Device and method for dynamically adjusting an operating voltage for an electric motor (9), which voltage is provided by a voltage source (1) formed in a modular manner, wherein modules (2-i) of the voltage source (1) formed in a modular manner are switched in dependence upon a rotational speed (n) of the electric motor (9) such that the overall degree of efficiency of the electric drive train is improved in a low rotational speed range of the electric motor (9), wherein the modules (2-i) of the voltage source (1) are switched by means of a switching network (3) which is composed of controllable power switches (5) and can be actuated by control signals which are generated by a switching controller (4) in dependence upon the rotational speed (n) of the electric motor (9).