Sunday, July 29th, 2012 10:49 am GMT -7 Sunday, July 29th, 2012 10:49 am GMT -7Sunday, July 29th, 2012 10:49 am GMT -7
Very large electric brushless outrunner motor

A run down of the tools you can use to measure the electric brushless outrunner motor constants.




Power Source

To measure the motor constants, you are going to need some tools. The first is a power source.

You can of course use one of your LiPo battery packs, but there is a serious drawback with doing that. The problem is that the voltage coming from a battery pack is always dropping. Every time you take a reading, you need to write down several values. Having constantly changing values makes for a very frustrating experience. Trust me, I know!

Years ago a good friend gave me a converted PC power supply. He installed two power posts on it, making it very easy to use. I get about 12 volts out of it and I believe up to a maximum of 8 amps. I have never come close to needing that many amps out of it.

To measure the no load current, the power source should be at least 8 volts. But I’ll bet anything that your speed control is happy with will work. This normally means the equivalent of 2 to 4 LiPos in series (8 to 16 volts, roughly). For measuring the no load current all you need is a couple of amps of current. Depending on which method you use to measure the constants, that might be all you need. However, for some of the methods you might need to supply the motor up to its maximum current capacity.


Since we are talking about measuring motor constants, I hope you realize that you will need some way of measuring the motor’s RPM.

Using an optical tachometer would not be a bad way of doing it. Many years ago I got one from Tower Hobbies that costs about $15 and is still working fine. Sometimes even wattmeters have tachometers, though they do not always work well.

There are a couple of problems with using optical tachometers. First, you cannot be using a fluorescent light source. These lights pulsate quickly and throw the reading off. The real problem is that you need to mount a propeller to the motor to be able to take a reading. There are obvious safety concerns with that. Remember, we are talking about running the motor indoors on your work bench.

What I like to use is my Eagle Tree eLogger. I have the brushless outrunner RPM sensor for it, that works by connecting a couple of wires to two of the motor wires. The only wrinkle is that you need to tell it the number of poles in the motor. They mean the number of magnetic poles, which is the number of magnets that spin around. It is not too hard to peek inside the motor and count them up. All of my motors seem to use 14, but you should count them in your motor anyway to make sure.

Voltage and Current

You also need some way of measuring a voltage and a current at the same time. If the first thought that just popped into your head is “wattmeter”, you would not be far off. Using a wattmeter is certainly convenient, but they are not the most accurate instruments.

I own two multimeters. One is a high end unit that I paid about $350 for, and the other is a decent standard unit that is worth about $50. I use one to measure the voltage and the other to measure the current.

Note that multimeters normally cannot measure currents higher than about 10 amps. This is fine for measuring the no load current, but if you plan to push your large motor to its current limit, you will need a clamp ammeter.

Clamp ammeters cannot be beat for convenience. Just clamp the wire and get an instant reading of current. But they have two problems. First, units that cost about $50 or less cannot measure direct currents (DC). They are useless for our application. Meters that cost less than about $100 can only measure with an accuracy of 0.1 amps. That’s not great.

The least expensive clamp ammeter that I can recommend is called the GTC CM100 Low Current Clamp Ammeter. It costs about $125 and can measure down to one milliamp.


To hook up all of this stuff together, I created three short jumpers with alligator clamps at the ends. The alligator clamps only cost me about a dollar at Harbor Freight. I used some high quality heavy duty (10 gauge) battery wire and soldered them all together.

Eagle Tree Systems eLogger website.

Click to buy the GTC CM100 Low Current Clamp Meter.

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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