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-   -   Another brushless 'myth', lower KV = more torque (http://www.rcuniverse.com/forum/rc-car-general-discussions-179/7137044-another-brushless-myth-lower-kv-%3D-more-torque.html)

mattnin 02-14-2009 01:32 PM

RE: Another brushless 'myth', lower KV = more torque
 

Quote:

ORIGINAL: Argess

Hey, hey, hey.....this is a discussion forum. Access is discussing like a gentleman. So should we all. I really appreciate the post Access has made. It's making me think, and I've been bored lately.

Anyway, here's what I get out of it. With EVERYTHING else being equal, and having a 10000kv motor with 1/2 the turns of a 5000kv motor, the torque will be the same. Why?

The current in the 10000 kv motor is double that of the 5000kv motor due to the # of turns (reactance) being 1/2, so it sounds like twice the torque.

But the number of turns in the 5000KV motor is twice that of the 10000kv motor, so that multiplies the torque by 2.

So, in the end, the torque is the same in both motors, howver the 10000kv motor is drawing twice the current.

Guess our low-turn motor isn't too effecient, as it's drawing twice the power from the battery to produce the same torque as the higher turn motor.
You have many errors in these statements for the same reasons I have been discussing to Access. You are not comparing torque at all, just power output which isn't the same. For the most part, you can replace 'torque' with 'power' in your post and you will be much more correct.

mattnin 02-14-2009 02:09 PM

RE: Another brushless 'myth', lower KV = more torque
 
Access, would you agree and say it is a true statement that....

A higher KV system with a higher current draw can have the same torque as

A lower KV system with a lower current current draw?

Argess 02-14-2009 02:40 PM

RE: Another brushless 'myth', lower KV = more torque
 
Mattnin: I have no doubt I am making mistakes. Electric motors are not my thing. I am only trying to learn a bit more about them. However you are incorrect. We are NOT talking power. We are talking torque. Basically locked rotor torque. And we can move up from that a bit, and talk low rpms, but we still arn't talking power because as HP = Torque * rpm /5252, we havn't any rpms's yet, or hardly any, so there is no power to speak of. But there is still torque, even at zero rpms. The charts prove that. In case you are wondering, a locked rotor situation is more like a transformer, where a rotating motor is a whole different kettle of fish.

You really ought to get off your high horse on this. Basically, it appears to me that RC brushless is essentially a 3-phase induction motor with a squirrel cage, or solid rotor. I can tell you that it's a complicated thing to determine the equivalent circuit and make forecasts on operation. Access has tried to simplify things by stating certain assumptions based on his experience and his attempts to substantiate with theory. He should be commended on this regardless if some of it sets you (and me) astray.

Please don't drop your education again. I can tell you mine is likely better, but I never assume I know everything. All you do is lose the respect and credibility you are trying to obtain.

dacaur 02-14-2009 03:02 PM

RE: Another brushless 'myth', lower KV = more torque
 
Access is right in only one thing, which is that any brushless power system can have x amount of torque, the thing he seems to be unwilling to admit is that the battery power required to get there is often unacceptable, even if the motor/ESC combo can handle it, when compared to higher volt/lower KV systems, or even simply lower kv systems.

You can get 300hp out of a 4 cyl engine (low volt/high KV), or you can get 300hp out of an 8 cyl engine(high volt/low KV). but the 8 cyl will run cooler, be more reliable, and last longer.
Access, you said that motor/esc heat was the # 1 problem when trying speed runs. Well, that heat comes from the # of amps running through the motor, not the voltage, so if you up the voltage, you lower the amp draw, to up the voltage, there comes a point you need to lower the KV, or the motor wont have the necessary torque without drawing huge amps.

What it comes down to is this. In any given setup, a lower KV motor will produce more power/torque at the wheels with less battery power than a higher KV motor. The reason it is doesn't have to work as hard trying to get to high RPM's, so runs cooler and pulls less amps, so gets less voltage sag.

