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Author Topic: Motors efficiency  (Read 1686 times)

Offline Matt Piatkowski

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Motors efficiency
« on: December 27, 2018, 06:19:20 AM »
Hello,
Say, I run my MVVS 8.0 on 6S at the take-off RPM = 10,100 using Igor's 12x5x3 N carbon composite electric propeller.

This motor has Kv=680.

6S average voltage during flight is assumed to be around 3.95 Volts/cell [(3.7+4.2) / 2 = 3.95 V].
The total average voltage of the 6S pack is therefore 23.7 Volts.

No load RPM at this voltage = (680 x 23.7) - 2,000 = 14,116 RPM. 2,000 is the RPM loss due to internal mechanical (friction) and electrical resistance (copper and magnets related losses) of this motor. This number is assumed and is approximate. I would be grateful to anybody who can provide the data showing how such loss can be calculated or measured.

Assuming that the losses in this particular motor convert actually to 2,000 RPM, the efficiency of the system in flight is 10,100 / 14,116 = 71.6 %.

I have not noticed excessive heating of the ESC (Spin66) or / and the motor after the flight but it would be better to bring the system to about 85% efficiency.

Let's assume now 5S pack for this setup. 5S average voltage during flight = 19.75 V. 680 x 19.75 =13,430 RPM - 2,000 = 11,430 RPM.

10,100 / 11,430 = 88%.

This efficiency is very good and the 5S pack is usually about 2 oz. lighter than the 6S of the same capacity.

In my case, the average current in flight with 6S was 27 Amps with many current spikes reaching 32-35 Amps when the Igor's active timer reacted to the up-the-hill parts of the maneuvers.

6S drain after 5 min. pattern was 79-83% depending on the ambient temperature and wind.

My question is: why many of us use 6S batteries when 5S seems to be a better choice not forgetting of course that some of the world's class fliers use 4S and higher Kv motors.

Your opinions, comments and friendly critique are appreciated.

Happy Flying New Year,
M



 

 


Offline John Rist

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Re: Motors efficiency
« Reply #1 on: December 27, 2018, 08:02:57 AM »
I am not sure I follow all of your reasoning.  But it appears to me that you are confusing head room with efficiency.  The speed controller varies the voltage applied to the motor to control the RPMs so RPMs have little to do with efficiency.  Power out of the battery vs power delivered to the prop is efficiency.  The difference is power loss that shows up as heat.  Higher cell count provides higher voltage and lower current for a given power requirement (P=I*E).  The main power loss in a system is due to copper resistance losses.  So if you lower the resistance of a system or lower the current in a system power losses go down, system heating goes down and efficiency goes up. 

Not sure what you were getting at but this my take on why 6 cells are better than 5 cells in large airplanes.   D>K

By the way mechanical losses in a motor are very small.  Ball bearings have very little friction.
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Offline Matt Piatkowski

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Re: Motors efficiency
« Reply #2 on: December 27, 2018, 08:53:05 AM »
Hi John,
Could you please specify the head room in simple terms?
I am not familiar with the meaning of this term as far as the electric power is concerned.

You wrote: " ...RPMs have little to do with efficiency".
                 Please consider the shape of the electric motor efficiency curve and you will see what I meant.
You wrote: "Power out of the battery vs power delivered to the prop is efficiency". Yes.
You wrote: "The difference is power loss that shows up as heat". Yes. ESC or/and the motor can get hot.
You wrote: "Higher cell count provides higher voltage and lower current for a given power requirement (P=I*E).Yes.
You wrote: "The main power loss in a system is due to copper resistance losses.  So if you lower the resistance of a system or lower
                  the current in a system power losses go down, system heating goes down and efficiency goes up. 
                  It impossible to lower the copper resistance losses for the given motor. It is one of the constant of the motor.
                  The current is as it must be to provide the RPM for the desired lap time.

I am trying to understand if I can use Plettenberg 15-22 and/or MVVS 8.0 with 5S batteries and Igor's 12x5x3 N electric carbon composite propellers and I am collecting opinions. I will run the flight tests anyway using 5S batteries and we will see.

The whole idea of the exercise is to move the operation of the power system along the power efficiency curve to reach the maximum or close to the maximum efficiency. Then and only then the heat (read: waste) produced by the entire system is minimized.

Best Regards,

Offline Tim Wescott

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Re: Motors efficiency
« Reply #3 on: December 27, 2018, 09:02:17 AM »
...
2,000 is the RPM loss due to internal mechanical (friction) and electrical resistance (copper and magnets related losses) of this motor.
...
Assuming ... 71.6 %. ... but it would be better to bring the system to about 85% efficiency.

