VLT® HVAC Basic Drive FC 101

Introduction:
Variable Frequency Drives (VFDs) often need to control an AC motor at a precise speed, which can be set via a keypad, speed potentiometer, or analog input. Many VFDs include a built-in Proportional-Integral-Derivative (PID) controller to provide more precise process control.

PID Controller Function:

• Objective: The PID loop maintains a process variable, such as motor speed, at a desired setpoint.

• Error Calculation: The setpoint (desired speed) and the actual speed are compared at a summation point. If these values differ, an error signal is generated, which indicates how much and in which direction the actual speed deviates from the desired speed.

• PID Actions:

  • Proportional: Responds to the current error value.

  • Integral: Addresses the accumulation of past errors.

  • Derivative: Predicts future error based on its rate of change.

PID Output:
The PID controller processes the error signal through these three functions to adjust the VFD output frequency. The goal is to minimize the error by achieving the desired speed with minimal overshoot or oscillation.

Tuning and Autotune:

• Tuning: Adjusting gain and time settings to enhance performance, ensuring a fast response with minimal overshoot.

• Autotune: Some VFDs include an autotune function to simplify the tuning process by automatically adjusting PID parameters for optimal performance.

This setup allows the motor to quickly and accurately reach and maintain the desired speed.