2.8.5 Control Structure Closed-loop
The internal controller allows the Adjustable frequency drive to become an integral part of the controlled system. The
Adjustable frequency drive 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 pump application where the speed of a pump is to be controlled so that the static pressure in a
pipe is constant. The desired static pressure value is supplied to the Adjustable frequency drive as the setpoint reference. A
static pressure sensor measures the actual static pressure in the pipe and supplies this to the Adjustable frequency drive as
a feedback signal. If the feedback signal is greater than the setpoint reference, the Adjustable frequency drive will slow
down to reduce the pressure. In a similar way, if the pipe pressure is lower than the setpoint reference, the Adjustable
frequency drive will automatically speed up to increase the pressure provided by the pump.
Figure 2.24 Block Diagram of Closed-loop Controller
While the default values for the adjustable frequency drive’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. It is also possible to autotune the PI constants.
Introduction to VLT® HVAC D...
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HVAC Drive Design Guide
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