Efficient Networks Siemens Sinamics S120 - Manual (Page 115)

Drive Functions
Function Manual, (FH1), 07/2007 Edition, 6SL3097-2AB00-0BP4
115
Vector control
4
Compared with vector V/f control, vector control offers the following benefits:
● Stability vis-à-vis load and setpoint changes
● Short rise times with setpoint changes (–> better command behavior)
● Short settling times with load changes (–> better disturbance characteristic)
● Acceleration and braking are possible with maximum available torque
● Motor protection due to variable torque limitation in motor and regenerative mode
● Drive and braking torque controlled independently of the speed
● Maximum breakaway torque possible at speed 0
Vector control can be used with or without an encoder.
The following criteria indicate when an encoder is required:
● High speed accuracy is required
● High dynamic response requirements
– Better command behavior
– Better disturbance characteristic
● Torque control is required in a control range greater than 1:10
● Allows a defined and/or variable torque for speeds below approx. 10% of the rated motor
frequency (p0310) to be maintained.
With regard to setpoint input, vector control is divided into:
● Speed control
● Torque/current control (in short: torque control)
4.1 Sensorless vector control (SLVC)
In sensorless vector control (SLVC), the position of the flux and actual speed must be
determined via the electric motor model. The model is buffered by the incoming currents and
voltages. At low frequencies (approx. 0 Hz), the model cannot determine the speed. For this
reason and due to uncertainties in the model parameters or inaccurate measurements, the
system is switched from closed-loop to open-loop operation in this range.
The changeover between closed-loop/open-loop controlled operation is controlled using time
and frequency conditions (p1755, p1756, p1758 - only for induction motors). The system
does not wait for the time condition to elapse when the setpoint frequency at the ramp-
function generator input and the actual frequency are below p1755 * (1 - (p1756/100%))
simultaneously.