Starting torque of brushless DC (BLDC) PC fan motors?

Short answer: about 1 g*cm per gram of weight.

I measured a BLDC fan motor from ADDA, model number AD1505LX-90, by taping an arm on the fan, and taping a US penny to it (cut in half, each half at 1 cm from the motor axis.)  At full rated voltage of 5V, the motor would start the arm rotating.

A brief pulse of electricity (say one tenth of a second) would NOT start the motor.  The arm would twitch, but fall back into detent (between permanent magnets of the rotor.)

A little longer pulse (say one third of a second) would rotate the motor say half a rotation (past one or more detents.)

The fan weighs about 2 grams.  It moved a penny weighing 2.5 grams on a 1 cm moment arm (a torque of of 2.5g*cm.)

So the starting torque per unit of mass is about 2 g*cm per g.

I know this is fuzzy, but:

  • the manufacturers don’t publish the starting torque for these fans
  • there is a research paper about the torque of these fans but you must pay to see it (thanks, but no thanks, Prof. Fussell)
  • most posts don’t quantify, just saying “not much torque.”

I believe this will extrapolate linearly to larger motors.

Look for another post about the design of BLDC motors.



Tested a ADDA0205MX-K53-LF weighing 5g.  It has about 20g*cm of starting torque.  So it has about 4 g*cm of starting torque per gram of weight.

These motors are very thin and wide (like what is called a capstan motor in VCR’s and floppy disk players.)   This flat and wide configuration probably has much to do with its high starting torque: the permanent magnets are at a long moment arm from the center of the motor.


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