Short answer: Heat,efficiency and cost.
The motor I use, the ME0913 is open-frame air cooled. The motor has a cooling fan on it, connected to the output shaft on the opposite side. At lower RPM, it doesn't move enough air to cool it under heavy load.
Efficiency
The motor and controller are both more efficient at higher RPMs. The motor may have full torque at lower RPM, but those Nm cost more per watt. As a sailor, I'm a cheap bastard.
Cost
See my earlier post about the magnomatics pseudo direct drive gear motor. That would be able to directly drive a prop shaft at very low rpm. There is also the ST37 from Solomon Tech that does something similar but with machined planetary gears. These are very expensive compared to belt and pulley or even industrial fixed gear reducers. There was a vendor here, Advanced Marine Electric Propulsion, who had a direct drive offering but he used lower voltage systems which meant higher amperage on the battery bank. This means bigger, more expensive cabling, bigger battery banks in order to deal with the increased demand efficiently (peukert effect). Again, I'm a cheap bastard. I don't want to spend money where it doesn't directly benefit my sailing. But I also hate spending money more than once on something, like replacing a burnt, overloaded motor.
Cheers,
/Jason
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Jason Taylor
v:514-815-8204
e:j@jtaylor.ca
On Feb 22, 2016, at 9:36 AM, johnrhines@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
Jason,
Thanks for the detailed reply, but I'm am still confused. Let me ask the question a different way. You said to get a cruise speed of 4.7 kts with optimal conditions, your prop spins at 650rpm. With a gear reduction of 2.55, that means your motor is turning 1657rpm. Since electric motors have full torque at all rpms, what is the problem with spinning the motor at 650rpm and eliminating the gear reduction? Is it just a measure of efficiency? How would you calculate the load on a motor at 650rpm versus 1657rpm.Thanks,John
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Posted by: "Jason (Electric Boats) Taylor" <jt.yahoo@jtaylor.ca>
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