47
VLT
®
3500 HVAC
MG.35.B1.02 – VLT is a registered Danfoss trademark
Section 1Section 1Section 1Section 1Section 1
Section 2Section 2Section 2Section 2Section 2
Section 3Section 3Section 3Section 3Section 3
Section 4Section 4Section 4Section 4Section 4
3.5.1 PID contr
oller
How to use the PID controller
The PID controller integrated in the VLT
®
frequency
converter comprises a feed forward function and a
traditional PID controller.
The feed forward function
The feed forward function transmits the desired set-
point past the PID controller. Any changes of the set-
point will therefore directly affect the motor speed.
The PID controller
The PID controller maintains a constant process out-
put (pressure, temperature, flow etc.). The PID con-
troller adjusts the motor speed on the basis of the
setpoint and the feedback signal.
The feedback signal is a transmitter’s feedback indi-
cation from the process, showing whether the desired
setpoint is being reached. The feedback signal varies
as the load of the process changes.
An error (e) occurs, which is then regulated by the PID
controller by adding or subtracting from the feed-
forward signal.
The band width determines how much the PID con-
troller must add to or subtract from the feedforward
signal. The band width will thus ensure that the
controller keeps close to the setpoint.
The bandwith can only be reduced if there is a
correlation between the feedback and the motor
speed.
■
The setpoint is given as a percentage of the range
between the minimum and maximum feedback sig-
nal.
The feedback signal can be adjusted to that of the
actual transmitter. If the transmitter is a pressure
transmitter with a range from 0-10 bar, this can be set
as the feedback range to be displayed, but as stan-
dard it is set in percentage of the chosen voltage, cur-
rent or pulse signal.
Normal or inverse control
The control is called normal, if the motor frequency is
reduced, when the feedback signal goes up, and
increased when the feedback signal goes down.
The control is called inverse, if the motor frequency
increases/decreases with the increase/decrease of
the feedback signal.
