there is a sort of cascade effect to over sizing motors.... if you put in a larger motor than you can use and WANT to be able to use them then you will need a larger battery bank and larger gen sets to recharge them in the same amount of time.... it seems to me that running a motor at 60% (30 out of 50 possible amps) isn't that much harder on it than running it at 30% (30 out of 100 possible amps)... boat manufacturers over estimate hull speeds to the point that the sweet place is probably about 70% what they claim... being as electrics have all that torque for maneuvering and such they have a tremendous advantage around the marinas where i would mainly use them anyway... if i have to cruise at 6.0 knots rather than 8 and in the process double my cruising distance it seems a very good trade off... it's a personal choice of course but I am going to have to do some serious thinking/testing to determine against the idea of using smaller motors... i think that everybody's knee jerk is to go larger but what i really want is bullet proof... i'd rather carry a spare motor, controller and adjustable dc-dc converter while combining this with two 7.5kw gens and enough batteries for about 2 hours cruising than have to oversize everything for that 2 knots that i don't care that much about.... it would be a very expensive two knots... the back emf regen problem also rings true a bit as i have now heard more than once that you will actually regen more IF you run the motors at very low amp rather than set them for no power at all.... blathering a bit, sorry about that... thinking with my fingers... input gratefully accepted
From: luv2bsailin <luv2bsailin@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Friday, September 9, 2011 10:53 PM
Subject: [Electric Boats] Re: why you don't want to oversize electric propulsion motors...
Mike,
I agree that using a 100A motor when you'll never run more than 30A it probably overkill. If, however, your normal cruise is 30A and you want good efficiency plus some reserve for maneuvering then the 100A motor makes good sense. There is no real disadvantage to going over-sized (to a point) other than cost. A 100A motor operated at 30A will be more efficient and last longer than a 30A motor run at it's limit. I don't see motor inertia or weight being a significant factor on a boat. I say go large if you can.
This is not to say a motor can't be operated near it's limit, but when you start "pushing the envelope", efficiency starts to drop and things like component sizing, thermal management, gear ratios, and all the other engineering choices become more critical.
Jim
--- 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?Â
>
>
I agree that using a 100A motor when you'll never run more than 30A it probably overkill. If, however, your normal cruise is 30A and you want good efficiency plus some reserve for maneuvering then the 100A motor makes good sense. There is no real disadvantage to going over-sized (to a point) other than cost. A 100A motor operated at 30A will be more efficient and last longer than a 30A motor run at it's limit. I don't see motor inertia or weight being a significant factor on a boat. I say go large if you can.
This is not to say a motor can't be operated near it's limit, but when you start "pushing the envelope", efficiency starts to drop and things like component sizing, thermal management, gear ratios, and all the other engineering choices become more critical.
Jim
--- 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?Â
>
>
__._,_.___
MARKETPLACE
.
__,_._,___
No comments:
Post a Comment