformer bring the applied voltage to the point for which the transformer is designed. The wattmeter indicates directly the core loss, which includes a very small loss due to the current in the copper. Cautions.—1. Make sure of the voltage and frequency. The manufacturers' tabulated statements refer to a definite voltage and frequency and these have a decided influence upon the core loss. 2. The high tension circuit must remain open during the test. The voltage between any main wire and the neutral will be 57 per cent. of the voltage between any two main wires. For general distribution this system is desirable, requiring less copper and greater flexibility than other systems. Three phase 200 volt motors may be supplied from the main wires and 115 volt lamps connected between each of the three main wires and the neutral; if the lamp load be very nearly balanced the current flowing in the neutral wire will be very small, as in the case of the ordinary three wire direct current system. How to Test Transformers.—The troubles incident to gas or water service have their parallels in electric power distribution. Fig. 2,013.—Method of determining copper loss. Connect ammeter and wattmeter to high tension side of transformer short circuit secondary leads, as shown in the illustration, and by means of a variable voltage, adjust current to the full load value for which the transformer is intended. The wattmeter reading shows the copper loss at full load. The full load primary current of any transformer is found from the following equation.
EXAMPLE: To find proper full load current on a five kw. 2,200 volt transformer, divide 5,000 watts by 2,200 volts, the full load current will then be 2.27 amperes. A slight variation in primary current greatly increases or decreases the copper loss. Remarks.—Copper loss increases with temperature because the resistance of the metal rises. Do not overload the current coil of the wattmeter. For greater accuracy the I2R drop of potential method should be used. Companies engaged in the former, credit a large percentage of their losses to leaky valves and defective mains. The remedy may involve heavy expense and the loss is often tolerated as the lesser of two evils. In electric power distribution the transformer takes in part the place of the valve and pipe system. An inferior or defective transformer usually treats both the central station and its customers badly, being in this respect more impartial than the gas or water pipe which may annoy but one of the interested parties at a time. Like a neglected or defective gas fixture a transformer can menace life, failing, however, to give the warning the former gives, and with a more hidden threat on account of its location. Fig. 2,014.—Diagram of connections for regulation test. Connect transformer under test to high tension supply circuit. A second transformer with same or other known change ratio is also to be connected up, as illustrated. By means of a double pole double throw switch, the voltmeter can be made to read the pressure on the secondary of either transformer. Supposing the same change ratio it is evident that if both remain unloaded the voltmeter will indicate the same pressure. A gradually increasing lamp load up to the limit of the transformer capacity, will be attended by a drop in pressure at the terminals. This drop can be read as the difference of the voltmeter indications, and when expressed in per cent. of secondary voltage stands for "regulation." Remarks: The auxiliary transformer is necessary in order to make sure of the high tension line voltage. A large transformer under test may cause primary drop in taking power. This must be set down against it in testing regulation. The second transformer gives notice of such drop, whatever be the cause. Figs. 2,012 to 2,014 used by courtesy of the Moloney Electric Co. Apart from this, corresponding to an exasperated customer who complains at home and to his friends of dim lamps, blackened lamps, you will find in the power station the manager, who, also worried and in no better humor, contemplates the difference in meter readings at the end of the line. His business does not increase and would not increase even if he could lower the rates, which he cannot do because of these meter readings. He may be confident of his engines and generators, and that his line is up and all right, but he very seldom knows what the transformers are doing on top of Perhaps the transformers were purchased because of their attractive prices and never tested. Water, plumbing, gas and steam fittings are subjected to test. Why not transformers? Even more so because transformers take constant toll from the company installing them, while gas and water fittings, once passed, are off the contractor's hands. The busy manager has little time for complicated treatises and monographs on electrical measurements and even handbooks confront him with forbidding formulÆ. Accordingly the methods of transformer testing, which are very simple, are illustrated in the accompanying cuts. Managers of electric power and lighting companies should study them carefully. Fig. 2,015.—Wagner central station core type transformer repair unit consisting of one half set of primary and secondary windings together with the section of the iron core upon which the coils are wound. An ammeter, voltmeter and wattmeter are required to make the tests. Losses are small in good transformers and hence the instruments should be accurate. For the same reason instruments should be chosen of the proper capacity to give their best readings. If there be any doubt about the testing instruments being correct, they should be calibrated before being used. The testing circuits should be properly fused for the protection of the instruments. It is hardly logical, but a very common practice is to mistrust meters and to watch them closely, while the transformers are guilty of theft unchallenged, and keep busily at it on a large scale. Fig. 2,016.—Moloney tubular air draft oil filled transformer. The case is made of cast iron, with large steel tubes passing from the bottom through the top. In operation the air in the tubes becomes hot and expands; a draft is thus produced which carries away considerable heat. Transformer Operation with Grounded Secondary.—The operation of a transformer with a grounded secondary has been approved by the American Institute of Electrical Engineers, and by the National Board of Fire Underwriters. This method of operation effectually prevents a high voltage occurring upon the low tension wires in case of a breakdown or other electrical connections occurring between the primary and secondary windings. Fig. 2,017.—Moloney pressure transformer adapted for switchboard work in connection with voltmeters, wattmeters the, etc., in sizes from 25 to 500 watts. In case of a breakdown without the secondary grounded, any one touching a part of the low tension system, such as a lamp socket, might receive the full high pressure voltage. With the low tension grounded, the fuse in the high tension circuit will blow and the fault be discovered upon replacing it. Transformer Capacity for Motors.—The voltage regulation of a well designed transformer is within 3 per cent. of its rated voltage on a non-inductive load such as incandescent lamps, but when motors are connected to the circuit their self-induction causes a loss of 5 per cent. or more, and if the load be fluctuating, it is better to use independent transformers for the motor, which will prevent considerable fluctuations in the incandescent lamps. Arc lamps do not show slight voltage changes as much as incandescent lamps. The proper rating of transformers for two phase and three phase induction motors is given in table on the next page. Fig. 2,018.—Moloney current transformer switchboard or indoor type. It is used ordinarily for insulating an ammeter, a current relay, the current coil of a watt meter or watt hour meter from a high tension circuit, for reducing the line current to a value suitable for these instruments. A three phase induction motor may be operated from three single phase transformers or one three phase transformer. While the one three phase transformer greatly reduces the space and simplifies the wiring, the use of three single phase transformers Fig. 2,019.—Diagram showing a method of operating a three phase motor on a two phase circuit, using a transformer having a tap made in the middle of the secondary winding, so as to get the necessary additional phase. While this does not give a true balanced three phase secondary, it is close enough for motor work. In the above arrangement, the main transformer supplies 54 per cent. of the current and the other with the split winding 46 per cent. It is well to allow one kilowatt per horse power of the motor in selecting the size for the transformers, excepting in the small sizes when a little larger kilowatt rating is found to be the most desirable.
