a signal from table 132 and when value f e is greater than high threshold value f eht output line 150 is linked to second input 142 and command carrier signal f c is passed as carrier signal f c for use by pwm generator 108 see also fig7 the look up table 132 may take many specific forms but in general specifies a relatively low intermediate carrier frequency f c u2032 when the operating frequency f e is relatively low and specifies higher carrier frequencies as the operating frequency is increased specific examples of carrier to operating frequency are useful and are provided in graphical form in fig9 and 11 referring to fig8 and 9 in one embodiment when the operating frequency f e is below a low threshold operating frequency f elt the carrier frequency f c u2032 is set to a minimum carrier frequency f cmin when the operating frequency f e is greater than the low threshold frequency f elt the carrier frequency f c is set to a value above minimum frequency f cmin in one embodiment generator 110 generates carrier frequencies f c u2032 starting at minimum frequency f cmin and linearly increasing therefrom as illustrated in fig1 in operating assume frequency f cmin is 2 khz that f elt is 2 hz and that f eht is 8 hz also referring to fig9 assume command operating frequency f e is 1.