How to measure resistance of a motor - RC Groups
Oct 24, 2008, 10:38 PM
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# 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
Oct 25, 2008, 12:59 AM
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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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 Oct 25, 2008, 08:27 AM Registered User Bruce, you definately da man! Thanks so much.
 Oct 25, 2008, 10:02 AM Member 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!
 Oct 25, 2008, 10:03 AM homo ludens modellisticus Links to Mumtats and Drive Calculator in this e-flight calculator compilation: https://www.rcgroups.com/forums/showthread.php?t=943538 (sticky in this forum) Prettig weekend Ron
Oct 25, 2008, 02:54 PM
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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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Oct 25, 2008, 05:01 PM
Registered User
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.
Mar 13, 2017, 11:43 PM
Registered User
Quote:
 Originally Posted by Bruce Abbott 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).
can you please explain how the dynamic resistance works or anything about it. I read it's like half the losses due to Rm. So Rm, simply the winding resistance, will change based on load?
 Mar 16, 2017, 11:17 PM Suspended Account What about winding resistance?
 Mar 17, 2017, 11:15 AM homo ludens modellisticus See second post.
Mar 17, 2017, 11:38 AM
Registered User
Noticed a typo:

Quote:
 Originally Posted by Alan Hahn ...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
Bolded part should be ---note its value (="Voff")
 Mar 17, 2017, 11:52 AM Registered User is this an explanation of dynamic resistance? found this on another site: A brushless motor has more than resistance. Because the voltage is rising and falling it will have inductance and capacitance which act similar to resistance but are frequency dependant. The brushless motor is closer to an AC 3 phase motor than a brushed DC motor. The DC resistance of the wire is in many cases insignificant and will tell you nothing about power or current when operating. Speed is based solely on the frequency of the DC pulses so, as speed changes, so does total reactance (oposition to current flow) because induction and capacitance are frequency dependant.
 Mar 17, 2017, 02:24 PM Registered User Hi In theory there is no "dynamic resistance" Only resistance (DC or zero frequency) depending of the copper temperature And reactance (from inductance and capacitance) depending of turns, nuts geometry, iron and ... position of the rotor The BEMF (when the motor is driven) takes into account all that parameters. If you input between 2 phases a rectangular pulsed signal (some voltage 0-Y volt and some time duration) or a "step" you could graph the voltage and the current (oscilloscope + V and I sensors) and deduce information regarding resistance and reactance. Value Y volt (step or pulse) should be small to avoid too much dissipated power (for example 2 .. 3V) That could depend of the original position of the free rotor Another test could be made with the rotor locked on different positions (electric angle) Some guys are "modifyng" the measured resistance (ohm) to "compensate rpm computings" and to try to take into account, real behaviour of the ESC and algorithms + the motor at different loadings. Completely experimental and empiric compensation Louis
 Mar 17, 2017, 02:55 PM homo ludens modellisticus Measuring winding/phase resistance For calculations/calculators and checks. Too high/low phase resistances, or short, or open circuit, or differences between the phases indicate a problem.The phase resistance is very low (typical <0.1Ω), too low to measure accurately using a standard multimeter. Measuring Brushless Motor Resistance - WFF Using a car tail brake light and two digital multimeters. Measure Rm Accurately - RCG Using a simple 100mA diy constant current source. More methods in thread. DIY milliOhmmeter - RCG Based on a PIC18F2580 programmable controller. Prettig weekend Ron
 Mar 17, 2017, 06:40 PM Registered User Its still Copper Loss = I^2* R * t. Resistance will increase as temperature increases through time. You really need to find the resistance of the wire with temperature http://hyperphysics.phy-astr.gsu.edu...ic/restmp.html. You will find its the same as Rm at 250 C. Magnet wire insulation has a melting point. And the wire is not in free air, its wound and has hot spots. Laminates do transfer the heat to the cooling media to regulate temperature. Just how much wattage that needs to be transferred the cooling media is the copper loss - eff. Dynamic resistance is reactance. Switching frequency causes inductive reactance (inductive (mH) reactance (ohms)) in a coil. Xl=2pi * f * L Last edited by p901P901; Mar 18, 2017 at 03:56 AM.