Why is modified square-wave bad on extremely cheap R-C series powersupplies?

[AC/DCdude17]

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I have a plug in power meter that had a warning sticker warning you not
to use it on a generator or an inverter. I did it anyways with extreme
caution. The power meter can read the correct true RMS voltage from
modified square wave. The meter is powered through a series
resistor-capacitor power supply and the resistor gets extremely hot when
used on modified square wave.

What causes this?

Secondly, I noticed shaded-pole motors run slower on modified square
wave from inverter compared to powering from sine wave from the outlet.
Why?

 

[Don Kelly]

X-No-Archive: Yes

I have a plug in power meter that had a warning sticker warning you not
to use it on a generator or an inverter. I did it anyways with extreme
caution. The power meter can read the correct true RMS voltage from
modified square wave. The meter is powered through a series
resistor-capacitor power supply and the resistor gets extremely hot when
used on modified square wave.

What causes this?

Secondly, I noticed shaded-pole motors run slower on modified square
wave from inverter compared to powering from sine wave from the outlet.
Why?

Harmonics. These lead to losses (at higher harmonics the capacitive
reactance is lower and there will be relatively high harmonic currents - all
adding extra losses. In the motor case there can be some backward harmonic
torques.