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The Swamp
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Generator Inverter Weirdness - Need Advice

Mon Mar 15, 2021 9:07 am

Hi Guys,

This problem doesn't really fit into the other categories, but I figured y/all might be able to assist. I finally broke down and bought an inverter generator. I have a pair of non-inverter gensets that I use since I live in Hurricane Alley and power loss can be for several days if we get hit in the wrong way. However, non-inverters don't produce very clean power. Harmonic distortion can get uncomfortably high (25%) for electronics. An inverter is supposed to make clean power that's safe to use for anything. I found a good deal on a Wen unit, the GN400i. I tested it out this weekend for the first time. Started right up. Checked AC voltage which clocked in at 124.5 volts stable. So far, so good.

I checked to see the Hz output. I have a True RMS DMM that I used. I was expecting a clean 60 Hz signal. Well, that's not what I got. The DMM started jumping from 1 KHz all the way up to 9 KHz. That could not be correct, obviously. Thinking that the DMM might be malfunctioning, I tested the nearby wall outlet. It showed 60 Hz. I fired up my non-inverter genset and it showed a steady 59.7 Hz. So I knew the DMM was working fine.

I added a small load to the inverter to see if that would settle the Hz down so I could get a reading. Nope, no luck. I tried all of the outlets on the inverter, same reading. I tried different holes in the outlets, no difference.

I've seen DMMs have trouble reading Hz on gas generators before. Using a non-TRMS DMM on a non-inverter makes the meter go nuts when set to Hz. However, a True RMS meter is not supposed to be affected by that. I do have a non-TRMS DMM I could try to see if it gives me a stable Hz reading. But typically, high harmonic distortion is what makes reading frequency on non-inverter gensets difficult. The inverter should not have a lot of that, though.

I'm really stumped here. I don't know if I have a bad inverter board or bad cap or if something is not connected that should be, or if inverters can confuse TRMS meters (although I don't think so), or what is going on here. Anyone have any ideas they can share? Thanks!

ETA: I got off the phone with Wen. They were... yeah. Unhelpful. I could tell their expertise was in getting the engine started and that's about it. Any hard technicals about output are will beyond their scope. Sigh.
I wish I had gone to med school.
 
Wirko
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Re: Generator Inverter Weirdness - Need Advice

Tue Mar 16, 2021 7:37 pm

Hi,

I can't see any other explanation than a horrible amount of switching noise at the generator's output due to poor filtering. That could confuse a frequency meter because there would be many large voltage swings inside each period of the 60 Hz AC voltage. The meter would detect and count all of them or at least many of them.
Why does the type of multimeter matter is hard to say. It may be because true RMS meters often have a wider frequency range while cheap non-true-RMS ones may be limited to 50-60 Hz and mostly ignore everything that's higher. However "true RMS" is related to voltage measurement, not frequency measurement.
If you have any kind of power transformer with a 120V input and a suitable resistor (incandescent lamp or heater) to fully load the transformer at the secondary, you can connect those to your generator and measure the frequency at the secondary side. With some luck it will act a good enough low pass filter and remove most high-frequency noise. Do it with caution, all wiring must be solid, you don't want loose wire ends carrying 120V to fly around.

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