Flux and torque hysteresis comparators are two and three level respectively. anyone please tell me how these levels have been decided. Flux levels are 1 & 0 and torque levels are 1, 0, -1. Thanks in advance!
The reason is the zero voltage vector that stops the stator flux vector variation. As it also stops the change of the flux variation its use does not help getting the flux back to hysteresis band.
On the contrary, zero voltage vector in motor operation decreases the torque. Naturally a non-zero voltage vector can also be selected to decrease the torque, but not using zero voltage vector increases unnecessarily the switching frequency as the torque derivative is higher with non-zero vectors. However, near zero speed and especially with reversing of the rotation direction the zero voltage vector alone does not allow rapid enough torque response and non-zero vector has to be used.
Thus clearly for torque you have three main alternatives to choose a voltage vector: clockwise, zero and counterclockwise ones. This is usually indicated by numbers 1, 0 and -1 in the optimum selection tables that with the flux bit to increase/decrease flux will select the correct vector from the seven possible voltage vectors of a 2-level inverter.
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