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In the rathing of the resistors we
generally encounter the following cases:
NATURAL COOLED RESISTORS FOR CONTINUOUS DUTY
In
this case the number of elements depends essentially
on the power to be dissipated and on the specified
maximum operating temperature.
From the curves in figure 1 we can obtain the specific
rated power of a given temperature.
NATURAL
COOLED RESISTORS FOR CYCLIC DUTY
This
is the most frequent case when sizing braking resistors.
The power is applied for a number of seconds varying
the
interval length (fig. 2).
NATURAL
COOLED RESISTORS FOR IMPULSE DUTY
This
case deals with resistors used in the charging and
discharging of capacitors. In these cases the power
is instantly applied and subsequent pulses are generally
far enough apart to allow the resistor to start again
from the initial condition.
FORCED COOLED RESISTORS
Once the value to be attributed to the a
coefficient
of heat transmission has been determined, the rating
essentially follows
the criteria that we previously used for the natural
cooled resistors (fig.3).
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