MPPT for Stirling Engines

[NerdyEE]

Hi Vamoose!

I agree on your observations of various home brew generating systems and had fun trying to get something to couple to the JS002 Toys SE setup. As I told you privately, I bought the JS002 instead of the units with LED’s built in, because I wanted to couple it to a Maxon motor I used with my older daughter to demonstrate electrical generation and current direction (anti-parallel w/ different color LEDS; turn motor one way one color, other way other color). The SE was to be with my younger daughter for her Science teacher. The Maxon simply would not work due to the gearing and friction…

Regarding copper losses and other listed inefficiencies, this is more critical on large power systems or if you plan on making the generator from scratch. Definitely something I didn’t have to worry about with my 1/40 Watt system. I was however criticized for not doing my own generator on the flywheel. I don’t have a nice home workshop, just my home office that my wife has allowed me to play with this “MESSY” habit. So there is a lot to be said about using “off-the-shelf” or out-of-trash motors as generators…

Further points to ponder (or things I discovered in my experiments)…

Mechanical Coupling/Transmission- This is one of the games you have to play as speed and torque need to get coupled to the generator. And losses here were critical for my sub-watt system. I was unable to use a belt and by luck found I could use a contact point on the flywheel and spin a DVD spindle motor.
Generator Power –v- SE Power – Bottom line here, the motor used as a generator (or backwards) must be able to overpower the SE or the MPP point cannot be found/achieved! If the generator is underpowered, the MPPT will just get stuck at max duty cycle. So in my setup, because shorting the motor windings would most often stall the SE, knew I had the right combo to challenge the VMPPT…
MPPT Power (input voltage and current) – Motors/generator windings will still need to be chosen because input speed = output voltage. But the MPPT will also need to be sized and have the inputs specified. Since we would rely on Solar MPPT units, we have to conform to their input limits on voltage and power. For me I was stuck with a motor that I couldn’t directly power a LED. The voltage was really low and I doubted I could ever design something that was self powered on this low of power. But in my MPPT knowledge quest, I found the ST SPV1040 chip that had input specs that magically matched the outputs I was getting with the motor.

Actually I got very lucky on all of the above with my tests…

So for example, say I have a SE with 58cc of displacement (hmm, where does this number come from?). Despite the 2BTU claim, using Ian’s rule of thumb I might expect 58Watts of mechanical (WAY more than I need for one of my applications, borderline another). So I would look for a motor/generator size that would generate that much mechanical power (skip past unit conversions!). Think someone also posted that ½ open loop RPM is near MPP and my experiments confirm this as a good approximation. So that has me at approx 500RPM but must handle the open loop 1000RPM. Now I need a MPPT of approx 60Watts (doubt I need this much power as you mention on other mechanical to electrical efficiency posts). Can you guess what I have been gathering hoping a miracle happens and it actually arrives?????

I will see if I can bugger the contact coupling of the JS002 motor to effectively change the gearing ratios. Or maybe I can find more scrap motors…

Cheers,
-NerdyEE

[vamoose]

Howdy NerdyEE,

So you know, from my perspective you’ve done an awesome job 'Full Stop'.
Let alone with the the components you had at hand. There is no way I could have attempted this and I would challenge anybody to achieve such results working off the cuff. I look forward to seeing the results of any buggerisation of components that you undertake. As I do with all your posts.
I will continue to put my random thoughts and ideas in as they come to mind. Most of them will just be in general relation to the topic as I think of them, and also to raise discussion on concepts for whoever reads them and thinks it pertinent to respond..

I really hope that 58cc's comes your way. I can only imagine how much fun you will have playing with it!!

vamoose

p.s. I am still blown away and coming to terms with the results, and think they are Epic!!