Thread: Help! how to find motor rpms
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Old Apr 17, 2009, 10:44 AM
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You drive the motor at a known speed (with the help of a drill press)... measure the generated voltage. Will likely be be a volt or three on say, 3600 rpm. Just set your VOM accordingly.

Then calculate the motor constant ('kV" value) as RPM divided by measured volts.

Measured 2v at 3600rpm? 3600/2 = 1800 ... 1800 is the motor's "kV" value.

How to use kV?
Just mutliply it by the voltage you will apply to get the FREE RUNNING speed of the motor.
7.2v RC pack? 1800 x 7.2 = 12,960rpm (free running speed on 7.2v)

Of course, a prop will slow the motor down some.
- A tiny prop won't be noticed by the motor, and it will run near the no-load speed.
- A huge prop might slow it down to 50% of free running, in which case you'll suck amps and want water cooling.
- A well matched prop will slow it down to no less than 75-80% no-load speed, and you'll likely be ok without water cooling.
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