A Motor Module comprises a self-commutated inverter with IGBTs. It generates a variable voltage with variable frequency from the DC link voltage that feeds the connected motor. Multiple Motor Modules can be operated on a single DC link. This permits energy to be transferred between the Motor Modules. This means that if one Motor Module operating in generator mode produces energy, the energy can be used by another Motor Module operating in motor mode. Motor Modules are controlled by a Control Unit.