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Generator/Inverter?


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  #1  
Old 12-15-2015, 12:17:49 PM
George Andreasen George Andreasen is offline
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Default 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!)?
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Old 12-15-2015, 12:27:13 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Generator/Inverter?

Great question, I also would like to know!
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Old 12-15-2015, 12:29:05 PM
len k len k is offline
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Default Re: Generator/Inverter?

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.
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Old 12-15-2015, 01:00:13 PM
Big Bird Big Bird is offline
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Default Re: Generator/Inverter?

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 ?
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Old 12-15-2015, 01:21:20 PM
brianh brianh is offline
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Default Re: Generator/Inverter?

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
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Old 12-15-2015, 01:47:48 PM
pegasuspinto pegasuspinto is offline
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Default Re: Generator/Inverter?

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....
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Old 12-15-2015, 02:18:23 PM
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BigBlockChev BigBlockChev is offline
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Default Re: Generator/Inverter?

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
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Old 12-15-2015, 02:28:20 PM
len k len k is offline
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Default Re: Generator/Inverter?

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.

Last edited by len k; 12-15-2015 at 09:51:07 PM. Reason: (if you want to keep rpm constant, 60hz)
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Old 12-19-2015, 01:16:11 PM
Jim Marcozzi Jim Marcozzi is offline
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Default Re: Generator/Inverter?

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.
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Old 12-19-2015, 01:51:03 PM
len k len k is offline
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Default Re: Generator/Inverter?

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

Last edited by len k; 12-19-2015 at 02:21:00 PM.
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