Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 269)

Functional Description 5-15
7000 “B” Frame 7000-UM150I-EN-P – June 2013
Machine Converter Feedback
The function of the machine converter feedback block is to process
(scale and filter) the raw voltage and current feedback signals to the
form required by the drive control software. It represents most of the
analog portion of the machine side Signal Conditioning Board
(SCBM) and the Drive Control Board. The machine converter VFB
provides a total of six voltage feedback signals representing the three
ac (V
a1
, V
b1
, V
c1
), two dc (V
dc+
, V
dc-
) and one machine side filter
capacitor neutral voltage referenced to ground. The motor line to
ground voltages are subtracted from each other and filtered to
produce the three motor line to line voltages (V
ab1
, V
bc1
, V
ca1
).
The two dc voltages are subtracted to determine the machine side dc
link voltage (V
dc
), which is used for hardware dc link over-voltage
protection. The motor line to ground voltages are summed to
determine the motor neutral to ground voltage (V
ng
) and is used for
motor neutral over-voltage protection.
Hall Effect Current Sensor (HECS) provide stator current feedback
in two of the motor phases (I
a3-out
, I
c3-out
). These currents are then
filtered and processed by a variable gain stage (I
a3
, I
c3
) before being
sampled for protection. Inverting and adding the two current
feedback signals reproduces the current in the remaining phase (I
b3
).
The motor line voltages and currents are further used to calculate the
motor flux (F
ab
, F
bc
, F
ca
) using a hardware analog model. The
measured flux (V
d
and V
q
) is then used in the motor model block
(described in the next section) for synchronization and drive control.
For drives with Synchronous Transfer option, an additional VFB is
used for sensing three line-to-ground bypass voltages. These line-to-
ground voltages are subtracted from each other to yield two line-to-
line voltages (V
abxfer-out
, V
bcxfer-out
). These are further filtered (V
abxfer
,
V
bcxfer
) and sampled by the software for synchronizing the drive
output voltage to the bypass voltage.
If drive is installed with an optional tachometer, the feedback cable
is plugged into the SCBM. The motor speed is then determined by
counting the tach pulses in the FPGA on the machine side Drive
Control Board.