Pid - Danfoss VLT HVAC Drive FC 100 Series Design Manual

2. Introduction to VLT HVAC Drive

2.8. PID

2.8.1. Closed Loop (PID) Controller
2
The frequency converter's Closed Loop Controller allows the frequency converter to become an integral part of the controlled system. The frequency
converter receives a feedback signal from a sensor in the system. It then compares this feedback to a setpoint reference value and determines the error,
if any, between these two signals. It then adjusts the speed of the motor to correct this error.
For example, consider a ventilation system where the speed of the supply fan is to be controlled so that the static pressure in the duct is constant. The
desired static pressure value is supplied to the frequency converter as the setpoint reference. A static pressure sensor measures the actual static pressure
in the duct and supplies this to the frequency converter as a feedback signal. If the feedback signal is greater than the setpoint reference, the frequency
converter will slow down to reduce the pressure. In a similar way, if the duct pressure is lower than the setpoint reference, the frequency converter will
automatically speed up to increase the pressure provided by the fan.
NB!
While the default values for the frequency converter's Closed Loop Controller will often provide satisfactory performance, the control
of the system can often be optimized by adjusting some of the Closed Loop Controller's parameters.
The figure is a block diagram of the frequency converter's Closed Loop Controller. The details of the Reference Handling block and Feedback Handling
block are described in their respective sections below.
The following parameters are relevant for a simple PID control application:
30
MG.11.B7.02 - VLT ® is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide