Siemens SINAMICS S120 Function Manual page 170

Vector control
4.1 Sensorless vector control (SLVC)
Closed-loop control without changeover between closed-loop and open-loop speed control is
restricted to applications with passive load:
A passive load only has a reactive effect on the drive torque of the driving motor at the
starting point, e.g. high inertia masses, pumps, fans, centrifuges, extruders, travel drives, or
horizontal conveyors.
Standstill without a holding current is possible for as long as required. Then, at standstill,
only the magnetizing current is impressed in the motor.
Note
Generator operation
Steady-state generator operation at frequencies close to 0 Hz is not permissible in this
operating mode.
Figure 4-3 Vector control without an encoder
Blocking drives
If the load torque is higher than the torque limiting of the sensorless vector control, the drive
is braked to zero speed (standstill). In order that the open-loop controlled mode is not
selected after the time p1758, p1750.6 can be set to 1. Under certain circumstances the
"Motor blocked delay time" (p2177) must be increased.
Note
Exception for reversing drives
It is not permissible to use this setting if the load can force the drive to reverse.
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Function Manual, (FH1), 01/2013, 6SL3097-4AB00-0BP3
Drive functions