Hitachi WJ200-001S Instruction Manual page 117

332
In standard operation, the inverter uses a reference source selected by parameter 
for the output frequency, which may be a fixed value (), a variable set by the front
panel potentiometer, or value from an analog input (voltage or current). To enable PID
operation, set =. This causes the inverter to calculate the target freq, 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
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  is a scale factor that relates the external process variable
units to motor frequency. The figure below is a more detailed diagram of the function.
Standard setting
Multi-speed

POT meter on
V/I select
[AT]
[L]
[O]
[OI]
Error
PID
+
Calculation
SP
PV
Setpoint
(Target)

Scale factor
Reciprocal
setting
1


to
ext. panel
Process variable (Feedback)
Analog input scaling (OI)
A GND
Voltage
Current

PID V/I input select
Freq.
Inverter Motor
Process Variable (PV)
Frequency
source select

SP
+



 
External
Process
Sensor
Scale factor
 
P gain

Frequency
setting
I gain
+

D gain

Scale factor Monitor
 