Colin Girvan wrote:
>
>
> I have a hypothetical question that I hope you electrical engineers
> out there can give me an answer to.
>
> If we have a boat that takes 5kw of power to do 5kts in calm
> conditions, so this becomes a constant.
>
So no matter what the hardware is capable of, 5kw is the power required
to move the boat. It as you say, a constant.
>
> We have a 5kw motor providing the power at 48 volts.
>
The motor would then run at 100% of it's rating to provide the power
you want.
>
> Now my question is, if we change this motor to a 10kw motor at 48
> volts would the draw from the batteries be the same to do the same
> 5kts assuming nothing else has changed.
>
5kw is what your boat requires, so 5kw is what the motor will output,
the motor will run at 50% of it's max rating. The current drawn from the
batteries will be the same - assuming of course you are using the same
prop turning at the same RPM as above. If the more powerful motor wants
to turn faster you could be wasting power trying to push the boat past
it's efficient cruising speed and the extra power will just disappear
into creating extra wash.
>
> Secondly if we used a 5kw motor at 96 volts would the power draw from
> the batteries again be the same to do 5kts.
>
Assuming the RPM and prop are the same again, given that power equals
volts x amps, since the volts has increased, the amps is lower.
Depending on equipment factors the lower current might gain you some
electrical efficiency in the system.
> I'm trying to get a better understanding of the consequences of using
> larger motors other than extra weight and cost.
>
Larger motor is just more weight. The higher voltage might gain you a
small amount in electrical efficiency as the 'i squared r' losses will
be lower.
My own boat uses about 2.5kw to reach it's cruising speed. (measured)
The actual motor can output 13kw in it's maximum 'short term' power
configuration. (72v, 200a), although it's continuous rating is about 8kw.
I'm running on 36v where the maximum power is about 6kw, but the
controller has been programmed to further limit the maximum output.
So why a motor that can run at 13kw instead of one that runs say at 4?.
I wanted a high efficiency permanent magnet motor, and that's how it
came. I could have found a 4kw golf cart motor or something but it
probably would have been a series-wound motor or similar and would have
had higher loses and possible even weigh more. The motor I have could
also be used in direct drive in my application as well, so a very simple
transmission, (no reduction gear).
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