VAMP 59 - Thermal Overload Protection T> (49)

Technical description
2 Protection functions
2.10 Thermal overload protection
T> (49)
VM59.EN001
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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:
22
2
2
ln
aI
II
t
P


 
Alarm:
alarmIkka
Mode


(Alarm 60% = 0.6)
Trip:
Mode
Ikka 

Release time:
22
2
ln
Ia
I
Ct
P




Trip release:
MODE
Ika  95.0
Start release:
alarmIka
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,
NP
IkI  
(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
.