What I have been saying, and what everyone else is trying to tell you access, is that lower KV motors are chosen because they can produce more torque with a given amp draw, and give you a realistic RPM to work with. Put a 10,000KV motor in ANY rc vehicle, and you will either 1) need $500 worth of batteries to run it, since it will pull so many amps, (but the motor is still unlikey to ever reach full rpm), 2) melt your batteries, or 3) gear it way down to prevent the ESC going thermal, motor meltdown, and/or battery meltdown etc...

What you (access) are saying is like telling someone the above mentioned 300hp 4 cyl is just as good as the 300hp 8 cyl. Its not. While the motors might produce the same power, there are a lot of trade offs to get there. In the case of a brushless motor, the trade off is amp draw.

Your power system doesn't heat up because of voltage, it heats up because of amperage. You could run 600 volts @ 1 amp through the wires on your ESC and they would be ice cold. but try running 200 amps at ANY voltage, and they will heat up FAST.

Look at ANY 1/8 scale electric conversion, and you will NEVER see one with a high KV motor. Why? because they run on high voltage systems. you will never seen one that runs on 7.4 volts because it would have a stupid high amp draw, and run very very hot. higher voltage/lower kv is the future.

Airplane guys figured this out a looooong time ago. You can get more thrust with less amps using a lower KV motor. Yea you could use a higher kv motor and get the same thrust, at a much higher amp draw, but why would you?

I will say this. If you have a magical battery that can supply unlimited amps, then sure a high KV motor will put out just as much torque as a lower KV motor. But until that day comes, lower KV will give you more torque on any given setup, (unless you you gear it stupid low)

Access 02-14-2009 03:13 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: mattnin
Access, would you agree and say it is a true statement that....

A higher KV system with a higher current draw can have the same torque as

A lower KV system with a lower current current draw?
Not quite enough info... Each one has the same voltage? Torque at a given throttle setting? If so, the higher KV one is going to spin that much faster at any given throttle setting. The torque-per-amp will naturally be less (it's spinning faster), so yes, with higher current draw, you can have equivelent torque.

"He is not talking about the torque at all and only describing power output, so the title of this thread is misleading."
One type of power is torque at RPM... if we know the power we're putting in, and we know the RPM, we can figure out the torque.

"A heavier vehicle like a monster truck or 1/8 truggy is powered by 4S or 5S lipos and a lower KV motor, not a 2S lipo and higher KV motor. As matter of fact, there is a grass-roots movement underway which is trying to get a higher voltage/lower KV systems used in racing due to the increase in efficiencies."
Not an increase in torque... an increase in efficiency.
Are we really disagreeing here?
And now that Novak has started to come out with 1/10th scale 3s-capable ESCs, like their Havoc 3s edition, I'll bet it's probably only a matter of time before ROAR gives in.

It's the size of the motor that's used for the 1/8th scale that makes it more powerful. I don't, for instance, take a 540-size, 17.5-turn, 2200KV and put it in my monster truck, powering it with 6s LiPo. I use an XL-size motor or one like the motor that comes with the Castle MMM (even larger than XL-size).

Larger motors naturally tend towards lower KV, if I go from standard size to XL-size, same number of turns, the KV drops. A 4.5-turn Novak "HV" size motor is lower KV than a standard 540-size 4.5-turn.

Access 02-14-2009 03:46 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: Argess
incorrect. We are NOT talking power. We are talking torque. Basically locked rotor torque. And we can move up from that a bit, and talk low rpms, but we still arn't talking power because as HP = Torque * rpm /5252, we havn't any rpms's yet, or hardly any, so there is no power to speak of. But there is still torque, even at zero rpms. The charts prove that. In case you are wondering, a locked rotor situation is more like a transformer, where a
Actually...
if you want to talk about locked-rotor torque, again for motors of a specific family, more turns = more torque, almost always (as long as resistance is not a factor). It's a static system. I was always talking about torque at RPMs. When the rotor isn't spinning, the vehicle isn't doing any work, hence the power delivery is zero. You're either pushing into a wall or 'just getting started'. It's useful for rock crawling, truck pulls, and such.

