How do I read motor power ratings?

[Olle P]

Most serious motor manufacturers provide power data for their products.
Unfortunately they rarely provide more than a simple value, like "98W".
(At least Novak also provide the bit of info that it's "at 7.2V".)

How do I interpret that value?
- Is 7.2V the common standard voltage, and implied unless otherwise noted?
- Is it input or output power? (I'd guess output.)
- Is it the maximum output (at 50% of stall torque), or the optimum (at about 20% of stall torque)? (I'd guess maximum.)
_________________________________________

I do know that the output power can be described as "torque times revs". But I also want to put that knowledge into use to determine what motor power I need, and to compare different motors.

 

[DRTMI]

More often the motor makers will list the number of turns, 18.5-21.5 etc., and possibly the kv rating which is how many revolutions per volt, so a 2000kv would be 14,400rpm at 7.2 volts. The Watt rating I haven't seen very often on motors but have a few. The 7.2 volt is a mean value for a 6 cell NiMh sub c battery pack, where a 2 cell LiPo is 7.4, I think they do that from the old days before LiPo's. Hope I helped.

 

[EddieO]

You are not going to really find what you want.

While we used watts as the standard in the brushed motor racing days. However, as we moved to brushless......for whatever reason, everything was rated in KV pretty much. I am guessing mainly because KV is VERY easy to measure, while typically the main thing the plane guys needed to know to figure out the prop size.

Dynos have the flaw in their ratings too, as dyno to dyno the results can vary, even on the same brand.......along with the fact there is not a standardized setting to use (volts, amps, etc).

I guess my question is, why do you need said specs? Is there something specific you need to know the power ratings for?

Later EddieO

 

[Meatwad]

In the past I've used the ampsXvolts=watts idea to safely get an esc/lipo packs. Real world losses factored in as well. So at 98watts with a 7.4vdc input you'd get about 13amps. Amps are my driving force for decisions about esc/lipos.

 

[Olle P]

I guess my question is, why do you need said specs? Is there something specific you need to know the power ratings for?

My next motor! Running an LCC the power is everything when it comes to speed, given that so much power get lost in the worm drives.
But with more motor power comes even less power efficiency in the motor itself, so finding the best balance between power and efficiency is a big deal.

Being an engineer with a habit of doing all the relevant maths beforehand is of course also part of the equation...

In the past I've used the ampsXvolts=watts idea to safely get an esc/lipo packs.

The input is to a great extent a matter of efficiency.
I want to know more about the outputs to optimize my choice of motor to the most efficient in cost and power.

Just a couple of random picks to show that the power values are present:
Holmes Hobbies: 400W continous
Novak Ballistic Crawler
RC4WD provides a full chart for their 80T motor.
Tekin Redline Sensored Motors (But not the sensorless!)
... and so on.

I guess my main question is if the power values provided by HH, Novak and Tekin are fully comparable to each other?

 

[Meatwad]

Efficiency is a whole 'nother issue entirely. Quality is efficiency, if you know you're buying a not so quality motor just buy too much motor for your application. Only major issue would be longevity and durability.

 

[Charlie-III]

My next motor! Running an LCC the power is everything when it comes to speed, given that so much power get lost in the worm drives.

Being an engineer, you will hopefully QUICKLY come to the conclusion that you have 2 choices:
1-Buy lots of brushed motors
2-Buy a brushless motor/ESC system

Worm drives WILL eat brushed motors for breakfast/lunch/dinner, even great brushed motors. Brushless is the best route for worm drive comp rigs.
Worm drive "fun rigs" can get use out of brushed motors, but they're still short lived.

One of the biggest hang-ups of BL motors is lack of drag brake, but the worm drive negates that, so it's NOT an issue.

Keep in mind, this is a comment from a "many year senior onsite/field service guy" to an engineer.
Take it as you will.;-)

"thumbsup""thumbsup"8)

 

[Duuuuuuuude]

Being an engineer with a habit of over thinking everything is of course also part of the equation...

