Generators and non-linear loads

Sun Dog · July 2011

So I thought I had a handle on how to size the generator for my system and then I find out that charging batteries is a non-linear load and is hard on the generator head. My original goal was to use the smallest 1800rpm diesel generator I could find that would allow me to charge my battery bank and keep the house running. If I do that it sounds as though I will likely end up giving the generator head a pretty hard life.

The best solution seems to keep the non-linear loads to a minimum (don't charge the batteries at the max rate) and get a generator with an oversized head to deal with extra heat and run it with a smaller motor to keep the fuel consumption down. I'm guessing this will mean a higher price vs an off the shelf design so one could argue to buy the off the shelf unit and use the savings to pay for a lot of diesel. Any other solutions out there?

Cariboocoot · July 2011

Re: Generators and non-linear loads

Ever heard the phrase "over-thinking a problem"? :roll:

Buy a gen set that meets the needs. Where maximum loads are 50% - 75% of the capacity. Don't run it for long times lightly loaded (especially if it's diesel). Stop worrying.

A large number of off-gridders use generators for back-up/supplemental charging with no problems.

Bill will be a long in a minute to link to the choosing a generator thread.

BB. · July 2011

Re: Generators and non-linear loads

Actually "AD" and I have been having a long email discussion about this issue...

For me, I believe there is a big difference between the average non-power factor corrected battery charger (PF down to 0.6 or so) and a Power Factor Corrected battery charger (PF around 0.95).

I was looking over some generator head specifications from Hardy Diesel and it appears that the kWatt rating of a genset at PF not equal to one derates really quickly... 0.8^2 = 0.64 kW derating. 0.6^2=0.36 kW derating.

And this confirms what one dealer was telling Arctic... To run a genset at 3x the battery charger watt rating...

But this gets us to the whole issue of "wet stack"/coking/cylinder wall glazing when the genset is operating at 40-60% or less of engine power rating.

Their suggestion was to put some electric heaters on the genset to help better load the unit (OK in winter, not so good in summer).

Any way, I suggested he come here and ask his questions. AD already has a Schneider/Xantrex XW 4048 on order (does have PFC charging). So--looking for others real life experiences and suggestions for 6-10 kW diesel gensets. The two vendors (that I know of) that he has spoken with:

www.northern-lights.com
hardydiesel.com

Obviously, he has some dark winters in the Arctic (as I recall, he is losing his Grid Power from the Utility--and it is beyond his power to change that decision)--So a reliable installation is probably even more important than raw fuel use.

-Bill

Cariboocoot · July 2011

Re: Generators and non-linear loads

Change the order from the XW to a Magnum which has the PF corrected charger? Cheaper solution, I would think, than a custom-fit generator.

Any generator is going to need maintenance. Just keep on top of it. Have a back-up gen if your really generator dependent. Heck, I've got four generators! All work, all have lots of hours (one ridiculously so), all have had minimal servicing. None are diesel, though.

I suspect the whole wet stacking issue is one of using diesel gens insufficiently loaded as prime power: long hours of running without getting up to temp. I used to have a diesel that needed to be revved up before you shut the fuel off or else you'd not get it started again due to the excess fuel in the cylinders (acts as coolant and keeps the temp down so it won't reach combustion temps). Similar problem.

One fellow here, oil pan 4, has experimented with propane fumigation on diesels to keep combustion temps up. Might be something to consider, especially in arctic climes ('cause you can't by ether in 55 gallon drums

).

Sun Dog · July 2011

Re: Generators and non-linear loads

Morning guys,

I fully admit to the paralysis of analysis, it's just how I learn. I have followed the lead from both of you and picked up a EU2000i and already have another 5.5kW gasoline generator. As Bill mentioned, it just gets a little confusing when a generator manufacturer says something that appears to contradict with the direction I was planning on going.

