Mythbusters. Lower KV= more torque? - RC Groups
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Aug 17, 2009, 05:21 AM
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vintage1's Avatar
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Mythbusters. Lower KV= more torque?


Oft quoted as fact, in fact completely untrue.

Rationale of the myth
Torque is proportional to magnetic flux density which is proportional to ampere turns therefore more turns is more flux per amp is more torque.

Reality of the situation
Flux is limited by saturation in the motor core for any decent efficiency. Therefore peak flux cannot be increased.

In addition, to rewind a motor with twice as many turns, means that in order to fit, the cross sectional area of the wire will be halved..half the area, twice the length - 4 times the resistance. So, for a given amount of heat, heat being proportional to current squared times resistance, the current has to halve as well. So torque, for a given copper loss, doesn't change.
In fact the results of rewinding a motor to a different KV can be summarised as follows.

- RPM goes down as KV goes down for a given pack.
- Peak torque for a given rotor and stator assembly does not change.
- Peak torque for an allowable heat loss does not change
- Power goes down as RPM goes down as KV goes down. (torque times RPM is power)
- Iron losses go down with KV but these don't normally dominate.
- Efficiency goes down (at peak torque), as the same copper losses happen, but actual power input is less.


Conclusion. If you want more torque, use a bigger stator and rotor. Period. Or a gearbox.

Further comment. More poles don't really help that much. If you double teh poles, you half the amount of iron in each, and halve the amount of copper in each one.

Last comment. The above implies that power is more or less proportional to how fast you rev the motor. This is largely true until the iron losses start to become very significant.

Which leads to another conclusion.

A given size of motor has no single efficiency or power rating. Within extremely broad limits, its power is a function of its RPM and its efficiency is a function of the load on it.
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Aug 17, 2009, 05:35 AM
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rich smith's Avatar
Quote:
Originally Posted by vintage1
Oft quoted as fact, in fact completely untrue.

Rationale of the myth
Torque is proportional to magnetic flux density which is proportional to ampere turns therefore more turns is more flux per amp is more torque.

Reality of the situation
Flux is limited by saturation in the motor core for any decent efficiency. Therefore peak flux cannot be increased.
Vinnie, where do you get this stuff? I know increasing turns allows me to use bigger props whether you want to call it torque or not. And since when is saturating the core necessary for efficiency?
Aug 17, 2009, 06:22 AM
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vintage1's Avatar
Quote:
Originally Posted by rich smith
Vinnie, where do you get this stuff? I know increasing turns allows me to use bigger props whether you want to call it torque or not.
Its not torque. Its just less power that's all. Same torque.

Quote:
And since when is saturating the core necessary for efficiency?
NOT saturating the core is necessary for efficiency. And it represents a point at which losses increase dramatically.

So it represents a practical maximum for torque. You might in fact be limited by heat build up at a lower flux density anyway. Whichever is the case, increasing the turns changes nothing.
Aug 17, 2009, 07:28 AM
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Bruce Abbott's Avatar
Quote:
Originally Posted by vintage1
to rewind a motor with twice as many turns, means that in order to fit, the cross sectional area of the wire will be halved..half the area, twice the length - 4 times the resistance. So, for a given amount of heat, heat being proportional to current squared times resistance, the current has to halve as well. So torque, for a given copper loss, doesn't change.
Twice as many turns, at twice the voltage, drawing half as much current, equals the same power, rpm and torque! Rewinding a motor simply changes the required voltage (and current). However...

In practice the available voltage and current is often determined by your choice of battery and ESC. It is all very well to say that a 1 turn 5000Kv motor produces the same torque as a 5 turn 1000Kv motor (when run on appropriate voltages) but which one will swing a 10x6 prop at 8000rpm on 3S, and only needs a 15A controller?

On 3S @ 15A the 5000Kv motor is limited to spinning a 3x2 prop, and therefore its torque will be much lower. Of course, it could run a 10x6 prop (producing the same torque as the 1000Kv motor) on lower voltage, but where do you get the required 2V battery and 75A extra-low-voltage ESC?
Aug 17, 2009, 07:59 AM
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Information... We want information....

