Energy recovery a.c. dynamometers provide fast payback by regenerating engine power directly to the plant a.c. distribution system to reduce utility cost. These dynamometers are inherently universal, and can motor as well as absorb, to provide cranking, overhauling loads, and friction testing. This paper describes a successful system based on the variable voltage inverter design principal. Subsequently, additional development using current source inverters was completed. CSI technology provides a more economical system at higher ratings, currently 500 hp maximum. Future developments are expected in the area of newer, more economical inverters for lower horsepowers (150 and below) using variable voltage (VVI) techniques as described in this paper.