Fixed it. Nothing personal, just how you guys are. :ror:

Consider all advertised measurements as approximations. Not all motors are the same, even if they are the same.

Know the basics, figure out what you want, then whittle down the semantics of your choices.

Think too hard and you'll hit diminishing returns very quickly.

 

[Olle P]

Know the basics, figure out what you want, then whittle down the semantics of your choices.

I think that's my hole point!
- I will go BL, as soon as I can afford it. But I do not want to spend ~$150 (~$100 plus shipping, taxes and fees) on the wrong motor!
- I do know what I have, and I know what I want in relation to what I have.
- I'm looking for the available alternatives that might fit what I want.

My current stall torque is sufficient, and the power efficiency is okay.
I pretty much just want to double the top speed (to at least 4 mph) without degrading the performance in the very low speed region.
To double the top speed I need to at least double the power.

 

[Olle P]

Worm drives WILL eat brushed motors for breakfast/lunch/dinner, even great brushed motors. ... Worm drive "fun rigs" can get use out of brushed motors, but they're still short lived.

You should read my article on the subject...

I've used up one brushed motor in my LCC, but that was when I had it mostly wrong:

• Pinion too tight to the spur.
• Wheels too tight on the axles.
• Wheel bearings incorrectly seated.
• Too large pinion.
• The motor was well used with blackened commutators, even before I began using it...

After I fixed those errors I've got no motor problems. The car is just too slow.

 

[Charlie-III]

You should read my article on the subject...


I've used up one brushed motor in my LCC, but that was when I had it mostly wrong:

• Pinion too tight to the spur.
• Wheels too tight on the axles.
• Wheel bearings incorrectly seated.
• Too large pinion.
• The motor was well used with blackened commutators, even before I began using it...

After I fixed those errors I've got no motor problems. The car is just too slow.

I did read it.....if you gear it for speed, you will kill brushed motors, geared for slow brushed motors will last but then you lose speed.
This is the whole point of the exercise.
When funds allow, brushless is about the only way to have speed AND motor life for a worm drive.

 

[Duuuuuuuude]

I pretty much just want to double the top speed (to at least 4 mph) without degrading the performance in the very low speed region.
To double the top speed I need to at least double the power.

Have you seen the sensored brushless outrunner that Holmes Hobbies is working on? It might fit the bill perfectly.

 

[JeremyH]

I ran my LCC with the HD worms (a little higher gearing and more efficient power transfer) on a Xcelorin 17.5T brushless with a Mamba Max One Cell (with the boost enabled) and had good results with that setup. The truck was fast, had plenty of torque and the motor is sensored, so I noticed zero cogging at low speeds.

I know this doesnt answer your questions, but it might give you a good starting point for a powerplant in your truck. "thumbsup"

 

[Olle P]

Have you seen the sensored brushless outrunner that Holmes Hobbies is working on? It might fit the bill perfectly.

Yup! I'm pretty sure the 540 sized one will be my final pick, at least for now... ;-)

I ran ...
I know this doesnt answer your questions, but it might give you a good starting point for a powerplant in your truck. "thumbsup"

I've read plenty of "reviews" like that. And I think I've got a fairly good idea.

Still, it doesn't answer my original question:
Is there a common standard to how the power ratings provided by the manufacturers are measured/calculated? (And if so, what is it?)

 

[JohnRobHolmes]

Short answer, there is no common test method. You can take any ratings with a grain of salt. The basic jist of motors is that a bigger motor makes more power. INrunners have higher power density, outrunners have higher torque density. Brushed motors are at the bottom of the barrel for power. For your LCC a brushless solution will be better suited than brushed because of the power needed to turn the axles. Even the most high dollar brushed motor wears out fast getting past those worms.

For something available now, I would suggest the run of the mill sensored motor, 13.5t seems to be a good choice for most people. A Tekin 4 pole would be a great choice too.

