The boxes or blocks which contain these fuses may be attached to or combined with the transformer as illustrated in Fig. 128, or they may be entirely separate from it as shown in Fig. 143. In either case the fuse itself is usually inclosed in or carried by a tube or plug of porcelain which is easily inserted and withdrawn through a hole in the box in order to facilitate the inspection or renewal of a fuse. Fuse-blocks are made either double- or single-pole as represented in the two illustrations cited. The presence of a fuse in the primary circuit protects the secondary circuit also, since an abnormal current in the latter causes a corresponding increase in the primary current which will blow the fuse and open the circuit. In most cases the secondary circuit is further protected by fuses inserted in the local or house wiring. Testing Transformers. For determining efficiency various methods have been employed. That used by Professor Ryan* consisted in tracing out by means of instantaneous contacts the curves of primary and secondary E.M.F. and of primary current, the secondary current being measured by an ammeter. Having obtained these curves, the power in each circuit was calculated, and the ratio gave the efficiency. This method also has the advantage that the exact form and phase relations of the several waves are brought * Trans. Amer. Inst. Elec. Eng., Dec., 1889. out. Mr. W. Mordey* proposed to find the efficiency of a transformer by running it at the given load until a constant temperature is reached as determined by a thermometer or by a resistance test. Direct currents are then passed through the coils of such strength that their heating effect maintains the same constant temperature. It follows that the direct current power (= IR=EI), which is easily measured by volt- and ampere-meters or by a watt-meter, must be equal to the total losses with the alternating current. Calorimetric methods have been used by Dr. L. Duncan, † the total losses being determined by placing the transformer in a water, oil, or ice calorimeter. Both of these last methods, depending upon heat measurements, are laborious and liable to error. Volt- and ampere-meters may be employed to measure the pressures and currents in the primary and secondary circuits. If the load is non-inductive and more than one-tenth of full value the product of secondary volts and amperes, divided by the product of primary volts and amperes, is the efficiency. With very light load or with inductive load the current lags behind the E.M.F., and the voltamperes must be multiplied by the power factor (cos ) to get the true watts. By means of one of the various three-instrument methods, the true power can be determined; but the simplest plan is to measure the true watts in the primary and in the secondary circuits with wattmeters. Stray power methods are convenient and accurate, the losses being determined individually. The iron losses, which we have seen are constant (page 154), are determined by a wattmeter in the primary circuit when the secondary is open. The copper losses may be calculated for any load by (69) if the primary and secondary currents as well as resistances are known or can be measured, which is usually an easy matter. Since the efficiency is always found for a definite load, the secondary current is fixed by that fact. The primary current I' is in which I" is the secondary current, k the ratio of transformation, and I, the exciting current which flows with open secondary. If * Jour. Inst. Elec. Eng., London, vol. XVIII. p. 608. † Electrical World, vol. IX. p. 188. I, is assumed to be 3 per cent of I" + k the error in the efficiency will be very slight. Having determined the iron and copper losses the efficiency is equal to the secondary watts divided by the secondary watts plus the losses as given by (72). Potential Transformers are used to furnish current for voltmeters or wattmeters. They are small transformers (Fig. 144), usually mounted on the switchboard, their function being to convert high voltages to lower values that are more convenient and safer to measure. With a definite ratio of transformation, a volt- or wattmeter supplied from the low-voltage secondary circuit, can be calibrated to indicate the original or primary voltage. If the currents consumed produce a certain percentage of drop in the secondary voltage a corresponding error is introduced, unless the instrument is specially calibrated to allow for this. A simpler plan is to use a transformer having sufficient. capacity so that the drop. is insignificant. One should not connect additional instruments or pilot lamps to a potential transformer until it has been ascertained that they do not cause an objectionable fall in secondary voltage. Auto-Transformers. In these devices the primary and secondary currents both flow in a single winding. The circuits of one form of auto-transformer are represented in Fig. 145, A B being a coil of insulated wire wound upon an iron core as in an ordinary transformer. When the coil AB is supplied with alternating current from the primary circuit on the