Sector Determination Algorithm

To determine the sector which a given stator voltage belongs Vs to, some algorithms have been proposed in the literature which generally require many arithmetic operations and are based on the coordinates of Vs in the Concordia plane or in the a-b-c phase space. When this voltage is deduced from a V/f control principle, its modulus Vsm is computed by the V/f law recalled i the previous application note, and its phase θ is deduced from ωs by a discrete-time integrator. To implement this sector determination algorithm efficiently, we manage θ' and k instead of θ in a dedicated integrator shown on Fig. 6. The sector number is the output of a modulo six counter activated each time θ' exceeds π/3 , and θ' is confined to lie between 0 and π/3 (see Fig. 7).

The resulting dataflow diagram, shown on Fig. 8, can be used to build a speed control loop (Figure 8.), in which the difference between the desired speed and the measured speed feeds a PI controller that determines the stator voltage frequency. To decrease the complexity of the controller, the input of the V/f law and of the space vector PWM algorithm is the absolute value of the stator voltage frequency. If the output of the PI controller is a negative number, two of the switching variables driving the power transistors of the inverter are interchanged.