Narrow pulses with equal impulse but different shapes have the same effect when applied to the link with inertia. According to the PWM control principle, the waveform is divided into 6 equal parts, which can be replaced by 6 square waves equivalently. There are many classification methods of pulse width modulation, such as unipolar and bipolar, synchronous and asynchronous, rectangular wave modulation and sine wave modulation, etc. The unipolar PWM control method means that the carrier only changes in one direction within half a cycle, and the resulting PWM waveform only changes in one direction, while the bipolar PWM control method changes the carrier in two directions within half a cycle, and the resulting PWM waveform The waveform also changes in both directions.

According to whether the carrier signal is synchronized with the modulation signal, PWM control can be divided into synchronous modulation and asynchronous modulation. The characteristic of rectangular wave pulse width modulation is that the output pulse width columns are of equal width, and can only control harmonics of a certain order; The output waveform is close to a sine wave. Sine wave pulse width modulation is also called SPWM. Generating pulse width according to control signal is the key of this technology. Commonly used triangular wave comparison method, hysteresis comparison method and space voltage vector method.