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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Crist Rigotti on September 17, 2010, 09:38:28 PM
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I have a Turnigy 4250-580kv motor that is rated at 4 to 5S according to the HobbyKing website. I want to run a 6S pack and keep the rpm around 10K. What do you think?
Motor:
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8506
Battery:
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=9462
BTW, the Pathfinder I was given at the FCM contest is ready to go with the above and a CC ICE 50 Lite. 56 ounces! 620 square inch ARF electric at 56 ounces. It'll be a good one....I hope! y1
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Hi Crist,
It has been my understanding that as long as we don't exceed the maximum rated watts of a motor we can vary the cell counts up and down.(That is as long as we have a speed control that can handle the voltage.) I ran that setup in the Castle link demo mode and it told me that you are good from 8200 RPM to 11000 RPM on a 6 cell battery. It looks like you will be fine.
William
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Hi Crist,
It has been my understanding that as long as we don't exceed the maximum rated watts of a motor we can vary the cell counts up and down.(That is as long as we have a speed control that can handle the voltage.) I ran that setup in the Castle link demo mode and it told me that you are good from 8200 RPM to 11000 RPM on a 6 cell battery. It looks like you will be fine.
There's some funky electromagnetic effects that can cause the iron in the armature to overheat with too much voltage. But long before that happens you'll spin the motor so fast you'll damage the bearings or tear it apart with centrifugal force or otherwise do it violence from purely mechanical forces. With some speed controllers, before that happens the speed controller won't be able to keep up, and it'll be a moot point anyway.
I suspect the "cell rating" of the motor has more to do with trying to get you into a range where a normal sport plane with a normal prop will be happy, although it could be Turnigy's way of telling you how fast the motor can spin before it smokes the bearings or bends the bell or whatever.
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Hi Crist,
It has been my understanding that as long as we don't exceed the maximum rated watts of a motor we can vary the cell counts up and down.(That is as long as we have a speed control that can handle the voltage.) I ran that setup in the Castle link demo mode and it told me that you are good from 8200 RPM to 11000 RPM on a 6 cell battery. It looks like you will be fine.
William
Thanks Bill. That was basically what I was thinking. As long as I kept it at a reasonable rpm and amp draw the voltage sholdn't really matter. Looks like 10k is a good place to be!
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You will do best if the full throttle rpm is around 75% of the rpm calculated from the Kv times the battery voltage.
Almost all or out outrunners are most efficient if the FULL THROTTLE rpm is in the range of 70% to 85% of the Kv calculated rpm.
With this rpm requirement met you will get longer flight time for a given battery size or can use a smaller (lighter) battery for a fixed flight time.
Experimentally Determing if this condition is met is difficult experimentally but a calculator like moto-calc should work.
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Thank you Jim. I'm well aware of the 75% rule of thumb that's why when I want to run a 6S pack I'm running a 580kv motor.
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That has to be the cleanest installation I have ever seen on a profile. I studied that first pix for a long time before I figured out how you mounted the motor. The looks like a really secure battery strap too.
FORGOT! Back to your first post: On my Pathfinder I have an Arrowind 2820 turning a 12x4.5P (clipped 13") turning 9700 RPM on .015x63' This dovetails nicely to your 10K target - you ought to have plenty of headroom and a very efficient set-up.
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When you refer to a pathfinder thats a big pa model and the smaller one 1/2 one is a half -A pathfinder?
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When you refer to a pathfinder thats a big pa model and the smaller one 1/2 one is a half -A pathfinder?
The full size profile of 620 square inch wing area.
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That has to be the cleanest installation I have ever seen on a profile. I studied that first pix for a long time before I figured out how you mounted the motor. The looks like a really secure battery strap too.
FORGOT! Back to your first post: On my Pathfinder I have an Arrowind 2820 turning a 12x4.5P (clipped 13") turning 9700 RPM on .015x63' This dovetails nicely to your 10K target - you ought to have plenty of headroom and a very efficient set-up.
Dennis,
Thanks. After looking at it I think I might cut the nose off about another inch aft. Glue on the 1/8 firewall then add the 3/8 balsa and 1/16 basswood nose block. This would move the moter aft and let me fair the 2" spinner in a lot more cleaner. This airplane is a test bed to allow me to test next year's power system on an airplane of about 615 sq. in. and...you guessed it 56 oz. I'm going to add some blocks fore and aft, and 2 dowels above the battery to help keep it in place. I think it would only be prudent to do so.
The prop is a clipped 13x4.5 down to 12. I have the CC ESC set to 10K for the first flight. It'll be only a minute because I use my "test" JMP timer. It'll help with laps times and to figure out waht my total current draw will be on a full flight before I try one.
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Crist,
When you cut the nose off, be sure and get all the oil and fuel residue cleaned up real good! LL~ LL~ LL~ LL~ LL~
Will
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Crist,
When you cut the nose off, be sure and get all the oil and fuel residue cleaned up real good! LL~ LL~ LL~ LL~ LL~
Will
Ha, Ha! You kill me man! #^
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Ha, Ha! You kill me man! #^
My goodness, I hope not!!! ~^ ~^ ~^
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Hey all,
I just want to add to this thread and to say that I used Crist's setup on my pathfinder and it works perfectly. I had 2 Arrowind 2820/7 motors lying around wondering what to do with them and now they have a purpose. Having trouble with my Turnigy G25 motors got me to stumble accross this thread. I am using 5s 3000 batteries on the Arrowind motor rated for 4s with no ill effects. I had to use an Eflite E15 on the Pathfinder (which is too small of a motor for that size of plane) when I figured out the 10 pole g25 motors will not work with a KR timer. I also had to go to the E15 because I didn't have a G25 replacement readily availible at a price I was willing to pay. The only thing I am doing different is using the stock APC 12x6 electric prop as is.
Hope this helps out with the 4s vs 5s vs 6s on a 4 or 5s rated motor.
Cheers.
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There are 2 main things determining number of cells for our BLDC motors:
1/ maximal allowed RPM which is usually somewhere at 15000 on our full stunter motors (smaller will be higher). This does not limit us as Will already wrote, because we use governor and it will not allow high RPM even without prop. But be carefull if you calibrate ESC, some ESCs try to run motor without prop to its max RPM.
2/ However point 1/ is not valid on LOADED motor and that is real limitation, if you do the math, then you will see that on FULL THROTTLE and loaded motor under its max RPM limit you are going to exceed not only current of good efficiency, but also highest allowed current. Simply too high voltage will not allow RPM in safe range and also save current at the same time.
So those 2 things determine rated cell count. Howeverer we need some head room for voltage, especially with active timers and our ESC is never open for full 100%, so usualy we use max num of cells +1. If you go higher, you will face troubles mentioned by Tim, especially ESC overheating.
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Christ, I say go do it. I recently did something similar, I put a 580kV (50-50) motor on 6S into a mid 80's pattern ship, as I wanted something that would act like a screaming 61.
swinging an 11x9, it was RIDICULOUS, but worked fine :)