Re: [electricboats] New prop 10KW motor

As long as the "water force on the prop" is greater than the "throttle input", it should regen?

 

It's not that simple.

Let's say you throttle up and are cruising at 5knots and that it takes 1kw to do so.  Then the wind comes up and you transition to sailing and you are able to throttle back.  As that happens, at some point the prop may be driven backwards and regen 'could' occur.

Without knowing the specs on the controller, the motor and the prop, we can't say much more than this.  We DO know that there's roughly a square relationship between power required and speed for a displacement craft.  And that's a square loss both ways.  If it takes 1kw to drive the boat 5knots, you should not expect more power in regen than sqrt(sqrt(1kw)), or less than 6watts.  That's a trivial amount of savings.  Far greater is the savings you get by simply being able to throttle down, saving power significantly.

 

May I take this one step further?  What happens if at the same time you are solar generating say at 10 amps and regen at 5 amps?  Would this simply add up to 15 amps charge?

 

Sure, that's a start.  But it depends.  If the regen current alone causes your pack voltage to get near its rated peak, you'd expect regen current to "back off" if another source also pushes charging current into the pack.  And assuming that BOTH of these respect voltage limits, then it's a race condition, but suffice it to say that no, it will not simply add to 15amps of charge.  Now, IF that happens, you could consider pulling out of regen by pushing the throttle forward until you maximize power from the solar panels while getting greater speed due to less drag.  Overall, a win-win.  A similar thing should be considered if running a genset with battery charger while underway.  As you begin 'sailing', the throttle requirements drop.  As that happens, the pack voltage rises (especially if you are using lead-acid batteries) and that leads to the charger(s) detecting max charging voltage and backing off on charge current.  And that can cause the genset to back down.  This can result in waves of up/down current and genset speed.  Again, if speed is of value, it might be best to increase your throttle to the point that "net" current draw from the pack is zero or some acceptable level of net draw---pack stays charged and you're essentially just running on generated power.  I've done this.

 

Would the expected regen current at say 500 rpm be roughly the same as the current required to push to 500 rpm - minus friction and "throttle input" ?

 

As noted above, no.  Again, let's say you need 1kw to spin the prop 500RPM.  We know there is a square relationship between power and speed.  Sqrt(1000) = 31.6

Now, you are driven by the wind and the prop is now spinning 500RPM backwards.  We expect, due to this square law, sqrt(31.6) or 5.6watts of power might be able to be captured.

Now, the devil is in the details.

We know that a prop's loss is greater with speed and load.

So, if the water is spinning the prop at 500RPM, while the speed is the same, the load depends on the motor/controller and would ideally be set to deliver max power to the pack (e.g. via MPPT).  Still, I can't imagine in this scenario that you'd be able to get more than 5% of the power that it took to drive the prop at the same speed.  I'd like to be wrong… J

 

In case this helps…

 

-MT

 

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Mich Pop
Sent: Monday, January 11, 2021 1:44 PM
To: electricboats@groups.io
Subject: Re: [electricboats] New prop 10KW motor

 

Hi everybody

 

Thanks, Thomas, for that great explanation.

Am I correct assuming that the same applies to an induction motor?

As long as the "water force on the prop" is greater than the "throttle input", it should regen?

 

May I take this one step further?

What happens if at the same time you are solar generating say at 10 amps and regen at 3

5 amps?

Would this simply add up to 15 amps charge?

 

Would the expected regen current at say 500 rpm be roughly the same as the current required to push to 500 rpm - minus friction and "throttle input" ?

 

 

Cheers

Mich

 

On Tue., 12 Jan. 2021, 03:05 THOMAS VANDERMEULEN, <tvinypsi@gmail.com> wrote:

Carsten and Peter:
The amount of regenerated power, as Peter suggests, would be approximately the NET of the amount generated less the amount input.  If you've tried turning the motor shaft by hand, you've observe that it seems to want to "step" through its rotation.  This is the effect of the alternating resistance and attraction felt as the permanent magnets on the rotor pass by the stator windings.  To free-wheel a sailboat propeller, power is sent to the motor in order to energize the stator and overcome the latent resistance.  Once the resistance if equaled, the additional force of the water passing by the prop will turn the motor faster than it would otherwise be turning given the low power input, and -- voila -- the motor becomes a generator, transforming the surplus of energy into electricity flowing back into the battery.
I see this same effect in my battery-electric Chevy Bolt when coasting, and especially when going downhill at moderate speeds.
Although I haven't seen any scientifically oriented research to support it, my thought is that in an electric boat scenario, it makes more sense to have a three or four-bladed prop, as large as will comfortably fit the boat and with moderate pitch to allow some amount of regeneration with the ability to make the prop "transparent" as Matt stated earlier (clever metaphor, Matt!) when sailing at slower speeds.  Two-bladed, variable pitch, and folding props simply won't work as well *because* they're lower drag!
Have I tested these thoughts and theories out in practice ... NO, not yet!!  But I'll be back with you on the topic later in the Great Lakes sailing season!
[-tv]