So you are pulling a figure for motor losses out of your -- ear -- and then you're getting bent out of shape at how big it is?  If you're going to make totally unfounded assumptions, why not make optimistic ones?

...
6S pack
...
 and the 5S pack is usually about 2 oz. lighter than the 6S of the same capacity.
...

You cannot equate an "X" mAh 5S pack with an "X" mAh 6S pack.  The amount of energy in the pack is proportional to the number of cells times the charge capacity.  So, all else being equal, a 5S 3000mAh pack has about the same amount of energy as a 6S 2500mAh pack.  You can't equate things exactly, because the higher voltage of the 6S system will lead to less efficiency.  But while I haven't been flying electric CL, Fred Underwood has beet paying attention, and apparently thanks to market forces there's a sweet spot at the cell capacity that works out to 6S packs, and that sweet spot is pronounced enough that -- today at least -- the 6S pack is actually lighter.
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Offline Fred Underwood

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Re: Motors efficiency
« Reply #4 on: December 27, 2018, 10:49:40 AM »
Fred has been paying attention to Paul, Howard, Chris, Alan, and Igor and Rogerio.  The active timers and best props (Igor's), seem to work well and be designed for 6S, but will work with other systems.  Yes, the 2700 and 2800 mah 6s packs are reasonable in weight to mah or total watts, and price.  But, to give proper credit, I only copied others  :-)   Of course line length, speed, kv….. all figure in, but 6 cells work well.
Fred
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Offline Matt Piatkowski

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Re: Motors efficiency
« Reply #5 on: December 27, 2018, 11:37:40 AM »
Hi Tim,
I will send you the equation allowing to estimate the efficiency. I wrote "estimate" as nobody knows the accurate calculation method and I hope you know why.

Hi Fred,
Of course 6S works well. What I am trying to do is optimize the entire system.
Perhaps we all can use lighter batteries and more efficient power plants?

Like I wrote, I will run the flight tests in the spring of 2019 and will report the results on this forum.

Happy New Year Flying,
M
 

Offline Fred Underwood

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Re: Motors efficiency
« Reply #6 on: December 27, 2018, 11:51:36 AM »
Igor may answer, but will the active system work as well on 4 cells as 6, or is it designed/optimized around 6s?  Maybe with a different pitch, but similar mass distribution as the hollow Igor props?


Fred
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Offline Igor Burger

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Re: Motors efficiency
« Reply #7 on: December 27, 2018, 01:29:39 PM »
Sorry, I am not so often here and I did not see this thread before, however I answered to Matt in private mail, since he asked me also that way ... but now may be little more:

5cells are not enough for proper work of that system. Reason is simple - not enough voltage. calculation is simple:

5cells will deliver only 5 * 3.7V UNLOADED. As was written, the internal resistance will lower that voltage. Since we need highest power boost (read RPM) in worst conditions, we have to calculate available effective voltage in highest load at end of battery (on end of flight). The effective voltage is no load battery voltage which is 18,5V for 5 cell battery minus resistance loses in the system. Not only copper resistance  in motor, but also resistance in battery. Since motor internal resistance is approximately 40mohm and we have 5 cells with approximately 6mohm per cell, plus something in leads and ESC, so the complete resistance is 40+5*6=70 plus "something" in leads and ESC - say 75 mohms. So if peak current in highest load is only 40 A what is very conservative estimation, we have voltage drop 40*75/1000=3V what gives effective voltage 18.5-3=15.5V and will give us 15.5*680~=10500rpm.

We need 11000 for power boost so we are with new and healthy setup under that number, now imagine that we have little bit "used" batteries with higher internal resistance (when I throw away batteries with too little capacity, I often see 10mohms), or we need to discharge it to value when internal resistance goes up, it will hardly pull model also with constant RPM timer.

So that is reason why I use 6 cells - to have enough voltage head room (difference between necessary voltage and available voltage) for necessary power boost. Yes there is a way to make it little more efficient, but on cost of "repeatability". Means if I do not want to feel aging battery, or differences between new and old or good and worse - means if I do not want flying affected by quality of battery - means I want every flight the same, I simply need some reserve = that "head room", and therefore I use 6 cells. And when I hear some power drop, I know that battery is already KO and it must go away.

