Siemens 7SR105 Rho - Page 188

7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices Chapter 7 Page 15 of 31
minutes96.2
0.94.3
4.3
ln
8/60
II
I
ln
t
TauH
22
2
2
θ
2
2
=








−
=








−
=
(see Figure 2.1-3.)
As the safe stall time from cold (CSST) is 11 seconds, the TauH value could be set to 4 minutes e.g.
minutes4.4
0.864.3
4.3
ln
11/60
II
I
ln
t
TauH
22
2
2
θ
2
2
=








−
=








−
=
The safe stall time from hot (HSST) i.e. 7 seconds gives a T
HEAT
value of 2.8 minutes. For this lower value a
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
START
This time constant has not been provided set to 1.0 x TauH.
Cooling Time Constant T
COOL
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 100A
Actual motor current 80A
HSST 7 seconds
CSST 11 seconds
The thermal capacity used can be determined from the Hot/Cold Curve Ratio as follows:
( )
( )
%24%70%100
100
80
/%100
%64100
sec11
sec7
100/
=−×=
−×=∴
=×=×=
A
A
RATIOCURVECOLDHOT
FLC
currentmotoractual
usedcapacityThermal
ColdLRT
HotLRT
RatioCurveColdHot
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