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Cell count on brushless motors .

Old 04-14-2009, 02:47 PM
  #26  
BEX
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I did a quick trial on 5S , keeping to 600W . The esc was very hot after about 30 seconds, I did not realize that it would get that hot that quick. The motor was warm and it felt like it could be getting hot before long.
I left it for the moment to cool down, I will try it with a smaller prop at lower watts and with the esc outside in the propwash. If they still heat up too much ill stick to 3S and try a 10x10 prop to see if that gets me the speed that I want.
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Old 04-14-2009, 08:49 PM
  #27  
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The 10x10 did not give enough thrust for a good takeoff, I am going to abondon the tests for a while as i realize that the extra weight of my 5S lipos will be just too much , 3S will have to do for now.
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Old 04-14-2009, 09:53 PM
  #28  
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Two more things to consider.

One, the modeler must be aware of very high RPM's on the outrunner motors, and what happens if the rotor is just slightly out of balance. The higher RPM's would accentuate any vibration problems, especially with the cheaper brushless motors.

Two, what limit, if any do we have with just how fast the ESC's can switch the power to the motors windings?
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Old 04-14-2009, 10:29 PM
  #29  
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Originally Posted by BEX View Post
The 10x10 did not give enough thrust for a good takeoff, I am going to abondon the tests for a while as i realize that the extra weight of my 5S lipos will be just too much , 3S will have to do for now.
The 5cell should be similar weight to the 3cell one... same watts, lower amps... same flight time...

for instance...
3cell 2150mah 45amps is 499 watts with 2.86 minutes of flight time.... (186gram Rhino battery)
4cell 1750mah 35amps is 518 watts with 3 minutes of flight time.... (199grams Rhino Battery)


pushing 10x10 or similar props will definately give noticeably less thrust... try jumping on your throttle more for takeoff....

SK
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Old 04-14-2009, 10:31 PM
  #30  
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Originally Posted by kyleservicetech View Post
Two, what limit, if any do we have with just how fast the ESC's can switch the power to the motors windings?
hopefully, the limit is at least what the esc is rated for...LOL

SK
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Old 04-14-2009, 10:45 PM
  #31  
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Steve , my problem is that i use a 3300mAh 3S lipo , the smallest 5S that I have is a 3000mAh and I want to try get by with what Lipos i have.

Regarding the esc , from what i've read the limit on the esc will be well beyond the limit of the rest of the components.
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Old 04-14-2009, 10:51 PM
  #32  
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Originally Posted by ministeve2003 View Post
? I don't get what your trying to say Ron.... Bex and I are talking about adding a cell and Proping Down to keep same watts... Obviousely the prop selection wouldn't stay the same...
My remark was just a general remark about influence of changes in voltage only. Lots of people add an extra cell, not knowing that current goes up squared. And then they fry motor and/or controller.

Last edited by ron_van_sommeren; 04-14-2009 at 11:14 PM.
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Old 04-14-2009, 10:53 PM
  #33  
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Originally Posted by BEX View Post
Steve , my problem is that i use a 3300mAh 3S lipo , the smallest 5S that I have is a 3000mAh and I want to try get by with what Lipos i have.

Regarding the esc , from what i've read the limit on the esc will be well beyond the limit of the rest of the components.
I hear ya on that one... I don't like buying extra dif size stuff If I don't absolutely need it.... and 1 fast speed run just for kicks, when you know you'll just put a dif prop and run the other batterys on it later is hard to justify...LOL

Ps, how about paralleling a 2cell and 3cell packs togeather....

SK
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Old 04-14-2009, 11:01 PM
  #34  
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Originally Posted by ron_van_sommeren View Post
My remark was just a general remark about influence of changes in voltage only. Lots of people add an extra cell, not knowing that current goes up squared. And then the burn motor and/or controller.
your right about that... I've seen a few people add a cell without knowing they had to prop down... they smoked the motors in seconds...