"Access, you said that motor/esc heat was the # 1 problem when trying speed runs. Well, that heat comes from the # of amps running through the motor, not the voltage, so if you up the voltage, you lower the amp draw, to up the voltage, there comes a point you need to lower the KV, or the motor wont have the necessary torque without drawing huge amps."
You lower the KV so that the rotor doesn't tear itself to shreds or so that the bearings don't fail. If you're doing speed runs, you're running the motor near the maximum RPM that it can handle. You do like Nic Case did with his 8s LiPo and his Neu motor b'cos that gives you the best efficiency.

Anyways, look at some motor charts that show resistances. The lower the KV, the higher the resistance. There's a 'sweet spot' in there that you can find, if you know the resistance for every motor in a given family.

"What it comes down to is this. In any given setup, a lower KV motor will produce more power/torque at the wheels with less battery power than a higher KV motor. The reason it is doesn't have to work as hard trying to get to high RPM's, so runs cooler and pulls less amps, so gets less voltage sag."
I see what you're saying, but if you look at the potential energy of a rotor spinning at 60,000RPM, and a rotor spinning at 30,000RPM, is it really that great? I'd think it's hardly even a factor. Anyways it's true only to the point that the lower KV motor is more efficient than the higher KV motor. When you talk about torque to the wheels you start to have to consider gearing and everything else. And gearing means that a high-RPM, low-torque setup can function similar to a low-RPM, high-torque setup if both are geared appropriately. Sure the high-RPM setup is wasting a little bit of energy to get the rotor spinning that fast, but how much? By the time it gets to the spur gear, both setups could be very similar.

"What you (access) are saying is like telling someone the above mentioned 300hp 4 cyl is just as good as the 300hp 8 cyl. Its not. While the motors might produce the same power, there are a lot of trade offs to get there. In the case of a brushless motor, the trade off is amp draw."
No, think of it this way. The 4-cylinder engine is a 540-size motor. The 8-cylinder engine is an XL-size or larger motor. Has almost nothing to do with KV (other than that bigger motors will have lower KV).

"Look at ANY 1/8 scale electric conversion, and you will NEVER see one with a high KV motor. Why? because they run on high voltage systems. you will never seen one that runs on 7.4 volts because it would have a stupid high amp draw, and run very very hot. higher voltage/lower kv is the future."
And they do it for efficiency. Not for more torque, but for more efficiency. The biggest difference in efficiency might be like the difference between 75% efficiency (bad) and 85% efficiency (good). 85% is what Castle claims to have acheived with their Castle-Neu motor I think.

http://www.castlecreations.com/produ...a_monster.html

10% efficiency means 10% more torque, but, only 10% more. 10% isn't that significant from a performance standpoint. But 10% more efficiency, for the sake of controlling heat, that's huge.

I think we might be 'very close to being on the same page', I just don't know though...

Argess 02-14-2009 04:06 PM

RE: Another brushless 'myth', lower KV = more torque
 
oh my....just as I think I am getting a handle on this, Access writes something that seems to set me back a bit.....LOL.

I think Dacaur and I agree....more or less. I think Access and I agree on a few things. I have no idea where Mattnin stands on this, other than he seems to disagree with everyone.

Seeing how the plot is thickening with every post, I think I'll stand back and keep out of this for a while.

mattnin 02-14-2009 04:28 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: Access

Quote:

ORIGINAL: mattnin
Access, would you agree and say it is a true statement that....

A higher KV system with a higher current draw can have the same torque as

A lower KV system with a lower current current draw?
Not quite enough info... Each one has the same voltage? Torque at a given throttle setting? If so, the higher KV one is going to spin that much faster at any given throttle setting. The torque-per-amp will naturally be less (it's spinning faster), so yes, with higher current draw, you can have equivelent torque.
Voltage and throttle settings are not necessary to compare torque. Current and number of turns are, however. Maybe I should reword it such that:

Take two motors built identical, but one has 4.5 turns per coil and the other has 13.5 turns per coil. Everything else is completely identical. Wires, can, rotor, everything. Consider that the motors are operating at the same RPM, and before the torque curve drop off.

