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L100 Inverter
Operations
and Monitoring
4–27
PID Loop Operation
In standard operation, the inverter uses a reference source selected by parameter A01 for
the output frequency, which may be a fixed value (F01), a variable set by the front panel
potentiometer, or value from an analog input (voltage or current). To enable PID opera-
tion, set A71 = 01. This causes the inverter to calculate the target frequency, or setpoint.
A calculated target frequency can have a lot of advantages. It lets the inverter adjust the
motor speed to optimize some other process of interest, while saving energy as well.
Refer to the figure below. The motor has some eventual effect on a process of interest. To
control the external process, the inverter must monitor the process variable. This requires
wiring a sensor to either the analog input terminal O (voltage) or terminal OI (current).
When enabled, the PID loop calculates the ideal output frequency to minimize the loop
error. This means we no longer command the inverter to run at a particular frequency, but
we specify the ideal value for the process variable. That ideal value is called the setpoint,
and is specified in the units of the external process variable. For a pump application it
may be gallons/minute, or it could be air velocity or temperature for an HVAC unit.
Parameter A75 is a scale factor that relates the external process variable units to motor
frequency. The figure below is a more detailed diagram of the PID function.
∑
MotorInverter
Process
Sensor
Process Variable (PV)
Setpoint Error
PID
Calculation
Freq.
PV
External
SP
Monitor
P gain
A75
A11
F01
A76
A72
A15
A01
I gain
A73
D gain
A74
∑
A75 D04
A12
A13 A14
Analog input scaling
∑
Voltage
O
OI
L
Current
A GND
PID V/I
input select
Process Variable
(Feedback)
Scale factor
Frequency
setting
F01
Scale factor
Setpoint
(Target)
1/A75
Scale factor
reciprocal
Multi-speed
settings
Standard
setting
A20 to A35
Frequency
source select
Potentiometer
on keypad
Error
[AT]
V/I input
select
PV
SP
Technologies Inc.
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