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Technical description
2 Protection functions
2.10 Thermal overload protection
T> (49)
VM59.EN001
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71
2.10.
Thermal overload protection T> (49)
The thermal overload function protects cables in the feeder
mode against excessive heating.
Thermal model
The temperature is calculated using rms values of phase
currents and a thermal model according IEC 60255-8. The rms
values are calculated using harmonic components up to the
15
th
.
Trip time:
2
2
2
2
ln
a
I
I
I
t
P
Alarm:
alarm
I
k
k
a
Mode
(Alarm 60% = 0.6)
Trip:
Mode
I
k
k
a
Release time:
2
2
2
ln
I
a
I
C
t
P
Trip release:
MODE
I
k
a
95
.
0
Start release:
alarm
I
k
a
MODE
95
.
0
(Alarm 60% = 0.6)
T
= Operation time
= Thermal time constant tau (Setting value)
ln
= Natural logarithm function
I
= Measured rms phase current (the max. value of
three phase currents)
Ip
= Preload current,
N
P
I
k
I
(If temperature rise
is 120%
2
.
1
). This 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)
k
= Ambient temperature factor (Permitted current due
to tamb) Figure 2-9.
I
MODE
= The rated current (I
N
or I
MOT
)
C
= Relay cooling time constant (Setting value)
Time constant for cooling situation
If the motor's fan is stopped, the cooling will be slower than
with an active fan. Therefore there is a coefficient c
for
thermal constant available to be used as cooling time constant,
when current is less than 0.3xI
MOT
.
Содержание VAMP 59
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