Hi, Myles, thanks for that advice.
For my first fiddling with the dinghy, I am stuck with what I have,
which is a load of unknowns as far as the motor specs go.
But your idea is still applicable, that if I create too much load, I
will consume more electricity, presumably making heat instead of motion.
I was aware of this from my machinist days, heavy loading from high
volume metal removal made lots of motor heat.
I am expecting the dinghy tests to go well, like I have said before, the
posts I never see here are the ones saying how electric drives are a
failure, and a return to ICE is now the plan.
But that does not mean that I will end up with a practical setup for the
dinghy, nor do I have to. I might even be able to purchase the sailboat
motor and whatever else I need, and do the initial testing of the actual
equipment in the dinghy, who knows.
As far as the sailboat goes, I think that as long as I keep an open mind
on it, the only failure might be exactly what you warn about, load being
too high. But I can adjust that with prop changes, and ultimately, if I
have to, capability to go to a toothed belt drive, allowing ratio
changes.
At the same time, the real way to go if I have a choice is to have the
clean compact look of a direct drive. I like those white painted bilges
that seem popular with electric drives, too!
With the plan to aim for about 1/2 hull speed as a maximum, to save
electricity, I probably have to be careful not to lose the gains by poor
choices, and things get more critical than if I had power to spare, so
thanks for the caution.
John
5c. Re: Pros and Cons of Lower Voltage
Posted by: "Myles Twete" matwete@comcast.net electric_barge_boat
Date: Tue Apr 12, 2011 7:40 am ((PDT))
"In addition to what others have said, electric motors are happier
turning
at a higher RPM. Most include a fan which is more efficient spinning
faster
as well."
To expand on why this is, motors are referred to as having a "no load
speed"
for a given drive voltage. Current (and power) are minimum at no-load.
With torque loading, current increases proportionally. And power loss
to
heat in the motor increases with the square of the current. For energy
efficiency, we don't have many choices:
· Choose a motor that has torque/speed curves that fit your load
and
speed expectations
· Choose a motor that has minimal internal resistance
· Operate as close to no-load speed as possible (minimize
torque,
maximize speed)
So sure, you can run your motor at ½ no-load speed and avoid a 2:1
gearing.
But in doing so, the motor torque requirements will be 2x that you'd
need
with the gearing. And at 2x the torque, you'd expect 2x the current and
4x
the power loss. For a low resistance motor like the ETEK or other
permanent
magnet motors, the impact may not be that great, but the losses are real
and
need to be considered.
-MT
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