Hi Pat,
I completely agree with you that just halving the voltage will severely reduce the available power to a drive. Like you and Miles discussed, if one mearly halves the voltage and keeps everthing else the same, the power will be reduced by more than half.
But we rarely run our motors flat out, like direct wiring or a 100% pulse width. According to Mars, the ME0708 PMDC can take 190A (1850rpm) at 24V, but they rate the max current at 150A (3200rpm) at 48V. So there is some extra headroom in the 24V ratings.
However, my original answer focused on the many other cons related to going with lower voltage. I assumed that the user had aleady determined that the motor would still be able to deliver the required power to drive the boat, even at the lower voltage.
I believe that the general concensus is that there are few, if any pros, and many cons to the idea as presented.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "greenpjs04" <greenpjs@...> wrote:
>
> 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
>
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