left, differences of potential are established between the various parts of the coil. If connections are made to it at the points C and D, which divide it into three equal parts, the potential difference between D and E will be one-third of the total voltage applied at A and B, and between CE it will be two-thirds of that value. Assuming, for example, that 300 volts are supplied at A and B, then 100 volts may be tapped off from D and E and 200 volts between C and E. These might be used in almost exactly the same way as the common types of transformer with separate primary and secondary circuits, since a certain number of watts at one voltage may be converted into a nearly equal number of watts at another voltage. A D There is an objection, however, to auto-transformers, arising from the fact that the secondary is connected directly to the primary circuit, as at B and E in Fig. 145. Although the actual voltage between the secondary wires may not be high, nevertheless conditions may arise that will make the secondary circuits very dangerous. For example, an accidental ground anywhere on the primary conductor A will subject to the full primary voltage a person who is connected to the earth and happens to touch the secondary wire E. It is practically the same as if the primary current breaks through to the secondary circuit in an ordinary transformer, and we have seen in Figs. 138 to 142 what precautions are taken to make this danger as small as possible. On this account auto-transformers are not suitable for general use on high-tension systems. They are employed chiefly for series circuits in electric lighting, as described under that head in the next chapter. They are used also in place of dead resistance for starting alternating current motors. It is evident that they may be applied as compensators to subdivide the voltage in threeand five-wire systems instead of the machines described on page 79. E B The action of an auto-transformer is similar to that of the ordinary transformer. In either case the primary current sets up an alternating magnetic flux which induces an E.M.F. in each turn of winding. In an auto-transformer there is only one winding; but if any two points, as D and E in Fig. 145, are connected to a suitable circuit, a current will flow through it. This tends to produce a demagnetizing effect similar to that due to the secondary current of the common transformer; hence the primary current increases, in order to maintain the same magnetization, and automatically adjusts itself to supply the energy drawn in the secondary circuit. Reactive and choke coils, which are somewhat similar in construction and action, will be described as means of regulation in the next chapter. Standard Types of Transformers. The following table gives data concerning standard commercial transformers of from 6 to 50 k.w. capacity. It will be noted that the 125 cycle type has less core loss, and higher efficiency, but poorer regulation, than the 60 cycle type; the differences, however, are not very great. Regulation, GENERAL ELECTRIC TYPE H OIL TRANSFORMERS. ADAPTED FOR USE ON 50 TO 140 CYCLE Circuits. Data Based on 1040 or 2080 Volts Primary and 60 Cycles (Column A), or 125 Cycles (Column B). A B 600 25 1,000 32 25 1,500 38 30 2,000 45 35 2,500 50 39 20 70 95 125 16.7 2.93 3.50 93.5 94.2 92.9 93.8 91.1 92.5 85.2 87.7 27.4 2.80 2.87 94.4 95.0 94.0 94.9 92.8 93.0 88.1 90.4 37.5 2.63 2.90 95.2 95.7 95.0 95.6 94.0 95.0 90.392.1 50.0 2.58 2.80 95.5 95.9 95.4 96.0 94.5 95.5 91.2 92.9 155 54.0 2.23 2.50 96.0 96.4 95.9 96.4 95.196.0 92.1 93.6 195 3,000 55 42 62.0 2.13 2.40 96.2 96.7 96.1 96.7 95.5 96 3 92.7 94.2 220 4,000 63 49 85.0 2.19 2.40 96.4 96.7 96.4 96.8 95.9 96.6 93.6 94.8 270 5,000 70 51 105.0 2.17 2.40 £6.6 96.9 96.6 97.0 96.2 96.9 94.2 95.3 350 7,500 110 85 147.0 2.10 2.20 96.7 97.0 96.6 97.196.2 96.9 94.0 95.2 470 10,000 140 108 177.0 1.90 2.00 86.9 97.2 96.9 97.3 96.4 97.0 94.3 95.4 535 15,000 175 135 272.0 1.90 1.95 97.1 97.4 97.1 97.5 96.8 97.3 95.1 96.1 850 |20,000 190 147 356.0 1.94 2.10 97.3 97.5 97.4 97.7 97 2 97.6 95.9 96.7 995 25,000 220 170 460.0 1.98 2.10 97.3 97.6 97.5 97.8 97.3 97.7 96.1 96.9 1210 30,000 250 193 495.0 1.81 2.00 97.5 97.7 97.7 97.9 97.5 97.8 96.3 97.1 1500 40,000 390 300 590.0 1.65 1.90 97.6 97.8 97.6 97.9 97.4 97.8 95.9 96.7 1780 50,000 460 354 690.0 1.48 1.70 97.7 97.9 97.7 98.0 97.5 97.9 96.1 96.9 1900 Temperature rise not exceeding 45° C. for A and 40° C. for B in 8 hours full load. For comparison with data based on 1000 or 2000 volts primary, deduct 7% from the above core loss and add 0.1 to the per cent regulation. The above transformers are suitable for operation on circuits having voltage within 10% above or below the rating. Polyphase Transformers. Exactly the same types of transformers as those used for single-phase currents may be employed with two- and three-phase currents, each phase or branch having its own transformer or set of transformers. It is possible, also, to construct special polyphase transformers in which the magnetic circuits are combined in a manner analogous to that in which the |