Sure, there is a way to fly with 5 or 4 cells. Simply if you have 5 instead of 6 cells, what is approximately 1/5 of difference, you can use 6" pitch prop instead of 5" what will give you back that 1/5 difference.  And you will also lose 1/5 of that power and thus 1/5 of trust when slowed down. Simple math. And that is exactly reason why we fly 5" pitch props instead of 6".

Offline Igor Burger

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Re: Motors efficiency
« Reply #8 on: December 27, 2018, 01:59:25 PM »
There are 2 components of loses:

Iron loses which depends on RPM and we can evaluate them as effective voltage * no load current. That we can count as a constant (independent from number of cells), because with the same motor and the same prop it gives the same effective voltage to keep it at wanted RPM and it is maintained by ESC PWM.

Copper loses which are motor current ^2 * motor resistance. Unfortunately we do not know motor current. Basically it should be the same in both cases because the same prop at the same RPM needs the same torque and the torque = motor current / KV - so it should be also constant (its average value - just like the voltage), unfortunately that current is chopped and since the copper loses are I*I*R then chopped current make more loses (10A on 1 ohm will do 100W, but 20A in half period and 0A in the second will do 400W/2 = 200W instead of those 100W) and thus lower voltage really makes such system more efficient. But we simply need it, that is the only way how to regulate motor run. So the lower  the better, but only to extend that ESC can keep RPM as requested by ESC.

However there is also another source of loses - battery, ESC and leads. 6 cells will have higher resistance by one cell, but it has also lower current. But it has aproximately the same value as resistance of motor, so we have 2 x more loses in resistance than iron, so it is not bad to make current smaller if possible, so for me I like 6 cells more than 5 or 4, but I do not say it cannot work, it will, if properly configured - in meaning prop pitch, motor kv, battery voltage. There is no automatic or systematic reason to need more cells to have better efficiency.

Offline Tim Wescott

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Re: Motors efficiency
« Reply #9 on: December 27, 2018, 02:38:36 PM »
Hi John,
Could you please specify the head room in simple terms?
I am not familiar with the meaning of this term as far as the electric power is concerned.

Please don't shout.  We can hear you just fine without the bold face.

Head room means the difference between the voltage you need and the voltage the pack can supply.  The motor speed should be governed by the system, but that can only happen if the speed delivered at 100% throttle is faster than needed.  That 100% throttle speed is governed by the battery voltage.  If the battery voltage falls to the point where it is limiting the motor speed, then the airplane runs out of poop just when you need it to pop.

You wrote: " ...RPMs have little to do with efficiency".
                 Please consider the shape of the electric motor efficiency curve and you will see what I meant.

John has a lot of experience with flying electric.  I think what he says should be treated with respect.

...
You wrote: "The main power loss in a system is due to copper resistance losses.  So if you lower the resistance of a system or lower
                  the current in a system power losses go down, system heating goes down and efficiency goes up. 
                  It impossible to lower the copper resistance losses for the given motor. It is one of the constant of the motor.
                  The current is as it must be to provide the RPM for the desired lap time.

True, but your shouting seems to indicate that you simply cannot imagine that it's easy enough to swap out a motor for a bigger one.  That reduces the copper losses and increases other losses.  If you're going from the smallest possible motor to one or two sizes bigger, the extra weight and friction are often negligible compared to the drop in copper losses.
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Offline Matt Piatkowski

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Re: Motors efficiency
« Reply #10 on: December 27, 2018, 03:38:53 PM »
Hi Tim,
I was not shouting ( my wife forbids this), I was trying to emphasize my responses.

Here is the equation I promised: Efficiency =[ (RPM/Kv) x (I at these RPM - I at no load) ] / Input Watts.

For my Cobra 3515/18 (Kv=740) and 11.5 x 6 PolProp I was using with it while flying this summer in the EU on 6S, the I at 9,300 RPM was 25 Amps. average in flight.
From Cobra 3515/18 website test data, I at no load was 1.01 Amps.

After number crunching the efficiency = 52% what is about 30% too low. I have burned one of my Cobra 3515/18 because, at that time, I did not know that such a low efficiency leads to massive overheating of motor and/or ESC.

