the actual frequency of the mechanical variable speed gearbox

mechanical variable speed gearbox of the IGBTs in a PWM drive can range from 2 KHz to 15 KHz. Today's newer PWM designs use power IGBTs, which operate at these higher frequencies. By having more pulses in every half cycle, the motor whine associated with gearbox transmission is reduced because the motor windings are now oscillating at a frequency beyond the spectrum of human hearing. how each transition of the alternating voltage output is actually a series of short pulses of varying widths. By varying the width of the pulses in each half cycle, the average power produced has a sine-like output. The number of transitions from positive to negative per second determines the actual frequency to the motor.