It's interesting, but the chap has misunderstood some of the basics and reached a conclusion as to cause of the effects he's observed that isn't quite correct.
First of all, he talks about the very high torque of his double motor set up, and says that it's too much for the driveline. The simple answer here is that he doesn't seem to have a prop and reduction drive that is correctly matched to his motors, other wise why would couplings break?
Secondly, although the PM DC motors he has do generate pretty much constant torque vs rpm, the propeller can only ever produce a torque reaction (the thing that makes the motor have to deliver torque) in proportion to the cube of the prop rpm. This is a characteristic of all propellers, of any type. If the prop was correctly matched to his motor and reduction drive then it could only absorb the maximum motor torque at maximum motor rpm (this is normally how you optimise a drive system in terms of drive line strength).
Normally, you would optimise the system efficiency by selecting a prop that gives best efficiency at boat cruise speed, taking account of motor and prop rpm and maximum allowable prop diameter (normally this will be limited by the hull or draft considerations). In this case, I think that a prop has been selected that is a compromise, because the boat needs regenerative power power sailing. The prop would seem to be too high a pitch for the power requirement, but that is probably something that has been done to get it to work more effectively as a generator (the two requirements are very different in terms of prop pitch for best efficiency).
Finally, he makes an erroneous comment about the double motor system having twice the BEMF of the single system, and attributes this to the cause of some of his issues. This is wrong, the BEMF of one of those two motors will be exactly the same as that from two of them when coupled together, when running at the same rpm. He's right that the double motor system will have greater bearing drag and windage, so will require more torque to turn it, but I'm not sure the difference between two or one motor fitted to such a set up would be that noticeable.
One big advantage of having a motor that is too big is that it will generally be more efficient. It will have a lower winding resistance, so will give much lower losses for a given torque. It will run cooler for any given power, maintaining those lower resistive losses with prolonged running. It will also have lower losses when running as a generator.
If this chap wants to stop breaking drives and remove the adverse effects of having an over-propped system with motors that are able to deliver more torque than the system can handle then he should adjust the controller current limit down to limit it. This will have no effect on regeneration, yet will tame the behaviour that breaks components, whilst maintaining his current level of performance. Best of all, if he has programmable controllers (most are) it is a free modification.
Jeremy
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> this is a VERY interesting youtube done by a guy named Bruce Wilson and explaining that on his 41' cat he would probably be happier with two 4.5kw motors than the two 9kws that he actually has...
>
> with reference to direct connecting motors to each other and then the shaft, this is in effect what has been done with this 2003 boat
>
No comments:
Post a Comment