The article presents a standard electric power circuit of an AC locomotive of Ermak series operating in the mode of regenerative braking and reveals the circuit’s drawbacks. To eliminate them, it is proposed to use an improved power circuit where standard reversible converter is replaced with a more advanced IGBT-based counterpart and exclude ballast resistor blocks from the armature circuits of traction motors. A mathematical modeling of a standard system of AC locomotive regenerative braking is performed. The conducted mathematical modeling allowed to obtain the curves of the catenary current and voltage, generator current, inverter voltage and power factor. A method of regenerative braking without the application of ballast resistor blocks is proposed. Having analyzed the electromagnetic processes of AC locomotive standard power circuit operation in the regenerative braking mode, the authors have revealed a number of drawbacks that have an adverse effect on the efficiency of regenerative braking. A technical solution for improving the AC locomotive power circuit is introduced that enables a substantial increase of the locomotive energy performance in this operation mode.