Does a transformer make the waveform looks more like sine wave?

[bdunham7]

And an inverter powered from a 12V battery has plenty enough energy to kill you. 

Anything with mains output voltage and more than a few mA can kill you by direct contact, but saying that a typical battery powered inverter poses a danger of making your instrument (however crappy) explode might be overstating the case just a wee bit. 

[bdunham7]

The so-called “modified sine wave” can be dangerous for connected devices.

The one you're showing is especially bad for some linear power supplies because it has too much duty cycle and not enough peak-to-peak voltage.  Properly done modified-sine setups have to start with an output equal to the peak of the sine they are replacing and then shorten the on-time to bring the RMS down to spec.  IDK what the best arrangment is for induction motors, but I imagine they'll run hotter in any case.  A non-PFC SMPS is probably fine either way.

[Aldo22]

The one you're showing is especially bad for some linear power supplies because it has too much duty cycle and not enough peak-to-peak voltage.  Properly done modified-sine setups have to start with an output equal to the peak of the sine they are replacing and then shorten the on-time to bring the RMS down to spec.  IDK what the best arrangment is for induction motors, but I imagine they'll run hotter in any case.  A non-PFC SMPS is probably fine either way.

With motors such as those in a fan, you can hear that something is wrong with the e-ast CL300-12. It sounds sick.
But what surprised me the most was that my Voltcraft Energy Monitor 4000 Pro emitted a little smoke when it was connected to the inverter.
There is no problem with the e-ast CLS 600-12.

[BeBuLamar]

I have a small step down transformer to step it down from 120V to 12V. The waveform looks the same as when I connected it directly. This time I use the digital scope so I can make some measurement. Compared to a sine wave with the same peak value it has the same RMS value as well.

[Vovk_Z]

Power transformers in general don't work as good low-frequency filters. They have limited BW of cause but it is still to wide to be used as good filter.
I mean you possibly want 100-150 Hz LF filter but typical power transformer is  more like 1.5-15 kHz LF filter (depends on load and exact transformer). So it doesn't filter most needed frequency.

[Zero999]

I have a small step down transformer to step it down from 120V to 12V. The waveform looks the same as when I connected it directly. This time I use the digital scope so I can make some measurement. Compared to a sine wave with the same peak value it has the same RMS value as well.

Yes, that's exactly what you should expect to happen.

In my experience small mains transformers have a fairly flat frequency response up to a few tens of kHz, then there's a peak at nearly 100kHz. They can even be used for audio, if the voltage is reduced to account for the minimum frequency being 20Hz, rather than 50Hz. It will do nothing to smooth your modified sine wave into anything remotely resembling a proper sine wave.

[jonpaul]

no see the freq resp of a power trsf and spectrum of the wave in question

j