Solar direct drive kayak

I need some technical advice on building a custom buck converter. I would like to build one that is optimised for a very specific output load. I found the equations to calculate component values for a simple synchronous buck converter. What is the simplest way of driving the two MOSFETs?
I already have some 555 IC's so would prefer if the pwm circuit can be built using one (or more if required).

Just some background on the project:

I built a small electric drive using an 18V motor and gearbox from a cordless drill.
I modified my kayak to take 3x 50W panels - also 18V - and took part in a local solar boat race. The panels were wired directly to the motor and the aim was to select a prop size that would simultaneously load the motor and panels to their max efficiency.

With the gear reduction as it is, this results in a prop that is itself not very efficient. I need to lower the voltage to the motor so that I can turn a slower, larger prop. I also want to experiment with a small 3-cell brushless motor, so I am looking at an output of around 12V. With a possible fourth panel, output current will be around 17A.

I can unfortunately not justify investing in a 20A MPPT, hence me looking at buck converters as an alternative. I like the idea that the output voltage can readily be adjusted to any suitable value. I can use this to manually track MPP as conditions change.
When I eventually add a 12V battery to the system, it should also allow me to use a simple pwm charge controller without the usual loss in efficiency.

Off-the-shelf buck converters seem to cater for a large range of input and output values, so best efficiency is not guaranteed at the particular step-down ratio I require.

 

You'll probably get better response on an EV or electronics forum.

 

For what it is worth, I will continue to log project progress. Here are a couple of videos on the construction and first trials.

 

Tested an 8"x4" propeller today. Still no electronics but I managed to do 7.2nm in 2h20min.
That is 3.1kt average including all slow manoeuvres and a short stop. A 45% increase in speed equates to nearly three times the propulsive power of the little orange prop. And that is despite the large prop only running on partial panel voltage.

The voltage read near zero with the motor running, so clearly a problem with either the meter or some of the connections. It showed the correct open circuit voltage though. As a result I have no idea how much power I was getting from the panels.

A 1ft wind chop on the return leg had those little amas pitching like valve rockers. The front would submarine and the get flicked up, throwing spray everywhere. Is there an optimal position for placing the hinge axis relative to centre of gravity or centre of buoyancy? Photos of Meade Gougeon's sailing canoe as well as the B&B Expedition Canoe appear to have the hinge aft of "midships", but both have more volume aft and the amas appear to hang level.

 

Some more testing today.
I had ordered a couple of 9A buck converters and a 5A MPPT controller.
The electronics have arrived with no manuals or instructions, so it was really just a session of figuring out how to operate them.

I used a single 50W panel and another 18V brushed motor as a load. It is a longer can length than the motor I am using on the kayak and a higher no-load current. More than half the panel's short-circuit current, if my spec sheet is correct.

The MPPT certainly provides a huge improvement over no step-down conversion. It has separate output voltage and current regulation. Two trim-pots set a max value for each. I had set the current limit to 4A after getting whiffs of melting plastic at the max 5A rating. Voltage limit was set high and was not of concern since the controller will automatically reduce voltage under load. As long as the current did not hit its limit, power out was very consistent at different loads.
The power tracking on the input side still needs figuring out. It also has a trim-pot with which the input voltage under load can be limited. Any limit lower than MPP-voltage obviously limits power coming in.

I don't know what to make of the buck controller just yet. It has similar output regulation on voltage and current. With the current limit set just a fraction too high, the load would pull the panel voltage right down. The problem however is that unloading does not let the motor speed up again. With the panel voltage equal to the same low output, the current cannot be stepped up. So it gets stuck in a low power rut and the only way to get out is to disconnect and re-connect the panel.

The buck performed much better with two panels. It was still possible to to overload the panels but it would at least speed up again when unloaded.

