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Cell count on brushless motors .

Old 12-07-2008, 09:37 AM
  #1  
BEX
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Default Cell count on brushless motors .

Question about cell count for brushless motors.

Just been wondering , what determines the voltage that can be used in a motor.
What will happen if i use 4S on a motor of which the specs says 3S .
And what will happen if you use 5 S , and then 6S ?
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Old 12-07-2008, 10:21 AM
  #2  
slipstick
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It will turn faster and faster until something dies possibly the bearings collapsing. If you don't drastically reduce the prop size it will also draw more and more current so maybe the windings will burn out before the bearings die.

Steve
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Old 12-07-2008, 11:07 AM
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BEX
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OK , but how about a 750 KV 3S motor ? say you put that on 6S , the RPM will still be lower than a 1600KV on 3S , so will the bearings hold up then?
And if I prop it down enough to kep A low , will this motor still die prematurely?

I am more interested about the theory behind it all , just want to understand what makes a 3S motor different to a 4S motor.
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Old 12-07-2008, 12:22 PM
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groundrushesup
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Originally Posted by BEX View Post
OK , but how about a 750 KV 3S motor ? say you put that on 6S , the RPM will still be lower than a 1600KV on 3S , so will the bearings hold up then?
And if I prop it down enough to kep A low , will this motor still die prematurely?

I am more interested about the theory behind it all , just want to understand what makes a 3S motor different to a 4S motor.
Firstly - in my experience, motors are usually listed as compatible with a range of cell counts, like 2S-4S.

A motor spec'd for 3S at 750Kv is likely a pretty small motor - small stator diameter, etc. I think the first problem you would run into trying to double the cell count - even if you did keep the amperage low by propping down to a ridiculous extent - would be the motor's inability to dissipate heat fast enough to save itself. You would be putting in a lot of watts and getting less thrust, so the energy has to go somewhere.

Play around with this Motor Calculator, and pay attention to Amps, Watts In and Watts Out for a particular prop size/pitch. It can explain it far better than I can, especially at this hour.

GRU
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Old 12-07-2008, 02:36 PM
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Originally Posted by BEX View Post
I am more interested about the theory behind it all , just want to understand what makes a 3S motor different to a 4S motor.
Sorry, thought it was a practical question. Explaining the complete theory of motor design for specific voltages and other conditions is a bit beyond me but good luck with your quest .

Steve
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Old 12-07-2008, 03:52 PM
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You can put any voltage you want on a motor, there is no limit other than the mechanical construction of the motor and the electrical capability of the motor and supply.

From a mechanical point of view, the faster a motor rotates the greater the centrifugal forces on the windings, (trying to throw the windings off the stator). The bearing should be able to stand most things, (unless cheap and badly designed). For brushed motors, the commutator usually had a rotational limit before it flew apart, or because the brushes couldn't stay in good contact.
It's usual recommended to never run a motor flat out without a load, such as a prop, as it may exceed a safe design rpm.

From the electrical point of view. Watts is also heat. Windings that are held in place with some sort of bond, glue etc., when very hot may start to move or the insulation may break down and windings short out.

Manufacturers usually put some sort of recommendation on their motors, (volts/watts/rpm), so that people don't go and destroy a motor through misuse, and then keep claiming a replacement because it 'melted', or caused damage when it came apart.

It's up to the end user to determine what is safe to them, the manufacturer sets their recommendation.
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Old 12-07-2008, 05:41 PM
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On an brushless outrunne where the windings are stationary which eliminates a lot of the problems you mentioned above eflight-ray , would you say that an outrunner is safer to run at higher V than other ?
The stationar coils could be more difficult to cool on the other hand.

I did a check on that motor calc and what interests my most is that motor efficiency seems to get better the higher the V . But even though it gets better, because of the higher power use the amount of power disipated as heat also becomes more.
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Old 12-07-2008, 05:47 PM
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Originally Posted by BEX View Post
On an brushless outrunne where the windings are stationary which eliminates a lot of the problems you mentioned above eflight-ray , would you say that an outrunner is safer to run at higher V than other ?
The stationar coils could be more difficult to cool on the other hand.

I did a check on that motor calc and what interests my most is that motor efficiency seems to get better the higher the V . But even though it gets better, because of the higher power use the amount of power disipated as heat also becomes more.
Missing the most obvious issue here because it was 4am my time: The weight of a 6S battery compared to that of a 3S and how that will affect the AUW of your aircraft.

