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Old Nov 02, 2009, 09:23 PM
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Murman's Avatar
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Testing no-load rpm's with optical tach?

What is the proper procedure for testing rpm's with an optical tach? Currently I am using a popsicle stick mounted to the motor shaft in place of a prop. Is there a more accurate way to do this?
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Old Nov 03, 2009, 09:09 AM
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Quote:
Originally Posted by Flatulator View Post
What is the proper procedure for testing rpm's with an optical tach? Currently I am using a popsicle stick mounted to the motor shaft in place of a prop. Is there a more accurate way to do this?
I am not sure what exactly you mean by "accurate", but I am guessing you want to determine kV.

The accuracy for kV depends on how much drag your stick is making, how accurate you measure the applied voltage, and how bad the magnetic losses and bearing friction is on your particular motor. All these (except voltage measurement) depend to some extent on the no-load rpm you are running -----they get worse at higher rpm. So to minimize the effect, use the lowest cell count pack you have to run at the lowest rpm. Also using a disk instead of the stick might minimize the drag, but I am not sure how much that really matters if you keep the rpm's on the low side (e.g. 5-6000 rpm).

I also note that the kV depends on how the ESC is setup--timing for example.

So you see there are a lot of effects to worry about.

Another way to try and take this into account is to also run some props (all this is at WOT) in the rpm range you plan to operate, and then plug in the measured rpm's, currents, and voltages into a motor program, which then tries to extrapolate your measurements to a real no-load kV. Probably this is the most accurate way to go.

To be honest, super accurate kV knowledge doesn't matter that much to real world flying. It is basically fine to know at the 10% level --I think.
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Old Nov 03, 2009, 07:07 PM
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Chattanooga, Tennessee, United States
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Quote:
Originally Posted by Flatulator View Post
What is the proper procedure for testing rpm's with an optical tach? Currently I am using a popsicle stick mounted to the motor shaft in place of a prop. Is there a more accurate way to do this?
Inrunner... that's about all you can do, short of getting an Eagle Tree v3...but....

Outrunner....I wrap a file folder label around the bell, make two wide black stripes diametrically opposite....point a decent flashlight (I use a small Maglite) at the bell from the side... align your tach "along the beam"... read off the rpm using the reflected light, as if the tach is reading from a 2-blade prop.

(in this photo the prop is on....too lazy to remove the tape for prop testing... obviously for no-load.. the prop was NOT on there!).
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Old Nov 03, 2009, 07:13 PM
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For inrunners just marking the prop adapter black on one side is often enough. (Multiply the RPM reading by two of course)

Pat MacKenzie
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Old Nov 03, 2009, 07:39 PM
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Quote:
Originally Posted by Alan Hahn View Post
...to minimize the effect, use the lowest cell count pack you have to run at the lowest rpm.
Thanks, that makes a lot of sense.

Quote:
Originally Posted by Dr Kiwi
Outrunner....I wrap a file folder label around the bell, make two wide black stripes diametrically opposite....point a decent flashlight (I use a small Maglite) at the bell from the side... align your tach "along the beam"... read off the rpm using the reflected light, as if the tach is reading from a 2-blade prop.
Thanks Doc, I will have to try that. No drag from the popsicle stick would be a good thing.

Quote:
Originally Posted by pmackenzie
For inrunners just marking the prop adapter black on one side is often enough. (Multiply the RPM reading by two of course)
I'm gonna have to try that also. Thanks
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Old Nov 03, 2009, 08:20 PM
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I've always used a magic marker to color a couple quarters of the can black (opposite quarters of course ) or used a couple pieces of blue masking tape. The marker will clean off with lighter fluid.
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Old Nov 06, 2009, 06:36 PM
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Originally Posted by Beaverdam View Post
I've always used a magic marker to color a couple quarters of the can black (opposite quarters of course
Good idea, except for all these black bell motors I've been testing!
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Old Nov 07, 2009, 10:07 AM
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Got reflective silver striping trim
Pete
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Old Nov 10, 2009, 04:07 AM
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Good idea, except for all these black bell motors I've been testing!
That's where the masking tape comes in handy!
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Old Nov 10, 2009, 04:45 PM
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Antony (France)
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Hi
I have used thin aluminum adhesive tape ..
when I want to compare my Medusa Power Pro + rpm sensor
with my Mastech optical reflective tach (integrated light beam)
No problem
Louis
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