I also tested both units on a 3-cell brushless motor using only one panel. The motor turned a 10x5" propeller (in air). A servo tester was used to adjust power on the electronic speed control.
The MPPT had no problem handling sudden load changes or quick throttle adjustments.
The buck was again temperamental as panel current approached its max power point. Voltage would drop from 17.5V straight down to 10V until the ESC power was adjusted down a good deal. Panel voltage would recover to 19V and power could again be increased.

 

Nice project! I'd like to build something similar to learn.

Could you solder a poti to the mppt controller to regulate the out voltage to drive the motor directly without using an ESC?

What kind of RPM do you need for the propeller? I understand that slower and bigger propellers are more efficient generally, but slow RC motors with enough torque that are small enough are hard to find.

 

If you are using a brushed motor and want to reduce power you should be able to do that directly through the mppt.
When using a brushless motor you cannot do without the ESC as it is essential in converting dc to ac. To control the ESC without using a radio tx/rx you can simply use a servo tester wired directly to the ESC. Just make sure the ESC is able to supply power to the servo tester.

If my calculations are correct, my system would produce maximum speed of 4kts with a prop turning 1630rpm.
Starting with 150W and 18V at the panels, I assume an mppt will give me 14.5V output at 90% efficiency.
The reduction gearbox has a ratio of 9:1 and rough guess of 97% efficiency. So motor rpm is 14640 and running at 78% efficiency.
I calculated prop efficiency at around 69%, including a liberal reduction fudge factor to bring propulsive power down to match actual performance.
Overall efficiency would then be 47%.
It could very well be that the prop actually operates at higher efficiency and that my kayak drag calculations were too optimistic.
Either way, it will serve as a reference point for comparing other props.

If you have panels that put out 12V you can power a 3-cell rated ESC directly and eliminate the mppt losses. You will then have to manually adjust ESC output to keep panels at mpp if solar conditions change.
Assume a 300KV motor such as a Turnigy G60 outrunner. That gives no-load rpm of 12x300 = 3600rpm. Max motor efficiency under load should then be around 3100rpm. The ESC would lose around 4%. Assume a motor efficiency of 88%. A 2 blade, 5" x 3" drone prop would have an efficiency of around 53% with the boat doing 3.9kts.
Overall efficiency is 45%.

The scenario above is just hypothetical and might contain flawed assumptions. It illustrates the point that nearly equal performance can be achieved by different approaches.

 

Thanks for the info! 47% efficiency is pretty good, Torqeedo claims 55% efficiency.

Unfortunately I can't help you with a buck converter, except to say there are cheap ones on ebay from china. I spend some time trying to understand how the solar panel would react. All I could come up with is that if the motor is too stressed the resistance would drop (or back emf). Since the amps are fixed, the voltage would drop and the solar panel wouldn't produce maximum power. So you'd have to manually "throttle down" and lower the voltage to reduce the amp draw to get the solar panel back into maximum power point. Which could be a fun challenge

 

You have pretty much summed it up. The aim is to select a propeller that loads the motor such that it draws exactly the amount of current that matches the panel's MPP.
A buck converter uses pwm to throttle down (reduce the voltage, lower the rpm) while at the same time stepping up the current. It does this by making use of an inductor, a capacitor and return diode (or better yet, a MOSFET) downstream of the pwm switch. So during the pwm "off" time, the output capacitor keeps the voltage going while the inductor keeps driving the current. The diode (in parallel with the cap) forms the return path for the current. It therefore creates a temporary "flywheel" circuit until the pwm switches on again. The net result is that you do not have to throttle down as much as you would have to using just pwm.
The alternative is to use a variable speed gearbox or variable pitch prop.

 

Are you married to the "no battery" solution? A simple small 12V lead acid battery (motorcycle, UPS) would solve all your issues. 18V panels are actually designed to be able to charge lead acid batteries directly. Of course using a MPPT controller is better but you don't have to. The motor can be controlled by a cheap PWM, if the drill had adjustable rpm you can use that controller.
Using a motor designed for 18V nominal with 12V will increase the amps if you request the same power, wich the motor might not like without sufficient cooling. Consider filling the Al tube with transformer oil.

