Siemens SINAMICS S120 Function Manual page 543

Operation without an encoder
Operation without encoder is possible in both servo control mode and vector control mode. The
following applies:
● For operation without encoder, the total moment of inertia of the motor and driven machine
must be known.
● In controlled operation without encoder (speed < p1755), there is a risk of the motor stalling
in the acceleration phase due to inadequate parameterization of the moment of inertia.
Recommendation: Enter the greatest possible moment of inertia which can occur in the
current configuration of your drive in p1498 (load moment of inertia or load mass).
Additionally, ensure that the value "0" is entered in the parameter p1400.22 (speed control
configuration). This setting ensures that the start value for the moment of inertia estimator
is set to the parameterized moment of inertia (J = p0341 · p0342 + p1498) in the event of
pulse inhibit. The pulse inhibit should then only be set if the moment of inertia has changed.
The setting in parameter p1400.22 then ensures a return to the maximum moment of inertia.
● In controlled operation without encoder (speed > p1755), the speed precontrol or torque
precontrol requires the precise moment of inertia to attain optimum speed controller
dynamic response. An incorrect moment of inertia at the end of the acceleration phase,
results in an undesirable overshoot or undershoot.
Recommendation: Activate the "Moment of inertia estimator" function. This will prevent any
overshoot or undershoot.
Estimating the load torque
In order that the moment of inertia can be estimated, you must first estimate the load torque
M . Phases with constant speed ≠ 0 are required for calculation of the load torque (e.g. friction
L
force). At low speeds, the inverter calculates the load torque M
The conditions described as follows must be satisfied in this regard.
Figure 9-33
Conditions
The following conditions must be satisfied for calculation of the load torque:
● Speed absolute value ≥ p1226
● Acceleration · moment of inertia (r1493) < 0.3 · p1560 · p0333
Drive functions
Function Manual, 06/2019, 6SL3097-5AB00-0BP2
Parameters for calculation of the load torque
Function modules
9.12 Moment of inertia estimator
from the actual motor torque.
L
541