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[Mike Sokol]

Recently one of my RV readers emailed me about some new electronic appliances that won't run from his Xantrex modified sine-wave inverter. He also noted he's had a lot of electrical gear failures over the last 2 years in his RV ever since he's been powering everything from the inverter for extended times. I'm pretty sure it's the harmonics sneaking into the electronics and makes a mess of things, but wonder if it's possible to using something like lighting dimmer choke and a motor starting cap to create a 12-dB per octave low-pass filter in the few hundred Hz range that would pass 20-amps of current. Have any of you ever played around with something like that? 

[John Roberts {JR}]

If there is enough out of tolerance energy that it is causing problems it may not be trivial to passively filter it.

I would inspect his actual waveforms in use, to see if maybe it is something simpler like narrow inductive spikes causing over voltage that could be damped.

If the appliances he is having failures with are modest power, maybe a sine wave inverter just for them.

Technically all waveform distortion is harmonics, so that is too broad a description.

Surely the manufacturer of the inverter has received enough feedback to have a clue. If not a remedy.

JR

[Lyle Williams]

Won't that give you 6dB/octave rolloff?  Rolling off from 300Hz is probably a waste of time and rolling off near 60Hz needs some big components.

I think if there was a cheap and small circuit to smooth a stepped wave then it would already be in the inverter product.

[Mike Sokol]

Won't that give you 6dB/octave rolloff?  Rolling off from 300Hz is probably a waste of time and rolling off near 60Hz needs some big components.

No, a capacitor and an inductor forms a 2-pole filter, so it should have a 12-dB per octave slope. I think it's the higher order harmonics doing the bad stuff, but I could be completely wrong. Think light dimmer buzz...

I think if there was a cheap and small circuit to smooth a stepped wave then it would already be in the inverter product.

One would think so, but I've figured out lots of electrical safety stuff in the last few years, and was sure that EVERYBODY must know it already. RPBG outlets are one example. http://ecmweb.com/contractor/failures-outlet-testing-exposed

Perhaps the inverter industry has missed something simple, who knows. Maybe thinking about it as an audio filter offers a unique perspective. In any event it's an interesting brain teaser to consider.

[Steve M Smith]

Maybe thinking about it as an audio filter offers a unique perspective. In any event it's an interesting brain teaser to consider.

Perhaps something like a digital power amplifier will use on the output to remove the HF.


Steve.

[Mike Sokol]

Perhaps something like a digital power amplifier will use on the output to remove the HF.

Good idea. I'll look up a few circuits to see how they work.

[John Roberts {JR}]

I am surely repeating myself, but #1 figure out what the problem really is.

It is entirely possible that some very cheap inverter is doing something fast and loose, that less robust appliances don't like.

I would not expect a simple answer, while you might be able to damp an inductive transient spike with a modest passive approach, any significant energy will take significant power handling to scrub off and dissipate.

JR

[Lyle Williams]

Won't that give you 6dB/octave rolloff? 

Sorry, late night brain fade....

[Tim McCulloch]

Good idea. I'll look up a few circuits to see how they work.

Xantrex is one of the biggest supplier of inverters to the touring motor coach conversion industry, along with marine offerings.  I think contacting them would be a good place to start.

[John Roberts {JR}]

Good idea. I'll look up a few circuits to see how they work.

The digital audio amplifier output filters use a relatively large inductor and very high switching frequency. I suspect the (cheap) inverters use significantly lower switching frequency and cruder approximations of sine wave signals.

JR
 

[Frank DeWitt]

Just a bit off topic, but many RVs carry a generator with a very good inverter in it such as the Honda.  As I understand it they generate high frequency 3 phase AC Rectify it to DC, then invert it to 60 hz 120 VAC.   Could the generator be hacked to feed it DC either from it's own alternator, OR from batteries.  One inverter, two inputs.

[Lyle Williams]

I think the rectified DC voltage that feeds the inverter is up around 200v.

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