Pid Loop Operation - Hitachi SJ100 Instruction Manual

PID Loop Operation

In standard operation, the inverter uses a reference source selected by parameter A_01
for the output frequency, which may be a fixed value (F_01), a variable set by the front
panel potentiometer, or value from an analog input (voltage or current). To enable PID
operation, set A_71 = 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, 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
PV
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 A_75 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
F 01
Multi-speed
settings
A 20 to A 35
Potentiometer
on keypad
V/I input
select
[AT]
Voltage
O
A GND
L
OI
Current
Error PID Freq.
Calculation
Setpoint
(Target)
Scale factor
Frequency
reciprocal
source select
1
A 01
A 75
Process Variable
(Feedback)
Analog input scaling
A 12
A 11
A 15 A 13 A 14
A 76 PID V/I input select
SJ100 Inverter
Inverter Motor
Process Variable (PV)
Scale factor
A 75
P gain
A 72
Error
SP
I gain
∑
A 73
PV
D gain
A 74
Scale factor
A 75
4–39
External
Process
Sensor
F 01
Frequency
setting
∑
Monitor
D 04