Siemens 7SR105 Rho - Page 26

7SR105 Rho Description of Operation
©2018 Siemens Protection Devices Chapter 1 Page 23 of 57
The final steady state thermal condition can be predicted for any steady state value of input current: where t >>τ,
100%
I
I
θ
2
θ
2
EQ
F
×=
The Hot/Cold ratio setting determines the percentage of thermal capacity available for a motor running at thermal
equilibrium compared to that available when the motor is cold.
The thermal model is modified under normal load conditions (i.e. when I
EQ
< I
θ
) by multiplying the predicted final
steady thermal state by (1-H/C). The hot curve is modified as below:
100%)(1
I
I
θ
C
H
2
θ
2
EQ
F
×−×=
The hot to cold ratio (H/C) in the above “hot” equation is used to adapt the thermal model to incorporate the
thermal over design of motors to withstand start conditions rather than normal running conditions.
The thermal protection menu structure of the relay includes sub-menus for thermal overload, stall, start and phase
unbalance functions. Settings common to all of thermal functions are:
Gn 49 NPS Weighting: When set to Average the average true RMS current of the three phases is used
as I
EQ
. When set to Sequence Components the pps and nps current of the rated frequency are used to
calculate I
EQ
.
Gn 49 NPS Weighting Factor (K): Applied when above is set to Sequence Components.
Gn 49 Itheta Thermal Overload sets the overload pick up level (I
θ
).
Gn 49 Motor Start Type sets the criteria for determining that the motor has started. Defined by:
%Itheta: Current above ‘Gn 49 Motor Start Current’, or
Operation of a binary input configured as ‘Start Motor I/P’ (requires CB open/closed monitoring
connections).
(Note that a motor running condition is recognised by the relay when current increases from the
‘motor stopped’ level to a ‘NOT motor stopped’ level. For a VFD motor the running condition will
be recognised where the start current is not appreciably greater than the running current.
Gn 49 Motor Start Current sets the current level over which the motor has deemed to have started.
Gn 49 End of Start selects whether this is determined by measured current level or from an energised
binary input set as ‘Motor Running’.
Gn 49 Motor Stop Current sets the current level under which the motor has deemed to have stopped.
Motor stop condition is detected on the basis I
EQ
current.
Gn 49 Motor Stop Type sets the criteria for determining that the motor has stopped. Defined by:
% Itheta: Current below ‘Gn 49 Motor Stop Current’ AND Gn 49 Motor Stop Delay
% Itheta + CB Open: Current below ‘Gn 49 Motor Stop Current’ AND ‘CB Open’ binary input
AND Gn 49 Motor Stop Delay.
% Itheta + No Accel: Current below ‘Gn 49 Motor Stop Current’ AND BI programmed to ‘No
Accel’ energised.