Comparing DC alternators efficiency vs. RPMs.

[thomasonw]

Hello.   Has been some time that I had anything to post, but though this might of interest.  One of the longer term goals of my DC Controller project (http://www.microcogen.info/index.php?topic=2941.0) was to look at how different configuration options might impact overall efficiency.   By using the EGT as a control point proxy (consistent engine loading / output at the same RPMs) I 1st am looking at the difference between a mid-frame alternator spinning  around 6,000 RPMs vs. a very large frame one spinning at 2,600.

As shown in this graphs, Alternators are typically more efficient at lower RPMS: 





Last winter I swapped out the existing 130A alternator (Leece Neville 2700 series)  using a 2.5:1 drive ratio for a larger frame 200A alternator (4800 series) and a 1:1 drive ratio.  Here are the results (remember this is a 12v 'system'):

As a baseline using the 130A alternator I recorded the following:
       Alt:           135A using 2.5:1 drive ratio
       RPM:        2,550
       EGT:         943f
       Output:    1,610W

Swapping out the larger Alternator, changing the drive ratio and then increasing loading until EGT was back up in the 940f range; indicating the engine loading / HP being produced is the same as when driving the 130A alternator.   I now get:

       Alt:            200A using 1:1 drive ratio
       RPM:         2,580
       EGT:          944f
       Output:     1,920W


A 19% increase in DC generator output from the same engine loading.  More details here:

http://mvvikingstar.blogspot.com/2015/05/improving-dc-generator-efficiency.html

--or--

http://tinyurl.com/ktqjq5h


Perhaps next winter I will try to dig up a  Desno "Hairpin" Alternator from a junk yard and see what that gives me.  But for now, am very happy with the increased overall system efficiency this change represents.

-al-

[Dualfuel]

Thomasonw,
300w is a great gain. Why do you use EGT? Could you use fuel consumption? DF

[thomasonw]

Ya, I am happy with the gain in output, cutting the amount of run-time during 'bulk' by 19%.  When combined with the already existing ~5%-7% reduction in bulk run-time due to maintaining a consistent load on the engine at all points of bulk operation (actively managing amps/watts vs. a fixed amp output), this controller / alternator regulator should be reducing overall run-time by upwards to 25% vs. when I was using the more commonly available 'smart' regulator.

It would be interesting to connect up a graduated fuel supply and verify actual engine HP output (vs using the spec sheet values), but I do not have that equipment available - plus it is kind of a PITA plumbing things in.   I used EGT as a control point because I already had the EGT probe installed.  The controller allows me to actively regulator EGT to a target value, so it is simple for me to fix that one variable when comparing configuration changes: engine loading.

And it occurred to me there might be even more good news:  With the active throttle management, slowing down engine speed as load is reduced, we have found we will often let the DC generator complete a full acceptance phase more often then not.  Just so happens as the engine speed slows the system is pushed more into the point of alternator operation with even higher efficiency!

[BruceM]

Thomasonw, a really well done project, very nice data presentation! Surprising the lower rpm efficiency gain was so substantial.  Thanks for the education!   

[mobile_bob]

this confirms my earlier test results,  the 110-555jho large frame alternator has proven time and time again to be very hard to beat in efficiency, most especially when made to charge at 24vdc nominal.

what i found was the faster the the alternator spins the higher the frequency (of course) however the higher the frequency the bigger the losses, the iron core just can't handle the higher frequency without making more heat.

i suspect this is the compromise the oem makes when it decides on lamination thickness and other considerations, such as the clawpoles which don't do well at higher frequencies.

good work!  wish i had more time these days, sadly i am going to be out of play for the foreseeable future.  i am serving a two year term as mayor of this little town and there is just too much to do and no where near enough money to get it all done... so that means i get to clean toilets, mow parks and all sorts of other fun stuff.

what was i thinking! 

bob g

[buickanddeere]

A major factor that only addressed by mobile bob was hysteresis and magnetic path resistance. The lower rpm generator looses less energy to hysteresis . The larger machine also has much larger and larger resistance magnetic pathways to reduce energy lost in developing excessive magnetism in the fields which will be wasted.

[veggie]

Thomasonw,

Great post.
Thanks for documenting that data.
I was always under the impression that my Leese alternator was 50% efficient at it's "worst case", but looking at your graph I see that they can drift towards 40% at the higher speeds. I have my belt drive set to run the Leese at 2000 rpm when the engine is loaded so I assume I am running approx. in the 50% effy area.

