Wednesday, October 17th, 2012 11:29 am GMT -6 Wednesday, October 17th, 2012 11:29 am GMT -6Wednesday, October 17th, 2012 11:29 am GMT -6
Electric brushless outrunner motor

Got a motor without a label? Here is a strategy for figuring out what it can do.




Email Received

“First off, let me thank you for your RCadvisor website. I am learning much which I should have known years ago. My problem is that I bought a used electric model airplane equipped with a larger motor than recommended. All labels have been removed  from the outrunner can. How can I determine what the motor specs are? I want to know what the max/min voltage is. I currently run 3 cells at 11.1 volts.”

Simple Motor Identification

First, look very carefully at the motor to try and identify it. I have come across motors that look at first sight like they have no markings. But upon closer inspection, they do. The model number might be printed in small letters pretty much anywhere. Look carefully.

Look inside the can. You do not need to take it apart to do this. Does the wire in the winding look thin or thick? This is of course all relative to the size of the motor. Thin winding wire means a low Kv or motor voltage constant. If the wire looks unusually thick then it must be a high Kv ducted fan motor.

Get a feel for the build quality of the motor. Does it look like a top dollar motor, or an inexpensive no-name brand? That will help you gauge how hard you can push it.

Weigh the motor in grams. Leave the propeller adapter off. If the connector soldering job looks homemade, take a few grams off to account for them.

Try and figure out what brand it is and search for it online. Even if you are not sure which brand it is, look for a similar looking motor that weighs about the same as yours. This will not help you figure out the motor’s Kv and current capacity, but it is a start.

Maximum Current

You may have no choice but to test run the motor to figure out its specifications. The first priority then would be to estimate the maximum amount of current (amps) that it can handle. As a very rough estimate, assume that it can handle 0.25 amps per gram of motor weight.

Look at the shape of the motor. Is it short, with a large diameter? Good. That means that it gets excellent cooling as it runs. It will be able to handle a higher current than average. On the other hand, if the motor has a small diameter and is long, then it will be prone to overheating.

Carefully test run the motor with a propeller attached. Use a wattmeter to measure the current going through the motor. Start out with short bursts and slowly increase their duration. In between, use an infrared or other thermometer to measure the highest temperature on the motor.

You are more likely to permanently damage the magnets first before you melt the insulation on the winding copper wire. The quality of the magnets that manufacturers use vary, but to be conservative you do not want their temperature to go above 150 F (65 C). Note that the temperature on the outside of the can will be a little less than the temperature of the magnets.

If you do not have a thermometer, use your finger. Even better, ask a buddy to come over and tell him you need his help. No, do not tell him this was my idea. If the motor can is too hot to touch, then it is too hot to run.

Estimating the Voltage Constant (Kv)

This may not be easy to do, but measure the RPM of the motor when it is running at full power without a propeller. The easiest way to do this is by using an electronic RPM sensor like the Eagle Tree eLogger has. Divide the RPM by the input voltage and you will have an estimate of the motor’s Kv. The actual value will be maybe 10% higher than the value you just computed.

Measuring Motor Constants

The above procedure should be enough to let you estimate what the motor can do. At this point you have a choice.

One option is to just put the motor on a model airplane and try it. Bench run it first to make sure the propeller does not exceed the current limit of the motor. Also get a feel to see if the propeller will develop enough thrust to fly the airplane. Depending on the size of the motor, you are going to have to take a guess as to how many battery cells it needs.

Another option is to conduct detailed bench thrust measurements. Most of us are not setup to do this type of testing. I also do not like to recommend it because of safety concerns.

Finally, you can follow my detailed procedure for measuring the motor constants. Then you can use my free online calculator to really see what the motor can do. Here is a link to the first article in that series:

Measuring Motor Constants: Introduction (web)

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