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2.1.2.2
Start Time Greater Than Stall Withstand Time
Gn 14-n
Delay
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
A tachometric switch mounted on the rotor is used to signal that the motor is running. A relay binary input
is configured to be energised by this switch and is programmed to the 'No Accel.' function in the input
configuration menu. Select Gn14-n Control to 'No Accel'.
Under stall conditions, a trip will occur after Gn14-n Delay which should be set with a delay less than the motor
stall withstand time.
This can be supplemented by an additional Gn-14-n element (select Gn14-n Control to 'None') used to provide
protection against the motor running up but drawing starting current for an excessive time. This timer runs for
current above Gn14-n setting to provide excessive start time protection.
2.1.2.3
Over Torque DTL Element
The stall protection can be used to provide an over-torque feature which provides a faster trip where the motor
develops a gradual increase in load above rating. Typical causes are bearing failure or control malfunction.
Bearings may prematurely wear due to excessive vibration or loss of lubrication.
This feature allows the relay to remove the supply to the motor before further damage occurs to the bearing or
load. This function is primed when the current returns to normal load levels immediately after a motor start i.e.
Gn14-n Control is selected to 'Running'
The setting must allow for an increase in motor current from a system voltage depression or sudden acceleration
and to allow the relay to distinguish between backfed faults.
©2014 Siemens Protection Devices Limited
Running Motor
Thermal Withstand
Motor starting
characteristic
CURRENT
I
START
14-n
Setting
Chapter 7) 7SR17 Rho Applications Guide
Thermal Overload
Protection
(Gn 49 Char)
Excessive Start
Time Protection
(Gn 14-n)
Stalled Motor
Thermal Withstand
Stall Protection
DTL Characteristic
(Gn 14-n)
Page 13 of 36
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