Which raises a question for all...

We use a rule of thumb that 2HP can generate approx. 1kw of electrical power.
But that is when we are applying an AC generator which may have an efficiency of 90% or more.

What rule of thumb should we use for HP required to produce 1kw of DC Alternator power?
Should it be more like 4 engine HP to 1KW of DC power ?

veggie

[mobile_bob]

some real world testing numbers from 2007

a changfa s195 driving both a st7.5 (direct drive) and twin 110-555jho alternators, one was used for starter battery charging (which really didn't take much) and the other driven into a 24volt battery bank (12volt alternator via balmar controller into a 24volt battery bank)

the st head worked out to be very close to 78% efficient at making 240vac into a resistive load
the 110-555jho worked out to be right at the same efficiency producing power DC with the rectifier losses on a kw/kw basis.

the losses on that particular alternator are mainly do to copper losses at the driven speed of 3800rpm ,

when one considers the rectifier losses the alternator was working out to be in the mid 80's efficiency wise.

all testing was done on days where the ambient temps were 70 degrees' plus or minus about 1 degree and the tests were replicated many many times over several weeks of testing..

all testing was based on fuel consumption in lbs/kw/hr produced.

having said that, and having the need to go up to 48volts nominal, i was never able to get the alternator to produce power at the higher voltage with anywhere near the same efficiency, mainly due to having to run the alternator at around 6500rpm  and that is where all sorts of issues relating to higher frequency come into play. core losses, windage etc.

i finally decided to run the twin 555's (because the are ground isolated) in series to get the needed 48vdc nominal and maintain the 24vdc nominal efficiencies.

my conclusion was this,  there are certain alternator that can be made to do more than what they were designed to do, and do it with remarkable efficiency.  why folks insist on settling for using those other units that just can't get there is beyond me.

bob g

[thomasonw]

Thomasonw,

Great post.
Thanks for documenting that data.
I was always under the impression that my Leese alternator was 50% efficient at it's "worst case", but looking at your graph I see that they can drift towards 40% at the higher speeds. I have my belt drive set to run the Leese at 2000 rpm when the engine is loaded so I assume I am running approx. in the 50% effy area.

Which raises a question for all...

We use a rule of thumb that 2HP can generate approx. 1kw of electrical power.
But that is when we are applying an AC generator which may have an efficiency of 90% or more.

What rule of thumb should we use for HP required to produce 1kw of DC Alternator power?
Should it be more like 4 engine HP to 1KW of DC power ?

veggie

For 12v, the 'Rule-of-thumb' has been 1HP for each 100A produced when using a 12v alternator.  Or approx 1HP for 1.4KW? 

As Mobile_bob is pointed out, things change a bit when using a '12v' alternator to produce 24v.  Top end efficiency goes up, but one looses all output at lower RPMs (his comment about 48v needing 6500rpms).  There are several research papers out there from the 2000's looking at increasing efficiency of alternators, specifically targeting 42/48v deployments in anticipation of the Auto industry making a change.   Many of them use this basis of in effect a '12v' alternator to gain the efficiency at the higher end (ala, what Bob was seeing), while applying some other technology to boost up the output at idle RPMs w/o having the rewind the alternator and hence loosing  top end gain.

Lots happening here, bottom line I think is:  Our DC generators are a rather different set of needs vs. an auto - specifically with the RPMs required / allowed.  And as such one can easily think that there are hidden gems for us to dig out    Perhaps this one datapoint, and the work of Bob is a start..

[thomasonw]

A nice shiny new HAIR-PIN alternator!

This is a 220A larger stator (138.5mm) Desno unit.  Speced as an option for some of Dodge light and mid-duty trucks, was able to pick up this one on Ebay for under $180.

I am very interested to see how this unit's efficiency compares to slow-turing the large frame Leeve Neville 4800 series alternator I have been using for this past year.  I do know from one of the folks who is running my alternator regulators these Hairpin alts cut in at a very low field drive.










Couple of questions though.   Does anyone know if these alternators have the same charismatic improvement in efficiency at lower RPMs as more traditional designs?   Wondering if I should look to set this up for a 1:1 ratio,  or would it be better to do an RPM speed up.  (Try as I can, I have NOT be able to find any RPM/output curves for these alternators - let alone any RPM/efficiency plots).

