59 Frequency Converters 2.6.3 Asynchronous PWM Two asynchronous PWM methods are described below: • SFAVM (Stator Flux-oriented Asynchronous Vector Modulation) • 60° AVM (Asynchronous Vector Modulation) These enable the amplitude and angle of the inverter voltage to be changed in steps. 2.6.3.1 SFAVM Stator Flux-oriented Asynchronous Vector Modulation (SFAVM) is a space-vector modulation method that makes it possible to change the inverter voltage arbitrarily, but step-wise within the switching time (in other words, asynchronously).The main purpose of this type of modulation is to maintain the stator fl ux at the optimum level throughout the stator voltage range, ensuring no torque ripple.
Compared with the mains supply, a “standard” PWM supply will result in deviations in the stator fl ux vector amplitude and the fl ux angle. These deviations will aff ect the rotating fi eld (torque) in the motor air gap and will cause torque ripple. The eff ect produced by the deviation in amplitude is negligible and can be reduced by increasing the switching frequency. The deviation in the angle depends on the switching sequence and can result in higher levels of torque ripple. Consequently, the switching sequence must be calculated in such a way as to minimise the deviation in the vector angle.
Each inverter branch of a 3-phase PWM inverter can assume two switch states, ON or OFF, as shown in Fig. 2.15 Inverter switch states. The three switches result in eight possible switch combinations, leading in turn to eight discrete voltage vectors at the inverter output or at the stator winding of the connected motor. As shown below, these vectors (100, 110, 010, 011, 001, 101) mark the corners of a hexagon, where 000 and 111 are zero vectors. Fig. 2.15 Inverter switch states