This paper describes a step-down AC voltage regulator using an AC chopper and auxiliary transformer, which is connected in series to the main input. The detailed design of the AC regulator, logic, and Pulse Width Modulation (PWM) pattern of the AC chopper are described. The proposed AC regulator has many benefits, such as fast voltage control, high efficiency, and simple control logic. Experimental results indicate that it can be used as a step-down AC voltage regulator for power saving purposes very efficiently.

Recently, there is an increase in studies on Pulse Width Modulation (PWM) AC chopper because it offers numerous advantages such as sinusoidal input current with unit power factor, rapid dynamics and significant reduction in the filter size (Rufer, 1995; and Shenkman et al., 2004). One of the problems occurring when we use the PWM AC chopper is that the switching pattern of the AC chopper is critical and an alternative path has to be provided for the current when all switches are turned off. To solve this problem, many topologies are proposed by using additional bypass capacitors or RC snubbers (Rufer, 1995; and Shenkman et al., 2004). As the PWM AC chopper can be operated with high reliability without any problem of commutation, series compensation schemes using the AC chopper with a series transformer connected to the main input are proposed for line conditioning and AVR application (Kwon et al., 2007).

In this paper, a novel single-phase and three-phase step down AC voltage regulator using AC choppers with series transformers for the purpose of 10-20% voltage decrease, are studied. The operation principles of the proposed topologies and the design procedure of the prototypes are explained. To verify the performance of the proposed scheme, detailed experiments are performed, and experimental results indicate that the proposed AC regulator offers good performance.

When we make the switching pattern of the AC chopper, some considerations are essential for safe operation. First, it is important to ensure the freewheeling path in order to allow free flow of the current independent of the polarity of the applied voltage. If all the switches are on, short circuit is generated and if all the switches are off, the voltage spike across the AC switch damages the IGBTs in the AC chopper application. To solve this problem, the switching pattern of Figure 3 should be used for ensuring the current path of the dead time period by freewheeling switch. However, it is impossible to force the current in the dead time period of zero crossing point along the path of the input voltage. In this paper, to solve this problem, a simple RC snubber is used as shown in Figure 1. In this case, the power loss of the RC snubber is not greatly increased because it only makes the bypass circuit during the dead time of zero crossing point of input voltage.

Electrical and Electronics Engineering Journal, Single-Phase AC Voltage Regulator, AC Chopper Applications, Series Compensation, Input Transformer, Automatic Voltage Regulator, Series Transformers, Output Voltage Waveforms, Input Voltage Waveforms, PWM Switching Pattern, Series Compensation Schemes, Pulse Width Modulation.