Hi Igor,
I will experiment in Spring 2019 with 5S, your props. MVVS 8.0 and Plettenberg 15-22 and we will see.
Regards,
M

Offline John Rist

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Re: Motors efficiency
« Reply #11 on: December 27, 2018, 04:42:11 PM »
PS when I said lower the copper losses the motor resistance is constant.  However big wires, shorter wires, good connectors, and good speed controllers all can lower losses.  Batteries have internal resistance.  Good ones $$$$ have low internal resistance.   
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Offline Fred Underwood

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Re: Motors efficiency
« Reply #12 on: December 28, 2018, 12:25:38 PM »

Hi Fred,
Of course 6S works well. What I am trying to do is optimize the entire system.
Perhaps we all can use lighter batteries and more efficient power plants?


Trying to understand what you are wanting to do, not be argumentative.  It seems that you want to optimize motor, and battery weight and efficiency. Will you of necessity have an optimezed power system if/when that is accomplished?  It seems that the prop is a very important to actual delivery of power for flying,and motor and battery efficiency may need to place somewhere behind that.  The Igor props are at the top of the list to my knowledge and have 5" pitch and are running in the 10,500 - 11,000 rpm range depending somewhat on line length.  If that is a starting point why not use that prop and rpm and find the rest of the system.  Of course Igor has done a lot of the testing in developing the active timer and and then getting a motor developed.  Those props are low centralized mass with hollow blades, good for allowing rpm changes, and even without an active timer, good for gyroscopic precession.  It seems that the active timer does drive some of the other system needs and battery/motor efficiency may not be at the top of the list.


Are you looking just at motor efficiency, or do you have a whole new system in mind.


Fred
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Offline Tim Wescott

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Re: Motors efficiency
« Reply #13 on: December 28, 2018, 03:23:27 PM »
...

Are you looking just at motor efficiency, or do you have a whole new system in mind.

This reminds me of an entire rant that Brett Buck spelled out a couple of years ago, about how folks get stuck on optimizing for one thing or another, and lose sight of optimizing the whole package.  This seems to particularly happen to highly motivated people moving up the ranks, but also some people close to the top.

The problem is that you carve out one isolated part of the system and start optimizing away, and you end up making so many compromises otherwise that the system as a whole is degraded.  You end up with super-high aspect ratio wings that have really low induced drag and turn on a dime but won't fly well at all in the wind, or vastly asymmetrical wing panels so you don't need tip weight but the plane won't fly straight on takeoff, or the whole plane is built out of 1/32" thick contest balsa and is super light but it shatters on the first landing, or only lasts for 100 flights, etc., etc., etc.

It also reminds me of a rant that I occasionally (and usually gently) loose on young or over-enthusiastic engineers at work: if you don't ever ship product, it doesn't matter how optimal it would have been; the project will fail and the world will never see your design.  Sometimes it's best to start by just copying other successful designs, and then learn by tweaking.
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Offline pmackenzie

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Re: Motors efficiency
« Reply #14 on: December 28, 2018, 04:12:02 PM »
FWIW, in my Vector 40 I went from running an AXI/12x6 APC/ 4S TP2600 to a Orbit/Igor 11x5 3 blade/5S TP2600. (Both running governor mode)
Battery consumption was pretty much the same.  AUW was about the same as well, because I had a bunch of nose weight in the 4S setup.

So I actually dropped efficiency by ~80% with the change.

But nothing got overly hot (in either setup), and the plane flew a lot better. So well worth the drop in efficiency.
I suspect that a big part of the drop was the prop, so that does not go into extra heat losses of the electrical components.

If things are getting so hot they are failing then the problem is either poor quality components, insufficient cooling, or some other mechanical issue.

I don't recall ever seeing a judge look at how much battery was being used in a model  S?P

Pat MacKenzie

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Offline Guilherme Souza

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Re: Motors efficiency
« Reply #15 on: January 16, 2019, 09:40:19 AM »
Dear All

Just to share my experience to you

I try to use Igor `s Prop but my plane is heavy  and I use 5S zippy 2700 battery .
the flight was wonderfull , but I finish my flight with 10/12% of battery (using Hyperion meter) and hot battery

So I did a 6S using two 3 S battery's in series (Gens Ace 3000 15C ) . The flight was amazing and  after flight the meters shows 28/30 % of battery remaining .  Fast answer in the pattern very good response .

But I have 5  battery's 5 S  and need to use them , so I decided to use a 13x6 propeller  (2 blade) .
And happen exactally  Igor said , the motor works fine . RPM is very good , RPM change is good and after flight my worst battery shows 22% .

Now I have a new plane AXIOMA 2  and I will try 6S again with 2 and 3 blade prop .

The motor is the same COBRA 3520/14

I think this motor like low RPM and high pitch prop
Does Someone have  experience with this motor ?

thank you

Guil 


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