For something purchased in the future the Revolver S will likely be best suited to those axles. Better torque density than inrunners to get past the worm gears, and nice slow KV so you aren't using 30,000rpm motor speed to develop decent power.

 

[JeremyH]

Still, it doesn't answer my original question:
Is there a common standard to how the power ratings provided by the manufacturers are measured/calculated? (And if so, what is it?)

It seems like you are trying too hard to make a generalization about a product with quite a number of variables.

We know that KV = RPM/Volt. So, for example, if you get a 1000KV motor and run it off a 3S lipo (max charge is 12.6V), then your motor will spin at 12600 RPM.

However, specifics regarding motor construction (motor wire windings and type of wire as well as magnet specifics) will drastically alter the power output and torque output from the motor.

When all of these factors are considered, it would be quite difficult to set an industry standard. You have heard from two motor manufacturers on this thread and both have said that no industry standard exists.

 

[Olle P]

It seems like you are trying too hard to make a generalization about a product with quite a number of variables. ...

There aren't that many variables in play here!

• The motor design is given, and the combined effects of it is what results in the answer. Thus it's a constant and not a variable for the equation.
• For the given motor design the output power is the result of only three factors:
  1. Feed voltage. (Can easily be standardized, and 7.2V is the only "norm" I've found.)
  2. Mechanical load. (There are two logical choices here; the load that provides the maximum power (at about 50% of stall torque), and the one that provides best efficiency (at about 20% of stall torque).)
  3. Motor temperature. Higher temp means less power. (Can also be standardized, but will provide a problem since it's typically increasing while the motor is under load.)

To create an industry standard there's thus only a need to specify
- feed voltage,
- relative load level (ideally both of the ones mentioned above, side by side), and
- temperature conditions.
As an engineer I simply assume(d) that such a standard exist and is adhered to!

 

[sloppy]

You are not going to really find what you want.

While we used watts as the standard in the brushed motor racing days. However, as we moved to brushless......for whatever reason, everything was rated in KV pretty much. I am guessing mainly because KV is VERY easy to measure, while typically the main thing the plane guys needed to know to figure out the prop size.

Dynos have the flaw in their ratings too, as dyno to dyno the results can vary, even on the same brand.......along with the fact there is not a standardized setting to use (volts, amps, etc).

I guess my question is, why do you need said specs? Is there something specific you need to know the power ratings for?

Later EddieO

Short answer, there is no common test method. You can take any ratings with a grain of salt. The basic jist of motors is that a bigger motor makes more power. INrunners have higher power density, outrunners have higher torque density. Brushed motors are at the bottom of the barrel for power. For your LCC a brushless solution will be better suited than brushed because of the power needed to turn the axles. Even the most high dollar brushed motor wears out fast getting past those worms.

For something available now, I would suggest the run of the mill sensored motor, 13.5t seems to be a good choice for most people. A Tekin 4 pole would be a great choice too.

For something purchased in the future the Revolver S will likely be best suited to those axles. Better torque density than inrunners to get past the worm gears, and nice slow KV so you aren't using 30,000rpm motor speed to develop decent power.

What more do you need????

 

[JohnRobHolmes]

Those would be good standards Olle. I think the main problem is that few companies have a dyno sophisticated enough to make that data. Personally, I know of two motor companies in the US with full feature universal dynos. Only Astro shares the data, as they only test in house motors. The other company doesn't share because they test other companies products. I have a few dynos, but none are satisfactory for outputting what you want. That is why I'm building a better one.


I would go a step further and just plot the whole curve down to 75% stall torque at 7ish volts.

 

[JeremyH]

There aren't that many variables in play here!

• The motor design is given, and the combined effects of it is what results in the answer. Thus it's a constant and not a variable for the equation.

You dont think that motor design can vary? All 35T motors are exactly the same? You'd be wrong...