G'day John, All
At 02:10 AM 29/03/2011, John wrote:
>Hi,
>Just another of my quick questions to try to get a handle on all of
>this.
>
>Taking it that many people use less than full power in order to increase
>range, and assuming the batteries were regular lead acid wet batteries,
>are the two scenarios below going to give ROUGHLY the same running time
>at the same power/rpm?
>1 - A 10 kw motor set to run at a 5 kw battery draw.
>2 - A 5 kw motor running wide open.
>
>What I am trying to find out, is there some kind of hidden advantage to
>have the larger motor running under less stress, or are the performances
>proportional? I am assuming no appreciable heat buildup.
No appreciable heat build-up is the key to the answer. Heat IS the
limiter to the kW rating of a motor, a 5kW motor can only do 5kW
under certain conditions, and you can be certain that the marketing
salesweasel who wrote the spec sheet picked the absolute optimum
point to call the motor by its' maximum rating. Its' real world 100%
duty rating will likely be quite a bit less, depending on RPMs, load,
enclosure cooling, etc.
The electric drag racing guys are driving motors to many 10's of
times more power than their motor was rated to, as the rating is
usually a 1-hour rating (won't over heat at 'x' amps for one
hour). Their motors don't reach that critical temperature in the 10
to 15 seconds that they are over-driving their motors. However, if
you don't meet the designated datapoint, you may melt down a motor
well under an hour at less than the nominal power.
Heat is the tell-tale of efficiency, and in a motor, heat comes from
amps ( x the resistance leads us eventually to the watts) a bigger
motor has less resistance, if it has half the resistance, at a given
output power it will have 1/4 of the waste heat of the other motor of
twice the resistance, and therefore better motor efficiency.
However, the bigger (a.k.a. heavier) motor may (in a
light-displacement hull) be putting the wetted area of the hull into
an inefficient zone, reducing efficiency by more than the efficiency
gain of the larger motor, so it is an 'it depends' thing.
The bigger motor may have a better or worse torque/speed match to the
propellor, bringing in a more significant efficiency issue (and
adding to the attraction of getting a >properly engineered system
designed for your hull<. Which for most of us would not be available,
so "by educated guess and several prop changes" you will select your
best system.
In my opinion, buy as big as you can afford that will reasonably fit.
Allow budget to try some alternative prop pitch/diameters to maximise
efficiency at your typical performance point. One day you may find
that with wind and tide against you, you'll be glad you have reserve
power to get to a safe place, instead of trying to put in a system
that is 'just big enough'.
Just my opinion, and opinions are ... well, you know how that goes!
Hope this helps.
Regards
[Technik] James
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