Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 34)

5-16 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Line Converter Feedback
The function of the line 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
line side Drive Control Board. Since identical hardware is used for both converters, some functions are
not used on one or the other.
The two line 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 line side dc link voltage (VDC), which is used by the hardware dc link
overvoltage protection. The three ac voltages are summed to produce the neutral to ground voltage
(VNG) on the input transformer. This signal is not monitored by the drive control. The three ac line to
ground voltages are subtracted from each other to produce the three ac line to line voltages (V12, V23,
V31).
The first two line to line voltages are integrated and a signal which is the product of the corresponding
(fictitious) "delta" current and the commutating inductance is subtracted from each to compensate for the
commutation notches. The resulting reconstructed line voltages are called "flux" signals because the
corresponding signals on the machine side represent rotor flux. 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 proportional to the volts/Hz of the
supply and is used as the input to an A/D converter which measures the line voltage magnitude. The flux
signals are also converted to three square waves that are combined and used to synchronize the line
converter firing to the line voltage.
Current feedback is provided by current transformers in two of the ac input lines. The current in the
remaining phase (I2) is reproduced by inverting and adding the other two current feedback signals (I1, I3).
The line current feedback signals are subtracted and rescaled to create the two fictitious delta currents
(I12, I23) used in the reconstruction of the input voltages described in the preceding paragraph. The
three line current feedback signals are also half-wave rectified and added to produce the dc link current
feedback (IDC), which is used as the input to an A/D converter and to the hardware overcurrent
protection.
The preceding description applies to 6-pulse and 12-pulse converters using the original Drive Control
Board 80165-018, which has no voltage feedback from the slave bridge. For 12- or 18-pulse converters
using the new 18-Pulse Drive Control Board 80190-118, there is voltage and current feedback from all
bridges, which is combined to create an equivalent 6-pulse feedback.