| Doug McLaren wrote:
| Sure, a brushed motor may be
| > cheaper, but the added efficiency of a good brushless motor means you
| > can save money on your batteries, more money than the additional cost
| > of the better motor.
| That statement is, doing the maths, false, for most smaller sizes of motor.
You didn't read the post you're responding to very carefully. I
qualified my statement before the part that you quoted --
As for brushless motors, at this stage in the game, if your plane uses
more than say 200 watts or so, you really should have a brushless
motor, especially if you're using LiPos.
If you're going to correct me, at least try and read my post carefully
enough to make sure I'm wrong first.
| To get - say - 100W out of a 60% efficient can, takes 160W
| To get the same out of a 85% efficient brushless, takes 117W.
Which part of 160 watts is higher than 200 watts?
In any event, perhaps 300 watts would be a more definitive cutoff, but
I still stand by my statement -- the larger your setup, the more you
want brushless, even if it's just for economic reasons.
I was really aiming at setups where a speed 400 or speed 480 motor
wasn't adequate anymore. Your basic speed 480 can motor might be able
to handle 200 watts, but it won't last long at that power level. And
once you start getting into the larger brushed motors (larger than a
480) the price differences between them and the equivilent brushless
become smaller and smaller.
| The difference in cost between the (say) speed 480, at $10 with a $20
| speed controller is and a $50 Mega 16/15 brushless and $50 speed
| controller is about $70.
My main point was economics, so don't forget that if you are pumping
170 watts into it, that speed 480 motor won't last long, so let's
count a few of them (because the brushless motor will last much
Now, you could get a higher quality speed 480 motor, with rare earth
magnets and such, and you may even be able to replace the brushes
seperately, and the efficiency will improve, but it'll cost almost as
much as a brushless motor ...
| The TOTAL cost of a 3s2200 LIPO pack to power teh brushed motor is
| not $70 ...
... except that you're forgetting at least two factors :
1) if you put in a bigger pack, it'll weigh more. So you'll need even
more power, which will require an even bigger pack for the same
peformance. I'm talking about performance in a plane, not on the test
2) most people have more than one pack for a plane. Three sounds more
reasonable. So triple the difference ...
(Though to be fair, the brushless cost benefits will go down if you
have several planes that all use the same battery packs.)
| the difference between a 1600 and 2200 pack to 'make up for the
| brushed inefficiency' is almost negligible.
3 cell 1500 mAh LiPo pack at CBP = $52.
3 cell 2100 mAh LiPo pack at CBP = $79
Triple that difference and you get $81, which is *more*
than the added
money that you spent on the brushless setup. And this is a case where
we're looking at power levels *lower*
than the break-even point I'd
proposed, and I'm ignoring any additional capacity needed due to the
additional weight (perhaps I should be looking at the three cell 2500
mAh pack for $100 instead?).
| The only downside on the brushed setup is increased motor
| weight. That is an issue ONLY with ultra high performance
Well, it's a small issue, but it's not a non-issue. All that weight
adds up. Also, comparing that Mega 16/15 brushless to a speed 480 is
hardly fair -- the Mega can put out a lot more power. Comparing it to
a speed 500 or maybe 600 motor would be more reasonable -- and that
bigger motor will weigh a whole lot more.
Doug McLaren, firstname.lastname@example.org
That which does not kill me can still really hurt.
Click to see the full signature.