Sensorless Operation (Without An Encoder) - Siemens SINAMICS S120 Function Manual

Operating Modes
Servo control
3.1.10

Sensorless operation (without an encoder)

Description
This allows operation without an encoder and mixed operation (with/without
encoder). Encoderless operation with the motor model allows a higher dynamic
response and greater stability than a standard drive with V/f control. Compared
with drives with an encoder, however, speed accuracy is lower and the dynamic
response and smooth running characteristics deteriorate.
Since the dynamic response in operation without an encoder is lower than in oper-
ation with an encoder, accelerating torque pre-control is implemented to improve
the control dynamic performance.
It controls, knowing the drive torque, and taking into account the existing torque
and current limits as well as the load moment of inertia (motor moment of inertia:
p0341*p0342 + load torque: p1498) the require torque for a demanded speed dy-
namic performance optimized from a time perspective.
For torque pre-control, a torque smoothing time can be parameterized via p1517.
The speed controller must be optimized for operation without an encoder via p1470
(P gain) and p1472 (integral time) due to the reduced dynamic response.
At low speeds during operation without an encoder, the actual speed value, orien-
tation, and the actual flux can no longer be calculated due to the accuracy of the
measured values and the sensitivity of the parameters in this procedure.
For this reason, the system switches to current/frequency control.
The switchover threshold is parameterized via p1755 and the hysteresis via p1756.
To accommodate a high load torque in open-loop control, the motor current can be
increased via p1612.
To do so, the drive torque (e.g. friction torque) must be known or estimated. An
additional reserve of approx. 20 % should also be added. In synchronous motors,
the torque is converted to the current via the motor torque constant (p0316). In the
lower speed range, the required current cannot be measured directly on the Motor
Module. The default setting is 50 % (synchronous motor) or 80 % (induction motors)
of the rated motor current (p0305). When parameterizing the motor current
(p1612), you must take into account the thermal motor load.
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SINAMICS S120 Function Manual, 03/2006 Edition