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Ip
=
Preload current,
temperature rise is 120%
parameter is the memory of the algorithm
and corresponds to the actual temperature
rise.
k
=
Overload factor (Maximum continuous
current), i.e. service factor. (Setting value)
=
Ambient temperature factor (Permitted
kΘ
current due to tamb). See Figure 3.14
I
=
The rated current (I
MODE
=
Relay cooling time constant (Setting value)
C
τ
Time Constant for Cooling Situation
If the transformer's fan is stopped, the cooling will be slower than
with an active fan. Therefore there is a coefficient cΘ for thermal
constant available to be used as cooling time constant, when current
is less than 0.3xI
.
N
Heat Capacitance, Service Factor and Ambient Temperature
The trip level is determined by the maximum allowed continuous
corresponding to the 100 % temperature rise Θ
current I
MAX
the heat capacitance of the transformer. I
service factor k and ambient temperature Θ
and I
according the following equation:
MAX70
I
= k k
I
Θ
MAX
The value of ambient temperature compensation factor kΘ depends
on the ambient temperature Θ
Figure 3.24. Ambient temperature is not in use when kΘ = 1. This
is true when
I
40 is 1.0
MAX
Samb is "n/a" (no ambient temperature sensor)
T
is +40 °C.
AMB
Protection Functions
θ
=
×
I
P
or I
N
MOT
depends of the given
MAX
and settings I
AMB
MODE
and settings I
AMB
MAX40
3-47
×
k
I
(If
n
θ
=
1
2 .
). This
)
i.e.
TRIP
MAX40
and I
. See
MAX70
857-UM001A-EN-P – July 2009
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