Техническое чтение для энергетиков. Бухарова Г.П. - 60 стр.

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62
to lower voltages, suitable for use with small-motors and lamps. One could have
some other transformers in the system that reduce the voltage even further. All radio
sets and all television sets are known to use two or more kinds of transformers. These
are familiar examples showing that electronic equipment cannot do without
transformers. The facts you have been given above illustrate the wide use of
transformers and their great importance
Another alternating-current system of transmission and distribution is shown in
Fig. 9. You are asked to follow the whole process, that is, to describe it from
beginning to end.
TRANSFORMERS
Faraday's experiments of August 29, 1831, gave us the principle of the electric
transformer, without which the later discoveries of that fateful year could have little
real practical application. For to convey electric current over long distances, say to
supply a town, or feed an electric railway, it is necessary to generate it at a very high
voltage, or force. By means of transformers based on Faraday's Induction coil
discovery, it is simple for a current from the grid or direct from a power-station of
say 132.000 volts to be stepped down for the electric train to 600 volts and for
household use to 240 volts. Smaller transformers in individual prices of electrical
equipment, say a shaver or radio, may step the current down still further for special
purposes. Similarly, currents may be stepped up in voltage, if required, by the same
device. The procedure is quite simple. The current is fed into the transformer across
the primary, of input coil, which corresponds to Faraday's right-hand coil on his
induction ring. The resultant induced current is taken from the secondary, of output
coil, which corresponds to Faraday's left-hand coil. If this secondary coil has more
windings of wire than the primary coil, the voltage will be stepped down.
So the two related discoveries of 1831 provided not only the means of making
electricity easily and cheaply, on as large a scale as required, without any
cumbersome batteries, but also the way of using it in a safe and practical way.
PRINCIPLES OF OPERATION
Theory of Transformer Action.– A transformer may be defined as a piece of
apparatus* without continuously moving parts, which by electromagnetic induction
transforms alternating voltage and current in one winding into alternating voltage and
current in one or more other windings, usually at different values of voltage and
current. It consists essentially of an iron core on which are wound the primary and
secondary windings.
When an alternating voltage is applied to the terminals of the primary winding, the
secondary being open-circuited, the apparatus behaves like a choking coil. An
alternating magnetic flux is set up and this induces a back e.m.f. in the primary
winding. Neglecting losses, this back e.m.f. exactly neutralizes the applied e.m.f.,

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