7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices Chapter 7 Page 14 of 31
Thermal Protection - Common Settings
NPS Weighting
Negative phase sequence current has an increased heating effect on a motor, NPS Weighting should be set to
‘Sequence Components’ when applied to a motor.
NPS Weighting Factor (K)
Where machine data is available for the machine negative sequence withstand (NPS weighting factor), this figure
should be used. If no figure is available, it is recommended that K is set to the default value (K = 3). Should this
setting give nuisance tripping in service a reduced weighting factor can be applied, the relay meters can be used
and/or the motor manufacturer consulted to help arrive at a suitable value.
Gn 49 Itheta Thermal Overload
It is usual to choose a thermal overload setting of 1.05 x FLC, above this level the relay picks up and starts timing
out before issuing a trip output, This setting allows full utilisation of the output rating of the motor. Here a setting
of 1.05 x 0.86 = 0.903 is required, ‘Itheta Thermal Overload’ should be set to 0.90 x In.
Gn 49 Motor Start Type
The default setting ‘% Itheta’ can be used.
Gn 49 Motor Start Current
The default value (1.5 x Itheta) is chosen, this is below the motor start (locked rotor) current and above motor f.l.c.
Gn 49 End of Start
For a motor started DOL the end of the start can be defined as when the current returns to below the thermal
overload setting.
A settable End-of-Start current setting (default 1.05 x Itheta).
Gn 49 Motor Stop Type
The default setting ‘% Itheta’ can be used.
Gn 49 Motor Stop Current
Use default setting, 0.1 x Itheta.
Thermal Protection – Overload Settings
Gn 49 Characteristic
The following settings can be based on ‘IEC’ characteristic
Heating Time Constant T
HEAT
The motor thermal characteristic curve has not been provided so we can consider:
The locked rotor current of 5 x FLC (approximately the start current) and the run-up time of 4 seconds. Allowing
two consecutive starts i.e. 8 seconds total run-up time.
5 x F.L.C. = 5 x 0.86 = 4.3A
4.3/Itheta = 4.3/0.9 = 4.8 I
θ
The TauH value can be calculated from:
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