Clean up voltage on a modified sine wave inverter?

Okay, here's the deal. I'd like to set something up that will allow me to run my furnace for short (emergency) periods when the power is out. I tried an 1800w Coleman generator with no success. Some background: I have an Armstrong 90%+ efficient furnace with a "blue" smart valve. With the Coleman generator, the draft motor wouldn't even start. The voltage read over 130v going to the furnace. I figured it was either that or maybe the frequency was wrong. BTW, the blower would run just fine.

So now, I tried a Vector 1500w modified sine wave inverter, hoping that the voltage would be cleaner. The draft motor will run now. Blower runs, too. However, my so-called smart valve thinks it has an open limit, when I know for a fact that it doesn't. However, with a perceived open limit, it won't go through the ignition sequence... :candle: I tried putting the inverter under load, but still no success. No load voltage reads 91v, full-load (a 1500w heater) is 110v.

Is there anything I can do, such as an inline capacitor or something else, to clean things up so that my furnace will be willing to work with the inverter?

Valpo Hawkeye said:

The draft motor will run now. Blower runs, too. However, my so-called smart valve thinks it has an open limit, when I know for a fact that it doesn't. However, with a perceived open limit, it won't go through the ignition sequence...

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Please clarify exactly what you mean by "open limit."

Smart valve will probably be checking inputs from a flame rollout sensor, a flue draft sensor, an overtemp sensor, and possibly others. A no-go input from any of these will halt the ignition sequence.

My flue draft sensor fails occasionally, halting the ignition sequence. I learned to fix it using a short piece of bent coathanger wire that is kept handy, hanging inside the furnace. Once fixed, that draft sensor will be good for about a year of reliable operation - more or less.

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An LC filter on the output will help, but I'm not sure of the correct values to use here. The LC time constant is what matters first, and after that you need to choose an inductor beefy enough to pass 20 or so amps without getting too hot. It still won't be a pure sine wave, but rather a rounded square wave. However, it might be good enough for your uses. Also, "modified sine wave" is usually just sales speak for square wave. If you didn't pay much for this inverter, then it probably is just a square wave type. A 1500W pure sine wave inverter will run in the high 100s of dollars. There's a lot of extra components which go in there.

You can try the capacitor. Or try loading the inverter with a fluorescent shop light two 32 or 40 watt lamps (inductive load) at the same time or 200w of light bulbs (resistive load). It's worth a shot.

Personally, I have given up on non-sine wave inverters. They do bad things to electronics and motors. Take a look on e-bay for "sine inverter" and you will see some of the less expensive ones like the Prosine which have a great reputation for holding up and have a nice smooth output.

It sounds like your genny may be undersized for the start-up load. You might also look at the small electrically driven gen sets. An electric motor turns an alternator which is true AC. They were popular with service trucks (plumbers, locksmiths, etc) before solid state inverters became popular.

We used a monsterously large version for un-interupted AC at the phone company years ago. It powered the teletypes and things that were essential for maintaining the early electronic switching systems.

Daniel