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Way oversized motor--is this a problem?

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Way oversized motor--is this a problem?

Old 06-09-2021, 02:53 PM
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Default Way oversized motor--is this a problem?

Hi all-

Well--I ended up with a "spare" eflite power 160 that I had expected to use on a 30cc size plane but did not.

I am now looking at a 15cc size plane that recommends using an eflite power 60 or equivalent.

Assuming that physical size is not a problem and that the proper larger size propeller is also not an issue--what's the problem (if any) in using this large motor on this plane--

I think it is apparent I won't need full throttle and maybe would need to top out the throttle curve at something less than 100%, but can't I use it??


Larry J
Old 06-11-2021, 10:39 AM
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No problem, in fact since 'we' operate the motors beyond point of max. effiency, it will run at higher efficiency.
Chose the right prop for the plane and/so you don't have to reduce throttle.
Controllers work harder (warmer) at partial throttle, compared to full throttle.

The system determines current and power draw, installing a bigger motor of same velocity konstant Kv won't change current and power draw. Even seemingly small changes in power system can have massive effects.
The relationships are very simple.
Some expensive realistic Name:  frown.gif
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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.
Old 06-16-2021, 12:12 PM
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Originally Posted by ron_van_sommeren View Post
... Controllers work harder (warmer) at partial throttle, compared to full throttle. ...
If current at wide open throttle is too high for the controller then reducing throttle, using the stick or using end-point-adjustment, to get current below ESC max.current is not a good idea.
(Throttling down if current is too high for motor causes no problems).

In fact, reducing throttle is even harder on ESC than to keep overamping at full throttle.
Much safer to prop down a bit to reduce current.
Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
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Last edited by ron_van_sommeren; 06-16-2021 at 12:15 PM.

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