Gen head efficiency - big vs small?

[BioHazard]

I understand that with power inverters, they are most efficient when running near max load, rather than a small percentage of the max. Is the same true at all for generator heads? Let's say I put a 25kw gen head on a 10hp engine....would it still make roughly 5kw before killing the engine, or would the efficiency be nearly the same as using a 5kw gen head instead?

Additionally, could I expect an oversized gen head to last longer running at partial output than a smaller one running full out?

[LowGear]

Great question. 

Casey

[mbryner]

Without real calcs, the larger gen head will require slightly more fuel to rotate, so will be less efficient at lower load/smaller engine just because of that.  On the other hand, won't  it last longer because of heavier windings?

[WStayton]

BioHazard:

 I THINK that a large generator runnning at a small percentage of its output has to be less efficient than a smaller generator running at a larger percentage of its output, if for no other reason that cooling air flow.

 The larger generator has a larger cooling fan so it can flow engough air through it to keep it cool at a higher power output and that fan is DUMB, in that it doesn't know how much power is being made, so you have the whole, larger, cooling load all the time, whether you need it or not.

 But, having said that, I don't think it is an immense load, because for a 24 kW generator head, Gergis Generator specifies a miniumu of 35 HP which works out to just a smidge over 26 kW.  So, if it takes 35 HP to get 24 kW, about 2+ kW worth of heat is given off.  Of course, some of this is bearings and electrical resistance3, but I would expect those to be more nearly the same for different capacity generators operating at the same kW.  Maybe 50/50, fan-loss/heat-electrical?  

 For a 5 kW unit they specify 7.5 HP, which leaves a little LESS than 2 HP or 1.4 kW for non-electrical, so there is a differance of about 0.60 kW between the two of them - again if that was split 50/50, that would be .30 kW fan loss vs 1.0 kW for the 24 kW unit, so they are eating something like .7 kw MORE in the fan of the bigger generator??

 Of course, this was al based on a scientific wild-a$$ed guess - YMMV!   <grin>

Regardz,

Wayne Stayton

[Thob]

To find the answer, you need the generator efficiency curves for the particular generator you're interested in.  Here's a set of curves from one generator:

http://www.centralgagenerator.com/generators/brushless/yhg/manual/manual.pdf

The efficiency peaks around 50% load, below that it falls off like a rock (note the curve starts at 25% load).  In addition to the fan loss that Wayne mentioned, you also have the loss associated with powering the field winding.

Different style generators likely have different curves.

As far as lifetime, that depends on what fails.  Bearings, for example, might last a little longer due to lower temperatures in a lightly loaded generator, but not much.  And who knows about the voltage regulator?

My advice would be to stay between 50% and 80% of max load.

[Ronmar]

Mobile Bob and I explored this in some detail a few years ago.  Yes, a lightly loaded generator is less efficient.  The biggest kicker is probably going to be field excitation.  You need a certain ammount to get rated voltage output, and a larger genset with larger field windings will require more current to properly excite them.  In our fuel burn tests, we determined that the first KW of electricity is the most expensive, with each succesive KW costing less and less in terms of fuel.  Bob worked out a calculation for that, and was able to call, if I recall correctly, my last 2 full load (for my 6/1) fuel consumptions to within a small fraction of a GPH...

Another kicker is that a larger generator has a fixed cooling fan that delivers sufficient airflow for that head to deliver full power output and not overheat.  Lightly loaded, you are moving a lot of wasted air, and that costs fuel...

Also an inadvertent high load will probably stop your engine dead in it's tracks, and you may experience drive problems.  If I recall correctly Jens had a lot of issues with driving a really large head on his 2 cylinder(belt chirping and excessive wear ect...).

As mentioned, a genset loaded to 20 percent will probably last forever though:)

[cgwymp]

To find the answer, you need the generator efficiency curves for the particular generator you're interested in. 

Anyone have a set of curves for ST-family generators?

Cheers!

[Horsepoor]

I would think that if you know the large generator is goingto limited by a very small prime mover (I.e. 6/1 on ST 15 head), then why not block off 50% or 75% of the air intake so that the cooling fan has less "air" / mass to bit into and thus move?

Of course, it would be wise not to block off to much, but it seems to me some reasonable estimates on the effect of air flow reduction could be made by measuring the resulting temperature change or lack there of..

[WStayton]

Horsepoor:

  I don't think simply blocking off the inlet to the cooling fan is going to help much - you will be making the fan run in a "thinner" environment, so you will then increase the kinematic viscosity of the air that IS passing through it and probably not affect the power absorbed by the fan by much at all - in fact, if you sealed it off well enough, it MIGHT use more power, not less!  But no matter what direction the power change is, it will be pretty small relative to the total load of the fan so I don't think you will get much help by blocking off the inlet.

  If you REALLY think that you will never need the additional cooling, a better solution might be to chisel off half of the blades on the fan - of course that will introduce balance problems, so you've only really traded one problem for another! <grin>

  Again, based upon Scientific Wild A$$%d guess - YMMV.

Regardz,

Wayne Stayton

[mobile_bob]

here is the reality folks

on single phase st heads they range from about 78% efficient (st5/st7.5) to bit less than 80% (st 12/st15) and likely
are not much over 80% even up to the larger st20-30kva models

this follows the efficiency  curves of first world alternators of similar design that are single phase, its not until you get
to around 50kva and larger that you get to somewhere around the mid 80's in efficiency.

if you want higher efficiency, you gotta go with a modern design head (read that very expensive) or go with an stc three phase head.

there is just not much difference between the st heads we typically use, the field excitation is very hard to measure as it equates to bsfc
and the fan is even harder to quantify. 

in testing (and no i haven't duplicated or confirmed this) the st12 head is a bit more efficient than the st7.5, however it is very hard to measure
accurately, most folks will not be able to tell one way or the other.

what one can tell for sure, and it is measurable is the bsfc depending on engine load, the closer one gets to full rated load the more efficient their engine/generator will be (generally).

bob g

[BioHazard]

Great information everyone, as usual. 

if you want higher efficiency, you gotta go with a modern design head (read that very expensive) or go with an stc three phase head.

Interesting...are smaller sized 3 phase gen heads more efficient at producing 120v than a single phase unit?

[mobile_bob]

3 phase heads are almost always more efficient than a single phase head of similar kva and type

bob g

[WStayton]

mobile_bob:

  You Said: "3 phase heads are almost always more efficient than a single phase head of similar kva and type"

  Any idea of WHY this is true?

  Conceptually you are passing a coil, or three coils, through a magnetic field - its hard for me to understand why it takes less force, per coil, to wave three coils through a magnetic field than it does to wave one coil through a magnetic field . . .

  Remember, I'm a MECHANICAL engineer, so draw big pictures, carefully labeled!!  <grin>

Regardz,

Wayne Stayton

[mobile_bob]

better distribution of wire in the stator
giving a better power density per unit of iron

to be perfectly honest i am not sure of all the reasons, other than it is generally true

some things in life i don't have the time to question, and just accept as fact without proof.

sometimes i get bit, but i don't think this is going to be one of those times.



bob g

[WStayton]

mobile-bob:

  I'm not questioning that 3 phase heads are more efficient than single phase heads - I've seen that same information in other places, so it IS true - I'm just curious as to why!  <grin>

  If it has to do with the winding density, I would THINK (always dangerous) that the manufacturere's would have found some way to wind single phase heads to the same energy density as 3 phase heads - of course with ST heads I don't think that there has been a whole lot of engineering beyond "hang a coil on some bearings and go", so maybe I just answered my own question!  <smile>

  Thanx for indulging me!

Regardz,

Wayne Stayton