Frequently Asked Questions

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Question Answer
Q: Is there a difference between EMI and RFI?
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A: The terms EMI and RFI are often used interchangeably. EMI is actually any frequency of electrical noise, whereas RFI is a specific subset of electrical noise on the EMI spectrum. There are two types of EMI. Conducted EMI is unwanted high frequencies that ride on the AC wave form. Radiated EMI is similar to an unwanted radio broadcast being emitted from the power lines. There are many pieces of equipment that can generate EMI, variable frequency drives included. In the case of variable frequency drives, the electrical noise produced is primarily contained in the switching edges of the PWM controller. As the technology of drives evolves, switching frequencies increase. These increases also increase the effective edge frequencies produced, thereby increasing the amount of electrical noise.
Q: What causes unwanted high frequency electrical noise?
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A: These frequencies can be from either noise conducted through power lines or radiated through the air onto the power lines.
Q: What is the difference between Common Mode noise and Differential Mode noise?
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A: 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.