SK
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Old 04-14-2009, 11:12 PM
  #35  
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Originally Posted by Biffidum View Post
Does EMF (electromotive force) play into this? I am no engineer, but I believe that as the RPMs increase counter-EMF increases and this will rob you of efficiency. ...
Nope, BEMF does not come into play here. But, as RPM increases, both hysteris losses (proportional to RPM) and eddy current losses (proportional to RPM²) in the stator increase. Both losses lumped together are the 'iron losses' and togheter with the 'copper resistance losses' in the winding wire they make up most of the losses in a motor.

Because of the wire-resistance, there is a voltage drop over the wire (voltage_drop = wire_resistance*current). The voltage the motor sees under load, drops. That means that rpm will drop too. RPM of a motor with zero copper resistance would not drop under load at all, RPM would always be Kv * Voltage.

Have a look at Joachim Bergmeyers article, very enlightening:
http://www.rcgroups.com/forums/showt...ight=bergmeyer

Originally Posted by BEX View Post
... What Ron is saying is that the force required to spin a prop at higher rpm is will give an exponential curve ...
Torque needed goes up squared with rpm (ref. force needed goes up squared with speed)
Another way of looking at 'current goes up squared with voltage':
Torque and current are proportional (torque = Kt * current), current must go up squared with RPM. But since RPM and voltage are proportional (rpm = Kv*voltage) ...

Originally Posted by ministeve2003 View Post
... Less Heat = Better Efficency....
Better write it the other way round: higher efficiency means less electric energy converted into heat.

Higher efficiency does not only mean that the motor makes better use of the batteries' power, it also means the motor is able to handle a higher power input before hitting its maximum temperature mark i.e. a the power/weight ratio will be higher.
An example:
Say the motor has an efficiency of 70% and it can handle 50Watt input. That means it can get rid off 30%*50=15Watt excess heat. Now, by cramming in thicker wire (and/or using better stator-iron, segmented magnets), efficiency increases to say 75% (I'm a bit optimistic here). The motor's ability to loose those 15Watts has not changed (by radiation, convection and conduction). This means the motor now can handle 60Watt before it hits the 15Watt (25%*60Watt) losses mark. An efficiency increase of 5% gives an increase in the power to weight ratio of 20% (from 50Watt to 60Watt). That's why efficiency plays such an important role, in any motor design: efficiency governs maximum power. The motors weight may have increased a bit due to more copper.

Originally Posted by kyleservicetech View Post
... One, the modeler must be aware of very high RPM's on the outrunner motors, and what happens if the rotor is just slightly out of balance. The higher RPM's would accentuate any vibration problems, especially with the cheaper brushless motors. ...
Some manufacturers add a third bearing to prevent this from happening at high rpm's. E.g. the Kontronik 'Kora' and 'Pyro' series, the Torcman 'Triton' series, the Plettenberg Orbit, Xtra, Terminator(2huge, 3, 4?) and Predator(2huge, 3, 4?) series and a number of Scorpion motors.




Vriendelijke groeten Ron

Last edited by ron_van_sommeren; 07-03-2009 at 12:17 PM.
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Old 04-14-2009, 11:21 PM
  #36  
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Originally Posted by ron_van_sommeren View Post
That would spark a nice conflagration

Vriendelijke groeten Ron
do they have to be the same cell count? I'll have to look at how they're wired....

SK
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Old 04-14-2009, 11:32 PM
  #37  
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Originally Posted by kyleservicetech View Post
... what limit, if any do we have with just how fast the ESC's can switch the power to the motors windings?
Depends on the number of calculations per second the ESC can do: type of microprocessor/microcontroller used and its clock-frequency. To the controller, a 14magnetpole motor running at 30krpm is exactly the same as a 2magnetpole motor running at 210krpm.
Table listing maximum rpm per ESC make, type and number of magnetpoles in motor, by Christo van der Merwe (user 'skylar' on RCGroups, you can write him in Dutch )
www.bavaria-direct.co.za
-> Motor building Info
-> Max. RPM Limits (.pdf file)

Originally Posted by ministeve2003 View Post
... how about paralleling a 2cell and 3cell packs togeather....
That would spark a nice conflagration Call North-Korea and/or Iran.

