One issue that often arises when designing closed-loop applications is which type of feedback to use on the motors. In this article, Jeff Kardell of KEB America describes the different types of feedback devices in variable frequency drives (VFDs) and servo applications, as well as some typical examples of where they can be utilised. What is motor feedback? To answer this question, we should begin by considering an open-loop system that has no feedback. The VFD is given a speed command and attempts to run the motor at that speed by adjusting its output frequency. The advantage of this system is its low complexity and low cost. A control diagram would look like Figure 1.
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