Wednesday, July 21, 2010

By The Numbers: Brushless Motors

Brushless motors have lots of numbers associated with them.  Let's use the popular TowerPro 2408-21 as an example.  It's very well documented.  Here's the HobbyKing, TowerPro, and TowerPro Motor Spec pages.

Most Important Numbers

  • Kv -- measured in RPM, shows the maximum rotational speed per volt of input (bigger number = faster spin = more power required).
  • weight -- typically measured in grams.
  • max current -- measured in amps.  The maximum current draw this motor can handle for an extended period of time (important for picking your ESC.  The ESC Amps should be larger than this number).
  • max loaded current (60s) --measured in Amps, and usually given with a timeframe (60 seconds is common.)  The maximum current draw the motor can take for a short period of time without burning up.
Other Numbers
  • Battery - number of cells
  • dimension - the physical size
  • shaft diameter - more powerful motors need bigger, stronger shafts.  you need to know this number if you buy a prop saver. 3mm is a common value.
  • poles, magnets -- not sure why these are important, maybe highter quality motors have more?

Thrust Calculation
(still in progress)

Relationship of ESC numbers to Motor Numbers
  • ESC Amp number must be greater than motor Amp number, or you will burn up you ESC by pulling too much electricity through the circuit.

"Runner" Configuration

Brushless motors are either inrunner or outrunner. The Towerpro motor above is an outrunner.
  • inrunner -- the motor is stationary and the motor shaft spins
  • outrunner -- the motor shaft is stationary and the motor body spins
Motor Equivalency

If you need to see how equivalent two motors are, the two most important numbers to match are
  • Kv -- they should be close to the same power output
  • Weight - they should weigh similarly

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