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Setpoint channel and closed-loop control
7.4 Vector speed/torque control with/without encoder
7.4.1
Vector control without encoder
Description
For sensorless vector control only (SLVC: Sensorless Vector Control), 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 operation is controlled on the basis of time
and frequency conditions (p1755, p1756, p1758 - only for induction motors). The system
does not wait for the time condition to elapse if the setpoint frequency at the ramp-function
generator input and the actual frequency are below p1755 x (1 - (p1756 / 100 %))
simultaneously.
Transition from open-loop to closed-loop operation always takes place when the changeover
speed in p1755 (characteristic "1" in the figure below). If the speed increase is set very slow
and a changeover delay time >0 is set in p1759, transition takes place after the changeover
delay time (characteristic "2" in the figure below).
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Figure 7-9
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Changeover conditions
In open-loop operation, the calculated actual speed value is the same as the setpoint value.
For vertical loads and acceleration processes, parameters p1610 (constant torque boost)
and p1611 (acceleration torque boost) must be modified in order to generate the static or
dynamic load torque of the drive. If, on induction motors, p1610 is set to 0%, only the
magnetizing current r0331 is injected; at a value of 100%, the rated motor current p0305 is
injected. For permanent-magnet synchronous motors, at p1610 = 0%, a pre-control absolute
value derived from the supplementary torque r1515 remains instead of the magnetizing
current. To ensure that the drive does not stall during acceleration, p1611 can be increased
or acceleration pre-control for the speed controller can be used. This is also advisable to
ensure that the motor is not subject to thermal overload at low speeds.
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Drive converter cabinet units
Operating Instructions, 10/2008, A5E00288214A