How do I read motor power ratings?

[EddieO]

Originally Posted by Olle P

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.)

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!

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.

Does the term pissing in the wind mean anything?

Honestly, what I am telling you is there is NO way you are gonna get all the motor companies to come up with a standard for testing motors. It's just NOT gonna happen, especially with the Chinese companies, who many of which have little ethics or morals when it comes to selling something. They will do or say anything to move product (this isn't saying all chinese companies do this).

And even if by some miracle you got every company to sign up.....who pays for the 3rd party to come up with the dyno? Better yet, what happens when some new company comes along and decides to NOT be part of the program....

Remember, in RC advertising HYPE sells products extremely well. Pay for a big ad in RC Car Action....sell product. Guys who have been around RC long enough can tell you this. There is a reason we really call it RC Car Ad Action.

As for users using numbers well....is bigger better? Watts the motor produces is just one part. RPM, Torque and Efficiency all are big numbers too.

Originally Posted by Olle P

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!)

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.

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.

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

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.

While windings are one of the indicators of power, there are numerous other aspects of a motor that will not only effect power, but RPM, Torque and Efficieny too, along with their durability and thermal properties.

Part of the problem lies to in the fact that by simple putting some Horsepower number on every motor, users will get in the mindset of bigger is better....which can be bad news in crawling, regardless if we are using a brushed or brushless motor.

You can't compare this to power supplies for a computer. You don't plug an RC motor into the wall of your home, nor do you have to worry about it working with 10+ different electronic components. Better comparison, how about the fans in your computer. Are they all tested to some standard? Hardly....

Like I said, pissing in the wind.....what you are asking for, is NEVER gonna happen.....there is a better chance of me having a threesome with meagan fox and lucey pinder.

Later EddieO

[Charlie-III]

Olle P, I'm not trying to be a PITA (American English = Pain in the Arse...)

Peak power vs. usable power....here is an example.

Small 125cc 2-stroke motor makes "X" power, but in a 300RPM range.
Great if you have an 8 speed gearbox.
But, for the poor schmuck that has a 4 speed gearbox, the RPM jumps are so great, every time he shifts, he drops well below the power peak, waits until it hits the sweet spot (goes like a raped ape) then falls off the other side waiting for the next shift.

So, what motor is better?? "It depends". Are you following me??

Usable power means it fits your rig & application.

Way back when, I used to race carpet cars. We all bought site supplied "sealed spec motors" that had fixed timing.
Depending on the motor, whether we were running oval or road-race, gearing, car weight, etc., what I found as a great motor, sucked for someone else.

As an automotive modder, I look for specific mods that help with a broad power range. Why?, mostly because I run in snow and/or pull 1800 sailplane trailers. Thus turbos are out, but I need decent power over a long range.
For someone with crappy traction, no car weight, no trailers, they can do "paeky power mods" that I hate.

Sooo.......since I have dealt with this, where am I wrong?

You are an "engineer", you want perfection.
I'm a field service guy, I HAVE to deal with reality. I do it EVERY FRIKKIN DAY where either sales or engineering state things or make promises, I have to try & support them.

While I agree it would be great to apples to apples comparisons:

1-Who will pay for "3rd party, unbiased testing"?
2-Will the "buying masses" even have a clue (other than GREAT NUMBERS) of what all this crap means?
3-Try as you may, you can NEVER control how a site does dyno set-ups/calibrations. Thus there is an error. How much error?, I can say my NA 2.5 Legacy GT could outrun some modded FI Imprezas. Why? I had better usable power, plus I know how to drive.

Even in "calibrated testing" (UL, ETL, SAE, etc.) there are ways to fudge the numbers without getting caught.
Comment to the choir all you want, I highly doubt it will ever happen.
If you get reasonably close, great.

[Manning]

Olle, this is an opportunity for you to make a name for yourself. Make it happen! Come up with the tests and equipment, then convince everyone in the industry it will benefit them to follow the std and publish the results.

[Duuuuuuuude]

Originally Posted by EddieO

Does the term pissing in the wind mean anything?

Honestly, what I am telling you is there is NO way you are gonna get all the motor companies to come up with a standard for testing motors.

And even if by some miracle you got every company to sign up.....who pays for the 3rd party to come up with the dyno? Better yet, what happens when some new company comes along and decides to NOT be part of the program.

I was thinking today (stay calm, it happens) that probably the largest factor in all of this is cost. It costs money to develop a standardized test system and equipment, it costs money to have it set up properly, it costs money to train people to administer the tests, and it costs money to maintain the equipment. Plus I'm sure the motor builders would have to pay some kind of additional fee to place the official seal of approval on their packaging.

All those costs have to be recovered somewhere and would reflected in the price of the motors. How many people would pay an extra $5, $10, $15, or $20 per motor just for the peace of mind that it has been dyno'd at an approved industry standard? Especially considering that as soon as the motor gets some wear, or overheats, or gets ran through dirty water, all those expensive numbers become meaningless.

There are simply too many variables in the real world to make any kind of standardized and specially developed testing cost effective for anyone.

[sloppy]

Originally Posted by Duuuuuuuude

All those costs have to be recovered somewhere and would reflected in the price of the motors. How many people would pay an extra $5, $10, $15, or $20 per motor just for the peace of mind that it has been dyno'd at an approved industry standard?

Racers might.. and the morons on this sight that buy anything they are told to..

[Olle P]

Originally Posted by Charlie-III

Peak power vs. usable power....here is an example.
Small 125cc 2-stroke motor makes "X" power, but in a 300RPM range. ...

Usable power means it fits your rig & application.

