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Old 15-10-2007, 21:14   #1
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Generators: Clean 110 volt ideas?

I have a small genset -- 2kw/2.5 surge -- which should be enough to run my air and a few other things. I know that the sine wave off of the genset isn't appropriate for computers and other electronics, but upon researching the manual for mine, I see it is recommended that it NOT be used for air conditioners either.

Ok, why? lol

Also, I am thinking about getting a decent size UPC for my computer and TV, and using that for clean power while the genset runs to re-charge it. The UPC should supply clean power, and the genset can re-charge the battery in the UPC in an ongoing basis.

What do you guys think?
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Old 21-10-2007, 07:34   #2
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The problem with running sensitive electronics off of your generator isn't the sine wave, any generator by it's very nature should be producing a true sine wave, it's only inverters that make square waves. The bigger problem with a small generator is the voltage regulation which will jump around quite a bit as loads start and stop, which might be a problem for some sensitive electronics. As for the air conditioner I think it's just a matter of what is the starting current of the a/c? If it's more than 20-25 amps you'll trip the breaker on the generator when it starts.
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Old 21-10-2007, 08:47   #3
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Approximate Watts required to Start & Run a typical motor:
(Typical Starting current is roughly 4 times Running current)
HP 1000 Watts Start, then 215 W to Run
1/3 HP 2,000W / 400 W
HP 2,300 W / 575 W
HP 3,000 W / 750 W
1 HP 4,000 W / 1000 W

A smaller 5, ,000 BTU/hr Window type Air-Conditioner might have a H.P. Motor, requiring about 3 kW to start.
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Old 21-10-2007, 08:58   #4
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The UPC is an excellent idea, both at home and on board, to protect the electronics.
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Old 20-11-2007, 16:23   #5
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power factor

Another reason small generators have trouble with air conditioners is that heavy inductive loads like compressor motors have a very poor power factor. This means that even though it is only drawing 1 kw of power which would be 8.5 amps at a 1.0 power factor, at a 0.6 power factor it would draw 14 amps and the generator might overheat. Low Power factor results in high KVA (volts times amps.) KVA ratings are also on most generators and must not be exceeded.

Rule of thumb for motor loads is the genset max load (KW) must be three times the running load to be able to handle the starting load, and a power factor of 0.6 must be assumed when running air conditioners. More info about power factor is available by googling it. Dino
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Old 20-11-2007, 17:23   #6
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Some good things to think about have been cited above.

One further thing: all a/c compressors are not created equal! Some draw more than others when starting, even when their ratings are the same.

The newer rotary and scroll compressors draw dramatically less when starting than do the older piston-type compressors. The addition of capacitors or other phase-shifting devices can alter the power factor on startup.

Consider, for example, the Matsushita compressors used in Flagship Marine air conditioners:

(from their website...Marine Air Conditioning Systems - water, air and keel cooled.)
Very easy to start - Soft start capacitors, stratified start up sequence and time delay compressor circuits in combination with our rotary or scroll compressors make our A/C units start effortlessly - our 16,500BTU unit should be on a 25 amp breaker!
Very low start up surge -The start up surge of these compressors is a fraction of the dinosaur like piston compressors! Millions of these compressors have been used for many years in window units - the piston compressors simply cannot meet the minimum energy efficiency standards.


I have two of these a/c units on my boat, one is 16,500BTU and one is 12,000 BTU. These are very nice units, chosen by the USCG and the Canadian Coast Guard for their boats. A neighbor of mine, a very experienced Naval engineer, also has two units on his boat.

The above notwithstanding, it is true that a/c units will draw more than you think they will, and you definitely do not want to try to run one drawing anywhere near the rated output power of a generator. In my case, I have a 3.5KW diesel generator aboard, with the high-stability voltage option. I can start and run the larger a/c just fine (rated at 12.7 amps running). I can also start and run the smaller one (rated at 10.8 amps running), but not for long. Together, the two of them are just too much for the generator.

