2.16 Thermal overload protection T>
(49)
2 Protection functions
Technical description
82
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VM50.EN004
The value of ambient temperature compensation factor k
depends on the ambient temperature
AMB
and settings I
max40
and I
max70
. See Figure 2.16-1. Ambient temperature is not in
use when k
= 1. This is true when
I
max40
is 1.0
Samb is “n/a” (no ambient temperature sensor)
TAMB is +40 °C.
k
I
MAX70
I
MAX40
AMB
(°C)
AmbientTemperatureCompensation
0.6
10
20
30
40
50
60
70
80
0.8
1.0
1.2
Figure 2.16-1 Ambient temperature correction of the overload stage T>.
Example of a behaviour of the thermal model
Figure 2.16-2 shows an example of the thermal model
behaviour. In this example
= 30 minutes, k = 1.06 and k
= 1
and the current has been zero for a long time and thus the
initial temperature rise is 0 %. At time = 50 minutes the
current changes to 0.85xI
mode
and the temperature rise starts to
approach value (0.85/1.06)
2
= 64 % according the time constant.
At time=300 min, the temperature is about stable, and the
current increases to 5 % over the maximum defined by the
rated current and the service factor k. The temperature rise
starts to approach value 110 %. At about 340 minutes the
temperature rise is 100 % and a trip follows.
Initial temperature rise after restart
When the device is switched on, an initial temperature rise of
70 % is used. Depending of the actual current, the calculated
temperature rise then starts to approach the final value.
Alarm function
The thermal overload stage is provided with a separately
settable alarm function. When the alarm limit is reached the
stage activates its start signal.