Would the 4.5 turn motor need to draw more current than a 13.5 turn motor to get the same torque?

SAVAGEJIM 02-14-2009 05:48 PM

RE: Another brushless 'myth', lower KV = more torque
 

Quote:

ORIGINAL: Ttam Says Blarg

Holy cow I think my head is going to explode:D
And that is such when reading stuff written by those educated in EM physics.

It really is not all that bad once you learn the basics here. Mechanics and EM physics (of course the focus being in EM and the force creadted by them).

SAVAGEJIM 02-14-2009 05:59 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: mattnin

...You are not comparing torque at all, just power output which isn't the same. For the most part, you can replace 'torque' with 'power' in your post and you will be much more correct.
I am thinking that the fallacy being committed here is mistaking mechanical power as being equal mechanical power and setting those two to equal each other to factor out torque.

I remember this being said earlier in another above post:
"1) Electrical Power (Volts * Amps) in Watts
2) Mechanical Power (Torque * RPM) / 5252 in HP" (Dimensional units aside, I dont remember an electrical power-mechanical power theorem anywhere)

I guess it is like another fallacy many noob physics studensts fall for: thinking that moment and energy are equal because they have the same dimentional units.

EDIT: I must also add: to get torque out of mechanical power, simply making Mech power equal to Elec power is not enough. More steps are required to make this work.

An idea: instead of so much debate and disagreement, why not express all this mathematically and discuss the point in equations? This way it will be conclusive to see which are correct and which are fallacious.

HJJFFFAA 02-14-2009 06:04 PM

RE: Another brushless 'myth', lower KV = more torque
 
I want to understand but I can't.

mattnin 02-14-2009 07:00 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: HJJFFFAA

I want to understand but I can't.
This is actually such a simple problem, I am surprised there is still such a debate about it all. The title of this thread is labeled "Another brushless 'myth', lower KV = more torque". With extraordinary claims, extraordinary evidence must be presented, and it wasn't, and I enjoy setting out to set the record straight.

We can simplify all the variables except for one, the number of turns in the motor. To compare torques between motors of different windings, we have to make everything equal. In effect, this cancels out all the variables as well, but they are all considered.

We have to consider identical setups, so lets look at a real world example. Take an Associated T4 truck with a final drive ratio of 8.4, 2S Lipo Reedy Battery with a Novak GTB brushless system. We are going to test two motors, the 13.5 and the 4.5.

By looking at this test setup, we can see that the only thing different between the two is the number of windings in the motor, everything else is physically the same. So, this cancels out all the variables except for the motor windings themselves.

So now, we just have to analyze how coils in a brushless motor create torque which is given and broken down into the following equation:

Torque = N (number of turns) x I (current) x A (area of loop) x B (magnitude of the magnetic field) sin O (orientation of the normal to the coil with respect to the direction of the field)

With this equation, this can also be simplified except for N (number of turns). We can make the current in these tests equal, the area of the loop equal, magnitude of magnetic field equal, and orientation of magnetic field equal. So, lets make all the values = 1 for the sake of simplicity.

Torque = N (number of turns) x 1 x 1 x 1 x 1 = N (number of turns)
Torque = N (number of turns) simplified

So, the torque for a 13.5t motor is
Torque = 13.5 A*m^2
and for a 4.5t motor,
Torque = 4.5 A*m^2

To see how much torque the 13.5 motor produces compared to the 4.5, you simply divide
13.5/4.5 = 3

The 13.5t motor has 3 times the torque of the 4.5 motor, everything else equal.

So, based on everything being equal, in an Associated T4 truck with a Reedy 2S Lipo where we compare a 4.5 and 13.5 motor and we keep the current draw the same, and gearing the same, it is evident that the 13.5t motor will have 3 times more torque than the 4.5t motor.

Access 02-14-2009 07:16 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: mattnin
Voltage and throttle settings are not necessary to compare torque. Current and number of turns are, however. Maybe I should reword it such that:

Take two motors built identical, but one has 4.5 turns per coil and the other has 13.5 turns per coil. Everything else is completely identical. Wires, can, rotor, everything. Consider that the motors are operating at the same RPM, and before the torque curve drop off.

