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t
TauH
=
I
2
ln
I
2
−
I
As the safe stall time from cold (CSST) is 11 seconds, the TauH value could be set to 4 minutes e.g.
t
TauH
=
I
2
ln
I
2
−
The safe stall time from hot (HSST) i.e. 7 seconds gives a T
setting of 2.5 could be chosen, there is the possibility of nuisance tripping after the motor has been in service for
some time with longer run-up times and higher currents.
A setting of TauH = 4 minutes is preferred allowing two starts in quick succession.
Starting Time Constant T
This time constant has not been provided set to 1.0 x TauH.
Cooling Time Constant T
In this example, the cooling time constant has not been specified either as a time constant or as a multiple of its
heating time constant. We will therefore choose 10x, which is the default.
Hot/Cold Curve Ratio Setting (Hot Spot Weighting)
In our example the hot/cold stall withstand time ratio is 7/11 = 0.64 i.e. H/C setting = 64%. Note that at steady
state full load current the relay will stabilize at 'Thermal Capacity Used' of 36% with this setting applied.
Example:
Motor FLC
Actual motor current
HSST
CSST
The thermal capacity used can be determined from the Hot/Cold Curve Ratio as follows:
/
=
Hot
Cold
Curve
Ratio
∴
Thermal
capacity
used
80
A
(
)
=
×
100
%
−
70
%
=
100
A
Capacity Alarm Setting
An output can be configured to indicate that a selected thermal state level has been exceeded.
Typically this would be set to 95%. Of the thermal model
Load Alarm Setting
This current setting can be used to indicate abnormally high loading conditions e.g. 1.0 x Itheta.
Thermal Restart Inhibit Setting
This can be initially set to 50% in the absence of further information.
During commissioning, before starting the protected motor, check the thermal state at the motor by accessing the
"Thermal Capacity Used" meter and note the value. The motor should then be started with its normal load and,
when up to speed the "Thermal Capacity Used" value can be re-checked. The difference between these two
©2018 Siemens Protection Devices
8/60
=
=
. 2
4.3
2
ln
2
4.3
2
−
0.9
2
θ
11/60
=
=
4.3
2
ln
I
2
4.3
2
−
0.86
2
θ
START
COOL
100A
80A
7 seconds
11 seconds
LRT
Hot
7
sec
×
100
=
LRT
Cold
11
sec
actual
motor
current
=
×
FLC
24
%
7SR105 Rho and 7SR17 Rho Applications Guide
(see Figure 2.1-3.)
96
minutes
4
4 .
minutes
value of 2.8 minutes. For this lower value a
HEAT
×
100
=
64
%
(
100
%
−
HOT
/
COLD
CURVE
)
RATIO
Chapter 7 Page 15 of 31
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