Question about Mambas (or brushless motors in general)

[ss13t]

I know that the Kv rating on motors is the RPM per volt right? So the mamba 8000Kv makes 8000 rpm per volt and the 6800Kv makes 6800 rpm per volt. My question is, does a moter with a lower Kv rating make more torque? Is the mamba 6800 torquier than the 8000? Or is the 8000Kv just all around better in both torque and rpm?

[pinolelst]

From what I've read Torque is a factor of ampacity or the ability of a pack to deliver amps under load. That being said a 6800 motor @ 50 amps should have the same torque as an 8000 motor at 50 amps the difference is in the rpms per volt as you've already discovered

hope this helps

pinolelst

[tcolesen]

The 8000 will have less torque at 50amps than the 6800 will at 50amps. When kv increases, kt decreases. So to make up for the lack of torque, the 8000 will require more current.

[pinolelst]

The 8000 will have less torque at 50amps than the 6800 will at 50amps. When kv increases, kt decreases. So to make up for the lack of torque, the 8000 will require more current.
Is that a linear relationshp between kv and kt ?? :teacher:

pinolelst ;)

[splode]

Is that a linear relationshp between kv and kt ?? :teacher:

pinolelst ;)

The best way to answer your question would be in terms of back emf (the voltage that the motor generates in opposition to the voltage from the power source).
A motor with a lot of torque has a great number of turns, and is therefore a more efficient generator which will generate a large back emf at low RPM. Conversely, a motor with a small amount of torque has a fewer number of turns, and is therefore a poor generator which will generate a small emf at low RPM.This principle is what limits the overall RPM of the motor. Otherwise your motor would just keep going faster and faster.

[pinolelst]

my question has no answer. ;) bearings are the main rpm limiters in production brushless motors.

pinolelst

[splode]

my question has no answer. ;) bearings are the main rpm limiters in production brushless motors.

pinolelst

No, and no.

[ElectricThunder]

my question has no answer. ;) bearings are the main rpm limiters in production brushless motors.

pinolelst
That and the rotor's chance of exploding from centripetal force... :eek:

[splode]

Wow, look what took me less then a minute to find on the Internet via wikipedia. Seriously though, I think this might help you guys out a bit.


"The counter-electromotive force (abbreviated counter emf, or CEMF ) [1] is the voltage, or electromotive force, that pushes against the current which induces it, is caused by a changing electromagnetic field. It's represented by Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. Counter emf is a voltage developed in an inductor network by a pulsating current or an alternating current. [2] The voltage's polarity is at every moment the reverse of the input voltage [3] [4]

In a generator using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator.. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced. [5]

If it is assumed that a motor is 100% efficient with no friction or windage losses, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord."

[pinolelst]

wikipedia ??? :D

I'm no electrical engineer...just an electrical systems inspector...and only repeating what I've been told.In discussions with Castle Creations people and on forums the consensus I got was that bearings and rotors are the limiting factor in brushless...there is no real voltage limit and bearings and rotors can only take so much heat and centrifugal force before they blow up.Electro magnetic back fields never entered the discussion but if you're saying they will be of such magnitude as to hinder a brushless motors operation before a ceramic bearing gives up at 90k rpm or the rotor explodes at 80k...even though there are numerous tests done by people at those speeds..to destruction..then I have to bow to your mastery of things electro magnetic...
From someone who knows far more than I about brushless motors:" torque is proportional to current, speed depends on the voltage and the load torque."

Lenz's law assumes a %100 efficiency...there is no such thing and since it doesn't exist Lenz's law is mute.
what next..Google ???

Peace :wave:

[splode]

Well at least I got a good laugh out of you. However, so many of your statements are wrong at this point that I fear by continuing this conversation with you, other people will be mislead. I would suggest that you also do research from books, like I do, not just on the web. Then perhaps later on you can differentiate between the crap on the internet,random opinions posted by unknown sources, from the information you learned in those books.

"Lenz's law assumes a %100 efficiency" :rolleyes:

Are you for real? Just break open a book before you post; your Internet I.Q. might just break 100!

[glassdoctor]

No offense but I skipped all the posts except for the mention of wikipedia... :)


Short and sweet: There are more qualified people around here than me...BUT I believe that the torque will be the same, for a given "series" of motor, such as the 540 Mambas... all the same size, design etc... but different winds.

The available torque is the same, but the lower wind motor will suck more current and thus develop more watts = horsepower.

In practice, we never gear a 7700 the same as a 4600 though... not due to the lack of torque, but due to the heat and amp draw you would get trying to run a 7700 geared as tall as a 4600.

