This paper investigates the spectral effects that stator supply harmonics can induce in wound rotor and doubly-fed induction generators. The purpose of this study is to clarify the nature and the origin of the reported significant inter harmonic effects in these machines. To this end, the authors derive analytical expressions that define the spectral signature frequencies of individual supply harmonics in the current and torque signals. The expressions are first validated through comparison with the spectra calculated using a harmonic time-stepped machine model and then by spectral measurements on a laboratory test rig comprising a 30 kW machine excited by a programmable power supply unit. It is shown that the presence of supply harmonics can result in multiple and variable wide-band interharmonic effects in the current and torque signals. Finally, the interharmonic current emissions for the wound rotor and the doubly-fed mode of operation are experimentally quantified using the relevant IEC standard indices.