Vriendelijke groeten Ron

Last edited by ron_van_sommeren; 04-14-2009 at 11:48 PM.
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Old 04-14-2009, 11:34 PM
  #38  
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Thanks Ron , will take a bit of time to digest everything . I know Cristo thanks to your earlier introduction to SARFLY thanks.
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Old 04-14-2009, 11:37 PM
  #39  
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Originally Posted by ministeve2003 View Post
do they have to be the same cell count? I'll have to look at how they're wired....
YES!!! When paralleling packs, they MUST have the same cell count, same make and type, same capacity, same history (charging and usage). Always charge them in parallel too.

Automatic wiring diagram generator:
http://scriptasylum.com/rc_speed/_lipo.html
http://scriptasylum.com/rc_speed/_wiring.html

Vriendelijke groeten Ron
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Old 04-15-2009, 01:08 AM
  #40  
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Originally Posted by ron_van_sommeren View Post
YES!!! When paralleling packs, they MUST have the same cell count, same make and type, same capacity, same history (charging and usage). Always charge them in parallel too.

Automatic wiring diagram generator:
http://scriptasylum.com/rc_speed/_lipo.html
http://scriptasylum.com/rc_speed/_wiring.html

Vriendelijke groeten Ron
Am I missing something? here's a simplified picture....

it shouldn't matter as long as the capacity and make are the same, and as long as you balance charge them and don't over discharge them....

SK
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Old 04-15-2009, 01:14 AM
  #41  
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making a Serial adaptor is easyier to use because you can take it off to balance charge it with the balance taps already on the packs.... I do this with some 2cell packs I run on a stryker for a 4cell config...

SK

Last edited by ministeve2003; 04-15-2009 at 02:12 AM.
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Old 04-15-2009, 01:22 AM
  #42  
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LOL, I see what I missed... I ment serial ....LOL

SK
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Old 04-15-2009, 05:21 AM
  #43  
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Well you've actually stumbled upon one of my old Stryker secrets. Running low kv motors on high voltages. Watts are watts, how you get there for the most part is up to you. 10 volts x 20 amps is 200 watts. So is 20 volts x 10 amps.

Quality motors helps too. I ran Mega motors often the 25/16/4's. Rated at 25 amps.
I ran them all the time way over that rating, sometimes in the 50 amp range, and NEVER had an electrical failure. We won't discuss the other ways I've damaged them.
But back to the voltage thing and my trick. The low kv motors are designed to spin a big diameter prop at a low amp draw. Upping the cell count to sometimes 6 instead of 3, and dropping the prop size gave me similar speeds to the hot high kv set ups.

But I still had a low amp draw, and I could run a bigger pitch and diameter prop then the so called hot set ups. Meaning I could fly at the same speeds, but take off faster from the line, and go vertical all day long. If I got low and slow, I could punch the throttle and because of the bigger bite props and higher then normal rpms I could leave the hot set ups in the dust. They would have to wind up to get to speed, due to having a high rpm but low thrust set up.

Also I could cheat on the weight factor too. Lower amp draw meant I needed a lower capacity pack. So what I added in cell count weight, I could at the same time subtract in capacity weight. For the most part staying near the same weight as the lower cell count higher capacity packs the hot set ups used.

Like the hot set ups were using a 3S2100 or 2500 20C pack, I would run a 4S1800 20C, getting better performance and similar run times. And weight wise nearly the same.

And hey if you go heavier it will handle the wind better.

Last thing, I noticed you saying about running some tests to get specs, and the esc getting hot. Static run ups should be limited to quick bursts, just enough to get wide open readings, then allow a cool down period before more testing.

The esc needs airflow over it for cooling, and sitting still will make it hot quick. And you can cook an esc quickly this way.

