CHAPTER LII
TRANSFORMERS
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:
- 1. A primary winding;
- 2. A secondary winding;
- 3. An iron core.
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 the south pole through the surrounding air; and as the reluctance of air is much greater than that of iron, the magnetism will be weak.
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:
- primary voltage: secondary voltage = primary turns: secondary turns
- primary current: secondary current = secondary turns: primary turns
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
a. Step up transformers;
b. Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
a. Core transformers;
b. Shell transformers;
c. Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
a. Single phase transformers;
b. Polyphase transformers.
4. With respect to the method employed in cooling, as
a. Dry transformers;
b. Air cooled transformers {natural draught;
{forced draught, or air blast;
c. Oil cooled transformers;
d. Water cooled transformers.
5. With respect to the nature of their output, as
a. Constant pressure transformers;
b. Constant current transformers;
c. Current transformers;
d. Auto-transformers.
6. With respect to the kind of service, as
a. Distributing;
b. Power.
7. With respect to the circuit connection that the transformer is constructed for, as
a. Series transformers;
b. Shunt transformers.
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 ⅕ 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 exactly the same in amount as that furnished per second by a current of 400 amperes supplied at a pressure of 50 volts; in each case, the product is 20,000 watts.
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, require the delivery of the current at comparatively low pressures ranging from 100 to 250 volts for lamps, and from 100 to 600 volts for motors.
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, and for small sizes it possesses most of the advantages of both types. It can be constructed at less cost than can either a core or a shell transformer having the same operating characteristics and temperature limits.
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 there are two or more magnetic circuits through the core, and the fluxes in the various circuits are displaced in phase.
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 difference between the input and the output is equal to the sum of these losses. They are divided into two classes:
- 1. The iron or core losses;
- 2. The copper losses.
The iron or core losses are due to
- 1. Hysteresis;
- 2. Eddy currents;
- 3. Magnetic leakage (negligibly small).
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
- 1. Heating the conductors (the I2R loss);
- 2. Eddy currents in conductors.
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.