What exactly is pulse width modulation?

Yesterday, at the end, I mentioned PAM, PWM, and SPWM, saying that they are often seen in some manuals or reference books, especially in the manuals of frequency converters. So, what are they? Let's talk about this today.

They all belong to waveform modulation, but the implementation principles are different. As mentioned above, the most important factor for variable frequency equipment to drag the load to move is to ensure that the magnetic flux is constant, which requires that the frequency must change in the same proportion as the voltage changes.

So, how do we solve this problem? It's like solving a word problem in high school mathematics. PAM is the most conventional way of thinking, according to the formula.

While changing the frequency, the voltage also changes. With the thyristor technology, the DC bus voltage becomes adjustable, and with the inverter technology, the frequency becomes adjustable. Therefore, PAM can be realized. However, this technology must control the rectification and inverter at the same time, making the control circuit complex and redundant. The control of both sides must be carried out at the same time, which makes coordination difficult.

Although the solution was finally found, a large number of articles were written about it, so this method had room for upgrading and modification, and PWM was on the cover of Time.

PWM is the pulse width modulation technology, which divides the output voltage waveform of the inverter into multiple pulses per half cycle, and adjusts the average voltage by adjusting the pulse width and pulse period. Because the pulse can be controlled, the output waveform can be adjusted.

There is a concept of "duty cycle" here, which is the proportion of pulse width in each pulse cycle, which is D in the figure below, D=tp/tc. At high frequency, the pulse width is large and the duty cycle is high. At low frequency, the pulse width is small and the duty cycle is naturally small.

PWM only needs to control the rising and falling edges of the pulse on the inverter side, without having to control the DC side, which can greatly simplify the control circuit, thus reducing the length of the decomposition by half.

If the pulse width is changed according to the sine law, it becomes SPWM. When the sine quantity is small, the duty cycle is small, and vice versa. This converts the output voltage waveform into a pulse wave equivalent to a sine voltage.

Since the winding of the motor is inductive, although the voltage is composed of a series of pulses, the current flowing into the motor is close to a sine wave. In this way, the motor will feel particularly comfortable.

In this way, a complex application problem was solved perfectly.

If you understand it, you will understand the output waveform of the inverter. This is the SPWM pulse width modulation technology often mentioned in the inverter manual. At present, all inverters use this output modulation mode.