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Nov 22, 2005, 05:59 AM
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Visualizing Motor Data


A few years back, there were very few brushless motors available, and they were expensive. As a result, modelers tended to have a few choice motors and it was critical to understand the base motor parameters to see how the motor’s performance envelop could be extended with different props and gearing.

Today, the situation has changed quite a bit. Rather than a few motor makers, there are dozens, and each are offering an astounding array of motor combinations at prices that continue to astonish.

Yet we’re still looking at motors the old way: scratching around for basic parameters such as kv, Io, and R, and then attempting to validate projections with bench data where a bunch of props are run at a range of voltages and the results published for others to examine. Tables of data, though, are an almost meaningless way to compare motors because there are so many very non-linear terms that go into thrust and power calculations, and also because propellers from different manufacturers, even props that are marked with the same diameter and pitch, vary wildly in their performance characteristics.

Given this, I’m proposing we look at motors a bit differently:

1) There is really no excuse anymore for using motors in regions of lousy efficiency. Poorly selected motors dramatically reduce flight times, pre-maturely cook battery packs, and really inhibit the enjoyment of the hobby. Many manufactures are routines showing ads in which their motors are propped across a range that is way too broad. My guess is that few hobbyists are capable of determining what really is optimum for their model.
2) When we decide that we’re going to use a motor for a very narrow range of props, then we are freed from worry about kv and all those other parameters that send us running to spreadsheets to answer the question “what if.” Instead, we can move to more general representations of motor performance and in the process make it a goal to ensure that comparisons are simple.
3) Finally, we need to bring a sense of order to how the industry advertises and sells motors. Today, it’s not too far removed from the way stereos were sold in the early 1970s, with everyone measuring differently and the truth obfuscated behind columns of caveated data. In short, I want it to be easy to know why Company XYZ’s motor is so wonderful and I want to understand it in 5 seconds!


At the root of my suggestion is the request that whenever someone makes a new motor, they simply tell us what prop the motor is the best at turning without burning up. That’s it. They would express the motor as follows:

JustGoFly 400DF: 46g-3s-230W-5.7x3

From this description, we’d know the motor weighed 46 grams, and from a 3S pack we could pump 230 Watts into the motor with a 5.7x3 prop attached and we’d get really good results. What does “really good” mean? Very likely a heck of a lot better than you are getting today! There’s no need to publish thrust data, because that is implicit to the prop and power. There’s no need to publish RPM data, because that is implicit to the wattage figure anytime efficiency is very high. And since we’re talking about the “best shot”, efficiency is very high.

This approach also allows prop selection to be completely decoupled from motor selection. We all know the X watt per pound rule for flying, we can look on a prop chart and decide if we more interested in 3D or speed, intersect that with our target wattage and arrive at a prop diameter and pitch and wattage. From there, we’d simply look for a motor that had been optimized specifically for that particular prop and that was very close to our wattage range.

Finally, assume XYZ company came out with a new motor with the following:

XYZ Flier: 40g-3S-240W-5.7x3

Wow! That’s quite an improvement! It’s 10% lighter and can drive a 5.7x3 prop slightly harder. that improvement came from either improved efficiency or clever cooling (which is unlikely since the weight went down).

What about this motor:

ABC Speedster: 44g-3S-220-5.7x3

Hmmm. Let’s just say I hope it’s cheap 

This raises an interest question: what exactly is behind the wattage figure? Actually, a lot. The figure represents the maximum thermal dissipation, expressed in watts, of the motor when being driven by the rated pack (in this case, a 3S). The math behind this is a bit interesting, so it’s useful to go through it. The root assumption I’ve made is that all motors can tolerate a sustained dissipation of Pd = 0.4 * weight, where weight is expressed in grams and Pd is expressed in watts. So, a motor weighing 100 grams could tolerate 40 watts of wasted power in the form of heat. If a motor has a particularly innovative cooling solution, it might be a bit higher. If it’s a very closed motor, it might be a bit lower. But this is a critical parameter the motor manufacturer can adjust based on what they are comfortable with in terms of internal temperature.

From this Pd figure, determining the max permissible dissipation is simply a mater of calculating Pmax = Pd/(1-efficiency). So, in the above example, if Pd = 40 watts, and the motor is operating at 85% efficiency, the max power we can pump into the motor is 40/(0.15)=266 watts. Determining efficiency requires you to know output (prop) and input power. That is discussed below.

Back to the comparison side of things: Assume we needed a 40 watt solution and there were a few prop choices around 4-5 inch diameter that we were open too.

