Question about Inverter Electronics - Others
The rms voltage is what counts, because it tells how much power the output will deliver to a resistive load. Inexpensive multimeters on their AC ranges are usually average-responding rms-calibrated meters. This means they measure the average of the absolute value of the AC component of the signal, and display that average multiplied by about 1.11 (actually, pi over sqrt(8)), the ratio of rms to average value for a pure sine wave. That way, the meter will give the right rms reading for a sine wave.
If the signal is a square wave, where the average and rms values are equal, the average-responding meter will read 11% too high.
Many inverters put out a modified sine wave (MSW), which sits at zero for a while, goes to a constant positive level for a while, goes back to zero for a while, and goes to a constant negative level for a while to complete the cycle. The positive and negative parts of the signal have the same magnitude and duration.
The rms and average values of an MSW depend on its duty cycle D, the fraction of a cycle for which the signal is not at zero. In a well-designed inverter, the duty cycle will be adjusted when the DC input voltage goes up and down to maintain the nominal rms output voltage. If we use peak voltage Vp to mean the magnitude of the positive and negative voltages the signal goes to, then Vavg for an MSW is equal to Vp times D, and Vrms is equal to Vp times the square root of D.
The duty cycle for which an MSW will have the same rms to average ratio as a sine wave is 8 over pi squared, or 81%. For any duty cycle less than this, an average-responding meter will read a lower voltage than the inverter rms output, and for a duty cycle higher than this, the meter will read too high.
If your MSW inverter is putting out 120 volts rms and its duty cycle varies from 50% to 75%, the meter reading will vary from 94 volts to 115 volts. I avoid the problem by using a Radio Shack 22-174B true rms digital multimeter.
Posted on Sep 01, 2012
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