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Technical description
2 Protection functions
2.16 Thermal overload protection T>
(49)
VM50.EN004
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81
2.16.
Thermal overload protection T> (49)
The thermal overload function protects the motor in the motor
mode or 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
.
2
mod
2
2
2
)
(
ln
e
p
I
k
k
I
I
I
t
, where
t =
Operation
time
= Thermal time constant (Setting value)
ln
= Natural logarithm function
I
= Measured rms phase current (the max. value of three
phase currents)
Ip
= Preload current (corresponds to the actual
temperature rise). This parameter is the memory of
the algorithm.
k
= Allowed continuous overload, i.e. service factor.
(Setting value)
k
= Ambient temperature factor (See Figure 2.16-1)
I
mode
= The
rated
current
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
.
Heat capacitance, service factor and ambient temperature
The trip level is determined by the maximum allowed
continuous current I
max
corresponding to the 100 %
temperature rise
TRIP
i.e. the heat capacitance of the motor or
cable. I
max
depends of the given service factor k and ambient
temperature
AMB
and settings I
max40
and I
max70
according the
following equation.
e
I
k
k
I
mod
max