Little Screamers "do novo": 24g-2S-31W-5.25x6.25
Custom CDR Billet Bullet Hot Wind: 22g-3S-40W-5.5x4.5
Medusa Research Afterburner 4K: 15g-3S-44W-8x4-4:14


In about 10 seconds, we can see that out of the 3 above the Custom CDR will fit the bill, needing 3S, taking 40 watts, requiring a >12A ESC, and spinning a 5.5x4.5 prop. The Little Screamer power was low at this particular prop range, and the Medusa motor needed a gear drive and the larger prop was outside of our range.

Simple and quick!

Attached is a graph that really shows how this approach can speed analysis, and with some simple computer tools we could very quickly build a database of hundreds of motors consisting of measured data not calculated data. Want to drive a 3x3 motor at 25 watts? Just check the database!

But how do we get all this data?

At the root of this is that we all need to work on getting really good prop data. Folks over at Aircraft World have a start on this, but we really need more props measured by several folks throughout the world so that we can fully digest the prop data. Measuring props is tricky and requires a dyno that few have or could easily build. But once that prop database is established, the rest of the us need only a tach, an inexpensive power meter and a set of props that are in the database to fully characterize a motor. No more thrust measurements!

As you look at the graph, look how good the situation gets at the higher efficiency ratings, and look how dismal the situation is at lower efficiency ratings. This isn’t unique to this motor. Most all vendors are selling motors and advocating their operation in highly inefficient regions. Also, look how quickly you can assess a motor’s strength and weaknesses in this format. I’m very close to starting on some software to help enter and grind through and filter and view the data, but I’m not certain folks see value in viewing the data this way, or they just like the old way.

Thought from folks? Pieces that are missing? Is this potentially useful? I know many on the board already “get” the value of focusing on efficiency. My hope is that with easier tools other would too. Hardware + Software tools such as the emeter are definitely heading the right diretion.

Disclaimer: Absent the large prop database, and considering the inconsistency in published data on the web from 3rd parties and vendors, you should look at all the data shown in this doc is simply representative and illustrative rather than conclusive (and I'm sure there's a typo/error someplace my data) But That is the point of this post: let’s abandon the status quo and build something better.
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Nov 22, 2005, 07:58 AM
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Much better than what we currently have in terms of specs on most motors. The downside, however, is that manufacturers will continue to publish specs that put their motors in the best light by pushing the envelope in terms of wattage numbers. Perhaps it would be appropriate to have the manufacturer supply three different "solutions" - perhaps two different voltages and/or up to three prop selections that define the best operating envelope for their motor. One useful (and often limiting) parameter is the total watts of waste heat for a given configuration and the maximum rated current for the particular wind with "normal" cooling.

Ross
Nov 22, 2005, 08:39 AM
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Dr Kiwi's Avatar
Very educational and useful!

I must be missing something though - looking at (4) on your graph, I can't understand how the motor on 3s drawing ~23A (230W) can be running COOLER than it does on 2s with a smaller prop at only ~8A (55W).
Nov 22, 2005, 01:46 PM
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Thread OP

It is confusing...


Quote:
Originally Posted by Dr Kiwi
Very educational and useful!

I must be missing something though - looking at (4) on your graph, I can't understand how the motor on 3s drawing ~23A (230W) can be running COOLER than it does on 2s with a smaller prop at only ~8A (55W).
Yes, that's a bit confusing. The 55W figure is for the 3S which unfairly penalizes the motor due to excessive losses at the higher voltage. At 2S the motor efficiency gets over 80%. As currently shown, the red line is for the 3S case.

Maybe what would be ideal in a graph is to show the peak efficiency bar, and then show the voltage at which the efficiency falls of by 10% for other props????
Nov 26, 2005, 04:38 AM
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Thread OP
OK, I've taken another try at this and I think it'll make more sense. I've also got some c# code up and running, so now I can hopefully start cranking some more motors through.

The goal remains the same: provide a super quick means to digest new motor data and really highlight where a new motor might shine.

In the attached plot below, you'll find 4 colors in use: dark green and light green, and dark red and light red (pink).

The dark colors (red and green) represent what is possible from 2 Lipo cells, the light colors (red and green) represent what is possible from 3 Lipo cells.

The green colors represent areas of safe operation: the motor won't get too hot. The red colors represent areas of unsafe operation. It doesn't mean you can't operate there, you just can operate there 24x7. You can operate 24x7 in the green area, and the motor will stay at a a safe temperature. This temp is a function of motor efficiency, and is determined by wasted power and motor weight (0.4W/gram).

So, some examples: You can operate a 5.7x3 prop off of 2S and pump about 80 watts in the motor, or you can bump it to 3S and pump about 230 watts into the motor. Since it's green, you won't ever have to worry about overheating with this prop.