2nd question:  It comes with a nice Ribbed belt pulley.   I am under the impression ribbed belts require a spring belt tensioner, but I also have seen some installs w/o it.  In fact, that same person I mentioned above purchased a ribbed belt refit kit, seems the belt was a special version designed to not need a spring tensioner idler pulley.   Does anyone have any insight on this as well.  Or I might just change the pulley and keep the current dual V-Belt setup I have.  Thoughts / ideas???

[veggie]

With regards to the Ribbed Pulley and belt tension.
Proper procedure with serpentine belts is to use a flat surface spring loaded idler pulley on the back (flat) side of the belt.
However, in practice many people use a simple "adjusting slot" belt tension technique when driving only one device. EG: an alternator or generator head.
I have never had any issues with my several machines when using the slotted adjustment system instead of the idler pulley tensioner.
If the driven device (alternator) is close the the power source (engine) there is little room for belt whip and you should be ok with a simple slotted tensioner.
If the distances are excessive the poly-vee belt can have a tendency to whip or bounce a bit. This increases the chance of jumping a rib.
Greater center distances also create alignment issues, if not perfectly aligned the poly-vee belts tend to "climb" out of their ribs in an attempt to self align.
Many Listeroid users have experienced this manifestation as belt that just wont stay on the generator pulley and keeps jumping grooves.

cheers,
Veggie

[mobile_bob]

yes you see a lot of serpentine belts with tensioners, however
the first use of these belts on hd trucks going back to the 60's was in use
on 8v71 detroit diesels used in GMC trucks where the accessory drive was off the rear of the
blower drive, the alternator (a delco 11100080 and similar) was mounted above and driven
with a short serp belt using standard  slot adjustment.


many newer trucks  drive their alternators without using a slack/auto adjuster as well. L and M series don't use
them, as well as the big cat engine's... most that do use them use them in applications where a single belt drives the
water pump, fan drive and alternator.  the reason for using one in that application, comes down to the enormous shock load from the fan when the fan clutch is activated at full engine speed.  having a slack adjuster eliminates a lot of adjustment and the adjustment is assure to be correct via proper engineering and belt length. that way they know the belt is tensioned within reasonable limits and if the thing is still squealing then the belt needs replaced, not more tension which is hard on all accessory bearing.


without a tensioner:
yes you have to adjust them from time to time, but not that big a deal.

auto adjusters just make it so  you don't have to adjust the belt saving maintenance time/cost

there is no other reason to use one.

bob g

[thomasonw]

Hey guys, thanks.   I need to think a little, one reason for changing out the pulley to a dual V-Belt is it would match the other large alternator install on the main engine - and the spare alternator I carry with its pulley already mounted.

I also found some insight onto RPM/output here: blog.dcpowerinc.com/2013/11/01/the-truth-behind-denso-hairpin-high-output-alternators/
Looks like my alternator is an example of their 'Large Frame 270A' model.

It looks like keeping the 1:1 ratio will work, unless efficiency creeps up into the 80% range for this 12v application.  ( I run the small Kubota engine from 1600 - 2800 RPMs depending on the required charging load).

[mobile_bob]

your alternator looks like a pretty well designed unit
it looks alot like the bosch high efficiency unit.

anyway, something to consider

the prevalent losses in these alternators are
1.. stator resistance, so fewer turns result in lower losses
2. rectifier losses, these losses are the same whether you are rectifying 12, 24 or 48vdc
3. windage and friction losses, they are all about the same, however speed has affect
4. iron/core losses, due to hysteresis,  these losses speed has affect as frequency goes up. this is why i like alternators
with short stator cores, and thinner laminations.

just some random thoughts
bob g

[thomasonw]

your alternator looks like a pretty well designed unit
it looks a lot like the bosch high efficiency unit.

anyway, something to consider

the prevalent losses in these alternators are
1.. stator resistance, so fewer turns result in lower losses
2. rectifier losses, these losses are the same whether you are rectifying 12, 24 or 48vdc
3. windage and friction losses, they are all about the same, however speed has affect
4. iron/core losses, due to hysteresis,  these losses speed has affect as frequency goes up. this is why i like alternators
with short stator cores, and thinner laminations.

just some random thoughts
bob g

Hey Bob, thanks for the nice summary.  The alternator is a Denso 'Hair-pin' alternator, part #421000-7001    The larger 138.5mm stator version.

-al-