Source: {"pile_set_name": "USPTO Backgrounds"}

As shown in FIG. 1, alternating current (AC) chopping is one of the best techniques to achieve voltage regulating of commercial power. If an inductive load is connected to an AC chopping, because of an effect of inductive reactance, a freewheeling must be proceeding when the chopping switch is off. Currently, the commonly used solutions of freewheeling are realized by parallel connecting an AC chopping switch with an electronic switch for freewheeling. That is to say, regardless of the specific structure of the chopping switch circuit, parallel connecting a chopping switch 1 with a freewheeling switch 2 is essential. Both the chopping switch and the freewheeling switch adopt Insulated Gate Bipolar Transistor (IGBT) switch components. It takes some time to turn on and turn off the IGBT switch components. To prevent a short circuit on the chopping circuit that might be caused by turning on the freewheeling switch 2, control signals for chopping electronic switch and freewheeling electronic switch has to be accurately controlled, and an interval time has to be set between the turning off time of the chopping wave electronic switch and turning on time of the freewheeling electronic switch. The interval time is called as “dead time”. Because of the dead time, there is a delay with the freewheeling when the chopping electronic switch is turned off, and because of the dead time, there is a limit to further improve the frequency of the AC chopping. When proceeding three phase AC chopping, a more accurate and complicated controlling to a phase position of the control signals of chopping electronic switch and freewheeling electronic switch is needed.