Hi all!
The last couple of years I have been planning to convert my boat to electric. The plan was to use a 17kW induction motor from an electric car (the norwegian produced Think).
For my boat (27 feet, 7700lb, displacement hull) I have estimated a mechanical power demand of approx. 2kw at cruise speed (I pulled my boat at 400N and 4,5knots, and estimating 50% efficiency of the propeller). At this cruise speed it is important with high efficiency.
Recently I made a simple test bench and performed a efficiency test for the Think-motor. It turns out the efficiency is too low (approx 55%) when the motor delivers 1-2kW (in the ranges 20-40Nm and 200-800rpm). This is not very suprisng, yet very disappointing since I have put so much time and energy into making this motor work outside a car..! (If anyone needs a powerful motor for their conversion please let me now:-)
I expect much higher power demand in extreme cases, i.e. in harbors with strong winds, maybe more than 10kW (at two or three time the propeller speed) - but at very short time periods. Does this seem reasonable?
From my estimations above I conclude with this: At 2kW the motor should work at high efficiency, and at 30-50% of max speed, and at approx 20-25% of max power output.
But does such a motor exist?
When I look at the performance curves for the motors made by Golden Motor I cannot find information about the torque v.s. speed (only inverse). Therefore I dont know where the motor goes form the "constant torque" to the "constant power" region.
Another thing: these curves gives one point for each power out (with corresponding efficiency). But say for 2kW output power there is infinite combinations of torque and speed! 10Nm at 1910rpm, 9Nm at 2120rpm, 8Nm at 2387rpm and so on... Can I expect the efficiency to be approx the same at all these points?
One thing I remember form reading a book (Hughes) on electric motors is that the "constant torque" region of inverter controlled induction motors is decreased when the applied voltage is lower than rated. But what about the opposite case: increasing the applied voltage to increase the constant torque region? This will probably be a problem for the stator windings, but if this over-rated voltage only is applied for short periods of time (some seconds) maybe this will be a solution to make a smaller motor powerful enough for these 10kW power bursts..?
I take the 3kW, 48V (liquid cooled) from Golden Motor as an example. This one delivers 2kW at 4.5Nm, 4300rpm at 87.5% efficiency (http://www.goldenmotor.com/hubmotors/hubmotor-imgs/HPM3000-48V3KW%20Curve.pdf). Hopefully this also delivers 2kW at 10Nm, 1910rpm at the same efficiency (?).
If I power this motor with a 96V battery (and maybe a 5kW, 96V motor controller?), can I expect this motor to deliver 25Nm at 4000rpm?
Any comments or suggestions are very welcome!
Best regards
Bendik, Norway
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Posted by: Bendik Vignes <bendik.vignes@yahoo.com>
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