BLDC controller requirements

Discussion in 'The Projects Forum' started by lungfish, May 17, 2010.

  1. lungfish

    Thread Starter New Member

    May 17, 2010
    I'm trying to determine servo controller supply voltage requirements based on a given BLDC mode of operation.

    * mechanical point of operation (steady state)
    - rpm, torque
    * motor data
    - phase-phase-resistance [Ohms], phase-phase-inductance [Henries], torque-constant [Nm/A], speed-constant [1/(rpm*V)], number of pole-pairs
    * controller data
    - sine control, 20kHz PWM

    The problem:
    To estimate the required controller supply voltage, I would calculate the backemf=rpm/(speed-constant), the current=torque/(torque-constant)) and the voltage drop over the phase-phase-resitance for the current. Add those up - buy the controller.

    Is that all? I keep asking myself, how much voltage drop the PWM or commutation would cause over the coils. Unfortunately I could not find any quantitative answers to those questions while browsing for "bldc" "bldc control" "bldc losses".
    From a normal brushed DC motor I remember, that the coil inductivity is not visibible during steady state operation - only during accelartion (as electrical time constant).
    Asking the controller/motor manufacturer basically resulted in "do the estatimate and double the supply voltage, to have reserves for the current controller". This of course is quite unacceptable and unsatisfactory, since cost and size are very much dependant on supply voltage.

    Any hints would be appreciated,