Hi Dave, Thanks for responding.
When you say 'pushing the motor hard' are you say by high volts, or by
slowing it with a large load, such as too large or pitched prop?
Looking at the speed Eric gives for a 500 watt load, (10% of what he
uses to go only twice the speed) does this indicate that if I had his
boat I could use, say, a 1 HP motor, and have 200 watts to spare, as far
as heating goes? This assumes that I gear/prop it to run at a reasonable
speed, say, only as low as maybe 80% of no load speed?
What I am trying to say is, what would be a good guesstimate of good
starting points as far as motor HP spec to end up with a system that
draws about 500 to 700 watts continuous and does not waste resources by
making heat?
Sorry to ask questions that might have many variables!
I do not want spare power over what is needed to dock the boat, for
example. (I might regret this? But I want to try anyway.:-)
Thanks,
John
2a. Re: Water Cooling Of Motors
Posted by: "davecover" davecover@gmail.com davecover
Date: Mon Apr 4, 2011 7:27 am ((PDT))
With electric motors, and the rest of the components, think of heat as
inefficiency and something to be reduced, not just an unavoidable
byproduct. This is not a hard and fast rule, just a guideline. Most
parts have operating parameters and you really need to know and respect
them. But for powering a sailboat with reasonably priced components
(motor, controller, etc.) you should be able to design a system where
excess heat is not a problem. I'd bet dollars to doughnuts that if you
have enough heat that you need water cooling, you can find more
appropriate parts that only need air cooling.
A lot of the DC motors I'm used to will run hot because we're running 75
to 300 amps continuously with spikes in the 500-700 range. The motor
gets hot enough that you almost can't hold you hand on it. In this case,
the heat is a product of pushing the motor hard. And it's still within
specs (insulation rated for 180C.) But if you are only motoring in and
out of port, and your motor is getting hot, you may want to consider a
more appropriate motor. Remember also, you can use a larger motor and
not pay any price in efficiency or energy used.
Look for a motor that is designed to run at the speed your prop should
turn. You can also work backwards from the ICE motor specs and find an
electric that will fit the profile. And if you need to "put your pedal
to the metal", or run hard to get out of rough seas, an over sized motor
will give you a little more margin.
Dave Cover, still looking for my project to convert
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