If someone knows I screwed this all up, please by all means say so. I don't worry too much about the theory... I tend to just look at what happens in reality and try to learn from it.

[pinolelst]

Well at least I got a good laugh out of you. However, so many of your statements are wrong at this point that I fear by continuing this conversation with you, other people will be mislead. I would suggest that you also do research from books, like I do, not just on the web. Then perhaps later on you can differentiate between the crap on the internet,random opinions posted by unknown sources, from the information you learned in those books.

"Lenz's law assumes a %100 efficiency" :rolleyes:

Are you for real? Just break open a book before you post; your Internet I.Q. might just break 100!

hmmm...still no substantive answer...we call that "duck and weave" on the Jeep BB I moderate on :D
Here I was trying to be nice...give you some slack...and you insult me.Please don't worry about my internet IQ..my tested one is higher than the avg bears as it is... :roll2: I probably read more technical data every year than the avg college student does in 4....

You still haven't given any credentials or viable sources...beyond..wikipedia..nothing more than a database with a constantly conflicted point of view BTW .If you're going to argue with an inspector you better do it well because trust me..there's nothing I like more than a good argument.. :D


pinolelst :wave:

[splode]

Inspector Kluso, I though that you could easily extrapolate the answers with the given information. Seeing as you could not, I will try to explain it as simply as possible (me Tarzan you Jane).

Facts:

1) A motor's Absolute maximum RPM is determined by several factors, such as bearings and rotor strength. I guess for the mambas this is around 65K RPM.

2) A motor's Maximum RPM, given by KV, is determined by the number of winds. The greater the number of winds the lower the KV.

3) A motors torque is determined by several factors such as the number of winds, the amount of current running through those winds, and the RPM of the rotor.



Before I derive anything, let's play around with this information. (I will assume all motors have the same permanent magnetic strength)

i) Let's say you wanted a really fast motor.
You would buy a motor with very few winds, but you would not have much torque.

ii) Let's say you wanted a motor with a lot of torque.
You would buy a motor with a great number of winds, but you would not have much speed.

iii) You might ask the question at this point, "What wind gives me a high speed and a large torque?".
The answer is no wind will; it will always be a trade off.

Now you ask, "But I have a motor with more torque and speed then my other motors!"
The answer is that you are simply running more current at a higher voltage through that motor.
A practical example would be a caparison between the mamba 4600 and the mamba 7700. The 4600 has a great number of winds and therefore needs more voltage then the 7700 to get same the RPM of the 7700.
The 7700 has fewer winds and therefore needs more amps running though its winds then the 4600 to get the same amount of torque the 4600 has.

So far, this is all the information I assumed you already knew. But let us digress.

How can we explain all this phenomenon? Simply look at my last post about back emfs( I don't care to type it all again). But all you really need to know is that as the rotor rotates, using Faraday's law of induction with lenz's law (a negative sign), an emf(voltage) is generated that opposes the emf(voltage) of the battery.

By now I hope you have the sense that KV is in inverse proportion to Kt.
Also, you may need to read the above several times before you understand it. Do that if necessary then read what is below.


(I had to crack open an old book for these, University Physics since you must know)

I) KV is given by RPM/V

II) Stalled motor torque = n(IAB)

III) Kt ~ n(IAB) x f(rpm) (I pulled this one out of thin air but it should work)
where:

n = number of turns
I = current
A = area of each turn
B = magnetic field strength of magnets
f(rpm) = a function of RPM in terms of torque


Using and manipulating these formula and using V = IR we get:

v = rpm/KV and Kt/(nAB x f(rpm)) = I = V/R

Substituting:

Kt/(nAB x f(rpm)) = rpm/(R X KV)

or simply:

Kt ~ c/KV which is indeed an inverse proportion.


Obviously this is a terrible approximation, but it does confirm to us that KV and Kt are in inverse proportion.

I hope by now I have answered all of your questions.

[ElectricThunder]

wikipedia ??? :D

I'm no electrical engineer...just an electrical systems inspector...and only repeating what I've been told.In discussions with Castle Creations people and on forums the consensus I got was that bearings and rotors are the limiting factor in brushless...there is no real voltage limit and bearings and rotors can only take so much heat and centripetal force before they blow up.Electro magnetic back fields never entered the discussion but if you're saying they will be of such magnitude as to hinder a brushless motors operation before a ceramic bearing gives up at 90k rpm or the rotor explodes at 80k...even though there are numerous tests done by people at those speeds..to destruction..then I have to bow to your mastery of things electro magnetic...
From someone who knows far more than I about brushless motors:" torque is proportional to current, speed depends on the voltage and the load torque."

