Siemens 7SR105 Rho - Page 184

7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices Chapter 7 Page 11 of 31
2.1.2.1 Start Time Less Than Stall Withstand Time
TIME
CURRENT
Running Motor
Thermal Withstand
Stalled Motor
Thermal Withstand
Thermal Overload
Protection
(Gn 49 Char)
Stall Protection
DTL Characteristic
(Gn 14-n)
14-n
Setting
I
START
Motor starting
characteristic
Gn 14-n
Delay
Figure 2.1-6 Start Time Less Than Stall Withstand Time
Figure 2.1-6 illustrates the most common situation where the stalled motor condition can be effectively
distinguished from a healthy start by simple time grading. Select Gn14-n Control to ‘None’. A single definite time
characteristic can give protection during starting and stalling without causing mal-operation during a healthy start
sequence.
2.1.2.2 Start Time Greater Than Stall Withstand Time
TIME
CURRENT
Running Motor
Thermal Withstand
Stalled Motor
Thermal Withstand
I
START
Gn 14-n
Delay
Excessive Start
Time Protection
(Gn 14-n)
Motor starting
characteristic
14-n
Setting
Thermal Overload
Protection
(Gn 49 Char)
Stall Protection
DTL Characteristic
(Gn 14-n)
Gn 14-n
Delay
Figure 2.1-7 Start Time Greater Than Locked Rotor Withstand Time
In cases where the motor starting time approaches or exceeds the stall withstand time (e.g. motors driving high
inertia loads) then protection discrimination between starting and stalling cannot be provided by simple time
grading.
It is required that the stall protection is enabled only when the motor is running, this can be achieved either by:-
Select Gn14-n Control to ‘Running’, or