How do I read motor power ratings?

[JohnRobHolmes]

I think he means for a certain test motor. The motor design won't change mid test. Each change to the motor would require a new test.

[JeremyH]

Originally Posted by JohnRobHolmes

I think he means for a certain test motor. The motor design won't change mid test. Each change to the motor would require a new test.

Yeah, I agree...that would be the ideal way, but then the list would be a mile long and need to be updated once a month....

[Olle P]

Originally Posted by sloppy

What more do you need????

Is that really so difficult to read?

What I "have" is a (virtual) list of motors with their power ratings.
What I "need" is the values of the three variables mentioned in my previous post, for each motor.

Without knowing those three additional pieces of data the power rating is just about useless!
It's like saying a motor runs at 14,360 rpm, but not telling any of voltage, load and Kv rating.

[JeremyH]

Originally Posted by Olle P

It's like saying a motor runs at 14,360 rpm, but not telling any of voltage, load and Kv rating.

Just to clarify, when you have RPM, you know the KV.

[JohnRobHolmes]

There isn't any other way to do it! What good is having all the data if you aren't testing each tweak to see how they compare? Sure, there are 100's of ways we can build the 540 motor, but we wouldn't need to exhaust the whole list. Between 7.5mm comm, 10mm comm, slotting of various depths, web sizes, and two diameters we have 126 choices for just the armature. It would be cool to run them all, but really only a few combinations would be needed to show the differences.

[JohnRobHolmes]

Originally Posted by JeremyH

Just to clarify, when you have RPM, you know the KV.

If you know the voltage. sometimes it isn't specified. And is it really the no load speed, and does the ESC have auto timing advance, and blah blah blah... The difference between dyno tests with different ESCs and power supplies is enough to screw around with data.

[JeremyH]

Originally Posted by JohnRobHolmes

There isn't any other way to do it! What good is having all the data if you aren't testing each tweak to see how they compare? Sure, there are 100's of ways we can build the 540 motor, but we wouldn't need to exhaust the whole list. Between 7.5mm comm, 10mm comm, slotting of various depths, web sizes, and two diameters we have 126 choices for just the armature. It would be cool to run them all, but really only a few combinations would be needed to show the differences.

Yep, I know.....it sounds like a daunting task......do you have any freetime coming up soon?

If you want, you and Eddie can send me one of each of the different motors you build along with the best dyno you can buy and I'll test them for you and create the list....my payment will be to keep the equipment and motors.

[JeremyH]

Originally Posted by JohnRobHolmes

If you know the voltage. sometimes it isn't specified. And is it really the no load speed, and does the ESC have auto timing advance, and blah blah blah... The difference between dyno tests with different ESCs and power supplies is enough to screw around with data.

Well, yeah, that is true. I guess that equation will only work in the other direction. I was assuming that number was at 1V, but that was a stupid assumption to make on my part...

[Olle P]

Originally Posted by JeremyH

Yeah, I agree...that would be the ideal way, but then the list would be a mile long and need to be updated once a month....

Of course I'm talking about individual motor manufacturers presenting data for their own line of products, where I'd expect all manufacturers to adhere to the same standard.

It's not one list, it's one standard!

When I write that the design is constant, it's per article number.
One Novak #3618 (Ballistic Crawler 18.5) isn't expected to differ much from another, even if they're manufactured years apart.

And John, even if only few companies can do the required measurements, that doesn't prevent those companies to use the same test routine for all clients. The clients can then use the results for their marketing.

[JeremyH]

Originally Posted by Olle P

Of course I'm talking about individual motor manufacturers presenting data for their own line of products, where I'd expect all manufacturers to adhere to the same standard.

It's not one list, it's one standard!

When I write that the design is constant, it's per article number.
One Novak #3618 (Ballistic Crawler 18.5) isn't expected to differ much from another, even if they're manufactured years apart.

And John, even if only few companies can do the required measurements, that doesn't prevent those companies to use the same test routine for all clients. The clients can then use the results for their marketing.

Yeah, and as both of the motor builders on this board have explained, there are many differences between the testing equipment and setups.

Now, just buy you a 17.5T Xcelorin and MMP1C and go have fun with your crawler!

[JohnRobHolmes]

One dirty trick I have seen is to rate the motor no load with no timing, then dyno at higher timing or advancing timing. Makes the motor seem more efficient under load when efficiency numbers aren't given, less rpm drop.

[JohnRobHolmes]

Originally Posted by JeremyH

Yeah, and as both of the motor builders on this board have explained, there are many differences between the testing equipment and setups.

This is certainly a problem when trying to compare two motors down to the last 1%. The only way to really compare two motors would be identical test conditions, temp, humidity, etc.. But there could be a general idea of how motors compare. I plan to do this on the motors I carry once the nicer dyno is done.

[EddieO]

There will never be any form of a standard.........you will never get all the companies to agree. Why? Because of advertising.

