Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 36)

5-18 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Machine Converter Feedback
The function of the machine converter feedback block is to convert 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 Drive Control Board. Since identical hardware is used for both converters, some functions
are not used on one or the other.
The two machine converter voltage feedback boards provide a total of five voltage feedback signals
representing the three ac (V1, V2, V3) and two dc (VP, VN) line to ground voltages. The two dc voltages
are subtracted to determine the machine side dc link voltage (VDC), which is used by the hardware dc
link overvoltage protection. The three ac voltages are summed to determine the motor neutral to ground
voltage (VNG), which is used as the input to an A/D converter which measures this voltage for ground
fault protection purposes. The three ac line to ground voltages are subtracted from each other to produce
the three motor line to line voltages (V12, V23, V31).
A signal that is the product of the corresponding (fictitious) “delta” current and the stator resistance is
subtracted from the first two stator voltages to compensate for the stator resistance drop. These two
voltages are then integrated and a signal which is the product of the corresponding "delta" current and the
motor total leakage inductance is subtracted from each to produce the rotor flux signals. The third flux
signal (F31) is produced by inverting and adding the other two flux signals (F12, F23). The three flux
signals are half-wave rectified and added to produce the flux magnitude feedback (FLX), which is used as
the input to an A/D converter which measures the rotor flux magnitude. The flux signals are also
converted to three square waves that are combined and used to determine the stator frequency and to
synchronize the machine converter firing to the motor flux.
Stator current feedback is provided by current transducers (LEM's) in two of the motor phases. The
current in the remaining phase (I2) is reproduced by inverting and adding the other two current feedback
signals (I1, I3). The stator current feedback signals are subtracted and rescaled to create the two
fictitious delta currents (I12, I23) used in the reconstruction of the rotor flux described in the preceding
paragraph. A three phase to two phase conversion of the stator current feedback is performed by
designating phase 1 as the "alpha" phase, and inverting one of the delta current signals to provide the
"beta" phase which is in quadrature with the "alpha" phase. These two signals are used as the inputs to
A/D converters that measure the two components of stator current.