Hi Jason,
> The principal reason is efficiency.
<snip>
I appreciate your reply. These things take time and effort and it's always nice to see others helping those at seemingly the beginning of their journey, however I wish to put it this way...
This is on basis of your statement "The principal reason is efficiency" - if being in answer to the subject of John's message of "Why gear reduction?"
My answer would be that we gear (be that pulleys/belt or reduction gearbox) to mutiply torque to gain mechanical advantage.
But, say we have two suitable identical boats, with two identical motors, two identical controllers, two identical props and we agree to build the most efficient system for say a 4 knot cruise speed, but one chooses a reduction system and the other direct drive. Which one is more efficient if say comparing a 24V direct drive system versus a 48V 2:1 geared system?
Financial efficiency - Direct drive wins.
Power to weight efficiency - Direct drive wins.
Power in versus power out efficiency (converted to thrust to move the boat forward or astern) - Direct drive wins due to fewer system components and therefore less losses, due to friction and resistance which create heat.
I appreciate the above simplifies matters and efficiency is not something static and depends on each component - but I hope it illustrates my point.
I am though slightly confused by the intent of the queries in the body of John's message. I'm not sure where to start with that other than to say:
"The relationship between Power (in kW), rotational Speed (in RPM) and Torque (in Nm) is fixed. If any two of the three variables are known, the other can be calculated." ref:
http://mdmetric.com/tech/powercalc.htm
As for John's question "So why not just program the controller to max out at 1300 rpm?", I'd like to take a step back and illustrate a scenario.
Say I have an 84V rated motor and a controller to accept that voltage, in a direct drive or geared system, I can limit the shaft rpm based on the motor no load rpm/volt constant (which reduces with load) to give me whatever shaft speed I want subject to the prop and (if used) gearing. In other words I can have whatever system input voltage I choose up to 84V based on the indivdual battery nominal cell voltages (2V for lead/acid, 3.2V for LiFePo4). So if I want 30V at the motor I'd pick a multiplier of cell voltages (above that) if wired in series to achieve that.
For practical reasons though with lead/acid we tend to choose system multiplier voltages that correspond with readily available batteries and chargers. So in this instance I'd have 18 cells x 2V = 36V lead/acid or 12 x 3.2V = 38.4V LiFePo4, and I could use 3 individual chargers if a monobloc 12V battery lead/acid system. But maybe I don't need 30V with my LiFePo4 batteries (little Peukert effect, little voltage sag) and 25.6V input is enough using one charger and 2 x 12.8V batteries. I think my point here is we can decide on any system voltage and limit the voltage/throttle at the controller going to the motor to control max shaft speed.
John R.
http://www.elektra-yachts.co.uk
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