According to the operation principle of the asynchronous machine, the developed torque is greater when the slip

is greater, respectively, the difference between the speed of the rotating field w₀ and the mechanical speed of the rotor w_r, is greater.

The Direct Torque Control consists, as principle, in controlling the advancement speed of the stator flux phasor (which is right the speed of the rotating field w₀), respectively the next position of the flux, by knowing the actual one, depending on the necessary torque that the motor has to develop.

There are three possible situations:

• the developed torque is to small: the flux phasor must advance (figure 1a)
• the developed torque is enough: the flux phasor must keep its position (figure 1b)
• the developed torque is to great : the flux phasor must return (figure 1c)

 

Figure 1

 

In the same time, the value of the flux must be kept constant (for a certain operation regime). This means that the point of the flux phasor must describe a circle, having the radius equal to the necessary value of the flux (figure 2).

Figure 2

The stator flux phasor will travel this circle with variable speed, depending on the demanded torque.