In order to keep constant the amplitude of the stator flux and to guarantee that the motor will develop the required torque, the choice of the next topology of the inverter is performed accordingly to the principle of direct torque control, in each sampling period, depending on:

• the actual position of the stator flux phasor, respectively in which specific sector 1...6 is;
• the result of the comparison between the real value of the modulus of the stator flux phasor and the preset value, (flux "greater" / "smaller");
• the result of the comparison between the real value of the electromagnetic torque developed by the motor and the preset value (torque "greater" / "constant" / "smaller").

The preset value of the torque is obtained classically, as output of a PI speed controller.

Type of controllers

• 1. Taking into account the type of the decision (binary) that must be taken concerning the magnitude of the stator flux, what type of controller must be used?

• 2. Taking into account the type of the decision (tertiary) that must be taken concerning the torque, what type of controller must be used?

Structure of the control system

• 3. Having the information concerning
• the sector where actually the stator flux phasor is,
• the required evolution of the magnitude of the stator flux and
• the required evolution of the developed electromagnetic torque,

how could be generated the command of the inverter?

• 4. Justify the structure of the assembly of the control system.

The influence of the sampling period

• 5. Because the minimum value of one pulse of the PWM is quite the sampling period of the control system, study the influence of this value on the general behaviour of the drive.