An Improved Single-Phase Active-Front-End Rectifier System for Use With Three-Phase Variable-Frequency Drives

In many rural areas, due to unavailability of three-phase ac power, three-phase variable-frequency drives (VFDs) are often required to operate from a single-phase ac source. Powering up three-phase VFDs from single-phase ac source requires addressing many issues: higher RMS value of input diode current, higher ripple voltage across the dc bus capacitor, higher peak input current, higher input current distortion, lower power factor, and poorer system efficiency. All of these lead to severe derating of VFDs for single-phase applications. A new low-cost single-phase active circuit based on injecting current into the midpoint of the dc bus employing only one bidirectional switch is proposed here. The bidirectional switch carries current for less than a quarter cycle and is rated accordingly. The presence of a three-phase diode bridge rectifier lends itself to better distribute the higher input root-mean-square current so that the diodes need not carry current higher than their nominal rating. The proposed method shows that VFDs need not be derated to achieve rated output power. Test results from a 15-hp 230-V system are given to prove the concept.