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?
Great question.
Casey
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?
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
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.
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:)
Quote from: Thob on May 01, 2011, 03:12:28 PM
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!
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..
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
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
Great information everyone, as usual. ;D
Quote from: mobile_bob on May 01, 2011, 11:23:48 PM
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?
3 phase heads are almost always more efficient than a single phase head of similar kva and type
bob g
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
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
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
Hi All,
I'm going to propose that in a 3-phase, each phase is only energized 1/3 of the time so you have less heating losses, and less hysteresis losses.
While the rotor accounts for much of the above losses, the stator still has a portion of those losses.
Lloyd
Just thought this was interesting, here are replacement fans for the ST heads:
http://stores.ebay.com/Central-Georgia-Generator/Metal-Fan-/_i.html?_fsub=1076849012&_sid=54603752&_trksid=p4634.c0.m322
Looks like the same fan is used on 5-12kw, 15-24kw, and 30-50kw gen heads.
eny fool should know that a gen has to spin at a set speed .to get corect volts out.next the size of driving moter will give you usible amps.bigger the drive force in the more amps out.up to the size of the getset
Lloyd:
I don't think it has anything to do with the phases only being energized for 1/3 of the cycle, since each phase is a, more or less, perfect sine wave that goes from zero to + 120 to zero to -120 to zero, on each revolution of the rotor, or on each half rotation for four pole, 1800 rpm units - so if you just look at one phase, it SHOULD look exactly like the single phase in an ST head. For three phases, they just "stack: three of these "simple" sine waves at 120 degrees to each other.
The above was the reson that it seemed strange to me that a 3-phase should be more efficient than a single phase.
BioHazard:
If you look at all of the specs for a range of ST-STC generator, you will discover that they have more in common than just the size of the fan - like, for some lines, the 15 kW, 20 kW and 24 kW all have the same shaft size, like wise, lots of different size units have the same size brushes, etc., etc.
I THINK that this is just to make life less comjplicated for the guys assembling the units . . . so for the 15 kW / 20 kW / 24 kW example, they just used a shaft that was "adequate" for a 24 kW unit and way over "designed" (word used advisedly!) for the 15 kW unit - the cost of making the shafts "too big" for the smaller units must not be as great as the cost of stocking/having/inventorying exactly the "right" size shaft.
All of which lends credence to the theory that they don't really engineer these things to an exacting spec, but rather just use lots and hope it is enough - clearly demonstrated by the fact that a commercially made US generator is about half the weight of an ST/STC unit. If you were "real" concerned about the cost of materials, you could easily take 1/3 of the weight out of the unit - albeit at the sacrafice of some of the reliability of having the unit exceptionally solid/firm/unyielding.
That's my take, YMMV! <grin>
Regardz,
Wayne Stayton
Quote from: WStayton on May 04, 2011, 07:10:57 AM
Lloyd:
I don't think it has anything to do with the phases only being energized for 1/3 of the cycle, since each phase is a, more or less, perfect sine wave that goes from zero to + 120 to zero to -120 to zero, on each revolution of the rotor, or on each half rotation for four pole, 1800 rpm units - so if you just look at one phase, it SHOULD look exactly like the single phase in an ST head. For three phases, they just "stack: three of these "simple" sine waves at 120 degrees to each other.
The above was the reson that it seemed strange to me that a 3-phase should be more efficient than a single phase.
unyielding.
That's my take, YMMV! <grin>
Regardz,
Wayne Stayton
Wayne,
Stop and think for a minute. 2 gens of equal output 1) single phase, and 2) 3 phase.
The 3 phase you can make the same power on smaller size conductor windings, with less heat losses, and associated magnetic losses.
Why you ask, bc you are carrying the same volt/amps over three different circuits, each phase is only energized 1/3 of the time, to make the same power.
Lloyd
3-Phase Efficiency = 746 x Horsepower
Volts x Amperes x Power Factor x 1.732
Single-Phase Efficiency = 746 x Horsepower
Volts x Amperes x Power Factor
Lloyd:
Join me here for a minute in a little "thought experiment". . .
Suppose., on one hand I have a 15 kW STC 3 phase generator and on the other hadn, I take three eash 5 kW ST generators and "weld" their shaft together so that they are 120 degrees out of phase with each other.