With the 5.5x2.5 prop, the motor will consume up to about 80 watts off a 2S pack, and will consume nearly 220 watts from a 3S pack. However, lesser efficiency means you need to be really careful with throttle and average a hair over 2S pack voltage (3.7V * 2).

Does this make more sense? Do you think this conveys useful information? Where things get really useful is hovering your mouse over a bar can bring up things like "5.7x3 at 108W is 16600 RPM" and then as you move the mouse the wattage and RPM change.
Nov 26, 2005, 05:43 AM
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vintage1's Avatar
The trouble is, that you are going away from what is not only a very good convention - Kv Io, Ro, and weight - which ALMOST completely specify the motor, to one where you are attempting to make a motor - a thing that has no specific power voltage or current rating at all - into one that does.

If manufactirers were to specify one more thing - the thermal resistance of their motor, and we knew what the maximum internal temperatures allowable were, then it would be possible to very simply plug all that into something that would instantly tell you waht you needed to know.

I wrote it, and its called MUMTATS. You can download it. Except I simply set a figure for maximum power dissipation by hand.

Its possible to write software that will take those 5 basic parameters - Kv,Io, Ro, weight, max power dissipation and graph up ANY motor you want to show you what voltage and current and RPM levels you should use.

Why not spend yoir time on that instead? Then simly plug all the parameter from all the motors in, and have instant comparisons.

You can even guesstimate thermal resistance from the motor weight. The wieght is proprtional to the volume, but the thermal resistance is propeortinal to the surface area, which is the 2/3rds root of the volume/weight...to a first approximation anyway.
Nov 26, 2005, 06:22 AM
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Thread OP

kv, Io and Ro still matter....


kv, Io and Ro still matter.......I'm simply proposing that folks pop up a level to make the comparison simple and quick. If you want to still mess around with those parameters, then feel free.

A motor does indeed have a specific voltage and current and power rating IF you are primarily concerned with really good efficiency. You are right: in the general case motors can do anything. But that's not what we're after. And that's the case I make in the first post: let's not buy motors to use if they only have 55% efficiency on the prop we need. Let's identify a narrow range of props for a model, and then find the most efficient $40 motor to drive it. Today, we really don't have any way to quickly know how efficient a motor is given voltage, current and RPM measurements...pieces are starting to improve with emeter, prop databases, etc. Let's continue the push! I think a piece of that is helping folks to visualize the performance. Tables of data isn't cutting it IMO. The data is hyper-non-linear and humans absolutely suck at comparing that type of data.

The second thing I note above is that we need to get to a point where folks can easily measure and exchange parameters with others. Sorry, but it has been, what, 15 years since Astroflight wrote their excellent book and few manufacturers are consistently speaking in those terms when they roll out a motor. The very expensive motors come with those parameters largely becasue those motors will have a 5+ year shelf life, but as I note above, we're living in a world of self-wind and super cheap imports, and its critical that we be able to quickly measure and compare.

What I'm proposing isn't at all radical: Astroflight has for a long time sold motors by stating the preferred prop and wattage. Makes a ton of sense.

Specifically, what bits of information do you think are missing in the graph?
Nov 26, 2005, 09:04 AM
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vintage1's Avatar
If I get time I'll knock up one that shows how to derive what is useful from what is primary.

Peak watts is not possible to specify on a motir without invovling other issues. Chiefly RPM KV, voltage, Io, Ro and of course the weight.


For a given heating effect, a speed 400 at 10A and one volts is heating up the same as one at 7A and 20v..

10W versus 140W input...both are 'maximum safe power' figures..
Nov 26, 2005, 12:12 PM
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vintage1's Avatar
Well. Here you go.

This is a pretty accurate way to compare motors.

You can add the graphics yourself.

I have taken basic motor parameters, and added one more - an RPM limit,which is VERY relevant with brushed motors...

I have used a fudge factor to estimate the amount of heat the motor can dump in averagely well cooled use by taking the case area from the length and diamneter of the motors. This actually is coinciding well with my own tests on can motors, and seems not far off reality. If you want to extend this and add extra cooling factors for heatsinks or EDF use, be my guest..

The program then calculates the theoretical best voltage (white indicates a voltage limited by the motor max RPM limit, and is set to the MAX RPM divided by KV..this is safe for a motor that can completely unload in e.g. a dive), the theoretical maximum current to not exceed the power dissipation, and from those the Power in, out, RPM and efficiency at peak power and efficiency.
I have thrown in watts per pound of motor as well, to show some interesing factors.

A selection of motors is included...feel free to add others.
Nov 26, 2005, 03:06 PM
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Thread OP
Quote:
Originally Posted by vintage1
If I get time I'll knock up one that shows how to derive what is useful from what is primary.

