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Jan 23, 2014, 06:53 PM
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brushless motors kv?


When comparing brushless motors, is the lower kv better or worse?
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Jan 23, 2014, 07:03 PM
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Piece's Avatar
Depends completely on the application. Kv is the relationship between voltage and RPM in the motor in RPM per volt. A motor with a higher Kv value will want to turn faster for each volt applied, but will also produce less torque per amp of current used. This means that for a given voltage, a higher Kv will generally be better suited to a small load turning quickly, while a lower Kv is better for a bigger load turning more slowly.
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Jan 23, 2014, 09:45 PM
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Jan 24, 2014, 04:44 AM
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Quote:
Originally Posted by Cavi
When comparing brushless motors, is the lower kv better or worse?
No. Depends what you want to use it for.

Low Kv tends to work with larger slower props as on trainers and vintage models. High Kv tends to be used for small faster props and models like pylon racers, maybe warbirds etc.

Steve
Jan 24, 2014, 05:47 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
And the Kv constant is no indicator for the power (watt) a motor can handle.

E.g. doubling Kv and halving voltage will give you the same rpm and power. Current is doubled of course.

Prettig weekend Ron van Sommeren
Last edited by Ron van Sommeren; Jan 30, 2014 at 07:16 AM. Reason: current halved -> doubled
Jan 24, 2014, 06:06 AM
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Kv, while an absolutely critical part of the system, is actually the item one should choose last.

Decide your peak power requirement based on the weight of the model and how you want to fly it
Pick a preferred cell count (voltage) and pack capacity for how to deliver the power
Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate
Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.

Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.

The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.
Jan 24, 2014, 07:41 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by scirocco
... So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.
Excellent

Prettig weekend Ron
Jan 24, 2014, 07:48 AM
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Fourdan's Avatar
Hi
Excellent .. Yes
Kv is similar to the ratio of an AC transformer (primary / secondary turns ratio)
You could find Primary 110V or 220V
Secondary 5V or 12V or 24V or ....
It is not an indicator of a power nor quality
And Kv is also a function of turns/slot (+ config + size + serial or // + ....)
Louis
Jan 24, 2014, 08:19 PM
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manuel v's Avatar
The Kv is the last parameter to select in model.

For me:
First wing loading,
Power required depending on the type of use,
Propeller and RPM.
Voltage,
Weight of the motor and finally the Kv.

Manuel V.
Jan 24, 2014, 09:34 PM
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Rather than first and last - - -
The model has a great deal to do with what is appropriate.
Generally, the best propeller is one that has a "reasonable" pitch,
and has enough clearance to avoid "cutting the grass".
Next might be the number of cells in the lipo, due to voltage vs current considerations.
A small light model (foamy) might only need a two or three cell, while a ten pound model 5 to 8 cells. The usually is an ESC price factor when you get into high cell counts.
My models are in the 25 to 60 motor size, and use 4cell lipos for the most part. The heavy (10lb models) use a 6 cell.
For initial calculations, based upon 10" to 14" props, I usually use 10k RPM as a start.
The expected weight is the next factor I look at, since it determines the power needed.
Finally, I shoot for about a 1.25 to 1 thrust weight ratio for up to mildly aerobatic models.
(I really hate "underpowered" models.)
Jan 25, 2014, 04:47 AM
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Fourdan's Avatar
Quote:
Originally Posted by manuel v
The Kv is the last parameter to select in model.

For me:
First wing loading,
Power required depending on the type of use,
Propeller and RPM.
Voltage,
Weight of the motor and finally the Kv.

Manuel V.
Hi Manuel
I would place "weight of the motor" before "voltage & Kv"
Louis
Jan 29, 2014, 04:01 PM
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manuel v's Avatar
Hello Fourdan.

Yes of course. motor weight is determined by the power required.
In my excel, once declared the input power, RPM, w/oz motor, this calculates the input power and then the motor weight, and Kv different number of cells.

Thanks for the clarification.

Manuel V.
Jan 30, 2014, 12:32 AM
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Quote:
Originally Posted by Ron van Sommeren
E.g. doubling Kv and halving voltage will give you the same rpm and power. Current is halved of course.
Isn't the curremt doubled?
Jan 30, 2014, 04:32 AM
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Fourdan's Avatar
Quote:
Originally Posted by Martyn McKinney
Isn't the curremt doubled?
Hi
I agree with Ron
But you could also halve the Kv and double the voltage Vbat
The couple Kv & Vbat is acting by the product
Kt is torque / A (reciprocal of Kv)
In the real world the efficiency could vary "a little" (maybe 1 or 2%)
The limit is to find the right optimized motor and the couple (VBat, Kv)
given desired rpm and power (or torque & rpm)
Louis
Jan 30, 2014, 07:16 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Martyn McKinney
Isn't the curremt doubled?
Duh You're right Martyn. Thank you for the heads up, corrected it.

Vriendelijke groeten Ron


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