If I combine the output of the 3 ea 5 kW heads into a three wire bundle, do I not have 15 kW of three phase power???
There will be the same current in each phase of both set-up, presuming I l;osd them with the same load.
Why would the three generators welded together be less efficient than the single 3-phase generator, neglecting the extra bearing loads - presumably they would have the same size wire in their windings, etc., etc.
Doesn't any generator have the wire windings sized for the current that they are going to carry, so then, the losses SHOULD be the same, no??? If I run 30 amps through a 1,000 ft of # 10 wire or 20 amps through a 1,000 ft # 12 wire aren't the losses the same - more or less??
Next question: Where did the "1.732" cpme from in your efficiency eqution for a 3 phase generator??? If that is true, then a three phase generator should be 43% more efficient than a 3 phase unit - is that right?? Remember, I'm a MECHANICAL engineer and only just barely belive in electrons which I havn't ever seen (But have FELT!) <grin>
Explain please, I WANT to get less dumb!! <smile>
Regardz,
Wayne Stayton
Lloyd is correct. A 3-phase generator (i.e., a 3 wire polyphase system with square root of 3 power advantage...1.73) is more efficient than a 1-phase generator simply because a 3-phase uses significantly less conductor material than a 1-phase generator. Using less conductor material means less conductor resistive losses (i.e., less I**2 x R losses) to generate the same power...therefore, more efficient. Or another way of looking at and quantifying it, 3-phase transmits 73% more power but only uses 50% more wire...therefore, 146% more efficient. 3-phase was also invented by Nikola Tesla, so there is little doubt that it is a better approach for power generation and why it is so widely used worldwide. I am a mechanical engineer and a electrical engineer working for an aerospace company my entire career. ;)
Bob B.
Quote from: WStayton on May 04, 2011, 07:40:58 PM..... Why would the three generators welded together be less efficient than the single 3-phase generator, neglecting the extra bearing loads - presumably they would have the same size wire in their windings, etc., etc. .....
You now have 3 field windings, which is 2 more than 1. That's a good chunk of power.
rm /
Here's one way to look at the issue of efficiency of a three phase generator vs single phase. A certain amount of power is lost in the field winding, this power is heat generator by current flowing thru the resistance of the wire. If this is DC (see the thread on ST waveforms for what happens when it's not) then the power is constantly being wasted. In a single phase generator, you have a part of the time when the waveform goes thru zero that the generator is not generating any power, but power is still being wasted in the field. In the three phase setup, due to staggering the power windings, there is always some winding generating power. So slightly less waste.
In the case of using 3 single phase generators to make 3 phase, you have 3 field windings, each one wasting power, so you wouldn't get this advantage.
As for rectifying the output and using an inverter, yes that is done on small generators. Search for inverter generators. I think they usually employ permanent magnet 3 phase generators, rectify that, and then use an inverter to create the AC. The other nice thing about this is that the frequency is now controlled by the inverter, not by the shaft speed. So the engine RPM can be varied to match the load, and the inverter pulse duty cycle varied to get the proper voltage. But you have to include the inverter (in)efficiency in the equation. People also use a 3 phase head to charge batteries (thru a rectifier) and then run an inverter from that.
Quote from: deeiche on May 05, 2011, 08:23:49 AM
I'm pretty ignorant when it comes to generators so this most likely is a stupid question.
Given the greater efficiency of a 3 phase generator can you rectify it's output, then combine the 3 DC sources in to an inverter?
You just described an automotive alternator. They are a 3 phase AC generator, with the outputs rectified and combined. The beauty of combining and rectifying 3 phase is that because you have 3 times as many half cycles, once rectified, they overlap, so the valleys are much smaller, and require much less filtration to get a clean DC output. Here is what single phase looks like rectified:
(http://i270.photobucket.com/albums/jj85/rmarlett/th_singlerect.jpg) (http://s270.photobucket.com/albums/jj85/rmarlett/?action=view¤t=singlerect.jpg)
Here is what overlapping 3 rectified phases does to the waveform:
(http://i270.photobucket.com/albums/jj85/rmarlett/th_3phaserect.jpg) (http://s270.photobucket.com/albums/jj85/rmarlett/?action=view¤t=3phaserect.jpg)
But since it is pure DC below the bottoms of where the valleys meet, you are left with this waveform which is nearly pure DC with only a little ripple to clean up...
