|Representative alternator RPM vs. Efficiency graph|
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)