T-Pain Is On A Boat: Re: [Electric Boats] Regeneration

Sunday, June 14, 2009

Re: [Electric Boats] Regeneration

Regen does not use the straight volts/rpm ratio. Unless the motor is turning above the no-load speed, regen is initiated by pulsing the windings with a brief charge in "reverse", then releasing the current and damping the spike with the battery. The regen comes as the spike, the increased current reacting to the rotating field, is clamped by the battery's internal resistance. The larger the preliminary current pulse, the greater the resulting spike, and the greater the drag on the rotor. Regen has limitations on brushed motors, namely a large pulse magnitude can cause the current drain duration to extend beyond the commutator bar contact connection during rapid rotation, resulting in an un-clamped voltage spike that can burn through the enamel wire insulation on the armature. Motors designed for extensive braking are wound with "inverter grade" wire, designed to resist punching through the insulation. Brushless motors are often limited by the maximum
voltage for the FETs, as the ability to disconnect from the rotating perminate magnets is not possible without a clutch. A shaft brake can also prevent damage, or a folding prop. AC motors can rotate freely without fear of overvoltage, as no voltage is initiated untill the field charges the rotor lams.

Arby

________________________________
From: James Lambden <james@toolboat.com>
To: electricboats@yahoogroups.com
Sent: Thursday, June 11, 2009 1:38:46 PM
Subject: [Electric Boats] Regeneration

When a boat moves under propulsion the propellor typically turns at
up to 1,200 rpm depending on the reduction gear used and the
propellor size and pitch.

Lets say your pitch is 12 inches and your boat is moving at 5 knots.

5 knots per hour x 6080 feet per knot = 30,400 feet / hour

30,400 feet / hour x 1 hour / 60 minutes = 506 feet per minute

The theoretical distance that the propellor covers is 12 inches or 1
foot, so the theoretical rpm is 506 rpm of the propellor shaft, less
if the pitch is more, more if the pitch is less.

If we load down the propellor by regenerating, and we assume there is
inefficiencies in the propellor we factor in slip.

If the slip was 20 percent, this rpm would be around 400 rpm.

If the boat is using a 2:1 reduction gear the motor rpm would be
around 800 rpm.

Any numbers out there for rpm during regeneration? ??

Since most electric motors are permanent magnet, they feature an rpm
to voltage constant. With the Mars motor, this constant is
theoretically around 72, and more likely around 60 rpm per volt.

When operating under propulsion, the motor is turning at around 2,500
to 3,000 rpm.

Now under regeneration, the motor is now turning at 800 rpm.

This is dramatically different from a golf cart or fork lift that
under regeneration the motor turns at the same speed renenerating as
during propulsion.

This means that under regeneration the motor is now producing as a
generator 800 rpm x 60 rpm per volt or 13 volts, not nearly enough to
recharge a 48 volt battery bank.

So the controllers have to boost the regeneration voltage to the
battery voltage. This boosting is inefficient.

The controllers are optimized for a braking force, because the golf
car companies and the forklift companies are more interested in
saving the brakes of the vehicle than regenerating the batteries.

Increasing regeneration requires boosting in a more efficient manner,
or using regeneration for house loads instead of propulsion loads,
and skipping the boost function altogether.

James

www.propulsionmarin e.com

On Jun 10, 2009, at 7:00 AM, constancedraper wrote:

>
>
> Hmmm, I wonder what the answer is? On the face of it, there has to
> be a way to capture more of that kinetic energy. I've heard others
> say that turning the prop to generate electricity cuts speed by a
> half knot or more.
>
> If you had a 100% efficient system (impossible, of course) that
> would mean that at the end of 12 hours of sailing you'd have
> accumulated enough in the batteries to drive the boat 12 hours at a
> half knot. Maybe 4 hours at one knot.
>
> A 50% efficient system should give you half that.
>
> The electric motor/generator is one of the most efficient machines
> around, so that's not the problem. Evidently, as you've suggested,
> the prop design is crucial. This is a subject I know almost nothing
> about.
>
> I understand that electric motors have great torque; I'm guessing
> that it requires more torque to turn them than than the relatively
> slow moving water flowing past provides. Seems like there needs to
> be some way to reduce the load so the prop spins easier.
>
> md
>
> --- In electricboats@ yahoogroups. com, "aweekdaysailor"
> <aweekdaysailor@ ...> wrote:
> >
> > --- In electricboats@ yahoogroups. com, "constancedraper"
> <drmark.draper@ > wrote:
> > >
> > > Randy,
> > >
> > > Don't overlook the possibility of recharging your batteries by
> capturing the energy of the prop's turning while you're under sail.
> > >
> > > md
> >
> > Actually - do. It doesn't work. On a good day, you'll maybe get
> another 10 minutes of propulsion out of it.
> >
> > Unless...you can actually fit a 20" prop - I am stuck with 13"
> max. But most boats in this size range will have that problem. The
> shaft-angle would have to be too steep to hang a (much) bigger prop.
> >
> > -Keith
> >
>
>
>

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