Classification

Two vector control strategies were formulated during the late 60's and decoupling of the stator currents was achieved either by direct measurement of the flux or by estimation from the motor model.Both these methods are based on the space phasor modeling of the induction machines The measurement technique is also known

as flux detection control or flux feedback control since the flux is measured and fed back into the control circuit. On the other hand, the estimation technique is known as flux feedforward control or slip frequency control due to the fact that the slip frequency is calculated from the model for flux estimation.

The flux feedback control uses Hall sensors or search coils to sense air gap flux. Alternately the rotor flux is estimated based on the stator current vector, voltage vector and/or rotor speed. The flux feedforward control determines the rotor flux from the stator current signals and rotor speed. An alternate method uses the terminal voltages and line currents for determining the stator flux derivatives which are then integrated to obtain the flux. Or else, the terminal voltage, line currents and rotor speed are used as observer quantities to determine rotor flux.