Integy was famous for this back in the NIMH days. The turbomatcher was pretty much the only battery testing device out there to match the packs........but, you could run different settings. Integy would match all the stuff at 20 amps, while almost everyone else did 30. They also would use .95v as the cutoff instead of .90 like most did........results, WAY higher runtime numbers and higher avg voltage.

Motor specs were nearly as bad from some companies....but they finally figured out, bad numbered motors don't sell, so they stopped using it in ads and such.

And there is still big variances in brushless motors even of the same brand and type.....thats why there are rockets and practice motors. You never know what you are gonna get. It's not as bad of variance as we saw in the brushed days of racing, but still there.....and there is really little you can do to tune the motor like you could with brushed.

I still think you are massively over analyzing this. Its a well known truck with tons of kits sold and in use........pretty straight forward to pick out a motor for it.

As an engineer, I sure hope you would never assume anything......scary word to be using in your profession.

Later EddieO

[Olle P]

To me this thread isn't about picking a motor for my crawler, but for me as a consumer trying to get relevant data out of what's provided by the manufacturers.

What has struck me is that the one piece of data that is both easy to measure and at least as relevant as the "power" is the stall torque (at a defined voltage and temperature).
Knowing the stall torque, combined with the always provided Kv rating (or no load speed at a defined voltage), it's a very straightforward matter to calculate the power curve for all loads and voltages!

And Eddie, life is all about making assumptions! There just isn't enough hard data to live without it. The trick is to know what's based on facts and what is assumptions, and then step by step increase knowledge.
(If you can't assume that the air surrounding you isn't toxic you must hold your breath until it's been clinically and scientifically proven that it's safe to breathe. Then repeat the process before next inspiration..)

[Meatwad]

And this is where customer satisfaction/support comes into play. As well as company reputation. Either buy what works, buy a motor that almost works but a bigger version for your application or be a guinea pig. Welcome to the world of luxury industries.

[EddieO]

While we can argue how easy it is and what the data is for.....I can tell you, its simply NOT that easy. John has been working on his dyno for a while. I've been working on mine. We both have functioning dynos from other companies.....they all have inherent flaws to them.

On top of that, its a LOT of work.....that once done, I would say 99% of all users would have NO clue how to dechipher the data. Read through the forums here. We got people asking "where to solder the wires" "How do I take off the endbell" Not that I mind the questions, but in general these are fairly simple things, yet people get stumped on them daily.

So say we provide a bunch of dyno data....now the people are confused even more. In the racing days, even when people had the dyno sheets for their specific motors right in front of them, they still didn't know what to do with it.....

This still doesn't get past the fact there is no standard.....what guy A tests at doesn't mean guy B will. All guy B has to do is change ONE setting....BOOM his motor makes more power. John already gave you an example of dirty tricks that can be played. We've seen it in racing still with brushless, where guys will use a boost capable speedo with just a touch of boost in it to skew the results.

On my new site, multiple people told me I should remove power ratings....why? It just confuses people. I am still in the works to provide data on all my different motors.....but lucky for me, I got someone who volunteered to do it.

I think thats the main reason we switched to KV.....its pretty easy to understand....

As for assumptions.....well, thats one way to look at it. I look at it like this



Later EddieO

[Olle P]

Originally Posted by EddieO

... its simply NOT that easy. John has been working on his dyno for a while.

But dynos are there to measure power (with the motor rotating), aren't they?

- Stall torque is measured with the motor axle static, and can be done much easier. (Measure static force from a given leverage arm.)
- The no load rotational speed is also a breeze to measure. (Using a calibrated stroboscope or other suitable equipment.)
- With stall torque T[Nm] and no load speed V[rps] (at the same voltage), the output power P[W] can be calculated as P=T*V*(x-x^2) where x is the load given as a fraction of the stall torque. (0 Pmax = 0.25TV and P(peak efficiency) ~ 0.16TV
Variations in feed voltage influence the output power to the power of two. (Double the voltage and the power is quadrupled, halve the voltage and the power is down to a quarter.)

Originally Posted by EddieO

... once done, I would say 99% of all users would have NO clue how to dechipher the data. ...
So say we provide a bunch of dyno data... now the people are confused even more.

You mean they couldn't compare two values and tell which is bigger?
The entire idea behind using a common standard is to have the ability to boil differences down to a single (comparable) value, where the reader doesn't have to understand the underlying data.
(Then a power graph can be provided as well, for those interested.)

As of today I expect these almost clueless readers to happily compare power ratings between different manufacturers without understanding that the numbers aren't comparable!

Originally Posted by EddieO

This still doesn't get past the fact there is no standard... what guy A tests at doesn't mean guy B will. All guy B has to do is change ONE setting... BOOM his motor makes more power.

I'm new to the world of R/C, but know more about the computer parts industry...
Perhaps you're familiar with the 80Plus concept?
Power supplies (PSUs) are rated by their efficiency at some well defined settings. These values can be measured by just about any half whit using not so very expensive equipment.
The PSU manufacturers aren't trusted to provide the data though, but all 80Plus certifications are done by one single company!
As a concerned customer I can visit that company's home page and verify any claims regarding power efficiency provided by PSU manufacturers. (Some claim to have a rating when they don't, other claim to have a better rating than they do have, and some more still have opted to certify at a lower level than the product actually live up to.)

