Myles,
I was trying to keep it simple and made assumptions that everything was linear (like a resister). That, of course, it not true for boats and propellers in the real world. So, your points are well taken.
Eric,
You are correct that the throttle is a way to adjust the current, but it does that by changing the voltage (or the average voltage if PWM). My point started with "for a given voltage". Perhaps I should have said "at max throttle, there is no way for the average user to adjust current". In most cases, the user adjusts the voltage (or average voltage) and gets whatever current the motor takes at that voltage.
Let me try to simplify my point again...
You can't take a 48 volt motor and run it at 24 volts without seriously limiting the maximum power it can produce unless you do something else to increase the torque (like using gears, belts pullies, or a different prop). It will not automatically draw twice the current (amps) to achieve the original power level. This all started when someone suggested cutting their voltage in half as an alternative to others methods of reducing prop speed. I was just trying to explain why that doesn't work.
Finally, let me take this opportunity to thank both of you and all the others who post in this forum. I have learned a great deal about sailing and electric boats from all of you. It's about time for my pontoon boat to be returned to the water here in Ohio. I can't wait for another fun summer of quiet reliable boating.
Pat
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> Pat-
>
>
>
> Sure. But as another electrical engineer, it's not quite as you state.
> Motors are not simple resistors.
>
> For a PM DC motor, 2 equations suffice:
>
> Vm = Kb * w + Im * Rm
>
> Tq = Kt * Im
>
> Where Kt (newton-meter/amp) = Kb (volts/rad/sec)
>
>
>
> Indeed, as you suggest, running a 48v motor at 24v and driving a boat with
> it will result in less current. However, the reasons for this are not quite
> as you suggest. Given the motor no-load speed at 24v is approx. half that
> of 48v, and given the exponential nature of drag in water, at 24v we expect
> just over half the speed attainable at 48v in water. Given the propeller
> torque needs drop nominally by half for each knot of speed reduction, the
> effect may not be 50% as you estimate at 24v. Say that 48v gave you
> 4kts---with 24v and the same motor we'd expect something greater than 2kts
> of speed. At 2kts, we'd expect 4x less torque than at 48v and 4kts. Since
> torque ~ current, we'd expect 4x (25%) less current, not 50% less.
>
>
>
> In case this helps-
>
> -Myles
>
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of greenpjs04
> Sent: Monday, April 11, 2011 10:49 AM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] Re: Pros and Cons of Lower Voltage
>
>
>
>
>
> Hi Eric,
> Everything you say is true, but there is an implication that a boat user can
> easily adjust the current to achieve the desired result. For a given
> voltage, the main way to increase the current drawn by a motor is to
> increase the mechanical load. If you travelled from a normal lake to one
> filled with molasses, the current would certainly increase! To use examples
> similar to yours, if you have a motor that is designed to use 2400 watts of
> power at 48 volts, it will draw 50 amps. As you stated, a similar motor
> designed to 2400 watts of power at 24 volts will need to draw twice the
> amps, or 100. However, if you take that first motor and just lower the
> voltage from 48 to 24 volts, the current will drop in half (to 25 amps) -
> NOT automatically double to 100 amps. I know you already know that, but the
> original poster seems to be confused by that concept. (Of course, when using
> a PWM controller, you need to consider the average power, voltage, and
> current, but the theory is the same).
>
> I hope I am helping here. I am not trying to be difficult, but as an
> electrical engineer, I always get the urge to correct misconceptions.
>
> Pat
>
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