The overhead (cell packaging, wiring, etc) for a 6S battery makes running it with a low KV motor as opposed to running a 1500Kv motor with a 3S for the same application quite pointless.
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Old 12-07-2008, 06:15 PM
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Originally Posted by groundrushesup View Post
Missing the most obvious issue here because it was 4am my time: The weight of a 6S battery compared to that of a 3S and how that will affect the AUW of your aircraft.

The overhead (cell packaging, wiring, etc) for a 6S battery makes running it with a low KV motor as opposed to running a 1500Kv motor with a 3S for the same application quite pointless.
That is more a practical thing. With the higher V of the 6S you need less mAh for the same amount of power , but when you go smaller and lighter that is probable the true.

It is all very interesting , keeps me thinking while i am not building .
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Old 12-07-2008, 08:50 PM
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A touch of brain fade regarding rotating windings and brushless. :o

Which leaves heat being the main culprit for brushless motors. With inrunner possibly also suffering from the magnets on the outside of a rotating part and those centrifugal forces. Outrunner magnets being retained inside the rotating part.

As Watts = volts x amps (in it's simpler form).

Increasing the volts and reducing the current can give the same watts, the good trade off is the reduced current.
Lower current = longer flight from the same battery, (or possibly the same flight time, but with a smaller capacity, and lighter battery. Food for thought.)
It's just to reduce the current requires a smaller prop.

Now I don't have a clue if a low Kv motor on high volts turning a 10x6, is better or worse than a higher Kv motor on lower volts turning a 10x6.

I just stick a prop on and check my wattmeter.

I've now reached the point where my brain hurts, (what's left of it).
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Old 12-07-2008, 10:43 PM
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I took this motor and did some experimenting on calculators.
Axi 2820/12 goldline

The following result are interesting , the motor specs are given as 3S-5S

V .........................11.1 / 22.2
A .........................33.6 / 27.1
Input W ................314 / 525.9
Output W ..............243 / 454.6
Efficiency ..............77.5% / 86.4%
Prop .....................12x6 / 8x4
RPM ......................7415 / 17793
Power lost as heat W 71 / 71.3

I cold not get better efficiency on 5S . Granted , the RPMs are getting a bit high.
but it was a 750 kV then they would still be ok, I just could not get a 750 KV on the calculator.
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Old 12-08-2008, 12:40 AM
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I did this test myself on this motor.... its only recommeded for 3cell....
http://www.hobbycity.com/hobbycity/s..._23A_Outrunner

I'm now using it on a light stryker with a 1000mah 4cell pack with a 7x7 prop pulling 220watts... makes it fairly speedy, but extreemly light....

I also use it in a couple other things with 3cell packs on 10x4.7 props pulling 220 watts...

My theory is prop similar on watts recommended... and you should be ok... but that is just my viewpoint, and you can try it at your own risk....

SK
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Old 04-13-2009, 07:45 PM
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Reviving this thread because i am at the point of actualy using a motor rated as 3S-4S on a 5S Lipo. The reason for this is that I dont have any 4S lipos while i have a bunch of 5S lipos.
The motor that i plan to use like this is the Rimfire 42-40-1000
The motor can handle 45 A sustained on 4S , this gives 666Watt .
I plan to use 5S but prop it down for a max of 600Watt . This should give me some safety margin regarding heat , I only need about 550 watt for this aplication anyway.
Going to try it at own risk just want to hear what you guys have to say about it.
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Old 04-13-2009, 08:06 PM
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Originally Posted by BEX View Post
Reviving this thread because i am at the point of actualy using a motor rated as 3S-4S on a 5S Lipo. The reason for this is that I dont have any 4S lipos while i have a bunch of 5S lipos.
The motor that i plan to use like this is the Rimfire 42-40-1000
The motor can handle 45 A sustained on 4S , this gives 666Watt .
I plan to use 5S but prop it down for a max of 600Watt . This should give me some safety margin regarding heat , I only need about 550 watt for this aplication anyway.
Going to try it at own risk just want to hear what you guys have to say about it.
If your gonna risk it.... then do the same as I did in the post above yours... prop for the same watts as the 4cell, the amps will be down, so there will be less heat, and better efficency...

SK
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Old 04-13-2009, 08:24 PM
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Thats the plan Steve , got it from that post of yours tthanks.
did you notice increase in motor heat at higher V?
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Old 04-13-2009, 09:20 PM
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no, I actually noticed it had less heat, but If you push them too much passed the same watts, they burn the windings without getting very hot... I'm guessing too much voltage through them at once can be bad too....

