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What is the difference between Common Mode noise and Differential Mode noise?

Common mode noise is the electrical noise on all power lines with respect to ground. Differential mode noise is the electrical noise on one line with respect to another line. When considering a filter for use in a variable speed drive application, it is important to note that common mode noise reduction is the most important factor. Variable frequency drives produce very little differential mode, or line to line, electrical noise due to the presence of DC bus capacitors.