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2.9
Thermal Overload Protection (ANSI 49)
The thermal overload protection prevents thermal overloading of the stator windings
of the machine being protected.
2.9.1
Functional Description
Thermal Profile
The device calculates the excessive temperatures in accordance with a single-body
thermal model, based on the following differential equation:
with
Θ
Θ
τ
I
The protection function models a thermal profile of the equipment being protected
(overload protection with memory capability). Both the previous history of an overload
and the heat loss to the environment are taken into account.
The solution of this equation is in steady-state operation an exponential function
whose asymptote represents the final temperature Θ
temperature threshold´is reached, an alarm is issued for e.g. load reduction mea-
sures. If the second overtemperature threshold, i.e. final overtemperature = trip tem-
perature, is reached, the protected equipment is disconnected from the network. It is
also possible, however, to set the overload protection to Alarm Only . In this case
only an indication is issued when the final temperature is reached.
The overtemperature is calculated from the largest of the three phase currents. Since
the calculation is based on rms values of currents, harmonics which contribute to a
temperature rise of the stator winding are also considered.
The maximum thermally admissible continuous current I
of the nominal current I
I
Apart from the k factor (parameter K-FACTOR) the TIME CONSTANT τ and the alarm
temperature Θ ALARM (as a percentage of the trip temperature Θ
tered.
Overload protection also has a current alarm feature (I ALARM) in addition to the tem-
perature alarm stage. This may report an overload current prematurely (before I
exceeded), even if the overtemperature has not yet attained the alarm or tripping
levels.
7UM61 Manual
C53000-G1176-C127-3
– Current overtemperature as a percent of final overtemperature at
the maximum admissible phase current k · I
– Coolant temperature as a difference to the 40 °C reference temper-
K
ature
– Thermal time constant for heatup of the equipment being protected
– Current rms phase current as a percentage of maximum admissible
phase current I
:
N
= k · I
max
N
2.9 Thermal Overload Protection (ANSI 49)
= k · I
max
N
End
max
N
. After an initial settable over-
is described as a multiple
) are to be en-
TRIP
max
is
59
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