Alternating Current Transformer
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-427
2025-12-17
18
Since alternating dynamos are built to give high voltage, some method is necessary for changing this voltage to that which can be used in a lamp. A step-down transformer is used for this purpose; it is virtually a reversed Ruhmkorff coil, - reversed because the work to be done is to change a high potential current into one of a low potential. The principle of the step-down transformer is shown in Fig. 1. The current from the dynamo flows through a long coil of fine wire which has an iron core to increase the magnetic field. Surrounding the same core is a second coil, shorter and of larger wire. To the terminals of this coil are attached the lamps to be lighted. The proper size and length of the wire in each coil are determined by the respective voltages of the dynamo and of the lamps to be used. A step-down transformer reduces the voltage in the same ratio as that of the numbers of turns in the two coils. For instance, to reduce a 1000-volt current to one of 100 volts, the number of turns in the primary must be 10 times the number in the secondary. Very little energy is lost in the transformation; if the 1000-volt current sent into the transformer is of 1 ampere, the 100- volt current taken from it will be of very nearly 10 amperes.
Figure 2 shows in cross section the details of a step-down transformer with the relation of its parts, P being the windings of the primary coil and S of the secondary; and Fig. 3 shows the instrument as set up for use. This is the kind used on a line pole to reduce the 1000-volt current from the station to a lower voltage current for house use. When in use, it is filled with oil for insulation.
A step-up transformer changes a current of low voltage to one of high voltage; it has a greater number of turns in the secondary than in the primary.
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