

Oct 25, 2008, 12:59 AM  

You can measure your motor's DC resistance with a multimeter, but not directly, because the value will be very low (~0.05 Ohms). To get sufficient resolution you need to feed about 1 Amp into a motor winding, while measuring the voltage across it. If the current is exactly 1A then the resistance equals the voltage. If the current is not exactly 1A then you can apply Ohm's Law (Resistance=Volts/Amps).
The tricky bit is, how do you get a known current of about 1 Amp? You will need a stable power supply (eg. 12V car battery) and a high power resistor of the correct value (12 Ohms for a 12V supply). If you can't get an appropriate resistor then you can use a light bulb (eg. 10~15W auto bulb). To find out how much current the resistor/bulb draws, connect it to the power supply and in series with your multimeter set to its 10A range. Record the measured value. Now switch the multimeter to its lowest voltage range (eg. 200mV) and connect any two of the motor leads in series with the resistor/bulb. Measure the voltage across the motor leads. For greatest accuracy the multimeter probes should be placed directly onto the motor wires or bullet plugs (otherwise you will also be measuring the resistance of the connections!). However, DC resistance is not what Motocalc wants! It needs the Dynamic Resistance (Rm), which can only be determined by measuring Volts, Amps and rpm with different loads. Therefore I recommend using a Wattmeter and tachometer to obtain the necessary data. If this is not practicable then multiply the DC resistance by about 0.8 to get a reasonable guesstimate (eg. if you measured 0.05 Ohms then use 0.09 Ohms in Motocalc). 
Oct 25, 2008, 10:03 AM  

Links to Mumtats and Drive Calculator in this eflight calculator compilation:
http://www.rcgroups.com/forums/showthread.php?t=943538 (sticky in this forum) Prettig weekend Ron 
Oct 25, 2008, 05:01 PM  
Chicago 'burbs
Joined May 2006
1,708 Posts

Quote:
Here's how I do Lipo's. Set up the lipo with a Wattmeter, ESC, and motor. Choose a prop that loads down the motor, but I recommend something smaller than you normally fly with, just to make sure you don't overheat things. Run the setup at WOT for 1020 seconds, and note the voltage and current call them Von and Ion), then pull the throttle to zero. On the Wattmeter you will notice the voltage quickly jumps upwards to a more or less steady valuenote its value (="Von"). I calculate the lipo pack resistance (=Rpack) to simply be Rpack=(VoffVon)/Ion. For cells in series, the individual resistance is just the Rpack/#cells. For cells in parallel, multiply by the number of cells. I seem to get values which are similar to those given by the manufacturer. This is easier with a data recorder of coursejust because the battery voltage and current is changing pretty fast under load, but you should be able to get reasonable values with the wattmeter. 

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