Would the 4.5 turn motor need to draw more current than a 13.5 turn motor to get the same torque?
To turn at the same number of RPM, the 4.5 would need less voltage than the 13.5
To produce the same amount of torque at that (same) RPM, the 13.5 would need more current than the 4.5.
The power-in to torque-out is equal, b'cos power = Volts * Amps

The 4.5 might only need 2.5V to turn at 20,000RPM while the 13.5 might need 6V.
The 4.5 might require 60A to provide the same amount of torque which the 13.5 produces at 25A.

Look at it from the standpoint of the ESC, that is, ESC as a 'closed system'. If our timebase is large enough (ie. several cycles of the ESC's PWM waveform or more), the ESC can neither create or destroy energy. I say 'large enough' b'cos there are capacitors on the ESC. Any energy that goes into the ESC either goes to the motor or goes to waste heat. Perhaps a little bit goes to vibration and such, but not much. Modern, quality ESCs generally have very good efficiency, otherwise they would bake with these setups that draw hundreds of watts or more.

Say you have 12V at the battery in both cases. In one case the ESC is taking in that 12V and only putting out 2.5V, in the other case it is taking in that 12V and only putting out 6V. So what has to happen to the current? The ESC can't magically create or destroy energy. Power (electrically defined as V * A) is energy transfer over time.

I know people have been saying the PWM isn't like that, that it is basically drawing 12V, 60A for time T and then drawing nothing for ~5T, repeating.
Even if the ESC had no capacitors at all, you'd still be sending a square wave with a roughly 17% duty cycle into a big inductor (the motors coils) to ground. If you send a 8KHz square wave into an inductor like that, what's going to happen? So this effect, combined with the capacitors that really are there on the ESC, is enough to stop this kind of stuff from happening (unless the ESC is poorly designed, with not enough low-ESR capacitors, too low of a PWM frequency).

http://www.bourns.com/bu/bu08_02intro.html
See the link above to show you what happens when sending a square wave of high enough frequency through a series inductor...

dacaur 02-14-2009 07:56 PM

RE: Another brushless 'myth', lower KV = more torque
 
Seriously, I will give you that a 10000kv motor *could* produce the same tourqe as a 5000kv motor in a perfect world with a magical battery that put out the same voltage no matter how many amps you take from it.

But in the real, world, thats not how it is. Those of us that have been using brushless motors for years can tell you this from experience. Voltage sag under load is going to mean that even in the equal RPM setups you are talking about, the 10000kv motor thats pulling 2x the current from the battery is going to have less power becuase it has less voltage to work with. The end.

Access 02-14-2009 08:16 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: dacaur
Seriously, I will give you that a 10000kv motor *could* produce the same tourqe as a 5000kv motor in a perfect world with a magical battery that put out the same voltage no matter how many amps you take from it.

But in the real, world, thats not how it is. Those of us that have been using brushless motors for years can tell you this from experience. Voltage sag under load is going to mean that even in the equal RPM setups you are talking about, the 10000kv motor thats pulling 2x the current from the battery is going to have less power becuase it has less voltage to work with. The end.
What is the size, and weight difference of:
1) 6000mAH, 2s, 20C battery
2) 3000mAH, 4s, 20C battery

That's the only 'fair' comparison here. Battery (1) can provide twice the current that battery (2) can. There's nothing 'magical' about it. Battery (1) belongs in the 10000KV setup and battery (2) belongs in the 5000KV setup, if they are to be 'equal RPM'.

The size / weight difference becomes absolutely negligeable when you get into cell capacities so large you are comparing a 2s2p battery to a 4s1p battery. Either way you have 4 cells of the same size.

10C drain on a quality LiPo and the voltage doesn't really 'sag' too much.
http://www.rcuniverse.com/forum/m_83...tm.htm#8310589
The good LiPos easily provide 11V @ 10C. This drops to ~10.5V as they discharge. 20C drops twice this much, but honestly if you run a battery at a sustained 20C, you're only getting 3 minutes runtime...