Lenz's law assumes a %100 efficiency...there is no such thing and since it doesn't exist Lenz's law is mute.
what next..Google ???

Peace :wave:
...:D

[pinolelst]

...:D

Hey... :D Tomatoes Tomahtos...you understood.

As far as the lengthy diatribe by mr splode here..all those words to essentially agree with me..rotor and bearings are the main limiter long before any theoretical EMF failure :wave:
You're thinking is more in line with combustion motors....smaller pistons at high rpm have nowhere near the torque that large high volume pistons do.That's combustion engine physics.My 113" V-twin can pull away from a light in 3rd gear without breathing hard while the guy on the hayabusa will stall trying to do the same thing or burn his clutch up in the process....

oh yeah..that's Mr.Inspector Clouseau to you buddy :D

pinolelst

[splode]

rotor and bearings are the main limiter long before any theoretical EMF failure :wave:

What are you talking about? What is a emf failure?

[Quinton]

Here's what I've learned about the Mamba's from reading and learning from others on this board. I'm going to keep it simple for my simple brain.

A 7700 is faster in top speed then a 4600, but it doesn't have as much torque. But it still has GOBS more torque then we really need.

My question is why would someone buy the 4600 instead of the 7700 when they're the same price? Unless the 4600 is alot more efficient. Is it?

I'm hoping to buy one this summer but don't know which one to get (I'm leaning towards the 7700, like I did when I bought my Comp X for my rc18t).

Thanks in advance for any answers I may get!

[splode]

You hit the nail on the head. Both motors are essentially the same, one is just more efficient.

[glassdoctor]

Power comes at a price.... no free lunch. If you don't NEED what a 7700 can do then you are better off with a 4600 which is more efficient.

I still say the torque is the same for both 4600 and 7700.

[Mangokiwi]

I still say the torque is the same for both 4600 and 7700.

Yes, as long as the battery can supply the Amps needed.

[starluckrc]

I've had to answer this question many times. A few good points have been mentioned here. In a nutshell......4600, 5700, 6900, 7700......all the same
since the voltage constant is inversely proportional to the torque constant.
Just different ways of getting to the same place (speed, torque, power). In the real world though, lower kv motors tend to give a slight edge in efficiency due to a combination of several factors.

[glassdoctor]

Yes, as long as the battery can supply the Amps needed.


Right... I think I said that in my first post. :teacher: :D

[vashon10]

more thAn
greater thAn
less thAn
faster thAn
slower thAn

get it ? got it ? good.

[splode]

Well I'm glad to see that everyone has reached a consensus on this topic.

Though, I'm still dying to have "Mr. Inspector Clouseau" explain what an emf failure is; is it something you heard on Star Trek? :D

You also said you were an electrical systems inspector? I would like to know exactly what you inspect :eek: (cheap Chinese flashlights *cough*) JK

Don't worry, I'm sure you can "duck and weave" out of all of this. :cool:

Seriously though, I know that this is way off topic, but you said that you love a good argument. :p

[pinolelst]

Well I'm glad to see that everyone has reached a consensus on this topic.

Though, I'm still dying to have "Mr. Inspector Clouseau" explain what an emf failure is; is it something you heard on Star Trek? :D

You also said you were an electrical systems inspector? I would like to know exactly what you inspect :eek: (cheap Chinese flashlights *cough*) JK

Don't worry, I'm sure you can "duck and weave" out of all of this. :cool:

Seriously though, I know that this is way off topic, but you said that you love a good argument. :p


Still in insulting mode I see.That's what I expect from someone who's lost his argument and can't think of anything else valuable to add to the conversation....
I am a Building Inspector if you must know and hold certifications in Structural, Electrical and Mechanical systems inspections...Been doing it since I was 30 years old ,nearly 20 years now....so no..I don't inspect cheap chinese flashlights....so padwan..what makes you so smart.???
Please regale us with your electrical engineering degrees and many lifetime accomplishments which led you to believe that a theory requiring a motor that does not exist in the real world has any bearing whatsoever on this discussion. :roll2:

Oh btw..the consensus did not match yours if you bothered to read 1/2 of what was said...of course that's assuming you can get the blinders off and see the trees for the forest ...or have reading comprehension skills beyond 3rd grade level.That would help immensely :D

Peace

Pinolelst :wave:

[pauster]

I know that the Kv rating on motors is the RPM per volt right? So the mamba 8000Kv makes 8000 rpm per volt and the 6800Kv makes 6800 rpm per volt. My question is, does a moter with a lower Kv rating make more torque? Is the mamba 6800 torquier than the 8000? Or is the 8000Kv just all around better in both torque and rpm?

