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Generator/Inverter?

Being an old phart, I've only lately noticed the term "inverter" being tagged on to generators. To me, an "inverter" is a device, either rotary or solid state, to boost low D.C. voltage to a higher A.C. voltage........normally used to get 110 volts A.C. from batteries.

Can someone explain what an inverter does as part of a generator circuit? Is it desireable at all (that'll probably open a can of worms!)?
 

len k

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Last Subscription Date
12/12/2018
It's all about money.

Typical gens need to run at a fixed rpm all the time to make 60 hz.

To save fuel costs some gens make and rectify 3 phase power to pure DC then use an inverter to make 60hz. That way they can lower engine rpm when full power is not needed , thus saving fuel.

Down side is the inverter is very expensive to buy and if it fails usually it's a non-repairable $$$$$ modual. And being electronics it doesn't like heat so it needs a good cooling design or it can run hot and have a short life. Because the replacement modual can cost more than a used gen, I wouldn't touch one with a 10 ft pole.
 

Big Bird

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02/16/2018
If the generator set produces three phase power, and then is run through a rectifier to create DC , can you use the three phase power it produces to power a three phase motor ,like for an old Lathe ? :shrug:
 

brianh

Registered
Yes, but it is likely not 60 Hz. Higher frequencies = less filtering needed. You could measure one and see. I brought out the windings from a car alternator one time to try that. It made 180 Hz by the time I got it spinning fast enough to excite and make power :(
 

pegasuspinto

Registered
the alternator inside an inverter-generator type set will almost certainly be high frequency, they are more efficent.

3 phase variable speed drives with single phase input are now getting common and fairly cheap....
 

BigBlockChev

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Last Subscription Date
07/06/2019
I tested one a while ago and it was in the neighborhood of 240-300Hz depending on engine speed. We thought about rectifying the output and using the dc for something like batt charging , it was a few hundred volts can't remember exactly. A lot of work for not much gain. Cheers Dan
 

len k

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Last Subscription Date
12/12/2018
Problem with using just the genhead w/o inverter is genhead likely uses a permanent magnet for field, at least Onan's does in their HDKAxxx model. That means voltage output will be proportional to rpm and you can't change that. Most other gens use an electromagnet for field drive and you can change gen output voltage by changing rpm AND/OR field strength. In a permanent magnetic gen this also means that voltage will drop as you load the gen and you can't fix that (if you want to keep rpm constant, 60hz). In testing I've done on an Onan NHM at 60 hz this means you will likely be limited to < 1/2 the gen's rated output to still have an acceptable output voltage range.

So if you want 60 hz you've got to take whatever the voltage coming out of the windings happens to be. Other issue is since rpm will likely have to be low to get 60hz you won't get as much kw out of the engine, guessing 1/3 - 1/4. And that's in addition to not having a use for the kw of the 3-rd phase. So not practical to go that route.

Other issue is fuel consumption will be a killer compared to your small output. Oversized engine to genhead output issue.
 
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Jim Marcozzi

Registered
Works similar to these systems that we use on emergency vehicles.

http://www.meps.com/

It's a standalone alternator (VERY similar to a Leece NeVille) that puts out 3 phase at approx 40V+ and at over 300Hz. The 3p is fed into a remote node that rectifies it and stores it in a capacitor bank and is then regulated down to the constant voltage that the inverter uses (as i recall, 48V) then to 120/240.

These units are VERY efficient and small. They hold 60Hz better than most gensets can, too. On the 7KW system, we commonly see about 1200W at a 700RPM engine idle. Full output is typically achieved near 1300-1400 engine RPM. The beauty of the MEPS system is that you can generate 120/240 at 60Hz while driving, no matter the engine RPM.
 

len k

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Last Subscription Date
12/12/2018
Interesting that some of the smaller alternators look very similar to 12V car alternators.

In past doing some back of the napkin calculations I found that the regulator in a 12V car alternator throttles back it's voltage GREATLY when it runs at high rpm. That's because alternator is designed to make ~full power at engine idle rpm.

Remove/redesign the car alternator regulator and relocate the 3 phase diode rectifier to a place with better cooling and you can get a lot of power out of a car alternator, at high rpm. Think you can make ~110 V AC straight out of the winding
 
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Wow.........thanks for all the responses! So, in other words, the manufacturers have complicated an otherwise basic design in order to save fuel, BUT if the inverter portion goes "kaplooey" you're stuck with something that costs more to repair than just buying a new one.

Some engineer HAD to think that one up.......

As already mentioned, I'll stay away from them and stick with something I know. I don't mind paying for a bit more fuel.

Interestingly, I once had a small Onan single cylinder, cast iron engine generator that operated the man lift on my bucket truck. That was the first time I heard a generator running at 1800 rpm and it caught my attention immediately. I looked at the rating tag and although I don't remember the voltages, it was three phase! I was a little disappointed as it looked to be an ideal little standby generator for my homestead at the time.

I believe the truck was mid to late seventies vintage at the time, but that's the only information I can provide. Anyone have an idea what model it might have been!
 
If the generator set produces three phase power, and then is run through a rectifier to create DC , can you use the three phase power it produces to power a three phase motor ,like for an old Lathe ? :shrug:
To complement my 1920 Hendey 16" lathe, I mounted a 1920's vintage Century 2 hp motor.......3 phase, and I only have ordinary residential single phase available.

I looked at rotary and static converters, considered building my own out of a couple of motors, etc......then finally bit the bullet and spent a whopping $150 for a solid state drive instead. Best money I ever spent on a machine tool. Not only does it operate the motor on 220 volt single phase efficiently, but by installing a Radio Shack pot I have complete speed control.......from max down to crawl at the turn of a knob. You do lose torque/power at the lower speed, so I just engage the back gears instead.....no big deal and plenty of power.
 

len k

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Last Subscription Date
12/12/2018
...... BUT if the inverter portion goes "kaplooey" you're stuck with something that costs more to repair than just buying a new one.

Some engineer HAD to think that one up.......
More like marketing or a government requirement that they be more fuel efficient ( but not cost efficient)

Len K, MSME
 

Birken Vogt

Email NOT Working
...... BUT if the inverter portion goes "kaplooey" you're stuck with something that costs more to repair than just buying a new one.

Some engineer HAD to think that one up.......
More like marketing or a government requirement that they be more fuel efficient ( but not cost efficient)

Len K, MSME
The Honda EU series are pricey but I know a lot of contractors and off grid people who have them and love them. The EU2000s are very cheaply made inside but they seem to be made cheaply "right" in that they don't go kaboom very often, they wear out gracefully. The EU6500 I know of several that have thousands of hours (that is a lot for any air cooled) and show no signs of slowing down.

Now the cheap Chinese inverter sets are exactly as you say, they go poof and no parts are available.
 

BigBlockChev

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Last Subscription Date
07/06/2019
The big thing that inverter gens usually have is that they are quiet. Aside from that I can not believe that the money that they supposedly save in fuel is ever going to pay for the difference in purchase cost over the life of the generator. Pretty much everything on the market aside from Honda and Yamaha has a lifespan of a couple hundred hours or less so you are not going to save anything there. But they are quiet. Cheers Dan
 

len k

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Last Subscription Date
12/12/2018
Quiet is a good selling point. Low rpm ( at low load ) helps that a lot.
The rest is good muffler design and either quite mechanical design or good sound proofing case.

My Onan 7NHM's muffler is so good and quite, that what you hear is the mechanical engine noise. Guess I got to check the valve clearances.
 
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