Abstract: The invention concerns a system for actively reducing rotational nonuniformity of oa shaft, in particular the drive shaft (10) of an internal combustion engine (1) or a shaft which is coupled, or can be coupled, to the drive shaft. The system includes at least one electrical machine (4), in particular an induction machine or traveling wave machine, which is coupled, or can be coupled, to the shaft; at least one invertor (17) which generates the voltage and/nor current of variable frequency, amplitude and/or phase necessary to produce the magnetic field of the electrical machine (4); and at least one control device (31) which controls the invertor (17) and hence the electrical machine (4) in such a way that the machine counteracts both positive and negative rotational nonuniformity of the shaft. The invention also concerns a method of actively reducing rotational nonuniformity using the system.