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Old Oct 24, 2008, 09:38 PM
Registered User
West Chester, Ohio
Joined Nov 2006
1,382 Posts
Help!
How to measure resistance of a motor

Like the title states, i want to measue the resistance of a motor (and even lipo bats) cause I've got a chinese motor and don't know it's resistance. Could you break it down real simple. I've got one of those voltage/resistance things you can buy at a hardware store but have no clue how to use it properly. I'm not looking for "exact" figures but would like to have some value to put into Motocalc. BTW, it's a 600kv motor that's suggested to have 6S lipo with 14x9 prop for watts in the 1300 range. Thanks
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Old Oct 24, 2008, 11:59 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,169 Posts
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).
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Old Oct 25, 2008, 07:27 AM
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West Chester, Ohio
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Bruce, you definately da man! Thanks so much.
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Old Oct 25, 2008, 09:02 AM
jrb
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Edina, MN, USA
Joined Oct 1999
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Or, do a search for Mumtats; it a spreadsheet that calculates all of a motor's paramters from a few tests.

DriveCalc is very good too; but no Rm -- its not needed!
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Old Oct 25, 2008, 09:03 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
near Nijmegen, Netherlands
Joined Feb 2001
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Links to Mumtats and Drive Calculator in this e-flight calculator compilation:
http://www.rcgroups.com/forums/showthread.php?t=943538 (sticky in this forum)

Prettig weekend Ron
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Old Oct 25, 2008, 01:54 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Quote:
Originally Posted by jrb
Or, do a search for Mumtats; it a spreadsheet that calculates all of a motor's paramters from a few tests.

DriveCalc is very good too; but no Rm -- its not needed!
Jim,

Isn't "Rd" in Drive Calc the same as Rm: - I have used Motocalc and Drive Calc for certain motors and the Rm from Motocalc and Rd from Drive Calc are essentially the same.

Here are a couple of Drive Calc graphs;

For the 28-37-3 Motocalc gave Rm = 0.10 ohms: Drive Calc Rd = 0.103 ohms

For the 28-47-1.5 Motocalc gave Rm = 0.045 ohms: Drive Calc Rd = .0433 ohms
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Old Oct 25, 2008, 04:01 PM
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Chicago 'burbs
Joined May 2006
1,689 Posts
Quote:
Originally Posted by missle
Like the title states, i want to measue the resistance of a motor (and even lipo bats) cause I've got a chinese motor and don't know it's resistance. Could you break it down real simple. I've got one of those voltage/resistance things you can buy at a hardware store but have no clue how to use it properly. I'm not looking for "exact" figures but would like to have some value to put into Motocalc. BTW, it's a 600kv motor that's suggested to have 6S lipo with 14x9 prop for watts in the 1300 range. Thanks

Here's how I do Lipo's.

Set up the lipo with a Watt-meter, 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 10-20 seconds, and note the voltage and current call them Von and Ion), then pull the throttle to zero. On the Watt-meter you will notice the voltage quickly jumps upwards to a more or less steady value---note its value (="Von"). I calculate the lipo pack resistance (=Rpack) to simply be

Rpack=(Voff-Von)/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 course--just because the battery voltage and current is changing pretty fast under load, but you should be able to get reasonable values with the watt-meter.
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