eFoil DC motor used as a generator to charge battery

EFOIL.builders Propulsion System (Motor, Gears)
1

Maybe there’s a better way to use the wave energy like producing our own electricity. It could allow to decrease the battery case volume and therefore the board price …
Inspired from: Using a brushless motor for a generator

In addition to waves or a good pumping technique, what else do we need ?

  • Propeller: what is the optimum pitch for a propeller to be reversible ? 1 = 45 degrees ?
  • ESC: as it shouldn’t allow reverse currents, a diode should be set on each of the 3 phases to forbid reverse current in generator mode.
  • Bridge rectifier: the 3-phase AC will need to be rectified into a DC output
  • Motor: can the reverse voltage/current behaviour of a DC motor (generator mode) be deduced from the same motor (V,I,rpm) characteristics used in direct mode? Mounting it to an equivalent motor in direct mode (a drill ?) controlled by the efoil ESC would be a good mean to get that characteristics though …

Negative points to expect:

  • 10 to 20% efficiency loss
  • is direct drive compulsory or a gear will allow to max out the low speed ?
  • some sudden drag when going down the wave which could mean “loosing” it
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2

I’m making a build with two vescs 6 and two 6364 190kv outrunners on direct drive, the vescs have regenerative braking buit in, which was originally designed for e-skates. If we set a specific brake current that won’t “lock” the propeller and design a prop specifically designed for this, it will be simple to implement.

3

Did you check the propeller rotates at all if moved through water? I would expect it to block.

4

Still can’t test anything, I’m still waiting for the motors. If we can set a low regenerative charge current and design a specific prop (as SoEFoil mentioned, about 45 degrees pitch) we can spin the prop slightly and hopefully reduce the drag. I honestly think that it isn’t worth it until we can find a ideal prop design.

5

A very interesting topic, but the function of a generator is prevented by several very difficult obstacles.

First obstacle: The “generator” drive must be straightforward because the propeller torque does not exceed 4:1 (even 2:1) transmission resistance. This is because each propeller for e-foil has very little efficiency as a water turbine.

Second obstacle: The BLDC motor will produce a very low voltage (V) as a generator. I’ll measure it in a few days. But I have to buy 4 pieces of Schottky diode first and have to make a “generator” clutch to connect it to the motor. The measured voltage V and RPM speed will show. But it will be a great disappointment.