G'day Colin, All
At 06:27 AM 9/11/09, you wrote:
><snip>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.
Since firstly you need some basics, the answer is "it depends".
If you are comparing typical commutator-style DC motors, some things are
able to be said:
1) A motor that is rated 5kW continuous can peak at 25kW for as long as you
can keep it cool. What limits the power is the heat build-up. Amps causes
heat, as a function of the DC resistance of the motor and the effective
cooling. Most motors are class 'H' insulation, 130 degree celcius (if I
remember correctly) so keep the motor below that and it is still OK.
2) A 10kW motor has a lot less resistance than a 5kW motor, probably about
1/4 the resistance. Running a 10kW (output) motor at 5kW (output) should
use a little less energy from the battery pack (less energy lost as heat,
and in the mechanics used to extract the heat).
3) A 10kW motor will weigh around twice as much as the 5kW version. Since
we're talking boats, this is more drag. Will that much more drag be more
than the efficiency gain? Depends on the boat.
4) If your 10kW motor can only deliver 5kW with full battery voltage on it,
then is there advantage to putting a 10kW motor in? That is very dependent
on the particular motor and installation, various things can be done (field
weakening of a wound field motor, etc) to give more power when needed.
>Secondly if we used a 5kw motor at 96 volts would the power draw from the
>batteries again be the same to do 5kts.
Again, it depends on the situation, you'll probably need to change your
controller, the higher voltage controller will likely have more losses than
a lower voltage controller. Not a lot higher, but should be measurable. If
you use a higher-amperage controller, the controller losses may be less, in
which case the power drawn will be the same.
This question has a great many variables to influence the answer, but in a
nutshell, a 10kW motor will probably give your same speed with slightly
less battery consumption. Wether you can get more power is also an 'it
depends' question.
Hope this helps
Regards
[Technik] James
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