Sorry about the answer babun. I resisted going technical and calling a spade a spade. Smaller generators develop sinewaves with a lot of harmonic distortion. How's that? :)
But it isn't "square wave" or stepped sine wave. In theory a modified sine wave can mean two steps or twenty - the more steps the more sinewavelike the product will be.
Nevertheless when I am forced to use my Tektronix scope with a low-end generator it connects through my Sola ferroresonant line voltage regulator. An Isolation transformer would pretty much do the same waveform correction.
But all this does not apply toward hammer and chisel cheap generator waveform. Plugger in and go. Things should be fine. The enramada freezers work fine on a harbor freight generator whose waveform is uglier than something that fell out of an uglystick tree.