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Old 05-13-2009, 03:59 PM   #1
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Default Motor Ratings....

Hey guys, Im having a bit of trouble with the whole motor ratings.

Are the ratings provided by manufacturers trustworthy, or are they based differently like Nitro engines?

What determines a good motor? Is it RPM, RPM/Kv, Power (Watts) or Efficiency?
I'm asking this because I was comparing a X12 motor with a X11 motor, same number of turns and found something odd:

X12:
RPM: 45.360
Specific RPM/V (KV): 6.300
Power (W): 463W
Efficiency: 91%

X11:
RPM: 58.320
Specific RPM Per Volt, KV: 8100
Power: 393W
Efficiency: 91%

As you can see, the X12 has less RPM and therefore less RPM/V, but has more Power (W).

My guess is that the X12 will produce more Torque than the X11, but the X11 will have more RPM than the X12. Therefore for small applications, such as, 1/12, the X11 is better.

Can someone correct me please and shed some light?????
Thanks in advanced...
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Old 05-13-2009, 09:31 PM   #2
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No one?
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Old 05-13-2009, 09:39 PM   #3
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Best advice is forget about the label. Just check and see what the majority of the fast guys are running and why, not all motors have the same power band. I'm assuming you are asking about brushless motors, since the brushed motors are dead.
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Old 05-13-2009, 10:06 PM   #4
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Originally Posted by Serpenteer View Post
Hey guys, Im having a bit of trouble with the whole motor ratings.

Are the ratings provided by manufacturers trustworthy, or are they based differently like Nitro engines?

What determines a good motor? Is it RPM, RPM/Kv, Power (Watts) or Efficiency?
I'm asking this because I was comparing a X12 motor with a X11 motor, same number of turns and found something odd:

X12:
RPM: 45.360
Specific RPM/V (KV): 6.300
Power (W): 463W
Efficiency: 91%

X11:
RPM: 58.320
Specific RPM Per Volt, KV: 8100
Power: 393W
Efficiency: 91%

As you can see, the X12 has less RPM and therefore less RPM/V, but has more Power (W).

My guess is that the X12 will produce more Torque than the X11, but the X11 will have more RPM than the X12. Therefore for small applications, such as, 1/12, the X11 is better.

Can someone correct me please and shed some light?????
Thanks in advanced...
You basically got it. All else being equal, power and effeciency are the most important. In my opinion, the heavier the car will benefit from the extra torque more.
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Old 05-13-2009, 11:24 PM   #5
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You basically got it. All else being equal, power and effeciency are the most important. In my opinion, the heavier the car will benefit from the extra torque more.
I agree...

GIMME Power/Efficiency over RPM almost ANY DAY

I'll FIND a gear ratio for LOW RPM - but I dare you to find MORE Power/Efficiency!

(However...if Power/Efficiency is close to EQUAL - I'll do High RPM over LOW RPM)
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Old 05-13-2009, 11:54 PM   #6
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If you're interested in the physics of it here's some basic knowledge you need to refer to:

http://en.wikipedia.org/wiki/Torque

This explains the relation between mechanical torque, power and rpm.

One unanswered question is what power is reported in the info you get with your motor. Is it electrical or mechanical?

My suspicion is that the power reported is electrical power, i.e. the power consumed by your motor (out of the battery) to produce the parameters they report (or at least that would be a consistent way to report the data on the motor). I suspect so just because it is the easiest for the company to measure. Wether that is true or not is another discussion. The fact they don't report torque also points to the company just measuring some electrical parameters and deriving or reporting some as such.

But let's say they do it, and the power reported is then W=U*I where W is power, U is the battery voltage (I admit this is simplifying a bit, but it's way easier to treat then go into the electrical physics of it) and I is the current drawn out of the battery. This means then that you need to calculate the loss on the motor and that's where efficiency comes into place, because it gives you the information you need to see exactly how much useable power you motor will produce. Useable power then is W (power reported) times efficiency. This is then the mechanical power available to you to play with your car. With this then you can calculate the mechanical torque available at the rpm the other data is reported.

You will see straight away that you depend immensely on the data being properly reported. If it isn't, all the above goes out the window. So in fact what you can do is check everything and if something doesn't respect the physics of it, then they're not reporting the data properly (which I think it's the case most of the time).