I've been running brushless years before the Mamba Max came out, lots of people have. Back before sintered rotors were standard... I remember going 54mph with a 2s LiPo and LRP 4.5 motor, that thing was absolutely baking hot by the end of that run. The mamba max, 5700KV motor, I could do the same speed and the temperature was actually manageable.

mattnin 02-15-2009 02:09 PM

RE: Another brushless 'myth', lower KV = more torque
 
I cleaned up my previous post to show the math as SavageJim requested.

Also, it doesn't matter at which RPM the motor is operating at, it can be at 0 RPM, 1 RPM or 60000 RPM, as long as the coils in the motor are receiving the same current at 0 RPM as it would at 60000 RPM, it would produce the same torque. However, there is a phenomenon called back EMF that occurs when the rotor is spinning fast enough, the motor itself will begin to act as a generator, and the current produced by that action travels down the wires in the opposite direction toward the ESC and counter-acts the current coming from the ESC. This effect can be so great that it can cause the net current seen at the coils to become near 0 amps. So for example, if the ESC is outputting 100 amps to the motor, but the back emf is at 99 amps at 60000 rpm, the coils are only seeing 1 amp so the torque can be calculated at that RPM.

Argess 02-15-2009 05:40 PM

RE: Another brushless 'myth', lower KV = more torque
 
Well....I have a different way with the math, although the end result may be the same.

I'll ignore my "locked-rotor" for the moment and just talk about "power" in the common rpm range of the two motors.

In each case, (if we assume perfect effeciency), the mechanical power out is the same as the electrical power in. Conservation of energy....er...more or less.

So....Power = Volts x Amps = Torque x RPM x Constant

or RPM (speed) = (Volts x Amps) / Torque

So let's compare two identically geared vehicles, one with a low KV motor, and one with a high KV motor.

Let's look at them for a moment in time when they are both at the SAME speed.

Low KV motor has L with the symbols. High KV motor has H.

(VL x IL)/TL = (VH x IH)/TH

As we have the same battery voltage in each case, it can be dropped from the equation the same way the Constant was.

IH/TH = IL/TL

So, we know the higher KV motor has less windings. Therefore the current draw must be higher (IH > IL). In order for the above equation to be true, the torque (TH must also be higher). Therfore the higher KV motor with less windings has more torque and will accelerate quicker.

Since it is accepted that this doesn't happen in practise, I offer Decaur's explanantion, that the battery may not be able to deliver the current required by the higher KV motor

That's enough for the post at hand, but there is a bit more info:

If we go back to the two motor's Access described....identical, except the low KV one had twice the windings of the high KV one.

It is without a doubt that more turns equals more torque. However, as you reduce the turns, the inductance gets lower, and more current flows. This seems neglected in your math Mattnin....the current will go up as the turns are reduced.

So.....we could have the same torque in both motors if we compare a 20 turn coil with 1 amp running through it to a 10 turn coil with 2 amps running through it.

But what will we really have? Basically, cutting the turns in half, make the inductance, and thus the reactance 1/4 of what it was (L being proportional to N^2) and the current should increase 4 times, double that which is required to maintain the same torque. So being so much more than what we need, the higher KV motor with 1/2 the turns, should be quicker. Well, honestly, we can't tell from this.

Why? Because the motor can be thought of as having an equivalent circuit of a transformer, but with a rotating seconadary. So, there are other fixed inductances to be considered and as they arn't changing, the current won't be so drastically increased from a simple turns reduction with regards to only one of the circuit elements). Is it enough? Or is it not enough?

Well, it changes with speed. The rotor impedance, or equivalent transformer secondary impedance, increases with speed, hence the need for a higher stator current (and less turns) to get a higher speed. So we look at locked rotor....intersting as it lets us know when the wheels will "start to spin". But we really can't tell. It is inherent in the design (rotor size, material, etc.). That's why we have to look at power, and use the equations I provided above. Looks like Mattnin was on the right track after all when he wanted to talk power.

