In many rural areas, due to unavailability of three-phase AC power, three-phase Variable Frequency Drives (VFDs) are often required to operate from 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 this leads to severe de-rating of VFDs for single-phase applications. A new low-cost, single-phase active circuit based on injecting current into the mid-point 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 better to distribute the higher input rms current so that the diodes need not carry current higher than their nominal rating. The proposed method shows that VFDs need not be de-rated to achieve rated output power. Test results from a 15hp, 230V system are given to prove the concept.
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