Tuesday, May 19, 2015

Improving DC Generator Efficiency - selecting the alternator



Representative alternator RPM vs. Efficiency graph
We use our DC generator a lot**, 200+ hours a year.  As such, any improvement in efficiency will be well rewarded. For common alternators it is perhaps a little known fact that they are most effective towards the bottom of their RPM range.  That may not be the highest output, but it is the most efficient.   Look at the graph on the right - peak efficiency (55%) occurs just as the alternator is starting to provide output at around 1500RPMs with perhaps 60A.  A long ways away from max output (110A / 6000+ RPMS), but by that point efficiency has dropped to around 42%

Last winter I changed out the 130A Leece Neville alternator (2700 series) on our Kubota DC generator ( LINK ) for a 200A very large frame (4800 series) one.  I also adjusted the drive pulley ratio from 2.5:1 down to 1:1.  Over the past couple of weeks I have been doing trial runs.

By using EGT as a proxy for engine load at a given RPM (very reasonable for Diesel engines BTW) I was able to adjust the parameters in the integrated DC generator controller ( LINK ),  slowly increasing output of the alternator until I arrived at an equivalent load on the Kubota motor.

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
  •    Efficiency:  39%

(Using the Kubota spec sheet value of 5.6HP continuous at 2600 RPMs and 746W/HP gives me an assumed 4,178W output from the Kubota engine.   Comparing the 1,610W delivered by the alternator gets a calculated 'efficiency' of 39%)

Swapping out the larger Alternator and changing the drive ratio, I adjusted the controller parameters to reflect the new configuration.  I then  increased loading until the EGT was back up in the 940f range, indicating the engine loading / HP being produced is the same now as when driving the 130A alternator.   I now get:
  •    Alt:            200A using 1:1 drive ratio
  •    RPM:        2,580
  •    EGT:          944f
  •    Output:   1,920W
  •    Efficiency:   46%

Results of the slow-turning alternator?   A 7% increase in overall system efficiency, and a 19% increase in alternator output.  Or said another way: a 19% reduction in run time while consuming 7% less fuel to boot! 

Well worth the change out.  Next I might try to use one of the Desno "Hairpin" Alternators and see what more I can gain out of that.


** Running the Generator a lot:   Well, 200 hours a year might not seem like much, esp to those who need to run their AC generator every day.  I suspect this is a function of the DC concept of our boat, and that while running our generator it is working at peak efficiency and peak load, not running under a very light load.


And a final comment:  Yes this now has a 200A alternator, but it is in no way able to deliver 200A; at least not without damaging the engine.   The web is full of stories of these small Kubota engines being overloaded and destroyed due to overloading.  At 1,920W I was delivering 140A while operating a very large 35lb alternator at its most efficient point.  Be aware - most setups I have seen should not be trying to output more than 120A max..  (And I may back off to 900f as an EGT target)

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