The rotary drive system includes: a direct voltage source (102); an electric motor (104) having an axis of rotation (A), and including independent phases having directions about the axis of rotation; an inverter (106) for connecting each phase to the direct voltage source (102); and a control device (110) for supplying a command to the inverter (106). The control device includes: elements (118) for selecting a formula from predefined formulas, each predefined formula being intended to calculate either a homopolar voltage set-point, or a homopolar current set-point; elements (124) for determining a set-point intended to apply the selected formula to determine, according to the formula selected, either a homopolar voltage set-point, or a homopolar current set-point, and elements (126) for determining a command intended to determine the command of the inverter (106) based on the set-point determined.