2.9 Thermal Overload Protection
223
7UT613/63x Manual
C53000-G1176-C160-2
2.9.3 Overload protection using a thermal replica with ambient temperature
influence
Principle The calculation basis is based on those of the overload protection, according to
Section „Overload protection with Thermal Replica“, the ambient temperature, usually
the coolant temperature, is however taken into consideration.
The ambient or coolant temperature has to be measured with a temperature detector
in the protected object. The user can install up to 12 temperature measuring points in
the protected object. Via one or two RTD boxes and a serial data connection the mea-
suring points inform the overload protection of the 7UT613/63x about the local coolant
temperature. One of these points is selected and relevant for the temperature calcu-
lation in the overload protection.
The thermal differential equation in Section „Overload Protection using a Thermal
Replica“ is extended by one term that considers the ambient temperature ϑ
U
. For this
the „cold“ state with ϑ
U
=40°C or 104 °F is assumed (temperature without heating it-
self). This temperature difference is scaled to the maximum admissible temperature
and then designated with Θ
U
. The thermal differential equation is
Otherwise the function is the same, as in Section „Overload Protection with a thermal
Replica“. To create the relation between current and temperature, the device needs
the temperature at rated current of the protected object.
In the event of failure of the temperature input via the thermobox, the device works
with an accepted temperature of 40 °C or 104 °F. The result shows the same condi-
tions as with the thermal protection without ambient temperature (Section „Overload
Protection with a Thermal Replica“).
2.9.4 Hot-Spot Calculation and Determination of the Ageing Rate
The overload calculation according to IEC 60354 calculates two quantities relevant for
the protection function: the relative ageing and the hot-spot temperature in the protect-
ed object. The user can install up to 12 temperature measuring points in the protected
object. Via one or two RTD boxes and a serial data connection the measuring points
inform the overload protection of the 7UT613/63x about the local coolant temperature.
One of these points is selected to form the relevant point for hot-spot calculation. This
point should be situated at the insulation of the upper inner turn of the winding since
this is the location of the hottest temperature.
The relative ageing is acquired cyclically and summed up to a total ageing sum.
Cooling Methods The hot-spot calculation is dependent on the cooling method. Air cooling is always
available. Two different methods are distinguished:
• AN (Air Natural): natural air circulation and
• AF (Air Forced): forced air circulation (by means of ventilation).
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