Hi, Eric, and Pat, Everyone,
Great link Eric, I now know what a controller actually does. Clever.
Pulsating DC with variable gaps in the pulses.
What I like about it is that it would seem that if it failed while
underway, it could be bypassed, at least on smaller boats with low amps
involved. There is nothing scarier than the unknown!
Pat, thanks for the volts/amps info. clarification. I was actually,
until now, freshly confused! But now am back on track with the thinking
on it all.
What I was trying to do was figure out if, as I had always 'known' from
fiddling with models as a kid, I could reduce the battery drain by
lowering the speed by lowering the volts.
I think that the confusion comes/came from assuming that the speed is a
required constant, whereas the whole point in my thinking is to reduce
the speed and make use of the greatly extended battery life by only
reducing the speed, say, to 50% of flat-out. I am still just totally
impressed with the 500 watt figure to go at over 3 knots.
So, all I have to do is make sure that I do not overload things by using
too large a prop.
By fiddling with a low power dinghy setup first, I can gain more
knowledge, and at a comfortable cost.
Remember too, Eric, that the danger of having less speed control
decreases if the max speed is half of what might be 'normal'. Without
provoking side discussion, I suspect that with lower overall maximum
power, the acceleration would be slower, thus allowing more control than
with a situation where 'normal' full power amounts (5kw+?) are switched
right on instantly. To take the figure of, say, 500 to 1000 watts, and a
boat displacement of 6000 pounds, there is probably unlikely to be any
instant speed!
Myles probably has enlightened me too by enforcing the thinking about
the load having an effect, although for my personal experiments right
now the amount of gain is not as critical as knowing that the gain will
be there.
The issue that Matthew raises re the prop getting snagged is
interesting, too. An ICE would presumably get to a position of running
too slow to maintain it's own running, and stall, whereas an electric
would just increase the amps until the prop freed up, or the motor
burned out.
This has been really useful info, I don't know if it answers Jaks
questions, if I were him, I would try the 24 volts first, and then fix
it if it didn't work as well as expected. Just be careful of not
overheating the motor. He could presumably adjust the load by changing
the prop, too. In his case, the simple goal would be lower RPM, so I
suspect that most of this discussion applies.
Thanks
John
2c. Re: Pros and Cons of Lower Voltage
Posted by: "Eric" ewdysar@yahoo.com ewdysar
Date: Mon Apr 11, 2011 10:26 am ((PDT))
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
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