Flux vector control is another method of speed control and this feature is also available on some standard inverters which normally operate under V/F control.
There are two methods of flux vector control:-
Flux vector control principles:-
The torque producing and magnetising current vectors are independently controlled. Therefore providing accurate speed / torque control. This is enabled by motor equivalent circuit data of the motor. Motor parameters are entered in the drive followed by an auto tune function. VSD then uses the auto tune data to produce a software model of the motor, for a closely matched speed / torque control.
Vector control in comparison with V/f provides:-
Closed loop control Can provide full torque at zero speed With encoder feed back.
Sensor less vector control can provide accurate torque control down to 3 - 4 HZ
0.3% is achievable with open loop (Sensor less vector).
0.03% with closed loop (Encoder).
The closed loop vector option has additional cost elements i.e. encoder and its installation. Sensor less vector can be used for most applications, however when speed and dynamic torque control at very low speeds are required then must consider the application closely prior to selection of the of the system
Motor Current (I) is made up of two Current Components
(I) Reactive or Inductive Induces magnetic field in stator and rotor. This is the flux producing current (magnetising Current).
(I) resistive is the torque producing current.
VSD can vary the I Reactive, hence total current and power factor angle.
Switching sequence & rotating flux.
The switching circuitry of the vsd connects the transistor switches to the positive or the negative DC bus in such a sequence that the produced voltage resembles a 3 phase AC voltage.
As it can be seen from diagram below that the position of the rotating magnetic flux generated in this way can be determined from the switching sequence. This along with motor parameters (equivalent circuit data) cab be used to provide sensor less vector control of cage induction motors.