

Jul 08, 2006, 03:01 PM  

The easiest and best is to just download Drivecalc 3 from www.drivecalc.de . Measure I, V and rpm for noload and a few props and Drivecalc will correlate this data and give you everything you need.
I used to use the motor equation: rpm=Kv(Vm  RmIm). Rearanged to rpm/Vm = RmKv(Im/Vm) + Kv. You then plot rpm/Vm vs. Im/Vm. This will yield a straight line with a slope = RmKv (so Rm=slope/Kv) and yintercept = Kv. But I think Drivecalc does a slightly better job of it. 
Jul 08, 2006, 03:03 PM  
Joined May 2003
764 Posts

Measure the following on your motor under test:
Io at approximately the voltage you plan on running the motor. No prop. Just the motor. Take the measurement in a few seconds of starting the motor. V, I and RPM for two different props. When you make this measurement, try to ensure the following: 1) The values of I aren't too close together. Ideally you'd want them to be 30% or more apart for about the same voltage 2) The prop is being operated at an RPM well below it's max RPM. Let's call the V, I and RPM measurements V1, I1 and RPM1 for the first prop, and V2, I2, and RPM2 for the second prop. Your motor constants are as follows: Io = Io, just as you measured above. Rm = (RPM2 * V1  RPM1 * V2) / (RPM2 * I1  RPM1 * I2) Kv = RPM1 / (V1  Rm * I1) That's it. www.peakeff.com will do this for you here and overlay prop data. 
Jul 08, 2006, 07:40 PM  
Joined May 2003
764 Posts

Quote:
I definitely see the value of direct measurements if you are developing a motor, but once you are interested in knowing what it does under load it's not as accurate as the indirect measurements in my observations. 

Thread Tools  
Similar Threads  
Category  Thread  Thread Starter  Forum  Replies  Last Post 
MiniHowTo  DIY Test Fixture to Measure Motor Resistance (Rm)  LBMiller5  Electric Motor Design and Construction  198  Sep 03, 2012 09:41 AM 
Discussion  How to Measure Motor Resistance?  asm_  Power Systems  16  Oct 08, 2006 03:56 AM 
Discussion  How to calculate motor Watts ?  dtrip  Power Systems  17  Jul 26, 2006 01:39 AM 
How can you calculate the air resistance on a flat surface as its falling (in still a  rcplanecrasher  Electric Plane Talk  8  Sep 09, 2005 04:33 PM 