Why the triangle reference wave is used in PWM for sine modulation?
The "triangle reference" method that has some history as a standard or popular method was more formally known as the "triangle interception method." It was based on generating switching commands based on the interception of a triangle wave with a sine wave as shown below. It was not too difficult to implement and it provided reasonably good results for variable frequency drives (VFDs). Results were improved by changing the ratio of the triangle wave frequency to the sine wave frequency over the range of the operating frequencies.

After the invention of large scale integrated circuits and microprocessors, more sophisticated PWM strategies became preferred.



Image from Zubek, Abbondanti &amp; Nordby “Pulsewidth Modulated Inverter Motor Drives with Improved Modulation” IEEE Trans. Ind. Appl., Nov./Dec. 1975