Siemens SINAMICS PERFECT HARMONY GH180 Function Manual page 199

Figure 7-12
The plot in the above figure shows results from an experimental evaluation of the software
thermal model with the "straight inverse time" option (100% "overload" setting) for various
levels of drive current. A 4 kV, 300 Hp motor was used for this test. The experimental data shows
the time taken for the estimated motor temperature to go from rated temperature to 120% of
rated. This curve is quite conservative as compared to a Class 10 TOL that trips at 280 sec with
150% current and at 630 sec with 125% current.
Legacy Inverse Time Mode
This algorithm mimics the operation of a thermal overload relay commonly employed to protect
a motor from excessive thermal stress. It is not as precise as the existing thermal model described
above, but it can work over a broader range. It comes in two inverse time TOL choices, in both
the speed derated and non-derated types, to provide protection to motors that are beyond the
range of the NEMA table. Therefore use this algorithm on out-of-range motor sizes only.
This algorithm uses the integration of power over the nominal load capability to determine the
energy flow balance. At rated conditions the system will be at equilibrium and can run
continuously at this point provided the motor is running at or below rated current and voltage,
and the ambient temperature is within range of the motor tolerances.
If a load is above rated, or if cooling (proportional to speed) is inadequate, the motor will
eventually overheat. The algorithm provides a derating curve inversely based on speed when
the only cooling on the motor is the internal, shaft-mounted fan. For motors that use an external
blower, this derating curve is not required.
The algorithm requires an overload threshold and a timebase parameter, which is achieved
using the parameters listed below.
Note
When switching between TOL modes, ensure that the internal timebase (integrator) is set to the
appropriate value, otherwise it will reset to the default of the mode selected.
NXGPro+ Control Manual
Operating Manual, A5E50491925A
Drive current (in percent of motor rated current) vs. time taken for motor temperature
Operating the Control
7.7 Motor Thermal Overload Protection
197