« НазадПродовжити »
step-down type, which is far more common, the primary, P, is composed of many turns of fine wire, being connected to the highvoltage supply conductors D and E, leading from the alternator, A; and the secondary, S, consists of comparatively few turns of large wire, to which the local or secondary circuit, FG, and lamps, Z, are connected. The ratio of voltages of the two coils is substantially the same as the ratio of the number of turns of wire that they contain.
Construction of Transformers. — In practice the arrangement represented in Fig. 122 would not be satisfactory for supplying constant potential to lamps, etc., because the flux produced by the primary P would not all pass through the secondary S, the magnetic leakage across from C to H being considerable, especially when the current in the secondary circuit is large.
To avoid this magnetic leakage, the primary and secondary coils are placed close together, in many cases being wound one upon the other, as in Fig. 123, which shows the core and coils of a small General Electric type F transformer. In other forms the coils are subdivided and placed side by side alternately, as in Fig. 124, which illustrates a Westinghouse 25 k.w. transformer with the casing removed.
In these and other types of constant
Fig. 123. Transformer with J 1
Superposed Cons. potential transformers the object is to
reduce the magnetic leakage to a minimum, by so arranging the coils and magnetic circuit that practically all the flux produced by the primary must pass through the secondary. This condition is in conflict with the necessity for very high insulation between the primary and secondary coils to prevent the dangerous high-voltage current from breaking through to the lowvoltage circuit. To avoid magnetic leakage, the coils are closely sandwiched together; but to give the best insulation they should be separated. The insulation is obtained, however, by completely covering each coil with several spiral windings of tape and micacloth, as represented in Figs. 123 and 125. Sheets of fibre, micacloth, etc., are also placed between the coils, in addition to which special means are generally provided to avoid trouble through failure of insulation. These will be discussed under the head of Transformer Protective Devices.
The coils themselves consist of ordinary cotton-covered round or flat copper wire. These are wound and insulated separately; the proper number and arrangement of primary and secondary coils arc then assembled, being held in position by a suitable frame or support. The strips of thin sheet iron (8 to 15 mils = ..008 to ..015 inch thick) which form the core are next placed around the coils as illustrated in Fig. 125. One method of building up the core is shown in Fig. 126, being the plan followed by the General Electric Company in the larger type //transformers. Two different sizes of strips, A and B, are used, one being longer than the other. A layer of these is laid (Fig. 126); then the next layer is placed so that it breaks joints with the first, as indicated by the dotted lines; and so on until the core is completed, the coils shown in section
at CC being entirely surrounded by iron, which forms two closed magnetic circuits. The small round holes in the iron strips are slipped over vertical bolts, which serve to locate and hold them in place. A core of the form illustrated in Fig. 123 is generally made of sheet iron punched out in the shape shown in Fig. 127. Each layer consists of a single piece; but it is cut through at L and M, forming a tongue, T, which is sufficiently flexible to be put through the coils (Fig. 123). The next piece is placed in the opposite direction, as indicated by dotted lines, in. order to distribute the joints in the magnetic circuit.
For actual use a transformer is enclosed in a cast or wrought iron case to protect it from mechanical injury and dampness. In many instances this case is filled with oil to facilitate the dissipation of heat and to improve the insulation. The Stanley transi BO
former (sizes 2 to 10 k. w.) for out-door use, having hanger irons with hooks to fit over cross-arms on poles, is shown in Fig. 128.
Copper Loss in Transformers. — This quantity is simply the heating or / s R loss which always occurs whenever any current, /, flows through a resistance, R. If /, is the primary, and I" the secondary current, R' and R" being the respective resistances of the primary and secondary coils, then the total copper loss, IVC, in watts is:
Fig. 128. Transformer Coil.
From (69) it is seen that this loss varies as the square of the load in amperes'. Its exact value depends upon the design of the transformer and working conditions; but in most commercial types at full load it is about 3 per cent for 1 k. w., and about 1 per cent
for 100 k. w. capacity. In a very y~ ~J \ \ large General Electric transformer
having 1875 k. w. output, used at Niagara, the copper loss is a little less than J. per cent.* It is also evident from (69) that the copper loss increases with the resistance; and since the latter is greater with higher temperature, the loss is larger when the transformer becomes heated by the current or in any other way. The maximum allowable rise in temperature is 50° C. above that of the surrounding air (Amer. Inst. Elec. Eng. Standard); and since the resistance is increased about A per cent for each degree, the resistance and copper loss are about 50 x .004 = 20 per cent
* Electrical World and Engineer, Nov. 18, 1899.
Fig. 127. Transformer Coll.
higher at maximum temperature. This point will be considered more fully later under Regulation.
The copper loss injuriously affects the action of transformers in three ways:
1. Copper loss reduces the efficiency.
2. Copper loss produces heat that may injure the insulation.
3. Copper loss interferes with the regulation of constant po
have just seen that the latter vary as the square of load. These statements apply to constant potential transformers, these being by far the most common. The case is exactly the reverse for the constant current type, in which the copper loss in the secondary is constant, and the iron loss varies with the load. In most practical types of transformer the iron losses are about equal to the copper losses at full load. This is not a necessary condition, it being an easy matter to design a transformer in which the iron losses are much greater than the copper losses, or vice versa. In fact, they are mutually related, so that increasing one tends to decrease the other.