Not that it matters, but for just for clarity it is the XW4024 that I have gone with. Like the Magnum line it has a PFC charger.

dwh · August 2011

Re: Generators and non-linear loads

These guys build some pretty sweet gensets.

http://www.nextgenerationpower.com/

Their smallest I think is a 300cc Kubota single cylinder engine driving a 5kw Markon (Cummins) head. Using differential pulley sizes they reduce the engine RPM to 2800 and still get a 3.5kw continuous rating out of the unit. Between the engine's rather beefy flywheel and the oversized alternator, it can obviously handle surges up to 5kw without a problem until the flywheel momentum bleeds off and the engine starts to slow down.

Can be had in several variations, including marine:

http://www.nextgenerationpower.com/3kwlow.html

Here's some pics of both sides:

http://www.hackneys.com/mitsu/photos/buildup_album_16/image007.htm

http://www.hackneys.com/mitsu/photos/buildup_album_16/image008.htm

mike95490 · August 2011

Re: Generators and non-linear loads

dwh wrote: »

These guys build some pretty sweet gensets.

http://www.nextgenerationpower.com/

Their smallest I think is a 300cc Kubota single cylinder engine driving a 5kw Markon (Cummins) head. Using differential pulley sizes they reduce the engine RPM to 2800 and still get a 3.5kw continuous rating out of the unit. Between the engine's rather beefy flywheel and the oversized alternator, it can obviously handle surges up to 5kw without a problem until the flywheel momentum bleeds off and the engine starts to slow down....

The fine print taketh away:
3.5 kw, 120VAC, modified sine wave, electronic voltage regulation.

dwh · August 2011

Re: Generators and non-linear loads

mike90045 wrote: »

The fine print taketh away:
3.5 kw, 120VAC, modified sine wave, electronic voltage regulation.

If you got that from the caption under the pic on Doug Hackney's site - that's a typo on his part. There is no such thing as a modified sine wave *alternator*.

RCinFLA · August 2011

Re: Generators and non-linear loads

dwh wrote: »

If you got that from the caption under the pic on Doug Hackney's site - that's a typo on his part. There is no such thing as a modified sine wave *alternator*.

Possible on inverter-generator but I doubt the Hackney site picture is DC alternator.

The pictures do show a strange pully ratio. Engine pulley is slightly larger then alternator pulley.

This would put generator rpm lower then alternator. It is usually the other way around with 1800 rpm four pole alternator run from diesel at 2000 to 2400 rpm to get up to a more optimum HP on diesel. Pully ratio doesn't look great enough to be driving a two pole alternator at 3600 rpm's unless diesel is running close to 3000 rpm which would be a bit high for a diesel. Maybe this is the case for the small HP diesel.

I know the Chinese make 5-6 kW portable generators running 10 HP diesel at 3600 rpm but I just feel a diesel with high compression ratio running at 3600 rpm is not going to be a very long lasting engine. Does anyone own one of these? How do they hold up?

mike95490 · August 2011

Re: Generators and non-linear loads

dwh wrote: »

If you got that from the caption under the pic on Doug Hackney's site - that's a typo on his part. There is no such thing as a modified sine wave *alternator*.

Well, I'm just reading whats there. And it's not a typo, it would be a mis-statement.

Dave Angelini · August 2011

Re: Generators and non-linear loads

One of the best Marine diesels ever built was the Isuzu 4AB1/C240 and the whole series of 4, 3, and 2 cylinder engines spun at near 3000 RPM. all of these were less than 50HP. 10,000 hours easy!

Sun Dog · August 2011

Re: Generators and non-linear loads

Thanks for the input guys. FWIW I ended up going with an off the shelf 10kW Perkins unit from Hardy Diesel. Seemed like a reasonable compromise as it has a bigger alternator than their 8kW but uses the same engine. Surrette says the max charge rate for my batteries is just under 11% at the 20 hour rate. If that seems like too large of a non-linear load (assuming I can tell :roll:) I could it lower the current and run the genni longer. The minimum charging current is ~5% based on the 20 hour rate.

Generators and non-linear loads

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