How to calculate motor resistance(Rm)? - RC Groups
Jul 08, 2006, 03:18 PM
Renaud Ecalle Wannabe
Discussion

# How to calculate motor resistance(Rm)?

What tools does one need to figure the resistance a motor has and the procedure for doing it? I wish all manufactures would give motor constants under the same testing conditions. Some give "no load" currents at different volts and resistance numbers for one phase instead of two. Correct me if I'm wrong, but apparently motocalc needs a two phase resistance number for a given motor to achieve accurate data. And i've heard some manufacturers only give resistance for one phase. Obviously I dont know everything about brushless motors, and i dont quite understand this one phase, two phase mumbo jumbo, but i just wish manufactures could give accurate and true motor values instead manipulating the conditions to make their numbers look better. So, i've decided to take matters into my own hands. No load current is easy enough to figure out, but Rm has me stumped. Somebody please help! Thanks.
 Jul 08, 2006, 04:01 PM hot air rises... 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 y-intercept = Kv. But I think Drivecalc does a slightly better job of it.
 Jul 08, 2006, 04:03 PM Registered User 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, 04:15 PM
Registered User
Quote:
 Originally Posted by spit What tools does one need to figure the resistance a motor has and the procedure for doing it? I wish all manufactures would give motor constants under the same testing conditions. Some give "no load" currents at different volts and resistance numbers for one phase instead of two. Correct me if I'm wrong, but apparently motocalc needs a two phase resistance number for a given motor to achieve accurate data. And i've heard some manufacturers only give resistance for one phase. Obviously I dont know everything about brushless motors, and i dont quite understand this one phase, two phase mumbo jumbo, but i just wish manufactures could give accurate and true motor values instead manipulating the conditions to make their numbers look better. So, i've decided to take matters into my own hands. No load current is easy enough to figure out, but Rm has me stumped. Somebody please help! Thanks.
Rm determines the IR loss of a motor. For BLDC motors it is simply the resistance between any 2 of the three motor leads. I use a standard 10 milliOhm resistor in series with the motor leads and put about an amp through the circuit from a power supply. The ratio of voltages times the resistance of the the standard is Rm. If you don't have a DVM with the range, use a bridge instead.

I stay away from indirect measurements using multiple prop-volt-current readings.
 Jul 08, 2006, 05:20 PM Renaud Ecalle Wannabe Thanks for all the help fellas. I really appreciate it.
Jul 08, 2006, 06:08 PM
Registered User
Here is a couple of simple circuits to directly measure rm. I take no credit for these instead it goes to Mr. Takao Shimizu who frequents these forums.

### Images

Jul 08, 2006, 08:40 PM
Registered User
Quote:
 Originally Posted by INewton I stay away from indirect measurements using multiple prop-volt-current readings.
Do you find direct measurements are a better predictor of motor performance? I find just the opposite.

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.