Very small transformers should not be used, even when the motor is large compared to the work it has to do, as the heavy starting current may burn them out. The following tables give the proper sizes of transformer for three types of induction motor and the approximate current taken by three phase induction motors at 220 volts.
Transformer Connections for Motors.—Fig. 2,020 shows the connection of a three phase so called delta connected transformer with the three primaries connected to the lines leading from the alternator and the three secondaries leading to the motor. The connections for a three phase motor using two transformers is shown in fig. 2,021 and is identical with the previous The copper required in any three wire three phase circuit for a given power and loss is 75 per cent. that necessary with the two wire single phase or four wire two phase system having the same voltage between lines. Fig. 2,020.—Three phase motor transformer connections; the so-called Delta connected transformers. The connections of three transformers for a low tension system of distribution by the four wire three phase system are shown in fig. 2,022. The three transformers have their primaries joined in delta connection and the secondaries in "Y" connection. The three upper lines of the secondary are the three main three phase lines, and the lowest line is the common neutral. Fig. 2,021.—Three phase motor connections using two transformers. The voltage across the main conductors is 200 volts, while that between either of them and the neutral is 115 volts; 200 volt motors should be joined to the mains while 115 volt lamps are connected between the mains and neutral. The arrangement is similar to the The voltage between the mains should be used in calculating the size of wires, and the size of the neutral wire should be made in proportion to each of the main conductors that the lighting load is to the total load. Fig. 2,022.—Delta-star connection of three transformers for low pressure, three phase, four wire system. When lights only are used the neutral should be the same as the main conductors. The copper required in such a system for a given power and loss is about 33.3 per cent. as compared with a two wire single phase system or a four wire two phase system using the same voltage. Fig. 2,023.—Diagram of transformer connections for motors on the monocyclic system. Monocyclic Motor System.—Motors on the monocyclic system are operated from two transformers connected as shown in fig. 2,023. In the monocyclic system the single phase current is used to supply the lighting load and two wires only are necessary, but if a self-starting induction motor be required, a third or teaser wire is brought to the motor and two transformers used. The teaser wire supplies the quarter phase current required to start the motor, which afterwards runs as a single phase synchronous motor and little or no current flows through the teaser circuit as long as the motor keeps in synchronism; in case it fall behind, the teaser current tends to bring it up to speed instead of the motor stopping, as would be the case of a single phase motor. Fig. 2,024.—Moloney flaming auto type arc lamp transformer for 110 volts primary to 55 volts secondary. A hook in bottom of case provides means for suspension of lamp. The transformer may be operated on circuits from 100 to 120 volts primary, 50 to 60 volts secondary. The secondary capacity is 8 to 12 amperes. The voltage of the transformers should be tested by means of a voltmeter or two incandescent lamps joined in series, before starting up the motor, to see if the proper transformer connections have been made and prevent an excessive flow of current. If one of the transformers be reversed the voltage will be almost doubled; in fact, it is a good plan to check up all the transformer connections with the voltmeter or lamps which will often save a burn out.
Fig. 2,030.—Installation of a transformer on pole; view showing method of attachment and disposition of the primary and secondary leads, cutouts, etc. Fig. 2,031.—Diagram of static booster or regulating transformer. It is used for regulating the pressure on feeders. In the figure, B are the station bus bars, R the regulable transformer, F the two wire feeders, and T a distant transformer feeding into the low pressure three wire distributing network N. The two ends of the primary, and one end of the secondary of R, are connected to the bus bars as shown. The other end of the secondary, as well as a number of intermediate points, are joined up to a multiple way switch S, to which one of the feeder conductors is attached, the other feeder main being connected to the opposite bus bar. As will be evident from the figure, by manipulating S extra volts may be added to the bus bar pressure at will, and the drop along F compensated for. R is a step transformer, the total secondary difference of pressure being comparatively small. The above device possesses rather serious drawbacks, in that the switch S has to carry the main current, and that the supply would be stopped if the switch got out of order. Kapp improved on the arrangement by putting the switch in the primary circuit. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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