... site supplied "sealed spec motors" that had fixed timing.
Depending on the motor, whether we were running oval or road-race, gearing, car weight, etc., what I found as a great motor, sucked for someone else.

Sooo... since I have dealt with this, where am I wrong?

1. Combustion engines are completely different animals when it comes to power! DC motors are as close to perfectly linear as you can get when it comes to torque and speed, while combustion engines are non linear.

2. To me "usable power" is where the motor doesn't overheat, and since the typical work load should be kept at 10-30% of stall torque to stay within good efficiency "usable" is best described as 64% of the peak power. (What you get with a load equal to 20% of the stall torque).

3. Your example of good/crap motor also doesn't apply, since that same motor (you found very good) in the same car with the same setup but running on a different track could quite possibly be bad.
There just isn't a "one size fits all circumstances perfectly".

Originally Posted by Charlie-III

You are an "engineer", you want perfection.
I'm a field service guy, I HAVE to deal with reality.

Guess what? While I'm an engineer (actually MSc in engineering) by education, I work mostly as a service technician!
I (sometimes) even have to support devices (by repair or teaching the users how to handle it) that I've never seen before, and in rare worst case scenarios: if I fail to get it right promptly (within an hour or so) somebody dies!

I have to rely on: Service manual, if available; prior experience with similar products; applicable laws, regulations and standards, hoping the designers followed them; general technical and physics knowledge.

Originally Posted by Charlie-III

1-Who will pay for "3rd party, unbiased testing"?
2-Will the "buying masses" even have a clue (other than GREAT NUMBERS) of what all this crap means?
3-Try as you may, you can NEVER control how a site does dyno set-ups/calibrations.

1. The manufacturers pay the testers a reasonable fee per design tested. (Not much money, as described below.)
This cost is then spread on the customers.

2. Bigger value => more power => more speed.
Yes, I think they'll get it!

3. Traceable documentation in accordance with ISO 9001:2008. I see no problems here...

Originally Posted by Charlie-III

Even in "calibrated testing" (UL, ETL, SAE, etc.) there are ways to fudge the numbers without getting caught.

The most straightforward way to "fudge the numbers" is of course to have the test sample being different from the production run.
That type of cheating will get exposed if a random production sample is tested later on, as a follow-up.

Originally Posted by Manning

Olle, ... Make it happen!

I have an idea of how to do it, in a very simple and straightforward manner.

Simple test, power:
1. Verify/set the motor timing. (What timing to use remains to be defined. In case of zero timing the motor is tested in both directions, with the results averaged.)
2. Heat the motor to pre-defined temperature.
3. With the temperature held near constant measure rotational speed at no load at a pre-defined voltage supply.
4. At the same voltage, measure the stall torque. (Stall torque is easy to measure accurately!)
5. Report the results back to the customer and, once the tested design is launched, present them publicly.

Extended test, efficiency:
During tests 4. and 5. above, measure the electrical current as well.

Presentation:
Presented to the customer (motor manufacturer):
- Test conditions: voltage, temperature and timing
- No load speed.
- Stall torque.
- Peak power. (0.25 x [stall torque] x [no load speed])
- Power at 20% load. (0.16 x [stall torque] x [no load speed])
(If applicable
- No load current.
- Stall current.
- Current at peak power. ([no load current] + ([stall current]-[no load current]/2))
- Efficiency at peak power. ([peak power] / ([voltage] x [peak power current]))

Presented to the public:
- Test conditions: voltage, temperature and timing
- No load speed.
- Stall torque.
- Peak power. (0.25 x [stall torque] x [no load speed])
- No load current. (If measured.)
- Stall current. (If measured.)

The key to keep costs and error margins down is to not measure the dynamic power, but stick to measuring the static stall torque and then calculate the power.
The only interesting values that can't be derived from this are peak efficiency and peak efficiency load. (But the 20% load values are close approximations unless the bearings impose high friction!)

The downside of measuring stall torque is that the currents go waaay up with high power motors.
This can be circumvented though, by reducing the voltage for this measurement and extrapolate the voltage/current/torque up to the desired value, since there's total linearity. (For example: A high power motor can be decided to use 10.0V as the nominal test voltage. At stall it would use >1kA, which can't even be provided by the test equipment, not to mention that the motor would overheat in a second. Instead the motor is tested at stall with an 0.10V feed, and the measured torque and current is multiplied by 100 to get the corresponding 10.0V results.)

[Manning]

Brilliant! Now convince the industry it's beneficial and necessary.

Hint: until there are regulations mandating this, it's not gonna happen.

Coincidentally, I get to bore a ISO 9001 / 17025 auditor to tears next week. The guy had to work really hard to stifle yawns last year. We have to do it because of regulations. It's a crapload of work to get a piece of paper.

[JohnRobHolmes]

I could think of one case where a motor with lower peak power has higher usable power, considering permanent magnet construction. One motor with greater heatsinking than another! Case design, integrated fans, and what have you. I've been helping a fellow out that has redesigned an Ebike motor with forced air cooling. Stall torque and peak power is the same, but continuous power doubled.

Just wanted to throw that extra wrench in things

[Duuuuuuuude]

Oh man, I love listening to engineers trying to justify themselves.

[JeremyH]

Originally Posted by Manning

Coincidentally, I get to bore a ISO 9001 / 17025 auditor to tears next week. The guy had to work really hard to stifle yawns last year. We have to do it because of regulations. It's a crapload of work to get a piece of paper.

I've actually corrected a state inspector (and explained why she was wrong) when my facility was being audited/inspected...

Originally Posted by Duuuuuuuude

Oh man, I love listening to engineers trying to justify themselves.