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Small GWS motor attached to test stand. Bob can connect battery to digital meter and then to the motor. After briefly running the motor, he can compute its expected run time. |
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Close up of how Bob attaches motors to the test stand with nylon ties. Limit bench-testing these motors to the just the time it takes to read the current from the meter screen. |
Q-33: A local electric-power expert told a reader not to run his motor for long periods of time on a bench test stand to determine run time on a particular battery pack. The reader wants to know if this is good advice, and, if it is, what is an alternative to estimating motor run time.
A-33: The expert in this case is positively correct! Your motor depends on air passing over it to keep the temperature down. When you run the motor on a test stand indoors, all you have is the propeller wash for cooling, and that isn’t enough. This situation was bad enough when we used Ni-Cd or NiMH batteries that were capable of seven-, eight-, and 10-minute motor runs, but the new Li-Poly batteries have run times exceeding 30 minutes. You could literally cook some motors in that length of time. What is the alternative?
If you have an AstroFlight digital meter, you can measure the actual motor current. With that information, you can make a simple calculation that will approximate your motor run time.
To do this, mount your motor on a simple test stand. Install a propeller that is the size you plan to use on your model and charge the battery pack you plan to use. Run the battery into the right side of the AstroFlight meter. At that point the LCD screen on the meter will light up. Plug the lead coming off the left side of the meter into your motor. The ESC isn’t needed for this check. As soon as you make the connection to the motor, it will start running. Observe the motor current (in amps) and write it down.
Disconnect the cables at this point; you are finished with the motor and battery. Recording the current reading should take you only a few seconds, and that won’t hurt your motor.
Now for the calculation! Multiply your battery capacity, expressed in amp-hours, by 60. If your battery is rated at 340 mAh, that’s the same as 0.340 amp-hr. Multiply 0.340 by 60 to arrive at 20.4. Divide that number by your motor current, which in this case was 2.5 amps. 20.4 divided by 2.5 = 8.16 minutes.
Figure that your motor, propeller, and battery combination will run for approximately eight minutes in this case. The calculation is simple, and you don’t have to burn your motor out in the process.