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
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Here's a page that discusses how PWM controllers vary the voltage to a DC motor. http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html
> You can jump to section 2 for the basic premise.
>
> Remember that the power of the motor is the product of the voltage and the amps. So a 24V system at 100A will produce about the same power as a 48V system at 50A. If your boat needs 2500W of power to motor at a given speed (my boat motors at about 5kts at 2.5kW) then it would take 100A at 24V or 67A at 36V or 50A at 48V.
>
> Since it takes power to drive the boat, halving the voltage will require doubling the current for the same speed. If your system can't handle twice the current, then your power and speed will be reduced.
>
> Boat speed is not linearly proportional to power. As far as boat speed is concerned, a very rough rule of thumb is that each knot of speed takes twice as much power. So halving your power should slow you down about 1 knot.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, "John Green" <v_2jgree@> wrote:
> >
> > Hi, Eric, interesting info, including re belt efficiency.
> > I am now confused - I had some preconceived notions that it seems might
> > be incorrect:
> > Is it correct that any DC motor generally has the speed controlled by
> > lowering the volts?
> > Surely the 'given power' will be less, as the purpose of the lower volts
> > is to reduce the speed, therefore the amps might well remain at the same
> > level?
> > Can I assume that the numbers on this are not proportional, that the
> > speed will reduce more than 50% if the volts are halved?
> >
> > John
> >
> > 2b. Re: Pros and Cons of Lower Voltage
> > Posted by: "Eric" ewdysar@ ewdysar
> > electricboats@yahoogroups.com
> >
> > Date: Sun Apr 10, 2011 9:28 am ((PDT))
> >
> > Jak,
> >
> > It's kind of hard to answer the question without knowing your
> > installation and how you expect to use the boat, but I'll make some
> > broad generalizations. From my perspective, there are few Pros and many
> > Cons.
> >
> > The most obvious answer is that halving the volts, doubles the amps for
> > any given power. What kind of power are you looking to get from your
> > drive? If you're looking for only 2.5kW, then at 24V your wiring, fuses
> > etc. only need to handle 100 - 120A. But if your boat needs 5kW (7+hp)
> > then your electrical system will need to be sized for at least 225A.
> > Bigger wires are heavier and more expensive. Higher amp loads have a
> > higher potential for voltage drop in the wiring.
> >
> > If you're planning on using flooded batteries, those kind of amp loads
> > will push you deep into Peukert's losses. Even at lower power settings,
> > you'll find that your battery range will be much less than you expect,
> > higher amp draws (regardless of the voltage) lose energy to heat as a
> > symptom of the battery's internal resistance. You can mitigate this by
> > running a larger battery bank or chosing AGM batteries. AGM batteries
> > have a lower internal resistance than flooded batteries, and Lithium
> > batteries are better still.
> >
> > Aa far as just the difference of efficiency in the motor itself, you'll
> > find that the same motor at half the voltage will create more heat for
> > the same power setting (double the amps) so you'll need to address
> > supplemental cooling, especially at low rpm where the motor's internal
> > fan is much less effective.
> >
> > Finally, modern cogged belts, as opposed to common smooth v-belts, are
> > more efficient than you might think. Some products are more than 95%
> > efficient at transmitting power, so your loss would be minimal.
> >
> > For this type of question, I tend to favor higher voltage and I'm pretty
> > sure that the belted drive will probably outperform the lower votage
> > direct drive with that motor. But I could be wrong.
> >
> > Fair winds,
> > Eric
> > 1964 Cheoy Lee Bermuda 30 ketch - Serenity
> > 5.5kW Propulsion Marine drive, 8kWh LiFePO4 battery bank
> > Marina del Rey, CA
> >
>
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