26-11-2012, 11:43 AM
Transformer rating
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The transformers rating plate
The following data must be on the rating plate:
■ Maker: The manufacturer’s name or initials.
■ Serial number: Number for identification.
■ Current type: To be specified if DC.
■ Rated current: The rated value for the current
flowing through a winding’s line terminals at
rated load. The maximum current for which the
winding is designed.
■ Rated frequency: The frequency at which the
transformer is intended to work (AC).
■ Rated power: The apparent power in VA, kVA
or MVA for AC, W or kW for DC.
■ Rated voltage, primary: The value of the main
voltage which is intended to be applied to or
induced in the primary winding. The maximum
voltage for which the transformer is designed in
normal operation.
■ Rated voltage, secondary: The voltage which is
generated for the transformer’s secondary line
terminals with or without load (depending on
the standard).
■ Cooling type: A code which indicates
the cooling type.
Insulation class
The energy conversion in the transformer is not
ideal. Some of the energy supplied is converted
to heat loss. Among other things, IEC60726
specifies the maximum operating temperature
of transformers in relation to the insulating
material used. If the continuous operating
temperature is too high, the service life of the
transformer will be reduced considerably. That
is to say it is completely normal for transformers
to heat up. A transformer, which does not heat
up, is not working normally. At rated load and
an ambient temperature of 40°C it is not unusual
for the surface of the enclosure on a plasticenclosed
safety transformer, for example, to
reach 70-80°C. At this sort of temperature it is
difficult to keep your hand on the transformer
without burning it.