Inverter (not) Sine Wave

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I've been looking at an old APC BR500 Uninterruptible Power Supply which I've never used before.

There is a note in the instructions saying that the output is not Sine Wave, so I wanted to have a look at it.

I connected it up to a digital recorder via a voltage divider circuit and a transformer and am attempting to attach the output to this message.

(The digital recorder method of analysing the wave seems to work, as checking the wave from another inverter gives a lovely sine wave output.)

The output from the APC BR500 has each peak and trough spit into two separate distinct peaks & troughs.

My power meter indicates 230 volts but moving on to the mode where it should display Hz makes it go wrong and need reset.

My laptop charger works fine on it.

Is there something wrong with it or is this how it is supposed to work. (If so, is there a name for the wave produced?)

Any thoughts or comments?
 

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Hi,

I made a quick websearch. According to the product page the warform is a Stepped approximation to a sinewave.

What is the frequency of the signal if you are evaluating the falling zero crossing, assuming this is your time constant?

BR
 

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The wave is ++ followed by -- and if you count each ++ as one then frequency is 50Hz. Let me put that another way - looking at the wave in Audacity, if I select 10 of the double peaks, then I've selected 0.2 seconds.

I've tried recording the wave again without the transformer (using a higher top resistor in the divider circuit instead) and the pattern is the same.

For comparison, I've attached a screenshot of the wave from the first setup (with the transformer in place), when testing a "pure sine wave" inverter.
 

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  • inverter.png
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Looks like a "modified sine" output to me.
Basically a DC supply at about 60% of the rated RMS output fed to a bridge that reverses the output connections at line frequency. It gives an output alternating in polarity with a dead period between transitions. It works fine for most loads like your laptop charger but wreaks havok with highly inductive loads like motors or transformers.

Brian.
 

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Take another look at it - note the double peaks and double troughs.
 

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I see them but they are quite common and because most modified sine inverters have a filter in their output. Feeding a modified sine to a filter causes ringing on the edges unless it is optimally loaded.

Brian.
 

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I've added another image contrasting a sine wave, a modified sine wave and the loch ness monster wave from my UPS. It doesn't look like any of the representations of a modified sine wave that I can find.
 

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Without a manufacturer specification or a known good reference device it's impossible to decide if your UPS is operating regularly or just defective. I guess the latter.
 

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OK. I've asked APC support and will aim to get back here if they reply with anything of interest.
 

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