Question:
During operation without feedback in the operating mode 'vector control' (C0006 = 1) it can be observed that with controller release the motor shaft carries out a momentary and slight motion (< 5 °), although no speed setpoint value exists (e. g. QSP = active).
How can this be explained and which possibilities exist to prevent this behaviour?
Answer:
The exciting current for initially magnetising the asynchronous machine always flows through the same switch pair of the inverter (phase V and W), if no feedback system is in existence (C0025 = 1). For this reason the motor is excited with exactly the same reference vector of the PWM with the result that the vector control adjusts the rotating field.
Due to residual magnetism and in combination with this exciting current a torque can occur, which can lead to a momentary motion of the motor shaft.
Dependence:
- the magnitude of the residual magnetism influences the length of time between controller inhibit and renewed controller release, as well as machine-dependent parameters (motor type).
- the resulting torque influences the magnitude of the exciting current and
- the angle of the rotary motion influences the difference of the space vector of the last motor current and the magnetising current as well as the mass inertia of the entire drive influences the performance of the drive during the magnetising phase.
Possible remedy measures:
- V/f characteristic control
- application of mechanical brakes
- application of feedback system
