Am I getting caught up in the weeds worrying about THD on a generator?

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  #1  
Old 03-13-17, 07:39 AM
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Am I getting caught up in the weeds worrying about THD on a generator?

I am looking for an emergency-only generator with 240 V output--fairly small one (3000-5000 watts continuous) to power some of the key components in my house during a power outage. I had an outage recently and learned that my cheap car inverter would chug along for the fridge (barely) or some other things, but wouldn't power my computer UPS devices because its modified sine wave was too dirty.

So anyway, I like some of the $300 generators, and they are well reviewed. But, I know their total harmonic distortion (THD) is quite high, like in the 12% range, and google tells me that "sensitive electronics" should have <6%. To get a generator capable of power that clean I pay a massive premium.

I assume a cheapy chinese generator still won't run my computer UPS, which is fine, but am I running a risk I could cause damage to the furnace, fridge, etc.?

I'm thinking I have probably read too many threads online about people going on about THD, because I've asked a few people who have generators about this stuff and I get blank stares. It seems to me most people don't know what this is, and are not worse for the wear; is this one of those things that in the real world is just not as important as it seems to be in the theoretical one of internet forums?

 
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Old 03-13-17, 10:51 AM
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It all depends om what you're running. If it's a simple motor and some light bulbs, then no problem. The more semiconductors and processors are involved, the more trouble you get into. A plain refrigerator or furnace from 1995 should have no problem. A newer fridge with wifi capability and all that, or a newer furnace with X13 variable speed controls on DC motors might fail ($600+ for that driver an they are pretty sensitive).
 
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Old 03-13-17, 11:06 AM
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Cheese, in your experience or that you've read about, how often is this a problem? I see a lot of people mention it but not sure how many people have actually fried things.
 
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Old 03-13-17, 12:06 PM
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I have borrowed and played with some cheap generators and I've even varied the frequency and voltage from my big generator. It was only short term testing but I was surprised at how well modern electronics handle dirty power. There are some items that need a true sine wave and they won't work from modified sine waves from cheap inverters but they handle the voltage and frequency being off quite well. TV's, computers and many other electronics did quite well. Many modern electronics are designed to work anywhere on the planet without conversion except for the plug so they can handle 100-240 volts and 50-60hz.

I think one of the worst things is if your generator is not able to maintain it's output under load. You can burn up a fridge compressor if the voltage dips when it's trying to run under load. Still, I was only testing for short periods and not with the major appliances in my home. I personally have high quality generators as the last thing I want in addition to a power outage is a fried air conditioner or dead fridge.

As for cheap Chinese generators. I've seen them run good and I've seen ones that don't. Generally when they don't run there are few people that want to work on them as getting parts can be difficult/impossible.
 
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Old 03-13-17, 12:21 PM
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I used a 5kw Honda generator for quite a while and now have a generac 7.5kw unit. Have had no issues with UPS or electronics not liking the power quality. Granted, I probably haven't run on the generator more than 100 hours total.

Keep in mind when you size the generator that some motors, like a well pump for example, have a high starting current. In fact, that's why I went from 5k to 7.5 k when I moved to my current house. The Honda would run the well pump, but sounded like a dying moose for 15 or 20 seconds every time the pump started, and it's only a 2 horsepower 240 volt pump. The 7.5K just groans for a second or two until the governor responds.
 
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Old 03-13-17, 01:32 PM
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Sounds good!

I ordered an el cheapo generator and will be testing everything in the house on it. It won't be powerful enough to run my AC so that at least guarantees that it won't break the AC, too

I got a killer price on a dual fuel duromax on ebay for $299 (4850 surge/3850 continuous). If I'm not happy with it I imagine I can sell it for what I paid for it, given the price everywhere else, but it got good reviews.

Looking forward in particular in seeing how the electricity from this compares to the modified sine wave from the inverter I had going in my car (UPS power supplies hated that).
 
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Old 03-13-17, 03:40 PM
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I've not really ever used generators much but from working on them and the training I have had, as long as the generator portion is fully functional all it needs for the proper output be it voltage or frequency, is the correct RPM, which is dependent on the engine, and more particularly, the governor. The gening part of a generator is nothing but magnets and coils of wire, so as long as the RPM remains unchanged, so should the output.
A quick easy test I use has shown quite a bit of variance especially on smaller units. I take the biggest grinder or drill I have, plug it, fire it up and listen or check for RPM change, then put the tool under a load and check again. IIRC, the technical term is "droop" for how well and fast the governor responds and maintains constant RPM.
 
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Old 03-13-17, 04:28 PM
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I don't think I ever fried anything because of that, but I haven't used sensitive electronics with modern contractor style generators either. Most stuff uses a power supply that converts the power to DC anyway, and that supply I think is usually able to handle some spikes and noise with a transformer, some filter coils, and capacitors. I'm not an electronics tech, but I would suspect that if the supply was too dirty, it may possibly just result in the device you're trying to run simply not coming on.
 
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Old 03-13-17, 05:45 PM
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I don't disagree cheese but my experience with aviation they use turbine engines to supply power which can indeed maintain any instant load. Also my marine experience on yachts was the same issue, any time I worked with tech's they were more concerned about the engine than the gennie itself. As I mentioned, as long as all components in the genning part of the equipment are functioning properly, all that is needed is the correct and constant RPM. I am no electronics "tech" by any means either but one thing I do know, is it might be intermittent, but it either works, or it don't. There is very little margin. The variance is in the power driving the generator.
 
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Old 03-14-17, 12:36 AM
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The variance in hertz and voltage is determined more by the engine and it's response to demand.
The distortion in question here can't really be changed by the engine. It's related to the method of power production. The AC wave if you watched it on an oscilloscope, will show the shape of the wave. Spikes and raggedy sharp edges are not good. I believe this is likely caused by arcing brushes, cheap switching diodes, low shielding, etc... That is what causes static in the radio, specks or lines in the TV screen, and long term, it supposedly shortens the life of semiconductors. I don't think it will burn up your computer, but it might shorten the life of it depending on how bad it is and how well the power supply cleans it up. That's about the extent of my knowledge of it.
 
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