Peak watts is not possible to specify on a motir without invovling other issues. Chiefly RPM KV, voltage, Io, Ro and of course the weight.


For a given heating effect, a speed 400 at 10A and one volts is heating up the same as one at 7A and 20v..

10W versus 140W input...both are 'maximum safe power' figures..
Peak watts is indeed possible to specify if you know the efficiency. And knowing the efficiency of the motor is possible if you know the power going into the motor, and the power going into the prop. Right?
Nov 26, 2005, 03:09 PM
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Thread OP
Quote:
Originally Posted by vintage1
Well. Here you go.

This is a pretty accurate way to compare motors.

[snip]
I don't disagree, but this goes back to my premise: now what? Look at your data and tell me what prop will do the best with this motor? That's what I'm attempting to capture and convey: what prop/motor combination results in the most optimum operating region.

You still haven't answered the question I posed earlier: what information do you feel is missing from the graph???
Nov 27, 2005, 01:40 AM
We want... Information!
Bruce Abbott's Avatar
There is no one 'best' prop for a motor. For example, a motor I tested today delivered the same power to both an APC SF10x4.7 and a SF9x6. Which one is 'best'? What prop would be 'best' with a different gear ratio? How about if I wanted to use it in a helicopter, or EDF?

The motor constants Kv, Io, and Rm are usually the only electrical parameters you need to predict a motor's performance. Maximum power, current and rpm tell you how hard it can be pushed. If all manufacturers included this info it would make motor selection a lot easier.

Unfortunately, a lot of motors are being sold with inadequate or incorrect specs. Either the manufacturers and/or resellers are too embarrassed to admit how bad their motors are, or they simply can't be bothered actually testing their products! This only seems to be a problem with Asian sourced motors - established US and European manufacturers usually provide plenty of data, often also supplying comprehensive application charts and even dynamometer tests.

I try to test every motor that passes my way, even those boring old brushed 'can' motors. Recently I made a simple dynamometer to get a better idea of what's going on. My dyno chart gives a good visualisation of the performance of a motor. At a glance you can see its power curve, the useful range of current and rpm, and how efficiently it performs. Results can sometimes be suprising.
Nov 27, 2005, 02:07 AM
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Thread OP

Beautiful data


Bruce, central to my thesis is that with so many motors being available, and the motors being so inexpensive, is that we should be spending much more time focusing on maximizing efficiency of the motor/prop combo AND making it easy to quickly identify where that region is. Looking at your graph there's indeed a narrow range of props that satisfy that.

Could you please provide a few more sets of data at higher voltages for the Aurora 400T? I'd like to present it to you in a different format and get your opinion.

EDIT: If I'm reading your plot right, Kv is about 1250, Io is 0.8A, and R=0.22. I'll run a set from that for now.

Again, beautiful plots. I could only wish every motor manufacturer provided this.
Last edited by matttay; Nov 27, 2005 at 02:30 AM. Reason: Adding Kv, Io and R assumptions....
Nov 27, 2005, 03:26 AM
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Bruce, attached is another view of the motor you sent. I used Kv=1247, Io=0.8A, and R=0.22 and ran for 6 props.

As you can see in the attached graph, 7x6 and 8x4 would require no throttle management on 2S, while 10x4.7 and 11x7 would require aggressive throttle management on 2S (holler if you'd like clarification on how to read that as the color coding might seem strange). 2S on the 11x7 would peak at nearly 100W, but average power needs to be about 30W for thermal.

You asked if the 10x4.7 or 9.6 prop was better. Here we see we'll pump the same amount into those props, but we also see that the 10x4.7 will need a bit more throttle management on 2S (larger red area). From there, the normal tradeoffs apply: higher pitch lets you cruise faster...etc.

You can also easily see that off of 2 cells you'd get nearly 50 watts of power into the prop for 8 and 9" props, and that with all props operating on voltages over 2S means things will be getting hot (I'm assuming this motor is about 42g). Off of 3S, the 8x4 and 7x6 will hit about 60 and 65W of "safe" power, and can peak over 100 watts. So 3S with aggressive management is possible on 8x4 and 7.6

Anyway, remember what I'm after here: a quick way to rapidly compare motors without any other computational tool around. I want to see an ad for a motor and instantly know if it's better than what I already have. To me, the soundbite on the Aurora is 42-2S-50-9x5: It's a 42g motor and driven from 2S it can eat 50W all day safely while driving a 9x5 prop. Given that, it's very easy to see if something better comes out. Back up that soundbite with a graph that helps me visualize how the motor operates with 2S and 3S packs while taking efficiency and thermal into account and it's a powerful view on the data.
Nov 27, 2005, 03:35 AM
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Adding annotated graph.


Annotated graph attached.


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