Pid Loop Operation - Hitachi SJ300 Series Instruction Manual

PID Loop Operation

In standard operation, the inverter uses a reference source selected by parameter A001 for the
output frequency, which may be a fixed value (F001), a variable set by the front panel potenti-
ometer, or value from an analog input (voltage or current). To enable PID operation, set A071 =
01. This causes the inverter to calculate the target frequency, or setpoint. An optional intelligent
input assignment (code 23), PID Disable, will temporarily disable PID operation when active.
A calculated target frequency can have a lot of advantages. It lets the inverter adjust the motor
speed to optimize some other process variable of interest, potentially saving energy as well.
Refer to the figure below. The motor acts upon the external process. To control that 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).
Setpoint
SP
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 A075 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.
Standard
setting
F001
Multi-speed
settings
A020 to A035
Potentiometer
on keypad
V/I input
select
[AT]
Voltage
O
A GND
L
OI
Current
Inverter
Error
PID
∑
Calculation
PV
Analog input
Setpoint
Frequency
(Target)
source select
Scale factor
A001
reciprocal
1/A075
Process Variable
(Feedback)
Analog input scaling
A012
A011
A015 A013 A014
A076
PID V/I
input select
Freq.
Inverter
Motor
Output
Process Variable (PV)
Scale factor
A075
PID Enable
A071
P gain
A072
Error
SP
I gain
∑
A073
PV
D gain
A074
Scale factor
A075
4–71
SJ300 Inverter
External
Process
Sensor
F001
PID Disable
C023
optional
intelligent input
Normal
PID
∑
Frequency
setting
Monitor
D004