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Old May 15, 2005, 03:45 AM
Buyin', Tryin' and Flyin'
hilgert's Avatar
DFW, Texas
Joined Jan 2005
2,283 Posts
Question
Outrunners - more efficient at high or low RPM?

The title says it all: Outrunners - more efficient at high or low RPM?

I know this is a question that depends on many variables, but I am more interested "in general" of where outrunners are most efficient relative to RPM.

The application I have in mind would use 11.1v lipos, a small outrunner (eflight Park400 740kv) and a 10A ESC to drive the variable pitch tail of a heli. I would want the motor to spin at a continuous, constant speed of around 4800 RPM (it will be used in a 1:1 gear setup driving the tail shaft), so torque will be an important aspect of this as the tail speed will need to keep constant even with the pitch of the tail changes drastically.

My other choice was a Park 370 at 1080kv. I "assumed" that it would be better to run the lower 740kv motor at this constant speed in that it would be futher up it's RPM curve.

So, my current assumption is that it is better to select a motor, for this applicaiton, that will run continuously at higher RPMs rather than lower RPMs. Again, this motor will be set to run a specific RPM and will not vary in-flight.

EDIT: I guess I'd need to consider ESC efficiency as well. I know that the voltage will be 11.1 volts, and that the ESC uses PWM (pulse width modulation) to control the motor, so that's why I am assuming a higher RPM is "better" for brushless motors.

Ultimately, I want my 4800 RPM at the lowest current draw, however, to prolong flying time.
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Last edited by hilgert; Jan 16, 2006 at 01:39 PM.
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Old May 15, 2005, 06:33 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,193 Posts
Comparing two motors of identical construction except for number of turns (and thus different Kv), both should have similar efficiency at the same rpm. However, the motor with higher Kv requires less voltage and more current. This may cause poor efficiency if a higher than necessary battery voltage is used, as the ESC has to work harder.

In this case it appears the candidate motors differ in more than just Kv. I created the graph below from Motocalc simulations, using the manufacturer's motor specs. According to Motocalc, at a constant 4800rpm the Park400 outrunner gets considerably better efficiency at output power levels much above 10W (Note that my simulation does not take into account any additional losses in the ESC!).
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Old May 15, 2005, 07:48 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
10,924 Posts
Quote:
Originally Posted by Bruce Abbott
Comparing two motors of identical construction except for number of turns (and thus different Kv), both should have similar efficiency at the same rpm. ...
Not quite, they will have different resistance due to different wire gauge and diffent number of winds. One could make the wire gauge in the high Kv motor smaller of course to get the same resistance.

Vriendelijke groeten Ron
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Old May 15, 2005, 08:40 AM
I don't want to "Switch Now"
pmackenzie's Avatar
Toronto (Don Mills), Canada
Joined Dec 2002
17,099 Posts
Assuming the manufacturer "filled" the slots in both motors, the higher Kv motor would use heavier wire, the lower Kv thinner.
To develop the same torque the low Kv motor will require less average current than the high Kv motor.
If for example you use a 2:1 ratio, the higher Kv motor would use wire twice the diameter that the low Kv motor does. It will also be half the length, so the resistance will be 1/4.
The low Kv motor will require twice the current to develop the same torque.
It should all work out that at the same output level the copper losses ( I^2*R) are nearly identical. Iron losses will be the same at the same rpm as well.
If you look at the published values for the mega 16/15 series this is very nearly the case.
Taking ESC efficiency into account, you should choose the Kv that will allow it to run as close as possible to 100% full power under maximum load.
This will minimize switching losses.
Unfortunately for a helicopter tail you will almost always be operating at partial throttle.
Pat MacKenzie
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