Something to think about too, if you have a pack plugged into your bird and it's just sitting around waiting to fly. The esc's parts are dissipating heat as they bleed off the power so it doesn't go to the motor. (layman's terms) The esc will get hot waiting on you, as there's no cooling air flowing over it,even more so if you're running a bare minimum amp capacity esc. Lower capacity means less mosfets to dissipate the heat, kinda like running a really small radiator on an engine.
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Old 04-15-2009, 05:41 AM
  #44  
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all things being equal, would a 10" prop pulling say 500grams thrust, feel any different than a 9" prop with 500grams thrust... assuming you keep prop pitched for the same speed...?

Last edited by ministeve2003; 04-15-2009 at 08:14 AM.
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Old 04-15-2009, 10:43 AM
  #45  
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Originally Posted by Glacier Girl View Post
The esc will get hot waiting on you, as there's no cooling air flowing over it,even more so if you're running a bare minimum amp capacity esc. Lower capacity means less mosfets to dissipate the heat, kinda like running a really small radiator on an engine.
Hi Glacier Girl , thanks for the input. The esc is a 60 A and I ran it at 36A for 30 seconds, and it was very hot , it did not have good airflow , but i cannot see how even good airflow would dissapate that much heat. I am sure running it at 30 A and having it in a good airflow would be better .
I might still considder running it on 5S as the motor did not get too hot. My motor is a 1000 Kv , so at 18.5 V the RPMs are getting high.
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Old 04-15-2009, 01:09 PM
  #46  
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Better controller cooling:
www.mgm-compro.com
-> tech.info

Vriendelijke groeten Ron
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Old 04-15-2009, 04:43 PM
  #47  
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ron van summeren
Some manufacturers add a third bearing to prevent this from happening at high rpm's. E.g. the Kontronik 'Kora' and 'Pyro' series, the Torcman 'Triton' series, the Plettenberg Orbit, Xtra, Terminator(?) and Predator(?) series and a number of Scorpion motors.

Add to this list the Hacker A40- 50- 60 series motors. These are four ball bearing motors, per the literature enclosed with the motor. The fourth ball bearing is a large outside diameter ball bearing that directly supports the "Bell End" of the rotating magnet structure in their outrunner motors. The large diameter ball bearing also has a large "Inside diameter", to allow bringing the motor leads out from the motor stator through the inside of the large diameter ball bearing.

This large diameter ball bearing absolutely prevents the rotating magnet structure from ever hitting the stator, at least in normal operation. If you crash the model into contrete, anything can happen!
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Old 04-15-2009, 07:08 PM
  #48  
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Originally Posted by BEX View Post
Hi Glacier Girl , thanks for the input. The esc is a 60 A and I ran it at 36A for 30 seconds, and it was very hot , it did not have good airflow , but i cannot see how even good airflow would dissapate that much heat. I am sure running it at 30 A and having it in a good airflow would be better .
I might still considder running it on 5S as the motor did not get too hot. My motor is a 1000 Kv , so at 18.5 V the RPMs are getting high.
BEX,

Was the ESC rated for the number of cells you were running through it?

I've tried running an older brushless ESC that was rated for 12 NiCd cells on 4S LiPoly, and it went into thermal shutdown on me shortly after takeoff.
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Old 04-15-2009, 07:20 PM
  #49  
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The esc is rated 2S-6S , i ran 5S through it.
I ordered a couple of props to try get more from the motor , i think the 12x8 will be the ticket.
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Old 04-15-2009, 07:43 PM
  #50  
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Originally Posted by BEX View Post
The esc is rated 2S-6S , i ran 5S through it.
I ordered a couple of props to try get more from the motor , i think the 12x8 will be the ticket.
10x10 and 12x6 should pull very similar amp results... you might want to be careful and check temps after 1-2minutes on your first trail with a 12x8...

if you look at this motor, I've tested the 10x10 at 47amps 700 watts (4cell)...
his result was pretty much the same for a 12x6...
http://74.125.127.132/translate_c?hl...eiF7H4nvksBheQ
SK
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