kyleservicetech

10-12-2009, 06:55 PM

What is involved in measuring the winding resistance of our brushless motors?? You don't need an elaborate setup. All you need is a two or three cell Lipo, or 12 volt battery, a power resistor on the order of five ohms, at 25 watts, and a pair of cheap digital volt meters. (One meter could be a Astroflight Wattmeter or similar) Heck, even a 12 Volt automotive tail light, or several tail lights in parallel will work, anything that draws an ampere or three. Applying higher currents will allow somewhat more accurate resistance measurements, at least up to the maximum current rating of the motor, or the 10 ampere limit of the DC Digital volt/ohm meter. More expensive meters such as the Fluke 87 meter allows measuring the voltage down to the 0.1 millivolt level.

Connect the battery such that current flows through the resistor, and the motor winding under test. (The motor and resistor are in series with each other.) Put one meter in SERIES with the resistor and motor, on the meters 10 Ampere DC range. (No you can't put the meter in series to measure the current, than take it out to measure the voltage. The meter itself on its 10 Ampere range has some resistance, and removing it will change the current through the motor under test.)

Measure the voltage ACROSS the motor windings with the first digital volt meter. Measure the current THROUGH the motor with the second digital meter. (Or Astroflight Wattmeter)

Now, the motors DC resistance will equal the voltage measured across the motor, divided by the current through the motor or R=E/I. The attached JPG photo shows it all.

If you measure 0.072 volts across the motor, and 2.5 Amps through the motor, the motor resistance is R=E/I or 0.071/2.5 which is 0.0288 ohms. Be certain to measure the motor voltage directly across the motor leads, and not across your clip wires to the motor. Measuring on the clip wires will add the clip's resistance to your total motor resistance.

(DO NOT CONNECT THE BATTERY DIRECTLY ACROSS THE MOTOR, THAT WILL BURN IT UP! :censor:)

Once you have this set up, it takes more time to read this instruction than to actually do the resistance measurement.:tc:

In the USA, you can buy a meter such as the Cen-Tech model 92020 for about $3.98 in American dollars. A much better meter is the Etek 10709 meter, I found one at Walmart for some $18.00. If you need more accuracy, more expensive meters can be used, such as the Fluke 87V, but mine cost some $350 USA dollars. It is accurate to 0.05% though.:D And a pair of Fluke 87V meters will give you far more accuracy than you need.

Look for a digital multimeter with a 2.00 AND a 200 Millivolt range. The Cen-Tech only has a 2.00 volt range, and is much less accurate for our purposes. My Etek 10709 meter does have a 200 millivolt range. So if you measure 70 MILLIVOLTS and 2.5 Amperes, the resistance of your winding is 70mv/2.5 Amps, or 28 MilliOhms, (0.028 Ohms).

Just a note, even my $350 Fluke meter can not measure the winding resistance of a typical brushless motor. The resistance values are so low, that measuring them requires a "MicroOhmmeter, a device that can easily cost thousands of dollars. We had many of them at work, before I retired.

Connect the battery such that current flows through the resistor, and the motor winding under test. (The motor and resistor are in series with each other.) Put one meter in SERIES with the resistor and motor, on the meters 10 Ampere DC range. (No you can't put the meter in series to measure the current, than take it out to measure the voltage. The meter itself on its 10 Ampere range has some resistance, and removing it will change the current through the motor under test.)

Measure the voltage ACROSS the motor windings with the first digital volt meter. Measure the current THROUGH the motor with the second digital meter. (Or Astroflight Wattmeter)

Now, the motors DC resistance will equal the voltage measured across the motor, divided by the current through the motor or R=E/I. The attached JPG photo shows it all.

If you measure 0.072 volts across the motor, and 2.5 Amps through the motor, the motor resistance is R=E/I or 0.071/2.5 which is 0.0288 ohms. Be certain to measure the motor voltage directly across the motor leads, and not across your clip wires to the motor. Measuring on the clip wires will add the clip's resistance to your total motor resistance.

(DO NOT CONNECT THE BATTERY DIRECTLY ACROSS THE MOTOR, THAT WILL BURN IT UP! :censor:)

Once you have this set up, it takes more time to read this instruction than to actually do the resistance measurement.:tc:

In the USA, you can buy a meter such as the Cen-Tech model 92020 for about $3.98 in American dollars. A much better meter is the Etek 10709 meter, I found one at Walmart for some $18.00. If you need more accuracy, more expensive meters can be used, such as the Fluke 87V, but mine cost some $350 USA dollars. It is accurate to 0.05% though.:D And a pair of Fluke 87V meters will give you far more accuracy than you need.

Look for a digital multimeter with a 2.00 AND a 200 Millivolt range. The Cen-Tech only has a 2.00 volt range, and is much less accurate for our purposes. My Etek 10709 meter does have a 200 millivolt range. So if you measure 70 MILLIVOLTS and 2.5 Amperes, the resistance of your winding is 70mv/2.5 Amps, or 28 MilliOhms, (0.028 Ohms).

Just a note, even my $350 Fluke meter can not measure the winding resistance of a typical brushless motor. The resistance values are so low, that measuring them requires a "MicroOhmmeter, a device that can easily cost thousands of dollars. We had many of them at work, before I retired.