SK
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Old 04-13-2009, 09:21 PM
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I'd say if you've ran that motor to 666watts on 4cell, your prob safe doing it on 5cell...

SK
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Old 04-13-2009, 09:27 PM
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here's a usefull site I found awhile back... this guy posts some prop test for various motors... I'll give you some good info for using more cells...

http://translate.google.com/translat....com/index.php

Look on the right side and click "engines"

SK
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Old 04-13-2009, 09:34 PM
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This is a video of the plane that i have it on , http://www.rcuvideos.com/video/aerobatic-maiden-met-outtakes-w/theater#theater_title

This flying was done on 3S . It flew fine, but needs more speed to do what i want the plane to do.
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Old 04-14-2009, 02:45 AM
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A motor cannot be viewed/modelled as a simple resistor where current goes up linearly with voltage. RPM goes up linearly with voltage (rpm = Kv x voltage), motorcurrent wants to go up squared with voltage and power needed goes up qubed with voltage

Going from 3 to 4 cells, keeping everything else the same gives an increase in current of (4/3) = 16/9 = 1.8, 80% extra, almost doubling current!

Code:
cells factor extra
1->2   4.0     300% 
2->3   2.3     130%
3->4   1.8      80%
4->5   1.6      60%
5->6   1.4      40%
or

Code:
cells 1  ->  2  ->  3  ->  4  ->  5  ->  6
factor  4.0    2.3    1.8    1.6    1.4
extra   300%   130%   80%    60%    40%
Vriendelijke groeten Ron

Last edited by ron_van_sommeren; 03-25-2014 at 04:39 PM.
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Old 04-14-2009, 03:31 AM
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? 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...

SK
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Old 04-14-2009, 04:41 AM
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Default EMF

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. I should really check this before I post but I am feeling lazy. I believe that is why RPMs do not actually follow a linear increase with voltage as suggested earlier.
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Old 04-14-2009, 05:54 AM
  #23  
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I think the limitations are different between inrunners and outrunners. You have to remember that magnets are extremely fragile, because the internal magnetic forces are trying to rip them apart (break a magnet in half and the sections will repel each other at the location of the break).

Outrunners support their magnets with a firm frame, so any failure is thanks to overheating your coils or wrecking the bearings. This all depends on air temperature and airflow and a bunch of other things, but it's basically boring.

Inrunners are much more interesting, because the spinning rotor is put under a lot of stress. Voltage limits on these are chosen to keep the rotor below a certain RPM. If you go past the limit, the rotor will literally explode. It's actually quite impressive. I tried to find a picture, but was having some trouble; I'll repost if I find anything.
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Old 04-14-2009, 06:25 AM
  #24  
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I think the key to what Ron is saying is "keeping everything else the same " .
What Ron is saying is that the force required to spin a prop at higher rpm is will give an exponential curve, so when you try this without a wattmeter it is very important to keep in mind.

But when you put a wattmeter in line and change prop to keep power required the same then the cubed factor comes into play in prop size selection.

In our case where power is constant then
14.8 V x 45 A = 648 watt 648 x 75% = 486 so 162 watt going into heat
18.5 v x 35 A = 648 watt 648 x 75% = 486 so 162

As far as I could determine with V going higher efficiency gets better , but I am sure this is also not linear and could reverse at some stage, so i will stay safe and lower the watts a bit.
Say 600w x 70% = 420 so180W of heat.
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Old 04-14-2009, 06:55 AM
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Originally Posted by BEX View Post
I think the key to what Ron is saying is "keeping everything else the same " .
What Ron is saying is that the force required to spin a prop at higher rpm is will give an exponential curve, so when you try this without a wattmeter it is very important to keep in mind.

But when you put a wattmeter in line and change prop to keep power required the same then the cubed factor comes into play in prop size selection.

In our case where power is constant then
14.8 V x 45 A = 648 watt 648 x 75% = 486 so 162 watt going into heat
18.5 v x 35 A = 648 watt 648 x 75% = 486 so 162

As far as I could determine with V going higher efficiency gets better , but I am sure this is also not linear and could reverse at some stage, so i will stay safe and lower the watts a bit.
Say 600w x 70% = 420 so180W of heat.
I think that's what Dr. Kiwi Mentioned in another post... it has to do with less heat... Less Heat = Better Efficency.... The hotter the motor gets, the worse the efficency is....

SK
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