The amplitude and the position of the stator flux phasor can be controlled only by way of the supplying stator voltage.
But the motor is supplied by a tri-phased voltage source inverter, which can have only 8 distinguishable states (due to the operation of the elements as switches). It results that the phasor of the stator voltage
,
can take only 8 positions (figure 3).
Figure 3
To keep (with approximation) constant the amplitude of the stator flux and to control its speed, is possible only by choosing one (the proper) topology among the 8 possible. The selected state depends on the necessary tendency for the evolution of, both the flux amplitude and the electromagnetic torque.
There are two possible topologies that correspond to the null phasor (state 0: KA = KB = KC = OFF and state 7: KA = KB = KC = ON). The choose of one of them, depends on the actual state of the inverter so the null state to be achieved with minimum commutations of the switches.