Especially on hot days -- which is when you want a/c anyway -- you risk overheating. A hot 3.5KW generator isn't going to put out 3.5KW for very long (just as a hot 100A alternator isn't going to put out 100A for very long).

And, don't forget the other draws involved, especially the seawater pump(s).

Bottom line: with a good 2KW generator you MIGHT be able to start and run a VERY SMALL and very efficient a/c unit for awhile, but it could well be pushing the limits to add anything else to the load.



Bill
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Old 20-11-2007, 19:32   #7
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regulated voltage

Only very large gensets have sine waveforms. All small gensets have waveforms that have terrible 3rd harmonics and higher. This is not, however, a problem, per se, for powering computers, etc.

One genset type that is an exception to this is the alternator type that drives and inverter internally dedicated to making a sine wave regulated output. They can run at variable rpms according to load demand. Honda is one manufacturer of such types.

Reliable uninterruptible power supplies have been used for years that do not have sine waves. You do not need pure sine waves to reliably drive computers and other devices. You do need good voltage regulation in some cases. Laptop computers today use universal input switch-mode supplies that tolerate a wide range of input voltage and frequency variation. Vitually all of the marine inverter/chargers will drive your computer and other devices.
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Old 20-11-2007, 19:58   #8
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Do a little Google research on the RV/Trailer_camping forums. That is where you will find many long and detailed discussions about small generator running air conditioners.
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Old 21-11-2007, 14:12   #9
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Quote:
Originally Posted by GordMay View Post
Approximate Watts required to Start & Run a typical motor:
(Typical Starting current is roughly 4 times Running current)
HP 1000 Watts Start, then 215 W to Run
1/3 HP 2,000W / 400 W
HP 2,300 W / 575 W
HP 3,000 W / 750 W
1 HP 4,000 W / 1000 W

A smaller 5, ,000 BTU/hr Window type Air-Conditioner might have a H.P. Motor, requiring about 3 kW to start.

Some of the new generators with inverters can start a muxh larger motor.

My old Kawasaki 3000w peak generator can not start my 20 amp table saw. (I start it by lifting the motor up so the pully slips then when it goes I just drop the motor and the pulley snugs up.)

My new Honda 2000 W gen can start my 20 amp table saw.
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Old 21-11-2007, 14:19   #10
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Your 20 Amp Table Saw may not be 20A rated running current (RLA). It may be that the manufacturer specifies a 20 A circuit, which (by code) is sufficient for a 16 Amp continuous current.
Notwithstanding, there are significant differences in the performance and capabilities of differing generators (as your example illustrates).
Please don't take my tabulated starting/running currents as Gospel - they're just a rough guide.
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Old 21-11-2007, 14:29   #11
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yes, it is a 20 amp running load generator, the guy that sold it to me told me it won't start the saw.

The honda with the inverter can start it because it acts like a VFD soft starter, so the in-rush current is minimized.
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Old 21-11-2007, 15:21   #12
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Quote:
Originally Posted by Rick View Post
Only very large gensets have sine waveforms. All small gensets have waveforms that have terrible 3rd harmonics and higher. This is not, however, a problem, per se, for powering computers, etc.

One genset type that is an exception to this is the alternator type that drives and inverter internally dedicated to making a sine wave regulated output. They can run at variable rpms according to load demand. Honda is one manufacturer of such types.

Reliable uninterruptible power supplies have been used for years that do not have sine waves. You do not need pure sine waves to reliably drive computers and other devices. You do need good voltage regulation in some cases. Laptop computers today use universal input switch-mode supplies that tolerate a wide range of input voltage and frequency variation. Vitually all of the marine inverter/chargers will drive your computer and other devices.
^^^^^^^^^^^^^What *he* said.

Especially when it comes to the power supplies for laptops tolerating a lide range of input voltage and frequency. They are very reliable.
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Old 21-11-2007, 15:37   #13
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Radios Tv's are the devices most affected by unclean power, in my experience,,, unless of coarse the are isolated, with a computer style power supply.
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