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

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- AC/DCdude17
  
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posted
19 years ago

Sat, Jul 3, 2004 2:52 AM

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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?

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- Don Kelly
  
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posted
19 years ago

Sat, Jul 3, 2004 4:30 AM

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.

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