Ute R/C Association Come on, let's fly!

Aaron

They are basically the same except for the motors.
One says 12T the other says 21T

As you may know, the letter T stands for turns. That is the number of turns
the wire is wrapped around the stator. It all depends if you need Torque or
RPM.

The 12T (high RPM) will need smaller props but will spin at a faster speed.
The 21T (high torque) can use a bigger prop but will spin slower.
The net thrust between both setups will be about the same.

If you plan on reusing the motor for other things I would get the 12T motor
with the 25amp ESC. It can be used for more things and will easily power a
slow stick.

Mike Passey

Doug, are you using a 2S or 3S battery?

Thanks for everyone's responses!

I tossed around a few ideas about how to mount the Tower Pro brushless motor to the the included mount and I found an easy and seemingly correct method to use.

This photo shows the way I aligned and attached the motor to the mount. The beauty of doing this is that there are only two screw holes to drill, and that the motor and output shaft are concentric with the output of a brushed setup.




Here are some other photos showing the alignment. I found that the easiest way to make this attachment method work is to remove the motor mounting plate by loosening the two set screws with a 1/16" allen key, then line up the plate on the plastic mount and drill the two upper holes. Then, attach the plate to the mount with the included screws, washers, and nuts, and finally, re-attach the motor to the plate and tighten the set screws.

This method might be old news to many, but I've heard a lot of discussion about how to best mount these motors, so I thought I'd share my technique in case someone else found it useful.

-- Aaron Greer

Here is a link to the original slow stick motor thread:
http://uterc.org/node/522

OK, I got my new motor mounted as seen in my last posts, but ran into a problem when I tried to fly with it. I had a nice gentle first flight with a 2S LiPo, but then I switched to a 3S LiPo for a comparison flight. As I throttled up the motor, at about 3/4 throttle, the motor and/or ESC emitted a continuous high-pitched squeal and the prop stopped spinning altogether. I played with it a for a few seconds and the problem was consistent, every time I throttled up. The motor was quite hot and the motor windings turned dark (probably showing damage to the lacquer insulation). I saw online at RC Universe that having the ESC timing set too low can cause that type of problem, so I changed the timing to high and tried again. It works much better now, but still gets really hot with the 3S pack. It's frustrating because I got the motor, ESC, and prop together in a kit and apparently the prop is too big for 3S and the ESC isn't setup for 3S either. I guess this post is a PSA as well as a question. If anyone has the squealing problem, try increasing the ESC timing. The question is, have any of you guys run into these types of problems with the Tower Pro motor/ESC setups? What size prop should I down-shift to for use with the 3S? Thanks! -- Aaron BTW, I'm using the TowerPro BM2408-21T / 18A BEC ESC / 1047 Prop Combo from Hobby City.

One last note on ESC's: It's important to be aware that your ESC is just
varying the timing of full-throttle pulses to create an average amp draw.
But for the split-second of each ESC pulse, your motor is drawing the exact
same amount it draws at full-throttle.

A lot of flyers think they can skimp on the ESC and motor, and just fly
around at half-throttle most of the time. They typically learn too late that
you can't really short-change your electrics that way. If your ESC is rated
for 18A, and your full-throttle amperage for a given setup is 30A, you're
still drawing 30A... just in tiny, little pulses. If that amperage is too
far above the highest efficiency amperage for your motor, then you're
building up a whole lot more heat than if you'd just your system
appropriately, and, as mentioned in my last note, heat kills your magnets,
windings, and ESC.

Great response Matt. Glad we have members that will take the time and
put in the effort to guide those who are just trying to evolve!

Many thanks! Joe

On 3/21/2010 8:01 PM, Matthew P. Barnson wrote:
> Aaron,
>
> Sorry I didn't pay closer attention to this thread earlier. Based
> upon your description, you torched your motor.
>
> The TowerPro BM2508-21T specs:
> * Max efficiency current: 13.8A (this is its "efficiency peak", the
> current at which you get the most output watts for your input watts)
> * 1400 RPM per volt (Kv)
> * Max current: 16.2A
> * Maximum power: 113.4 watts
>
> Looking at each of those stats, you can figure a few things. First,
> you really want to aim your average-throttle settings somewhere around
> the max efficiency current if you can. That gives you the best "bang
> for your buck", the least heat for the amount of power you are pumping
> through the motor, and is a decent design goal to know if you're close
> to your target. The "Max Current" is somewhat variable... typically,
> you can overdrive this for a few seconds, particularly if your motor
> is hanging out there in the open air or otherwise has good cooling,
> but I prefer to regard that statistic as the "hard ceiling" for amps
> going through my motor.
>
> The last statistic -- "Maximum Power" -- is one that a lot of people
> have fun ignoring (and so does HobbyCity in their recommended prop for
> this combo). If you exceed the amp limit of your motor, you're gonna
> fry your windings or your magnets. But if you just run the motor too
> fast by applying higher voltage, there may be no down-side at all if
> the unit can mechanically hold together under that load. It all boils
> down to if the bearings can handle it, if the bell is strong enough,
> if the glue holding the magnets to the bell can handle it, etc.
> Anyway, to play with maximum power, just drop your prop size to ensure
> that you're within the amperage limits of the motor, and you're
> golden. That's one of the reasons the combo you bought even works on
> 2S... they are running higher than the rated wattage for the motor,
> but pretty much bang-on the maximum amps to the motor.
>
> Now, here was your problem. Here's a tool to help you out:
> http://adamone.rchomepage.com/calc_motor.htm
>
> I had to plug in a slightly different motor with similar specs. Based
> on the results, even on 2S, you're running that ESC and motor right at
> the limits of their performance. Plug in 3S, and suddenly instead of
> around 165w into the motor at 25A on 2S, the motor is trying to pull
> 400+ watts at over 40 amps on 3S!
>
> The only way to make that kind of change without frying your motor is
> to drop the prop size or adjust your gear ratio if running through a
> gearbox. Dropping down to a GWS 8x4 would have kept you right around
> the same wattage as before (still higher than I'd be willing to run
> with that motor/ESC combo), and within the limits of your 18-25A ESC
> and motor rated for 16 amps.
>
> I've made this mistake myself. Went to a higher voltage without
> dropping prop size and let the smoke out on a big, .40-sized motor,
> but it still seemed to run OK afterward. That is, until the day a few
> weeks later that I dead-sticked my bird and the motor never ran again.
> The damage had been done on the bench, but took a few weeks of usage
> to fully destroy the magnets.
>
> There are lots of good articles on how you can create a thermal
> runaway condition in your motor, but here is the basic outline. By
> over-amping your motor, your magnets -- not to mention your windings