I'm not sure anyone can really answer your question about how "close to it's rated power" other than you and your comfort level. Ratings are very subjective, just look at how the different vendors even here rate their systems that use the same Motenergy motors. I look at the power rating as more the motors ability to handle heat, as by throwing James' water cooler on the 5.5KW system (his rating, see below) it can become a 7.5KW system with all the same parts (maybe a bit bigger controller).
I have a Propulsion Marine 5.5KW system, and the rating comes from the max power that it can continuously run and keep the winding temperature at 100C installed in the boat. So, I can run the motor at 100% of its "rated power" for eternity (or at least until the batteries give out). Other manufacturers rate the same motor at 7.5KW and up in the same configuration, depending on how hot their willing to run the windings. I'm no genius, but I do know that the more current you put in, the more heat you generate, which is why folks can play volt games and get more "power rating" with the same gear.
Josh
-- BTW, as shown again this afternoon, I think one of the best things about my electric boat is the ability to spin the prop at any speed desired without worrying about stalling an ICE. It makes coming back into the slip a dream, even in iffy conditions. My wife took the boat out, and put her back in the slip, entirely without any prompting from me today, and looked like a pro doing it. All the other stuff like quiet, less maintenance, electric sailing, etc. is just gravy :-)
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> thanks Jeremy.... i have to get my head in gear to read your stuff but it is for responses like yours that i throw my thoughts out there... good replies to my suppositions
>
> so now I am coming around to thinking that the bigger motors are the way to go BUT that i should go ahead and stick with the smaller gen sets and battery banks... by doing so I will get only very slightly more from the motors but significantly increase their robustness by upscaling.... sometimes i feel like a ping pong ball but as i say, that is why i throw it out here :-)
>
> let me ask this, what would you consider to be running a motor at "close to its rated power"
>
> 60%, 80%, 95%???
>
>
>
> ________________________________
> From: Jeremy <jeremy_harris_uk@...>
> To: electricboats@yahoogroups.com
> Sent: Saturday, September 10, 2011 2:56 AM
> Subject: [Electric Boats] Re: why you don't want to oversize electric propulsion motors...
>
>
> Â
> Generally speaking it is far better to fit a bigger motor than required, rather than run one continuously at close to its rated power. It is true that the frictional and windage losses for a bigger motor will be very slightly higher, but the resistive loss (which is far and away the biggest power loss factor at normal operating rpm) will be much lower.
>
> For example, let's say you need 5kW with a 50V motor voltage. Your current will be 100A. Let's also assume that resistance scales linearly with motor size (it doesn't really, in practice bigger motors have lower resistance than linear scaling would give).
>
> If a 5kW motor has a resistance of 0.06 ohms then it will be losing around 600W as heat from resistive loss (about 12% of the total power being provided to the motor). This power loss comes from this formula: P = I²R, where P is in watts, I is in amps and R is in ohms.
>
> If you use a 10kW motor, with a resistance of 0.03ohms (half that of the 5kW motor, because its bigger and has thicker wire) then the power wasted as heat in the motor will drop from 600W to 300W.
>
> The penalty of using too big a motor is the extra size and weight, the small amount of extra bearing and windage drag, and the extra cost. If you can tolerate these downsides then bigger is pretty much always better.
>
> Jeremy
>
> --- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@> wrote:
> >
> > my original thoughts centered around no belts to break, being able to decouple the motors in the case of a problem 50% of the time and direct connecting because of the constant torque that a motor provides.... others reminded me that the motors would operate more efficiently if they were allowed to reasonably spin up and that cooling would be better served by spinning up.... just thought it interesting that motors had been doubled up
> >
> > as a sort of corollary question to my original one, IF an electric motor can, for arguments sake, utilize 100 amps at a given voltage but never uses more than 30amps because that provides so acceptably close to hull speed then isn't that motor over sized by more than two times? Wouldn't it be better to have half the motor as such a motor only only being able to absorb 50amps would be no detriment at all at the same power level of 30amps. Isn't 30amps at a certain voltage delivering the exact same power regardless of whether the motor could ultimately handle 50, 100 or 500amps. I realize that inertia and other factors would have a part to play but don't these other factors all tend to point to the use of the smaller motor rather than the larger?ÂÂ
> >
>
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