[EMAIL PROTECTED] wrote: > When we get finished with the "latching relay" discussion could we look > at electric propulsion? I would love to hear about the pros and cons > from you guys that know electricity. AC motors, DC motors, permanent > magnet motors, controllers, new battery technology (Lithium titanate > nano). What does "traction motor" mean? Does a larger diameter motor had > more torque than a skinny motor, other things equal? etc. There seems to > be brushless AC and DC motors. How do they do that? Does a controller > control DC current or AC frequency or what? I need educating. > > Ed Schwerin
I have given this a lot of thought and research, and have come to the conclusion that the best approach for a small cruising sailboat would look like this; A small diesel genset, (around 5 kw for my 25 ft boat would supply hull speed propulsion), running at 115 VAC or 240 VAC which would be able to supply the propulsion motor directly (either a permanent magnet brushless type or an Perm mag AC brushless type) through a very efficient controller, like at least 96% efficient. The genset could also supply power to the house bank, (a set of AGMs most likely, just enough battery capacity to supply 30 nm or so at 80% hull speed...this means I would NOT have a massive, heavy, expensive battery bank.) The house bank could supply power to the propulsion motor by itself through an inverter although in this mode the boat would not be able to achieve hull speed, only 70-80% of that, but since it would only be used in pure electric mode to get away from a mooring or slip or across the bar or maybe to back off a mud bank, or to move out of the way of the ship at night in a flat calm, or as a get home in calms with no fuel back up, this is fine. It would be set up so that it would be able to power under the electric propulsion motor drawing electric power from the genset, OR from the batteries, but not both at once, although the genset would be able to supply power to the propulsion motor and charge the batteries at the same time. This is just to keep things simple and cheap. The system would NOT have regen, as I think that slows a boat down while sailing more than running a wind genny does. The reason I would use high voltage is because it allows a more efficient controller to be used, the higher voltage electric motors are more efficient, and the cable size needed is much smaller. Another issue is that with a low voltage high current set up, any corrosion in connectors is more likely to cause a fire. I would use off the shelf gensets, controllers, and motors, so that if any module failed it would be easy to find at least a temporary replacement, and they would be much less costly than custom or proprietary units. I would make it so that I would be able to by-pass the controller for a get home mode in the event of controller failure, it would then have full power on or off, but that would suffice as a minute emergency powered mode. Some of the other advantages would be a huge genset capacity for a small boat (I could run AC at anchor or power sand the hull in a yard that had no power)and lots of redundancy, with the wind and solar power I have I would bet I could cruise for a very long time without needing fuel, and this set up would more than triple the fuel range compared to my atomic four. In fact it would get better fuel mileage than a standard diesel installation, since with this set up the engine would be running at a more fuel efficient RPM/loading most of the time. I would NOT use new batteries other than AGMs, as they are still too problematic. As far as diameter of motors, no, a smaller diameter is not unnecessarily less powerfully, it depends on many things, but if two motors are of the same design, and are running at the same RPM, then yes the smaller one would generally be less powerful. In any case in general if two motors use the same amount of current at the same voltage and both are reasonably efficient, they will be very close to the same power output, although the torque curves may well differ and the design RPM may as well. Controllers fall into several categories, the most common are pulse width modulation for DC motors, less common is freq modulation control for them where the number of pulses per unit time is what is controlled not how long the pulse is, and for ac motors, yes freq control is a common way of doing it but for smaller motors controlling the amplitude of the peak voltage is also common. Brushless designs mostly use semiconductors (hall effect magnetic type or optical type) to sense motor rotor position and then that controls a semiconductor switch that reverses the applied polarity just as a brush would do otherwise. There are many advantages to this besides no brush wear, among them being able to control the 'timing' of the commutation at different RPMs, and better control over the switching itself. It also makes a more compact motor. Brushless perm mag motors can be as high as 97% efficient with a very flat torque curve, meaning that as the motors speed slows the torque goes up, and at very slow speeds the torque can be huge unlike other electric motors and internal combustion engines, so if you snagged a net for example, you could rip the shaft out of the boat in the blink of an eye if you did not have peak torque limiting of some sort. This can be electric sensing in the controller or mechanical such as a toothed belt or keyed pulley, or both. The controllers can also be tremendously efficient, and when the entire system is considered over its RPM and torque range, it is quite possible that it can be more efficient than a standard set up. I say that to emphasize what has made such a design practical today...efficiency. Without this, you have a system that is just not as practical as a standard direct drive IC engine installation. Also the size and cost of the equipment has gone down a lot over the last 15 years or so. Hope this gives you something to think about. -Ken _______________________________________________ Liveaboard mailing list Liveaboard@liveaboardnow.org To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/liveaboard@liveaboardnow.org The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
