Техническое чтение для энергетиков. Бухарова Г.П. - 62 стр.

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64
voltage, V
2
, since the secondary resistance, R
2
, and the secondary reactance, X
2
, cause
voltage drops of I
2
R
2
and I
2
X
2
in phase and in quadrature with the current, I
2
,
respectively. The back induced e.m.f., Е
1
, is related to E
2
by the formula
.
2
1
2
1
ratioturnsn
T
T
E
E
===
The primary applied voltage, V
1
, is again slightly larger than E
1
, on account of the
voltage drops caused by the primary resistance, R
1
, and the primary reactance, X
1
.
These again bring about voltage drops of I
1
R
1
and I
1
X
1
in phase and in quadrature
with the primary current, I
1
, as before.
AUXILIARY EQUIPMENT
Methods of Cooling
Various methods of cooling transformers are adopted in practice, depending upon the
size and the local conditions. Very small transformers are cooled naturally by the
atmosphere, no special cooling arrangements being necessary. Slightly larger
transformers are oil-immersed, being enclosed in a tank for this puprose. The object
of the oil is twofold. As an insulator, it is better than air, and it also keeps down the
maximum temperature rise by setting up convection currents which tend to equalize
the temperature. These convection currents carry the heat away to the walls and lid of
the tank, whence it is dissipated into the atmosphere. Small tanks are made with a
plain exterior, sufficient cooling surface being obtained in this manner. Rather larger
tanks are made with a corrugated exterior, or are provided with fins, to increase the
cooling surface. A more popular arrangement, however, is to obtain the desired
increase in cooling surface by means of a number of tubes running from top to
bottom on the outside of the tank. The oil immediately in contact with the transformer
in the tank is heated by the transformer, and consequently rises. Convection currents
cause the oil to flow outwards at the top of the tank and so it enters the tubes at the
top. It is now cooled in the tubes, sinks, and re-enters the tank at the bottom. It is now
heated again by the transformer, and the cycle of operations is repeated.
The larger the tank, the greater is the number of tubes required. These are now
arranged in rows, one behind the other, but very little advantage is gained by adding
more tubes when they are already three deep. A fourth row of tubes is so shut in by
the tubes on the outside that very little additional cooling surface is obtained. When
this stage is reached, the simple tubular design is abandoned, and external radiators
are substituted for the tubes.
With this type of cooling the tank surface itself now becomes plain again. The
external radiators consist of a long horizontal chamber at the top, and another at the
bottom, these being joined by numbers of radiator tubes.
Other transformers designed for use with an external oil cooler have no radiators
fitted to the transformer tank itself, practically the whole of the heat dissipation
taking place in the oil cooler.
Water-cooling is also employed. A number of tubes are arranged in a helical coil
inside the top of the transformer tank, but underneath the oil level. A stream of cold

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