Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 32)

5-14 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Current Control
The function of the current control block is to determine the firing angles of the line and machine
converters. The inputs are the torque and flux producing components of the dc link current command
from the speed control and flux control blocks respectively, and the dc link current and line voltage from
the line converter feedback.
The square root of the sum of the squares of Ix Command and Iy Command is calculated to determine
the dc link current command. In dc current test mode, parameter DC Current Test Command is used as
the dc link current command. Parameter DC Current Reference Step, which is used to demonstrate the
step response of the current regulator, is added to the current command and the sum is clamped to a
minimum value of zero to produce the dc current reference. The dc current feedback is subtracted from
the dc current reference to determine the dc current error, which is the input to the current regulator. The
response of the current regulator is determined by parameters Current Regulator Bandwidth, DC Link
Inductance, and DC Link Time Constant. The output of the current regulator is clamped to a fixed
positive limit of 0.990 pu, and to a variable negative limit called the retard limit. Retard limit is calculated
from dc current feedback, line voltage feedback, and parameter Commutation Inductance, and its function
is to ensure reliable commutation of the line converter when regenerating. When the drive is not running,
the dc voltage reference is set equal to the retard limit to force the dc link current to zero as quickly as
possible.
The line converter firing angle is the inverse cosine of the dc voltage reference. The machine converter
firing angle is determined by taking the inverse cosine of the ratio of Ix Command to the dc current
command, adding 90 degrees and changing the sign.