critterhunter
Posts: 2531
Joined: 9/24/2004
From: Brook Park,
OH, USA
Status: offline
|
quote:
ORIGINAL: LlamaFragments
Well, I set up my motor and reciever, got the propsaver mounted. I tried spinning it up, but I found a few problems...
With the TowerPro BP-21T has anyone have trouble with:
1. The shaft wobbling slightly
2. The mount and the bottom of the motor getting really hot when run for ~45 seconds
I have a 36a speed control (on accident) and a 11.1v 2250mAh 20c li-po. Are those good for this motor or did I go wrong somewhere?
First, the 36 amp ESC and 2250ma 20 C (It is a 3 cell, right?) lipo are perfect for this motor. More amp ability than you need but it's always better to have more to keep the ESC and lipo cool(er). I normaly run 30 amp ESCs in all my park flyers, even if the motor is only drawing 12 or 14 amps. A 30 amp Tower Pro ESC from Hobby City is so cheap at $14 that it doesn't make any sense to shoot for a 20 or 25 amp ESC, and the weight difference won't really be much of anything. The only time I'd opt for say a 14 or less amp ESC is if I was building a park flyer on the real small end of size, like a plane that is small and light enough to fly in the front yard, indoors, etc. Even then I've seen people run much larger speed controls than they needed at the cost of some weight.
The lipo at 2250 3 cell (series) 20C is what is most comonly bought and used in most park flyers these days. It offers a lot of amp ability if you need it (45 amps, though it's never good to pull more than about 80% of amp ability to keep it easy on the pack). Sure, you could run a smaller pack, say in the 1000 to 1800 amp capacity range roughly, on this plane if you wanted, but the savings in weight would make it harder to balance the plane and would cut your flying time down. 2250 sized packs are the most universal to use in probably 75% of all park flyers. It's big enough to power a big plane (approaching sizes that wouldn't be considered park flyer anymore), yet small enough to use on park flyers that would be considered smaller than average...approaching the front yard sized planes. I'm using these packs to power my first EDF scratch builds real soon. Most EDF people tend to go 4 cell or more, yet I researched and found that the Hobby City 2.5 and 2.68" EDFs on a 3 cell and a 3900 k/v motor will put out some pretty amazing thrust numbers (23 to 25 ounces or so), while still keeping the amps well south of 40. These builds will be posted here when I start on them.
Remember this: A load (motor, light bulb, your TV, whatever) will only draw what it needs. What this means is that it doesn't matter if your battery is the size of a pack of smokes or the size of a garage, it can not cause the load (your motor) to draw more amps than it needs or wants and thus cook it. The only thing you need to remember is that the voltage must be what you want (a 3 cell versus a 4 cell, for instance), and that your motor shouldn't want to draw more amps than the battery can provide. If that pack of smokes sized pack can only deliver 10 amps but the motor wants 20 then you've got issues which aren't going to turn out well, more than likely you'll destroy the battery, but usualy the ESC simple shuts down the motor because the voltage drop caused by the excessive amp draw will go below LVC (low voltage shutoff you have the ESC set for). Same thing applies to the ESC. You want your input voltage (battery) to match what the ESC can handle, and you don't want the ESC being asked to provide more amps by the motor than the ESC is capable of delivering. It'll get hot and do a thermal shut down (if you are lucky). Now, ESCs can be "pushed" a bit beyond their rated amp draw provided you cool them real well, such as adding an extra heat sink and providing plenty of air flow. However, this is never a wise move. I do it on a 30 amp ESC because my 12T motor on the Stryker with an 8x6E prop is pulling close to 30 amps. I want to be sure the ESC stays cool as possible, and heat only causes resistance and wastes energy anyway.
Now, down to the problem with the motor. When you spin it you are seeing the shaft wobble only or is the bell also? If it's just the motor shaft then more than likely it's bent, and can be bent back into shape with some trial and error. Did you cut the shaft down so it is just long enough to allow the adaptor to seat properly? Extra shaft length will cause vibration and also the odds of bending it.
If it's the bell too then there is a fix for this and more than likely the shaft isn't bent as well. Remove the little C-clip on the back of the motor's shaft. There probably is a washer under it too. Now pull on the bell/shaft up front (it will be hard to do) and the bell and shaft should pull out of the motor. You'll notice a nut on top and bottom of the bell that holds the shaft in. Remove the top nut and bottom one and re-center them and the shaft in the bell. Spin it and see if you got it right. There might also be set screws holding the prop shaft inside it. If it turns out that the bell it's self is bent you can tweak it in a vise and strength it out. Spin it on a table or prop balancer or something until you get the thing straightened out. Once whatever you do above is done, stick the bell/shaft back onto the motor and spin it by hand, watching the back of the bell to see if it's magnets are wobbling and/or touching the stator. If so, remove and re-adjust. By the way, when removing/inserting the bell/shaft, make sure the bearings stay in the stator's shaft hole. There is one at front and back that the motor's prop shaft rides in.
If the motor is getting real hot after say 20 to 30 seconds of stationary running (with a prop on) then that's probably not normal. Either it's rubbing inside or you have too big a prop on it. Now, after roughly 30 to 60 seconds the motor can get hot regardless of if everything is OK. Remember that it doesn't have the air flowing by it that it would have in the air, and also that static thrust (on the ground) also doesn't allow the prop to unload (less resistance in the air), and so the amps will average 2 to 4 higher than what it would normaly draw. It's a fine line between when the motor should get hot, but I'd say after 15 to 20 seconds on a static run it shouldn't be hot. Warm, maybe.
I just had a BP21 that a friend re-winded (the one for So #4, in fact) run when I installed it back into the bell/shaft. You should feel a "cog" caused by magnet force as you hand spin the bell, but you shouldn't feel uneven "cog" or grab. That would be caused by something rubbing. Well, I hand turned the bell while looking in the back and couldn't see any magnets rubbing the stator. Removed the bell/shaft and didn't see any spots inside that might rub. So I decided to run the motor (without a prop) for about fifteen seconds and then checked if it was hot. It wasn't, so I did another run for about 30 to 40 seconds. Motor still wasn't even warm and when I hand spun it this time the grab was gone, so whatever was rubbing rubbed it's self away. Problem solved. Remember to pull the battery from the ESC completely before putting your hand anywhere near the motor because that's the only safe way, and this will also avoid any confusion if you have the ESC set to brake "ON".
Didn't mean to get so long in the message but it's a good overview for newbies to RC electronics anyway.
_____________________________
If you believe in Global Warming then shut your mouth. All that hot air is warming up the planet, remember?
|
Printable Version
All Forums >> Electric Aircraft Universe >> Foamies! - RC Electric Foam Aircraft >> RE: Here's A Simple & FREE First Foam Build Plan! (Part 2) 
RE: Here's A Simple & FREE First Foam Build Plan! (Par... - 
That's no lie. For instance, with the second maiden of So #4 I flew it without ever thinking about the rudders until I had already landed. Rudders are great but I think they get in the way of learning an aileron/elevator plane. It's better not to even have it on the plane until later when you have the other controls down well. 