You won't get it!

By hook or by crook we will...
Aug 17, 2009, 07:59 AM
Registered User
In reality I'd guess that the "myth" is just a misquoting of the verifiable fact that Kt (torque per ampere) is inversely proportional to Kv (rpm per volt). So lower Kv gives you a higher Kt and therefore more torque...but only per ampere. So if you want more torque (perhaps to swing a larger prop) from your 20A ESC get a lower Kv motor .

Of course it's true that, in the same way that Kv doesn't tell you the maximum possible rpm, Kt doesn't tell you the maximum possible torque. But who really cares ?

Steve
Aug 17, 2009, 08:29 AM
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microgyros's Avatar
Yes!
Myth verified
Aug 17, 2009, 10:47 AM
Extreme Flight Tech Support
exeter_acres's Avatar
what if we add a flux capacitor?
Aug 17, 2009, 11:37 AM
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freechip's Avatar
We go back in time.
Aug 17, 2009, 12:04 PM
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Neil Stainton's Avatar
For a given battery voltage:

Lower Kv = More torque for the same current - True.

Lower Kv = Lower RPM and bigger usually more efficient props - True.

Lower Kv = More torque for the same temperature increase - False.

Neil.
Aug 17, 2009, 01:15 PM
Registered User
Quote:
Originally Posted by Neil Stainton
For a given battery voltage:

Lower Kv = More torque for the same current - True. But you need a higher voltage to force that current through the motor.

Lower Kv = Lower RPM and bigger usually more efficient props - True.For the same voltage of course.

Lower Kv = More torque for the same temperature increase - False.

Neil.
I added the color sentences.

The issue of course is there are three to four parameters working here and focusing on only a single one can give answers which are not always true--which of course causes the controversies or "myths".

Some parameters would be kV (or kT), supply voltage, rpm and prop load. As long as the prop load is within the limits of the motor design, you have some flexibility of choosing 2 out of the 3 other parameters (I am ignoring running at less than WOT).

So I agree with the original premise is that if you are wanting to run the motor at the maximum---where the iron is just on the verge of saturation, you cannot rewind that motor and change battery supply voltage to get more. The "magnetics" set the limits.

My 2 cents.
Aug 17, 2009, 01:39 PM
Suspended Account
I'm confused.

Neil said "For a given battery voltage: Lower Kv = More torque for the same current - True."

Alan then said "But you need a higher voltage to force that current through the motor."

Isn't this a contradiction in terms? Or did Alan miss that part of Neil's post?

Chuck
Aug 17, 2009, 02:10 PM
Yes, yes I am.
cjbucher's Avatar
So is that why high kv inrunners geared down end up giving you more torque than a direct drive outrunner?

I think from what you said above, would this be true: a 5000 kv motor geared 5:1 will have more torque than a similar 1000 kv motor at the same amount of input watts?
Aug 17, 2009, 02:59 PM
Jack
jackerbes's Avatar
OK, I get it now. Instead of calling the inherent ability of low Kv motors to make airplanes fly better at lower speeds by turning larger props at lower RPM "torque", we need to call it something else. What term would we use for that?

How about a term like Lower RPM Bigger Prop Turnability. And for high Kv motors we could use Higher RPM Smaller Prop Turnability. Or just call them LRBPT and HRSPT? Just think, we would save outselves one letter by doing that because torque is a six letter word.

Is it just me or is all this a little silly? Like much ado about nothing? Or is it kind of like another Grinch trying to steal Christmas.

Jack
Aug 17, 2009, 03:59 PM
Registered User
No, it's not silly it's just not all that important. V1 is getting irritated about the incorrect use of technical terms. Seems reasonable to me, in most cases it's a lot easier to talk sense about things if you use the correct terminology.

E.g. I get irritated when people refuse to spell mAh correctly and even more irritated when they think current (which they often incorrectly call "amperage" or even "ampage") is measured in mHa or MAH or something like that. We are all entitled to pick what we get picky about

Steve


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