With a family of motors it is typically the case that a motor with a lower kv rating will have more torque and less rpm while consuming a similar amount of power.

There are many posts in this forum explaining the physics in detail - I won't repeat them here.

In general it is preferable to run lower kv motors at a higher voltage because
this increases the efficiency (same power at lower current but hight voltage) - IF the ESC can handle it. As always, it is a trade-off.

Here (among many other places) http://www.neumotors.com/20061222/Motor%20test%20data.html you can find the proper curves and lots of data that explains how voltage, current, rpm and efficiency are correlated.

[ljohnson]

As far as the lengthy diatribe by mr splode here..all those words to essentially agree with me..rotor and bearings are the main limiter long before any theoretical EMF failure

Huh? Limiter on what? They are the things that will break if you get too much rpm - by putting in too much voltage in with too little load. splode's just trying to give an overly accurate answer to this :
Is that a linear relationshp between kv and kt ??

Looked to me like you didn't understand his answer and couldn't handle the qualifiers on the accuracy of the theory.

splode - do you have a one line version of the answer to that?

[splode]

Yes, check out Pauster's Link.

BTW
From the data, those look like some great motors!

[Stuart B]

As an engineer with an Electrical and Electronic degree that included motor theory, plus experience with them my self....Splode is pretty much correct pinolelst.

I can see exactly what you are saying though pinolelst....but unfortunately its not the whole story...its nothing like the whole story. For example if you forces a motor to spin (by driving its shaft from anoth rotational source) you could poobably sping pretty fast before anything was damaged.....which I think is what you are saying....a motor might not be damaged by spinning it really fast.

What splode is saying is that the internally generated EMF in a motor (all permanent magnet motors do) reaches a level comparable to the voltage you are applying to the motor the current through the motor will reduce or stop of the voltages manage to match (unlikely in a free air situation due to drag, friction and so on). It reduces because the voltage difference is low.

The speed of a motor is pretty much proportional to the voltage applied (ignoreing drag, fricition...)...this means that the motors ultimate speed for a system is the voltage applied (usually battery voltage minus losses unless some sort of voltage boost circuit is employed).

This internal EMF doesn't fail, not sure what you meant my that.

Also, for a given current a motor with more turns will have more torque but will have a lower speed. One affect of few turns is that the wire is shorter and prbably also thicker too...so it will have less resistance. So the current that can flow in the motor can be higher if the current can be supplied with out voltage drop/droop. This will make up for its torue defecit, but tyhe motor will taking more electrical (and making more mechanical) power to make that torque.

Stu

[pauster]

One more thing to keep in mind - efficiency.

Search for efficiency in this forum and you will see that this is an often overlooked parameter.

A high-quality motor that is running the ideal range will have an efficiency >90% and therefore produce less heat (e.g. a motor running at 92% efficiency will transform 8% if the input power as heat).

That heat has to go somewhere - sometimes with the help of fins or fans (some built into the motor). If the heat does not dissipate fast enough you'll fry the motor eventually.

Cheap motors may run hotter due to their sometimes lower efficiency. Many reputable brushless motor makers post detailed data.

There is a huge difference between running a motor at say 80% and 90% efficiency. Although the 90% motor will only have 12.5% more power than the 80% motor, the better motor will only generate half the heat so you can run it closer to the max a lot longer.

So - you need to size the motor right - and don't forget the gearing. It takes time but should pay off.

[ss13t]

So umm... torquier or no? If the 8000 has just as much torque as the 6800 AND more rpm, why would anyone buy the 6800??

[tcolesen]

Shorter run times, less overall battery life, more difficult to control, harder on the car, etc.

[starluckrc]

There is more to the equation than the motor. Higher kv, higher current draw, more voltage suppression.......which generally means less efficiency and overall power. Why buy the 8000 if the 6800 is already too fast?

[littlej72]

I was told by the guys who make them that the 5700 is the best balanced system and more than I would ever use unless I have a lot of room, if you look there is a low cell limit on the 7700 which dosn't really allow it to be any better than the 5700(the 5700 would just be...working harder) either way for a 1/10 scale on or off road I was told it would be the best setup they offer. There the same price because there such a good company they dont charge there customers more for something that cost the same to make! just because its perceived as being better.

Just keeping it simple...no "fancy Shmancy " lingo, hehe.

[ss13t]

Well people always complain about the 8000 being too hard to control. I have one in a kyosho 1/8 bike with the esc set to extreme timing and max starting power and Its fine.