There were some good posts regarding choosing the PWM switching frequencies considering the needs of the motor control. Here I am focusing rather on understanding of how to decrease losses in the power bridges of the controller. Mainly analyzing two main components of these losses: the conductive losses (power MOSFETS + inductor + resistive load) versus switching losses in power MOSFETs (Miller effect between full open and fully closed state).
Consider example of 3000RPM motor , 50Hz outer loop frequency, having 4 pairs of poles whence 200Hz outer loop motor frequency. According to the accepted "rule of thumb" the required PWM switch frequency starts from 10 x 200Hz = 2kHz This number is thought of been able recreating the "natural sinusoidal" EMF of the motor coils needed for voltage control of the coils current.
Let's assume that I have other means to change the voltage applied to the half bridges MOSFETs in order to control the speed of the motor (in this case to maintain steady 3000RPM). Is it correct to assume that I can reduce switching losses in the power inverter if I just apply the same switching frequency 2kHz thus not recreating sinusoidal waveform at all but only switching polarity of motor phases to make motor run with constant speed ? (Again, assume for a moment that issues of the close loop, position sensing and fine tuning are passed to another block - the voltage controller , adjusted for a permanent speed.) Or are the conductive losses increase so dramatically that saving on switching losses will not help at all? What kind of losses prevail within say 100Hz to 10kHz switching range ? Would conductive losses increase dramatically in the lower end of this range so that eliminating switching losses don't matter ? Or is it more beneficial to use upper end of the range to have minimal total losses?
Yet one more way to put this question: Is PWM switching frequency chosen only to recreate sinusoidal voltage control of the motor or is it also helping to decrease overall energy losses in the inverter compared to the long period of the ON state conductive losses in the MOSFETs?
Anybody who had heavy practical experience with motor control please share your thoughts. JonRB please? Edit: To clarify the expression "to recreate sinusoidal voltage control" is only one purpose of using PWM, another one - more frequently used for - is to control voltage level below of the provided fixed voltage source. In any case, this post is not about voltage control but about power losses in the inverter : the balance between conductive losses vs switching losses.