An introduction to feedback devices for VFD and servo applications
01Nov
posted by: Jeff Kardell, KEB America
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.
The problem here is that the drive has no way of knowing if the motor’s actual speed is deviating from the intended speed. This can easily happen as the motor is loaded or if the rotor becomes locked.
We can improve speed regulation performance and even add torque control and positioning functionality by adding feedback to the motor shaft. The feedback goes back to the VFD, where the actual speed is compared to the command speed. The drive looks at the difference between these signals and tries to reduce the error to 0 (zero) by adjusting its speed controller...
Read the full article in PBSI's November issue
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