How is the max rpm calculated?

[Ug??r]

I am trying to understand the max rpm limitation. I previously thought that the theoretical max speed would be calculated by Kv * battery voltage. But in any case the real max speed at no load is a half of expected.

I use a STEVAL-ESC001V1 at 18V with a Hacker A40-12S bldc (8Pols, 1350Kv). If i understood right the theoretical max. rpm is 18V*1350Kv=21397 RPM. I would expect the real no load rpm with 90-95% of the theoretical max. speed but i get less than 50% (9960rpm). 50% i get also with other motors, esc's, lower battery voltages... so I guess I didn't understand something.

Can anyone explain what the real no load speed limits to a half? Are there other motor parameters that limits the max speed?

[Cristiana SCARAMEL]

You are right with "Kv x battery voltage" you have an estimate of maximum RPM (neglecting the motor phases resistance).

Max Speed = 1350 x 18V = 24.300 rpm

Anyway, the actual voltage applied to the motor depends on the PWM duty cycle.

Check the duty cycle value that you are using in your application.