View Full Version : Alternator load on small engines

10-22-2004, 10:44 AM
How much load does a large-frame alternator place on an engine?

For small inboard engines (say, 8-20 hp), one can always hear a change in the exhaust note when the alternator grabs and starts charging, but what does that really mean in quantitative terms?

Is the resistance of the alternator a constant, or does it increase as electrical load increases?

Is it possible to "load up" a large (100 amp or so) alternator enough for it to "load up" an engine enough to get it out of the "damaging unloaded idle" range of operation?

Dan McCosh
10-22-2004, 10:50 AM
A 100 amp alternator is making about 1,200 watts of power, which takes a shaft load of about 2 hp. This is a substantial load on an 8 hp. engine. Most alternators don't run for long at this kind of loading, however.

Henning 4148
10-22-2004, 01:27 PM
You better multiply the 2 hp by something like 2 or even 3 because of poor efficiency. With 1.200 watts make it something like 4 to 6 hp max (which at the same time means, that a 120 amps alternator is very very big for a small engine.)

To make things a bit more complicated, the shaft load depends mainly on the electric load, the fan load (most alternators have fans for air cooling) and some other losses (bearings, eddy current, ...)

An alternator may for instance deliver a maximum of 120 amps at 6.000 rpm. At idle speed it will deliver much less (perhaps only 10 or 20 amps, if it is running too slow it will not charge at all). It may be allowed to spin up to 15.000 rpm or something like that (don't rely on this figure, it is different from alternator to alternator!!!!) without putting out much more than the 120 amps (ok, say a bit more, but not that much). As the fan load increases drastically with the rpm although you don't need this additional cooling at high rpm, the actual shaft load will depend not only on the electric current, but also on the rpm the thing is running on.

Good efficiency (say around 50 or 60 % under full electric load) of an alternator can be expected on something like 6.000 rpm or perhaps a bit below. Above, it will get pretty poor because of high losses (fan and eddy current).

This means, that for best efficiency you got to match the belt drive ratio to suit the engine and the alternator.

On a boat with a Diesel this could mean something like 1:2 into the fast for the alternator, at idle speed (something like say 1.000 rpm for the engine) you could probably still get a few amps out and at 3.000 rpm which would be a good engine speed for passage making you would have 6.000 rpm on the alternator and get max. 120 amps. With the max. rpm of the engine assumed at 4.500 you would only have 9.000 rpm on the alternator which would still be ok.

Now, if you have an old 1 cylinder slow thumper, you need a higher ratio.

Also if you want to be able to charge the battery at idle or low revs, you would want a higher ratio. Say your engine has a max of 4.500 rpm, your alternator 15.000, then you are safe with a ratio of 1:3 and at 2000 rpm engine speed the alternator is already deliverying his 120 amps max. The price you would pay for this is higher losses (higher fuel consumption) when passage making under engine at say 3.000 rpm.

All the above are figures for alternators built for cars. If you start changing the belt ratio always check, that your model is allowed the rpm that will result at max. engine revs.

10-25-2004, 08:19 AM
I have a 4000 watt 8 hp generator which definately loads down when things start getting switched on. I dunno if this transfers to an aternator, but if 8 hp can run a 4000 watt generator I'd think an 8 hp inboard could handle a 1,200 watt alternator....

Dan McCosh
10-25-2004, 08:30 AM
I think the question was whether a belt-driven alternator on small diesel would soak up enough horsepower to keep the diesel from having problems related to extended unloaded idling. The answer is that 100 amps of charging would likely accomplish this. The problem I see is maintaining a relatively heavy charging load. As the batteries are charged, the rate diminishes, and the last half or so of the charging cycle isn't going to load up the engine much, regardless of the alternator capacity. I think sailors faced with this situation off shore just put the engine in gear and motorsail while the batteries are being charged.

10-25-2004, 08:40 AM
Yeah, Dan you've got a good grip on the question. It actually came about like this....

The boat (not mine) has already been hauled for the winter, but the engine needs to be brought up to temp and run under load for purposes of winterization.

If you load up the electrical system (turn on all the lights, run all the pumps, leave the refrigerator door open, and plug one of those 500,000 candlepower spotlights into the cigarette lighter outlet) will this make the engine work hard enough to open the thermostat?

My instinct is that it will, but I also kinda wonder if the alternator will be suffering undue damage in the process.

Dan McCosh
10-25-2004, 08:52 AM
If all that is being accomplished is opening the thermostat, how hard is it to take out the thermostat? Assuming the idea is to add antrfreeze, etc. The loads you describe still don't add up to a lot, compared to a deeply discharged battery bank of, say 180 amp-hours or so. The alternator itself should take this kind of load for hours, since that's what it is designed for. I have a 100 amp alternator, and it runs for a half hour or so at 100 amps before the charge rate drops with a fully discharged battery. The caveat here is that I have a heavy-duty marine alternator. A typical 100-amp automotive alternator usually is not designed for sustained loads.

[ 10-25-2004, 09:54 AM: Message edited by: Dan McCosh ]