Now I have to ask, just what is this myth? Two identical cars, one with a low KV motor, and one with a high KV one? Or are the cars geared appropriately for their respective motors? At identical speeds, it the math favours the higher KV motor. Or are you talking about identical currents? And if so, why? Electrical Current isn't apparent in judging vehicle performance by eyesight, but speed and accelration are.

OH,...and with regards to your back emf....yes, as that grows, your rotor speed gets limited. However, it is more general to talk of "slip". Slip is the difference in frequency betwen the applied stator frquency and the rotor speed. At synchronization, the slip is zero and the unloaded motor is at full speed (which never really happens as it takes some power to keep the rotor moving even when there is no load on the motor). At locked rotor, the slip is 100%, or normalized to 1. The more load, the more slip. That's why mortors slow down as you load them up.

Anyways, if you've read this whole thread this far, and made it though this particalur post, let me congratulate you on your perserverence. I still am not sure about the whole thing. I still don't know what the myth really is about, or under what conditions it exists, but I know I did learn a fair bit with this whole discussion.

mattnin 02-15-2009 07:28 PM

RE: Another brushless 'myth', lower KV = more torque
 
Argess,
Torque x RPM x constant
to get horsepower does not convert into watts or joules like that and is not the same as P = VI

Access 02-15-2009 10:28 PM

RE: Another brushless 'myth', lower KV = more torque
 
1 Attachment(s)
Quote:

ORIGINAL: Argess
So, we know the higher KV motor has less windings. Therefore the current draw must be higher (IH > IL). In order for the above equation to be true, the torque (TH must also be higher). Therfore the higher KV motor with less windings has more torque and will accelerate quicker.

Since it is accepted that this doesn't happen in practise, I offer Decaur's explanantion, that the battery may not be able to deliver the current required by the higher KV motor
I think this is what's actually happening here. You've assumed you have the same voltage, so... Here you're comparing two systems with two differing maximum RPMs. Let's just think about no-load RPMs for a sec, the reason a system "maxes out" in RPMs is b'cos at that point the torque at that RPM point has dropped to zero. So, if I'm comparing a system that no-load maxes out at 50,000RPMs to a system that no-load maxes out at 30,000RPMs, look at the theoretical max torque at any RPM (except 0), the 30,000RPM will have less theoretical max torque.

Also if you use the same equations you just used, but make the assumption the battery can only put out so much power, 1000W, just pick the same number in both cases, you'll see something closer to reality I think.

Look at what mattnin posted about back-EMF, that's another way to look at it. When the 30,000RPM motor is 'maxed out' at 30,000RPM, at the voltage you're working at, you simply can't force any more current into it with the voltage you're battery is providing. If you up the voltage of that 30,000RPM system it will be able to achieve more RPM, if you increase that voltage to the point where both motors are able to spin at the same RPM, then I think you'll find the results I was talking about.

Quote:

ORIGINAL: Argess
It is without a doubt that more turns equals more torque. However, as you reduce the turns, the inductance gets lower, and more current flows. This seems neglected in your math Mattnin....the current will go up as the turns are reduced.
At the motor level, more turns = more torque. But you have to qualify this statement in a few ways. If we were just connecting batteries directly to brushed motors and holding the rotors in place this is true. When you start talking about RPMs, it becomes much more complicated, and that statement isn't necessarily true anymore.

Look at my drawing below and think of something like Novak's 'L' series motors which are specifically designed to have less torque. If you use the equation you used, you have to be aware of the limits of the motor itself. In that equation, as you decrease the RPM, the torque increases. If you look at the torque as RPM approaches zero, it's infinite. In reality, one of two things is happening, either the electromagnets have fully saturated at some point, or you end up physically breaking the motor shaft.

For back-EMF, if you remember when Novak switched from bonded rotors to sintered ones, I remember some statement that due to the sintered rotors being stronger (magnetically), they would create more back-EMF, and hence you could expect to lose some KV if you swapped the rotor.

