Danfoss VLT HVAC Drive FC 100 Series Design Manual page 32

2. Introduction to VLT HVAC Drive
Parameter
Custom Refrigerant A1
Custom Refrigerant A2
Custom Refrigerant A3
2
PID Start Speed [RPM]
PID Start Speed [Hz]
On Reference Bandwidth
PID Anti Windup
PID Differentiation Time
PID Diff. Gain Limit
Lowpass Filter Time :
Analog Input 53
Analog Input 54
Digital (pulse) input 29
Digital (pulse) input 33
2.8.3. Example of Closed Loop PID Control
The following is an example of a Closed Loop Control for a ventilation system:
32
Description of function
par. 20-31 When par. 20-30 is set to
par. 20-32 value of coefficients A1, A2 and A3 in the conversion equation:
par. 20-33
Temperature
=
par. 20-82 The parameter that is visible will depend on the setting of par. 0-02, Motor Speed Unit.
par. 20-83 In some applications, after a start command it is important to quickly ramp the motor
up to some pre-determined speed before activating the frequency converter's Closed
Loop Controller. This parameter defines that starting speed.
par. 20-84 This determines how close the feedback must be to the setpoint reference for the fre-
quency converter to indicate that the feedback is equal to the setpoint.
par. 20-91 On [1] effectively disables the Closed Loop Controller's integral function when it is not
possible to adjust the output frequency of the frequency converter to correct the error.
This allows the controller to respond more quickly once it can again control the system.
Off
[0] disables this function, making the integral function stay active continuously.
par. 20-95 This controls the output of the frequency converter's Closed Loop Controller based on
the rate of change of feedback. While this can provide fast controller response, such
response is seldom needed in HVAC systems. The default value for this parameter is
Off, or 0.00 s.
par. 20-96 Because the differentiator responds to the rate of change of the feedback, a rapid
change can cause a large, undesired change in the output of the controller. This is used
to limit the maximum effect of the differentiator. This is not active when par. 20-95 is
set to Off.
par. 6-16 This is used to filter out high frequency noise from the feedback signal. The value en-
par. 6-26 tered here is the time constant for the low pass filter. The cut-off frequency in Hz can
par. 5-54 be calculated as follows:
par. 5-59
F cut
=
off
−
2π
Variations in the feedback signal whose frequency is below F
frequency converter's Closed Loop Controller, while variations at a higher frequency are
considered to be noise and will be attenuated. Large values of Lowpass Filter Time will
provide more filtering, but may cause the controller to not respond to actual variations
in the feedback signal.
MG.11.B7.02 - VLT ® is a registered Danfoss trademark
VLT
User defined [7], these parameters are used to define the
A
2
A
−
( ln ( pressure + 1 ) − A 1 )
1
T lowpass
®
HVAC Drive Design Guide
3
will be used by the
cut-off