Generators - understanding rated output

[Anchor]

I have a small 4 stroke generator called a Honda 2.0 EUi.

It says 2000W in the literature, but dont be fooled, it isnt really - well it is but it isnt. It is RATED to generate 1600VA for sustained periods, can do 1800A for a short period and overloads at 2000VA for more than a few seconds.

In Australia, at 240V that roughs out to 6.7A available.

Now, why am I writing about this in the prep area? I got caught out thats why - luckily I was only testing - please dont get caught with a generator that isnt man enough for the job.

At the weekend I decided I should service and test the generator. This can be a chore, so to make it interesing I decided that this time I would pretend the mains was out and that my big mains UPS needed charging and see if the theory that I could power the big 24V 75A charger from the generator would work out. Initially I had figured 2000W, so 2000/24 =83A - since my charger only goes up to 75A then there should be no probs and my charge current limiter can remain unused and set to maximum.

Wrong. For two reasons.

One the charging voltage is 27.6V, so its not 83A but 72A and secondly the generator cannot SUSTAIN 2kVA output for more than about 3 seconds! I was rather gutted to find out that the most I could sustain a charge at was 60A and that is right on the border line of overloading the generator.

Moral of the story: most generators dont generate as much power as you might think - read the manual and check the rated output.

If you have a big load and cannot limit the current, then you are sh*t out of luck as your generator will not be able to drive it and cut out the output (like mind did), or refuse to run, or try valiantly and take damage; depending on the quality of the unit.

Fortunately I have a current controller installed on the charger so I can lower the maximum demand that the charger can make on the generator. I found that it was comfortably charging at 60A (approx 1656 VA into the batteries) - the charger I have (Victron Energy Skyalla 24V 75A) is about 85%-93% efficient depending on where it is in the load range.

In this case it means that the generator was supplying 1840VA, which was just above its rated output, the manual alludes to the idea that it can sustain overloads for a duration - I only ran the test for 20mins and it was Ok at that. If I had to let it run for a long while I would only charge at 55A which more or less exactly matches the rated output.

Incidently if you know how, then you should also take into account the powerfactor of the load as that can further limit things - especially if you only have a small generator, but I find it hard to explain, and in anycase my charger is power factor correcting and therefore has a powerfactor of 1. So I'm alright jack and quality does pay off. However, lets assume I am driving my old PC with the generator. It uses 300VA but has a cheap (Dell) low quality power supply which has a powerfactor of 0.68, that actually means I need to be have the ablity to generate 441VA, even if the PSU only generates 300W of work with that power.



Confused? Read about powerfactors here: http://en.wikipedia.org/wiki/Power_factor

Bottom line
Measure your loads!
Ensure that your generator really can drive those loads!
Test everything before you NEED it.

A..

PS: You know those low energy compact fluorescent lamps - the ballasts in those lamps usually have poor power factors, this means you end up paying for more power (billed in kW) than you actually use, since the power company had to send you a higher kVA to make those kW that you really end up using !

[Carmen]

Thats good advice Anchor, I will keep it in mind, and do the homework.