To understand how a brushless motor works, you need to understand both these things, that and that you cannot just 'turn on' or 'turn off' a coil instantly (inductance), which would be there even if you had no rotor. The more turns, the more inductance. But the more voltage you have, the faster you can overcome that inductance. These things are what keep the motor from just spinning faster and faster in a no-load situation.

Also think of how this applies to batteries. When you have more turns on the motor, it takes less current to get to the maximum torque, or to fully saturate those electromagnets. If you have less turns on the motor, it takes more current to fully saturate those electromagnets.

mattnin 02-15-2009 10:58 PM

RE: Another brushless 'myth', lower KV = more torque
 

Quote:

ORIGINAL: Access
At the motor level, more turns = more torque. But you have to qualify this statement in a few ways. If we were just connecting batteries directly to brushed motors and holding the rotors in place this is true. When you start talking about RPMs, it becomes much more complicated, and that statement isn't necessarily true anymore.
Access, it remains a true statement. For example, the 13.5t motor will have more torque, but the 4.5t motor will have more power. That is why the 4.5t motor can rev higher, it has the power but not the torque. I think it is time we all research Torque vs. Power.

Access 02-16-2009 12:25 AM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: mattnin
Access, it remains a true statement. For example, the 13.5t motor will have more torque, but the 4.5t motor will have more power. That is why the 4.5t motor can rev higher, it has the power but not the torque. I think it is time we all research Torque vs. Power.
Could you draw a picture, or describe, what the torque vs. RPM curve of either motor looks like (if both are run with the same voltage)?

I did a search online and found:
http://www.fastelectrics.com/elecmotorbasics.htm

The second picture on that page.

When you have two setups running same voltage, different RPM, are the torque curves really going to cross at some point?

mattnin 02-16-2009 08:09 AM

RE: Another brushless 'myth', lower KV = more torque
 
Yes, the torque curves will cross at some point, and there will be an RPM where the 4.5t motor will have more torque than the 13.5t motor. However, before the motors reach that RPM, the 13.5t motor will have more torque than the 4.5t motor.

By looking at that link that you posted and reviewing the graph, they are comparing an 11 turn and 12 turn motor. The 11 turn motor has a torque of 246 N*m^2 and the 12 turn motor has a torque of 277 N*m^2. However, their torque curves cross at around 17,500 RPMs and and after that point, the 11 turn motor has more torque than the 12 turn motor.

It also appears that the max rpm of the 12t motor will be right about 35,000 rpm and the 11t motor max rpm appears to continue right off the graph into the 40000-45000 rpm range.

Access, if you would have the title as, "Another brushless 'myth', lower KV= more power, you would have been so right.

http://www.fastelectrics.com/helis/motordyno2.gif

*edit to show the picture Access is referring to*

Access 02-16-2009 10:36 AM

RE: Another brushless 'myth', lower KV = more torque
 
I wonder if there is a similar tool one could download for free, for running simulations like that.
(1) "Motor 1: Simulation: 7.5 Volt"
(2) "Motor 2: Simulation: 7.5 Volt"
If he were to increase the voltage on motor (1) to the point where both motors maxed out at the same RPM, then what would the torque curves end up looking like? If the reality is the motor can't provide more than 90 units of torque, it would look like the picture I uploaded enough (except the dotted lines/theoretical should be crossing at some point).

dacaur 02-16-2009 03:24 PM

RE: Another brushless 'myth', lower KV = more torque
 
Simulations are usless, because they dont take all factors into account. No one asks "so which will have more power in a sumulation?" Real world testing on real batteries is all im interested in.

Access 02-16-2009 03:47 PM

RE: Another brushless 'myth', lower KV = more torque
 
Quote:

ORIGINAL: dacaur
Simulations are usless, because they dont take all factors into account. No one asks "so which will have more power in a sumulation?" Real world testing on real batteries is all im interested in.
Simulations are often used to at least give you a 'starting point' though, ie. the max speed and gearing calculators. This can show you theoretical efficiency (vs. RPM) and that at least gives you a hint of how to gear it and such.


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