BLDC motor phase current calculation

[Techman_7]

Suppose for 12V, 120W BLDC motor, the input current from 12V is 10A.
How to calculate the phase current of this motor.
Is it 10A / (sq. root 3) = 5.77A per phase ?

Can anyone help here..

 

[KlausST]

Hi,

impossible to say.
I guess, when you say 12V, 10A, then this means the DC voltage to the BLDC driver chip. Again: DC.
But the motor wires (with respect to each other) carry AC.

To be able to calculate the motor current, you need to know
* the voltage
* and the phase shift.
Both are unknow and vary in the application. Vary because of driving method, torque, RPM .. maily.

But generally:
One can say: If the input power is 120W
* you have loss in the driver (let´s just calculate with 12W = 10% loss)
* then (on symmetric driving) the remaining (120W - 12W =) 108W will divide symmetrically to the coils.
Each coil will carry 108W / 3 (for a three phase BLDC) = 36W.

Now let´s say you have 6V RMS sine (motor wire to virtual star point) then the theoretical minimum current is 36W / 6V = 12A. Theoretically! Better use 15A as realistic value for this example... because of phase shift.

Klaus