Техническое чтение для энергетиков. Бухарова Г.П. - 43 стр.

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45
increased many times ever since its invention, there has never been any change of the
underlying principle. One of its drawbacks was that its speed was constant and
unchangeable.
Some years later a squirrel-cage motor with two-speeds –the most far-reaching
innovation since the invention of the induction motor was developed. The speed
change is achieved by modulating the pole-amplitude of the machine.
HEATING EFFECT OF AN ELECTRIC CURRENT
The production of heat is perhaps the most familiar among the principal effects
of an electric current, either because of its development in the filaments of the electric
lamps or, maybe, because of the possible danger from overloaded wires.
As you know, of course, a metal wire carrying a current will almost always be at
a higher temperature than the temperature of that very wire unless it carries any
current. It means that an electric current passing along a wire will heat that wire and
may even cause it to become red-hot. Thus, the current can be detected by the heat
developed provided it flows along the wire.
The reader is certain to remember that the heat produced per second depends
both upon the resistance of the conductor and upon the amount of current carried
through it. As a matter of fact, if some current flowed along a thin wire and then the
same amount of current were sent through a thicker one, a different amount of heat
would be developed in both wires. When the current is sent through the wire which is
too thin to carry it freely, then more electric energy will be converted into heat than in
the case of a thick wire conducting a small current.
Let us suppose now that a small current is flowing along a thick metal
conductor. Under such conditions the only way to discover whether heat has been
developed is to make use of a sensitive thermometer because the heating is too
negligible to be detected by other means. If, however, our conductor were very thin
while the current were large the amount of generated heat would be much greater
than that produced in the thick wire. In fact, one could easily feel it. Thus, we see that
the thinner the wire, the greater the developed heat. On the contrary, the larger the
wire, the more negligible is the heat produced.
Needless to say, such heat is greatly desirable at times but at other times we
must remove or, at least, decrease it as it represents a waste of useful energy. In case
heat is developed in a transmission line, a generator or a motor, it is but a waste of
electric energy and overheating is most undesirable and even dangerous. It is this
waste that is generally called "heat loss" for it serves no useful purposes and does
decrease efficiency. Nevertheless, one should not forget that the heat developed in the
electric circuit is of great practical importance for heating, lighting and other
purposes. Owing to it we are provided with a large number of appliances, such as:
electric lamps that light our homes, greets and factories, electrical heaters that are
widely used to meet industrial requirements, and a hundred and one other necessary
and irreplaceable things which have been serving mankind for so many years.

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