Do you have any form of spring or dampener on the pivoting amas?

 

Batteries are not an option for solar boat races, but for all other applications they would certainly help. I have a couple of 22Ah (13V) LiFePo batteries just waiting for the next test. I am also busy modifying the drill trigger pwm to act as a crude buck converter. The idea is to simply add a capacitor and return diode across the pwm output. The inductive nature of the motor will hopefully suffice as current driver. I will update once I have results.
I have since also bought an MPPT which I will use with the batteries. I am still glad that I first tried to do it the hard way. By trying to optimise the entire system without batteries I have learned a great deal about motors and electronics. For instance, trying to to draw the same power from an 18V motor at only 12V does not just cause problems with heat, it also reduces motor efficiency by as much as 30%.

I have already started a series of videos explaining the design process of a generic solar-electric propulsion system. The videos will demonstrate the theory by doing actual tests using a small toy motor and a 3W panel. The videos dealing with motors and propellers will include pointers on how to get best use from batteries and MPPT's.
I will start a new thread with links. Still learning as I go along...

 

If batteries are a no go then I would try following:
Buy (or build) a buck-boost converter capable of taking the full 150W (wich you will probably never actually see) of the panels. A MPPT is nothing else, but with an added computer to decide to what voltage to convert. This function is not actually needed for what you are doing. You set the voltage to a fixed value, for example 18V or 20V (powertool motors are designed to operate between this values, the batteries are 5S lithiums, I would use 20V to simulate full batteries). This you feed to your standard PWM controller. You are basicly using the buck-boost converter as a constant voltage power supply. The modular construction allows you to switch controllers (if for example you switch to a PM motor).

 

You can go way bigger on that prop diameter, depending on your RPM.
R/C airplane props are the way to go.
There are some adjustable pitch props made in Germany that are excellent quality.
Did you consider mounting the motor pod on your rudder?
Less drag, less weight and increased maneuverability.
But could you get it deep enough with a larger prop?
You may also want to try it as a "puller" not a "pusher" prop.

 

That is exactly what I did with my new motor.
I used helical bevel gears from a small angle-grinder. The puller configuration greatly simplified the design and assembly since the direction of rotation now makes all nuts and threads along the drive train self tightening.
This year's race was held on Sunday. I had all of ten seconds of propulsion before the electronics let out some smoke and ended my race. I was still busy advancing the throttle and was just starting to accelerate nicely when it went pop and then quiet.
I used an MPPT set to the desired output voltage for the ESC. The ESC was connected to the MPPT's battery terminals as you would an inverter. Wired in parallel to the ESC was a small buck converter which got power directly from the panels. This buck put a charge on the ESC's input caps so that the MPPT sees "a battery" before the panels are connected to the MPPT. I don't know why this sequence is important, I was just following the manual.
The thing that went pop was the small buck's MOSFET. It was the first time I'd used it and I had not set a current limit or resistor on its output.
Once the panels were connected the small buck was technically obsolete. What I still need to figure out is why the ESC died, though not immediately, after the buck failed. It should have been able to run just fine with power from the MPPT. Back on shore I tested the ESC with a LiPo battery with no response.
The next morning I had it back in the workshop and it was running just fine under battery power.

 

A few days before the race I also tested the original brushed motor with the small 50W panels and a buck converter.
It was partly cloudy which provided perfect test conditions with variable available power.
The buck performed exactly as you suggested Rumars. By adjusting its output voltage I could keep the panels at MPP regardless of available sun.
It actually worked much harder than intended since, unbeknown to me, the motor had started rusting due to a leak. The gearbox and bearings were stiff and the motor was taking on water. I never measured the actual output current but it must have been huge. The panels were delivering their full rated power at times and still the motor was struggling to pick up rpm.

The video only shows the first test using the drill pwm trigger as a means of speed control.