Sunday, August 5th, 2012 07:53 am GMT -7 Sunday, August 5th, 2012 07:53 am GMT -7Sunday, August 5th, 2012 07:53 am GMT -7
Very large electric brushless outrunner motor

Measuring this one is a bit tricky, but not hard to do once you get the hang of it.





The motor or winding resistance is literally the electrical resistance of one winding or loop in the motor. Our brushless outrunner motors have three loops of copper wire inside of them.


You might think that you can take a multimeter and just measure the resistance. After all, that is one of the functions of a multimeter. The problem is that our motors have very low internal resistance values. We need to be able to measure with an accuracy of one milliohm (0.001), and most multimeters cannot come close to measuring with that kind of accuracy. My expensive multimeter can, but it is very hard taking an accurate reading with it. The multimeter leads and probe tips have significant resistance of their own, for example.

A much better way to measure the winding resistance is to use Ohm’s law: Resistance = Voltage / Current. If you apply a power source to the windings and measure the voltage and current flowing through them, a simple division will yield an accurate value for the resistance.

But don’t do that quite yet! If you directly connect a power source across one of your motor windings, you are likely to just burn up the motor. So don’t do it!

Power Resistor

To protect the motor, you have to connect a power resistor in series with it. It would then limit the amount of current flowing through the motor.

I like to use two 8.2 ohm 25 watt power resistors hooked up in parallel. Radio Shack sells resistors like these for about a dollar each, for example. Together they are the equivalent of a 4.1 ohm 50 watt resistor. Using my 12 volt converted PC power supply, I have (12 volts)/(4.1 ohms) = 3 amps of current. My Park 480 motor is rated for 22 amps continuous across all three windings and can easily handle that much current across one winding.

By the way, these two power resistors are perfect for measuring the internal resistance on a three cell battery pack. I would buy a pair of them and use them for both applications.

A warning. The 50 watt rating on the combined resistors is the amount of power that they can dissipate without permanent damage. It is not the amount of power before they get hot. In fact, they will get very hot well before they reach their rated wattage. Be careful in handling them.

Take the voltage and current measurements right at the motor terminals, because that is what you want to measure. Do not make the mistake of taking the measurements between the power resistors and power supply.

As before, you can try and use a wattmeter to measure the voltage and current. Again, multimeters will be more accurate. Any multimeter should be able to handle the couple of amps of current that you need to measure.

More Accuracy

Motors actually have three loops of copper wire through them. If you measure the resistance across all three loops, I guarantee that you will get three different values. They all should be pretty close to each other. If you are paranoid about accuracy, I would measure all three and average the result. I normally only measure one and use that value. It is entirely up to you what you do.

Also, do not worry about the exact type of winding that the motor uses. So called delta and wye windings are common in our outrunners. The goal is to measure the electrical resistance, regardless of how the motor is wired internally.

Link to Radio Shack 6.8 ohm 25 watt power resistor.

Articles in Series

Measuring Motor Constants: Introduction

Measuring Motor Constants: Tools

Measuring Motor Constants: Power Factor

Measuring Motor Constants: No Load Current (I0)

Measuring Motor Constants: Winding Resistance (Rm)

Measuring Motor Constants: Voltage Constant (Kv)

Measuring Motor Constants: Rm and Kv Method 2


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