Siemens 7SR105 Rho - Figure 3-2 Logic Diagram: Motor Thermal Overload Protection (49)

7SR105 Rho Description of Operation
©2018 Siemens Protection Devices Chapter 1 Page 25 of 57
49 Alarm
49 Trip
En
Inhibit 49
Enabled
Disabled
&
Load Increase Alarm
Restart Inhibit
IA
IB
IC
I
EQ
Reset
X Th
Gn 49 Thermal
State
Gn 49 TauS
Starting Constant
X Th
Gn 49 TauC
Cooling Constant
Gn 49 Thermal
Overload
Motor Start
Gn49: NPS Weighting
Gn49: NPS Weighting Factor
Gn49 Characteristic
Gn49 TauH Heating Const.
Gn49 Hot/Cold Ratio
Gn49 Capacity Alarm
Gn49 Load Alarm
Gn49 Therm Restart Inhibit
Gn 49 Itheta Thermal OL
Gn 49 Motor Start Type
Gn 49 Motor Start Current
Gn 49 End of Start Type
Gn 49 End of Start
Gn 49 Motor Stop Type
Gn 49 Motor Stop Current
Stopped
Run
Start
PPS/
NPS
Filter
Figure 3-2 Logic Diagram: Motor Thermal Overload Protection (49)
Gn 49 TauH Heating Constant (Heating Time Constant)
Used during normal motor running conditions and overloads.
Gn 49 TauS Starting Constant
Multiple of heating time constant takes into consideration the reduced cooling available during motor starting.
Gn 49 TauC Cooling Constant
Multiple of heating time constant takes into consideration the reduced rate of cooling of a stopped motor.
Gn 49 Hot/Cold Ratio
The hot/cold ratio setting determines the percentage of thermal capacity available for a motor running at full load
current compared to that available when the motor is cold. It modifies the IEC255-8 hot curve as below:
2
θ
2
2
P
2
II
)I
C
H
(1I
τ.t
−
−
−
= ln
Where;
H/C = hot spot weighting factor = 49 Hot/Cold Ratio setting
A setting is available to switch this feature out of service, however this Hot/Cold ratio setting will normally be used
on all motors as it will assist with the accuracy of the thermal modelling. The hot curve is only operational when