There are other data reported in your info. RPM per volt for instance I think it's a very misleading parameter since it induces the idea that bumping the voltage up 1V will give you another x rpm. This just can't be true. My guess is that's at best an average value over the rpm range, and that in fact you might get an rpm increase but the rpm increase you get when going from 1V to 2V is one and the rpm increase you get when going from 4V to 5V is a different matter. At best, if they are honest they will average the rpm gain over the voltage range and report that. Now averaging is a difficult proposition and depending how you do it, it will give different results (it is not a simple adding and division, there are other factors that need considered, but I will leave it at that).

One thing that immediately shows they are not very straightforward with us, the customer is the current drawn which is not reported and qualified with voltage and rpm it is measured at, as well as the rpm at best efficiency. That would basically give you a hard data point (which is not much, but is a good start and something you could immediately check with basic tools which is what they probably afraid of).

There is also another problem of what sort of voltage were they using when measuring all the above? Continuous voltage (as delivered by a power source) or impulse voltage (as delivered by your esc)? And if impulse voltage was used, what was the frequency? This will affect the results directly and seriously.

In conclusion, to answer your question, I would say try to get some qualified feedback from users of both motors (qualified as in as controlled conditions as possible-same voltage, same load, etc), so you can compare the info. To compare the motors you are looking at if all the conditions above were satisfied (all parameters measured at the same voltage, same conditions, etc, etc), it appears obvious that the more revvy motor has lower torque. If you checked the wiki page linked above or your physics knowledge helps you, you know that W(power)=Torque*rpm divided by some constant factor (say all units are converted to match). That means you have for the first motor a torque equal to 463 divided by 45360 and for the second motor 393 divided by 58320. As I said above this makes it obvious without even operating the divisions that the first motor has higher torque.

Again, I have to mention this assumes all the data is compatible. If not, you're on your own (or that's the message I read when I see such data, anyway). Companies are too lazy to do it properly (and when they do and you get a dyno printout you have to pay serious money for your motor) and they probably think we are too stupid to investigate all this stuff. There is also the competing trend to report higher and higher this or that forgetting that a number is meaningless if not qualified properly (kinda like the engine power in cars without reporting the mass or SAE power figures which are meaningless since the engine si stripped of all the ancillaries when they measure it; people say SAE stands for "sans almost everything").

So I try to measure things myself. See the dyno thread here it may give you some ideas.

Hope this helps.
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Last edited by niznai; 05-14-2009 at 12:15 AM.
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Old 05-14-2009, 09:40 PM   #7
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Originally Posted by niznai View Post
There are other data reported in your info. RPM per volt for instance I think it's a very misleading parameter since it induces the idea that bumping the voltage up 1V will give you another x rpm. This just can't be true. My guess is that's at best an average value over the rpm range, and that in fact you might get an rpm increase but the rpm increase you get when going from 1V to 2V is one and the rpm increase you get when going from 4V to 5V is a different matter. At best, if they are honest they will average the rpm gain over the voltage range and report that. Now averaging is a difficult proposition and depending how you do it, it will give different results (it is not a simple adding and division, there are other factors that need considered, but I will leave it at that).
RPM is directly proportional to voltage, for both brushed motors and brushless (providing that the switching frequency doesnt change).

Shawn.
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Old 05-14-2009, 10:07 PM   #8
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Originally Posted by Shawn68z View Post
RPM is directly proportional to voltage, for both brushed motors and brushless (providing that the switching frequency doesnt change).

Shawn.
It is, but it is not linear as they suggest when reporting "x many rpm per volt". That's why I said the increase is not the same over the voltage range.
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Last edited by niznai; 05-14-2009 at 10:25 PM.
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Old 05-18-2009, 11:57 AM   #9
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Quote:
Originally Posted by Serpenteer View Post
X12:
RPM: 45.360
Specific RPM/V (KV): 6.300
Power (W): 463W
Efficiency: 91%

X11:
RPM: 58.320
Specific RPM Per Volt, KV: 8100
Power: 393W
Efficiency: 91%
In this comparison, the voltage ratings are self-consistent:

Voltage = RPM/KV = 7.2{V}, nominal voltage for 6 cell NiMH or 2s Lipo

Although the winds are the same, the diameter of copper wire may be different. Magnet strength may be different. Diameter of the rotor may be different. These differences account for the different torque-speed curves.

More resistance in the winds reduces output power from the same battery, since it reduces current draw. Less resistance increases current draw and power.

Stronger magnets increase torque but reduce top speed. Weaker magnets reduce torque but boost speed.

A small diameter rotor of the same density material has less inertia and spins up faster. A large diameter rotor has more inertia and spins up slower.

Manufacturer's data is commonly derated for all sorts of engineering products. See the Sentry Dyno thread for more insight.
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