There's no reason (other than financial spite) why a trusted third party shouldn't be able to come up with a standardized test procedure to use on all motors (of same basic type), and then perform those tests for the manufacturers.
These test results would be trustworthy and comparable.

[Manning]

What you are wanting is perfectly logical and doable in a perfect world.

BUT, you are too much of an optimist (I'm guessing you are young). People are in this industry to sell stuff and make money, this isn't lab equipment, they are toys. If people tested in a perfect way to a perfect standard and everyone understood the results, then the cheap junk peddlers would be out of business. And/or everything would be so expensive nobody would be playing with these toys.

I'm an engineer in the consumer goods testing industry. Industry standards are what people in the industry hide behind and point fingers at and say "but I meet the standard".

[Charlie-III]

Originally Posted by Manning

What you are wanting is perfectly logical and doable in a perfect world.

BUT, you are too much of an optimist (I'm guessing you are young). People are in this industry to sell stuff and make money, this isn't lab equipment, they are toys. If people tested in a perfect way to a perfect standard and everyone understood the results, then the cheap junk peddlers would be out of business. And/or everything would be so expensive nobody would be playing with these toys.

I'm an engineer in the consumer goods testing industry. Industry standards are what people in the industry hide behind and point fingers at and say "but I meet the standard".

Nicely put.

Olle P:

I will compare some of this to the automotive world where I do mods and am on several brand specific forums.
One issue is, "But with these parts, my car makes X power!". Then the pissing match begins.
Even using the same brand & model dyno, the numbers WILL (not can, WILL) vary.
Why?
There is the calibration of the dyno
The condition of the dyno
Air density, temperature & humidity at the time of test (correction factors are "best guesses")

This is just for same dynos & you get different numbers. Try comparing from one model/brand dyno to another totally screws you.

Then, you have to consider "peak power" vs. "usable power". A higher peak power may yield a slower car than one with better usable power.
But, try to explain that to a buyer that is just looking for big numbers.

We haven't even started with the places that just flat out LIE about the test/calibration/numbers.

Not saying anyone supplying RC electric motors would do that, just saying it happens in other things where you are measuring power and selling based on that.

As to your original question, I had tried to do a conversion list to show brushed turns, brushed KV & brushless KV so you had a starting point. But all the numbers I saw that had some comparisons were different depending on source. Thus, I could not tell what was closest so I could do a list.
I ended up feeling I was more likely to create more issues than answer questions and gave up.

[Olle P]

Originally Posted by Manning

... cheap junk peddlers would be out of business. ... "but I meet the standard".

There are plenty of standards met by the cheap junk even today, where the physical size and shape of the motors is the most obvious.

Even cheap junk motors can easily have high power, they're just less power efficient and durable, possibly less "heat resistant" as well.
Standardized measurement and presentation of one or two power levels can't break the marketing scheme. (And if it does there's always the option to not present any value!)

Originally Posted by Charlie-III

... Even using the same brand & model dyno, the numbers WILL (not can, WILL) vary.

Given proper calibration and typical lab conditions, how much? 10%, 20% or 50%?
I use expensive test equipment where proper calibration is to within 10% of the reference value. That's not reason enough to claim that my measurements can't be compared to those of my colleague in another town using the same type of test equipment. It's just reason to add a margin of error.

Originally Posted by Charlie-III

Then, you have to consider "peak power" vs. "usable power". A higher peak power may yield a slower car than one with better usable power.
But, try to explain that to a buyer that is just looking for big numbers.

The motor with the highest peak power will also have the highest "usable" power, at least as long as power efficiency isn't an issue...
The reason I argue for a standard is precisely to avoid having to compare "peak power" (at 7.2V) from one manufacturer to "usable power" (at 11.1V) from another, and as of today, without even knowing which manufacturer present what type of power.

Originally Posted by Charlie-III

We haven't even started with the places that just flat out LIE about the test/calibration/numbers.

Which is why I advocate that the final tests should be carried out by a trusted third party.

Originally Posted by Charlie-III

I had tried to do a conversion list to show brushed turns, brushed KV & brushless KV so you had a starting point. But all the numbers I saw that had some comparisons were different depending on source.

No arguments there...
A generic conversion list brushed to brushless seems like an exercise in futility, due to several factors.
I think I've now pretty much come to grips with 540-sized brushed motors, and for those its possible to make quite a few predictions simply based on the winding. Still with wide margins of (predictable) errors due to factors such as timing and quality, since my statistics are based mostly on mass produced cheap motors.
540-sized BL (inrunner) motors are in a totally different league, with parameters such as rotor diameter, rotor material and variable timing.
At the very best it's possible to take the data from a specific inrunner and say which brushed motor would be a somewhat close approximation regarding (unloaded) speed, power or (stall) torque.

The BL motors' kV value is, as far as I can tell, only good for providing a ball park number to a suitable pinion size... unless some other properties are also provided.