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Jun 14, 2021, 01:32 AM
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Skylar's Avatar
Quote:
Originally Posted by Draven
Instead, I bought a SunnySky Premium X series V3 4120 430kv. It is perfect size and weight, and performance specs is better than my top picks (Scorpion 4120, Xpower 22cc, Leopard 5055).
Advertised specs should often be taken with a pinch of salt. But I'm curious. Which specs are you referring to?
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Jun 14, 2021, 07:15 AM
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Draven's Avatar
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Quote:
Originally Posted by Skylar
Advertised specs should often be taken with a pinch of salt. But I'm curious. Which specs are you referring to?
Weight/KV/efficiency of the sunnysky is in the ballpark of what I was looking for, but also the dimensions perfectly match the motor box I already had created for a different motor.
Jun 14, 2021, 02:45 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Draven
Weight/Kv/efficiency ...
Note that the velocity konstant Kv says nothing about motor max.power.

Kv , measured/expressed in rpm/volt, effects current and power drawn greatly though, both are proportional to Kv

What are max.current and max.power?


E-motors and ic-engines have fundamentally different behaviour.
An engine tries to keep a constant torque: as load goes up, rpm goes down.
An e-motor on the other hand tries to keep a constant rpm, no matter prop-pitch and -diameter: as load goes up, torque must go up. Therefore current and power have to go up. Even if this would lead to a fiery demise of controller and/or motor.


Kv says nothing about maximum motor power, nor about a lot of other things.
Note that the velocity Konstant kv says very little about max.power capacity, nor about max. current a motor can handle, nor about max. torque, nor propsize, nor rpm, nor rpm range.
A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same kv = 1000pm/volt.
Kv is the velocity Konstant, a physical property (like mass, length and volume), expressed/measured in the physical unit rpm/volt (like kg, meter and m).


Kv is not a rating, not a figure of merit.
Not something the motor delivers like power and torque (explanation).
A kv=1000pm/volt motor on 10volt and a kv=2000rpm/volt on 5volt will give same no_load speed of 10,000rpm.

Kv, what the motor wants to do, versus power, what it can do.
Kv matches desired rpm and battery voltage, there's nothing more to it.
  1. Want, try
    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 and torque
    RPM and prop determine torque needed, which in turn determines current drawn
    (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.


Vriendelijke groeten en wees voorzichtig, Ron
Without a watt-meter you're in the dark ... until something starts to glow
E-flight calculators watt-meters diy motor tips&tricks Cumulus MFC
Jun 14, 2021, 06:01 PM
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Draven's Avatar
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Did I choose the wrong motor?

The SunnySky X V3 4120 - 430kv not right for this application? When I flew maiden with this motor is seemed okay, did come down warm.
Last edited by Draven; Jun 14, 2021 at 06:14 PM.
Jun 14, 2021, 06:38 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by Draven
Did I choose the wrong motor?

The SunnySky X V3 4120 - 430kv not right for this application? When I flew maiden with this motor is seemed okay, did come down warm.
Can you confirm the whole setup including prop and battery. Any power setup must refer to all 3 everytime as motor alone just isn't meaningful besides physical fit.
Jun 14, 2021, 06:49 PM
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Draven's Avatar
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nevermind...
Last edited by Draven; Jun 14, 2021 at 08:39 PM.


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