(http://i270.photobucket.com/albums/jj85/rmarlett/th_final3phase.jpg) (http://s270.photobucket.com/albums/jj85/rmarlett/?action=view¤t=final3phase.jpg)
I havn't tried it, but with the supposed efficiency of induction motors used as generators, a 3 phase induction motor with the output rectified should be a pretty efficient way to make DC. Inductions as generators do not have any way to easilly regulate the output, but with fixed caps to provide an acceptable output that could be rectified and fed to an inverter, that might be the ticket. With the variable input voltage of solar systems, the tech is already available to deal with this, you just need to select capacitors to get the output of the induction motor into a range that is acceptable to the inverter.
I sure wish someone had an off the shelf system that even I could hook up to my 8 HP Witte and produce about 350 VDC to feed the Sunny Boy. Its getting close to crunch time as the back hoe is coming in next week to start the generator building hole. If I get right on it I'll be producing electricity after dark in about two years. ;D
Casey
LowGear:
It sorta looks like you operate with the same "sense of urgency" that I do! <grin>
Glad to see that I'm not the only one who like to sit back, take aseond, and a third, and . . . look to make sure I'm not screwing up! And I STILL manage to screw-up occasionally! <smile>
About the increased efficiency of three-phase heads over single-phase heads: I THINK I am finally beginning to understand why a thrre phase is more efficient - now, don't anybody say anything and confuse me AGAIN!!
Thanx for all the input to my confused musings! See, if you point enough and shout enough and pat on the back (low enough!) even a mechanisal engineer gets it . . . eventually!
Regardz,
Wayne Stayton
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
http://www.allaboutcircuits.com/vol_2/chpt_10/5.html
Quote from: LowGear on May 05, 2011, 09:06:57 PM
I sure wish someone had an off the shelf system that even I could hook up to my 8 HP Witte and produce about 350 VDC to feed the Sunny Boy. Its getting close to crunch time as the back hoe is coming in next week to start the generator building hole. If I get right on it I'll be producing electricity after dark in about two years. ;D
Casey
Casey,
Are you sure the 350 is the Vmin, and not the Vmax? Is it required DC or AC?
If it is Vmin a rectified 240 almost gets you there 3 phase 240 = 345.6 Vdc add a 1..25-1 step up transformer before before rectification, and you'll be 518 Vmax
I think there are many ways to skin your cat, if you know what type cat it is? Do You?
Ya got an owners manual with V requirements, what amps out are you looking for and at what voltage?
Lloyd
Lloyd,
Ask and you shall receive. I don't why I settled on that 350 VDC but it should work as my attachment shows (I hope). Your review would be appreciated. Three phase generators are very plentiful and not really that more expensive.
Casey
As always when I ask a question here I get information I never even vaugely thought of before. ;D
I guess if you only plan on running 120v stuff from your generator it seems like a 3 phase generator has advantages over a single phase, and no disadvantages...am I wrong?
Oops,
I should have shown you the output as well on the SMA 6000US. As you can see this is a 240 VAC, in my case, inverter.
Casey
rm /
Quote from: Thob on May 05, 2011, 08:42:44 AM
Here's one way to look at the issue of efficiency of a three phase generator vs single phase. A certain amount of power is lost in the field winding, this power is heat generator by current flowing thru the resistance of the wire. If this is DC (see the thread on ST waveforms for what happens when it's not) then the power is constantly being wasted. In a single phase generator, you have a part of the time when the waveform goes thru zero that the generator is not generating any power, but power is still being wasted in the field. In the three phase setup, due to staggering the power windings, there is always some winding generating power. So slightly less waste.
In the case of using 3 single phase generators to make 3 phase, you have 3 field windings, each one wasting power, so you wouldn't get this advantage.
Very clever and astute observation!
Perhaps related is the advantage of running a motor at 240 Volts rather than 120 Volts. Half the Amps means less heat and less heat mean greater efficiency. You guys know you're way over my head but sometimes simple is good too.
Casey