Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 262)

5-8 Functional Description
7000-UM150I-EN-P – June 2013 7000 “B” Frame
Flux Control
The function of the flux control block (Figure 5.5) is to determine the
magnetizing component (I
sd
) of the stator current (I
s
) needed to
maintain the desired flux profile in the motor. The inputs are Flux
Feedback (306) and Stator Freq (448) from the motor model, Speed
Feedback (289) and Torque Reference (291) from the speed control
block and the measured voltage at the input of the bridge Vline
Bridge (696).
The Flux Feedback is subtracted from the Flux Reference (305) to
determine the Flux Error (307), which is the input to the flux PI
regulator. The gains are determined from desired Flxreg Bandwidth
(97) and motor parameters T Rotor (132) and L Magnetizing (131).
The output of the flux regulator is Isd Command 1(309). An open
loop estimate of the magnetizing current, Isd Command 0 (308) is
determined by dividing the Flux Reference by parameter L
Magnetizing. Isd Command 0 and Isd Command 1 are added to
produce Isd Command (310) which is the magnetizing component of
the stator current command. To calculate the magnetizing current
supplied by the inverter Ix Command (312), the current supplied by
the motor filter capacitor in magnetizing is calculated and subtracted
from Isd Command. It should be noted that as the motor speed
increases, Ix Command decreases. This is because as the motor
voltage increases more of the magnetizing current requirement of the
motor is met by the capacitor. At resonant point, Ix Command is
nearly zero and becomes negative at speeds above resonance. Iy
Command (from Speed Control block) and Ix Command are then
passed to the Current Control block to determine the dc link current
reference (Idc Reference) and the firing angles of the two converters
(Alpha Line and Alpha Machine).
The flux profile in the drive is adjusted by the parameters Flx Cmd
No Load (103) and Flx Cmd Base Spd (100). Using these parameters,
Flux Reference is adjusted linearly with the desired Torque
Reference. At light loads motor flux is decreased allowing reduction
in losses while full flux is produced at rated load. The maximum flux
reference is limited to Flux Cmd Limit (623). This limit is dependent
on the input voltage Vline Bridge and the motor speed (Speed
Feedback). If the drive operates at reduced line voltage, then Flux
Reference is reduced. Also if the motor is running above the Base
Speed, the flux profile is made inversely proportional to the speed of
the motor resulting in the field weakening or the constant power
mode of operation of the drive. This is accompanied by a decrease in
the motor torque capability.