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Old 01-19-2020, 04:02 PM
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dasintex
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Quick Question; the higher KV Motor the larger the Props that can be used?
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Old 01-19-2020, 09:44 PM
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quorneng
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dasintex
No its the other way round.
kV means the motor revs per volt applied so a 950 kV motor will turn off load at battery volts x 950. For example with a 3s LiPo that would be 11.1 x 950 = 10,500 rpm. A 3000 kV motor would turn at 11.1 x 3000 = 33,000 rpm.
Brushless are basically constant speed motors so the rpm at maximum amps is only about 15% lower than at no load.
Too big a prop on a brushless will cause it to try to take too many amps and likely burn out the ESC and/or the motor.
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Old 01-20-2020, 11:16 AM
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ron_van_sommeren
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Originally Posted by dasintex View Post
... the higher Kv motor the larger the props that can be used?
Kv says very little about max.power or max. current a motor can handle, nor about max. torque. A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same Kv.


Motors have only one Kv ,it is the velocity konstant, a physical property, expressed in the unit rpm/volt. Kv is not a rating, not a figure of merit (explanation).
A Kv=1000rpm/volt motor on 10volt and a Kv=2000rpm/volt on 5volt will give same (no_load)speed of 10,000rpm, provided the motors can handle the current and power.


It's all about what the motor wants to do versus what the motor can do.
Kv matches rpm and voltage, there's nothing more to it.
  1. Want
    Kv and voltage determine how fast motor wants/tries to run
    (rpm_noload = voltage Kv, or, in other words, Kv = rpm_noload / voltage.)
  2. Current
    RPM and prop determine torque, which in turn determines current
    (current = torque Kv, in SI units!, or proportional to Kv ).
  3. Can
    Max.current and max.power determine whether that battery/motor/rpm/prop combo can run without going up in smoke.
An engine tries to keep a constant torque: as load goes up, rpm goes down.
An e-motor tries to keep a constant rpm, no matter the load: as load goes up, torque must go up, and therefore current goes up.

Last edited by ron_van_sommeren; 01-20-2020 at 01:51 PM.
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Old 01-20-2020, 11:18 AM
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ron_van_sommeren
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Below an excellent quote about Kv and motor selection.
From
brushless motors Kv?
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. 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.
  4. 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.
  5. 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. On the other hand, 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, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
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Old 01-20-2020, 11:24 AM
  #5  
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Originally Posted by dasintex View Post
New to Electric Flight
Some well-structured reading for rainy/windy days, including two online books, and some handy e-tools as well.
Will save you, and us a lot of questions, and probably a crash or two. Notably the 'what went wrong?' kind of questions
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools - RCG

And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter.

A watt-meter will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
Keep battery-/-watt-/multi-meter wires short!
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds - RCG



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