Monday, April 9, 2018

Common Mode Overview and Reduction Guide

MTE, April 9, 2018

Understanding the creation, effects, and how to reduce common mode over voltage.

What is Common Mode?

To start off, common mode is bad. Common mode voltage is created by Variable Frequency Drives (VFDs) that serve as a way of controlling the speed of AC motors by varying the frequency of the power source using pulse width modulation (PWM). This is done by switching the transistors, IGBTs, or thyristors, on and off continuously.

The continuous generation of power pulses from VFDs prevents a smooth sinewave from being produced, which at any point is at a sum of zero (see Fig.1). Instead, the waveforms produced result in a sum at any point that is not always zero (see Fig.2). The result is damaging common mode over voltage, which can cause devastating effects to your equipment.

Destructive Effects of Common Mode Over Voltage

Common mode problems occur outside of the VFD, which is why they are difficult to diagnose.  The effects of common mode over voltage are extremely problematic for everyday operations. These problems negatively effect your bottom line, creating the need to replace equipment, increase repair costs, and can result in the loss of production.