How to Measure Resistance Rm in a Brushless Motor

The typical multimeter does have a resistance measurement function, however it is not sensitive enough to measure the internal resistance of high performance motors.
What we can do instead of directly measuring the internal resistance is measure the voltage drop and current through one phase of the motor. Taking these values to the radiocontrolinfo.com website will allow us to calculate what the internal resistance is within the motor.
What is Required:
2 Multimeters
Power Resistor
Brushless Motor
Alligator Clips
LiPo Battery
These components are placed in to a circuit allowing you to indirectly measure the resistance of the motor winding.

[@MinhLe-ug7dp]

finally, have heard you talking about this so much but couldn't find any information about this. THANK YOU

[@RCexplained]

Your Very Welcome!

[@interestingcan]

This is really good, thank you! However, how did you derive your formula? I would be very interested in seeing how you did it, especially the temperature correction. Thanks again!

[@RCexplained]

Thank you. Check out this site: www.radiocontrolinfo.com/brushless-motor-efficiency/brushless-motor-winding-resistance/

[@interestingcan]

@@RCexplained I have read through your article and used the equation, everything looks great! But it does show how the equation was derived, did you apply the Kirchhoff’s Laws to the circuit? How was the temperature correction done?

[@RCexplained]

@@interestingcan sorry, don't have the derivation readily available. It ultimately comes from the basic V=IR formula

[@interestingcan]

@RCexplained totally understand, thank you! That's what I've been doing, just trying to duplicate the math to check my sanity.

[@LeonardoBaltazar1]

Great video! But can I use a DC source, which measures current, to replace one of the multimeters and the battery? And where does the constant 0.0038 in the formula come from?

[@RCexplained]

Yes you can.

[@tkostang]

@RCexplained I’m confused. Why didn’t you just use a multimeter that measures resistance? Is it not that simple? I have a decent multimeter (Fluke 88) and I’ve been wanting to test resistance on a few of my brushless motors but didn’t want to pop the internal fuse….. it’s got 2 and one of them is quite expensive. Does using the Ohm function not work that way when measuring brushless motors?

[@RCexplained]

Typical multimeters are not sensitive enough for this measurement.

[@elisamuelhernandez3636]

Hi, can you make a video explaining how to make sure that C rating on a lipo is what I Purchase?

[@RCexplained]

Good idea, I'll have to think of that one and if I come up with a good method I will add the topic to the queue. The queue is at least 4 weeks long at this point.

[@anonymousjoni]

Great video! But what power resistor should I use for 6S battery?

[@RCexplained]

You should be able to use ratio of your pack vs the one used. Your resistor will be of a higher resistance.

[@xaviertaylor759]

Great information as always. Could you say a bit more about the 3 configurations of windings you mentioned - is that the 3 sensors?

[@RCexplained]

Thanks for the comment Xavier. A brushless motor operates on 3 phase power and also has 3 wires coming out from it. You could be powering any one of these 3 phases and should see identical resistance in each. For example you could wire your test circuit to either motor phase using either wire (wires a,b,c) AtoB, AtoC or BtoC. There is nothing to do with sensors in measuring Rm.

[@Aqaqqqqaaac]

Thank you for the good video. I am Korean, So Please understand if my english is bad. How much watts of power resistor did you use? Should I use resistor that can handle the rated power of the motor?

[@RCexplained]

The power resistors used are 25W. I also do not leave the circuit live for more than 5 seconds to keep the resistors from overheating.

[@windsurfer3329]

One important comment. You really don't know if this resistor is 10ohms or not. Perhaps, it is 9.8ohms, perhaps 10.2ohms. Its actual magnitude will affect the measurement of the internal motor resistance from two of the motor leads. In order to remove this ambiguity, I would remove the motor from the circuit, and measure the voltage across the resistor and current through the resistor. The ratio will give you the absolute resistor value. then you can use that value in calculating the motor resistance. Good video though. Thank you for posting it.

[@avrahamcohen2134]

What power resistor do I need for 6S battery? do I need a 15ohms? And how many W does the resistor should be

[@MrSafrdA]

Hi, thanks for your video. What is the type of BLDC motor? I would like to do some calculations with maximum current, voltage and measure resistance... Because I am interested about determination of maximum current - how to determinate? Is it even possible? Thanks...

[@RCexplained]

Hello, the best way to get this information is by measuring the current, voltage and temperature of the motor spun up and loaded.

[@mikevarela705]

Awesome video!! Thanks for sharing

[@RCexplained]

Thank You

[@baxrok2.]

Great info. Thanks!

[@RCexplained]

NO Problem!

[@furiouswolf777voodoo4]

Thank you good information 😊

[@RCexplained]

No worries!

[@zakariakhamees]

I only have one multimeter :(

[@RCexplained]

Do you have a watt meter that you could use? Unfortunately you do need to have the multi-meter measuring current in the circuit at the same time you are measuring the voltage at the motor.

[@thiphuongnguyen2158]

the sound is not good I am quite disappointed

[@RCexplained]

The sound in this video is loud and clear.