Rockwell Automation 23P PowerFlex DC Stand Alone Regulator and Gate Amplifier - Manual (Page 28)

28 Rockwell Automation Publication 23P-UM001D-EN-P - July 2012
Chapter 2 Stand Alone Regulator Installation, Wiring and Configuration
SAR Current Feedback
Configuration
Overview
The current feedback to the SAR, provided via current transformers (CTs) that
monitor the U and W phase inputs to the power module, is rectified via a three
phase diode bridge. The output of the diode bridge is connected to a burden
resistor. The voltage across the burden resistor is directly proportional to the
amount of current being drawn by the motor load. This voltage is used by the
regulator to monitor and control the current by comparing its value with the
reference command. There are two on board burden resistor settings to choose
from, depending on the variables of the application. In addition, an optional
external burden resistor may be used for very specific cases. However, consult
with Rockwell Automation Development Engineering prior to using this option
to ensure that the circuit board ratings, as well the power dissipation ratings of the
burden resistor, are properly considered. See
Figure 11 on page 37 for details on
the burden resistor circuit and Figure 13 on page 39 for burden resistor jumper
locations.
Note: The current limit of the on-board burden resistors is 1 Amp. This includes
not exceeding 1 Amp for the internal burden resistors in the event of an
instantaneous over-current (IOC).
For retrofit situations current transformers are often already in place on the L1
(U) and L3 (W) AC incoming line conductors to the power module. However,
the secondary current rating of the current transformers may not be 1 Amp and
not well suited for the current feedback scaling and burden resistors of the SAR.
For high current systems, current transformers with 5 Amp secondary windings
are not unusual. These cases can be handled by using an interposing set of current
transformers (cascading the current transformers) between the existing current
transformers and the SAR. The resulting turns ratio is the product of the existing
current transformer's turns ratio multiplied by the interposing current
transformer's turns ratio.
The type of transformer for this application is a wound primary current
transformer. The advantage of this implementation is that existing current
transformers can remain in place, which eliminates the need to disassemble/
reassemble heavy bus bars, and the wiring to the existing and interposing current
transformers is signal level wiring. Care must be taken in the implementation of
the interposing current transformers to ensure that all considerations of sizing the
current transformer configuration are met, such as the following:
• IOC Limits for the existing and interposing current transformers.
• Minimum magnetization currents required.
• Proper resulting turns ratio.
Consult with Engineering for assistance in implementing a current feedback
configuration with interposing (cascade) current transformers.
The voltage across the burden resistor must be normalized for the full load rating
of the power module. The regulator expects a value of 0.612 Volts at the current
feedback input at 100% load of the power module. To ensure that the regulator
receives 0.612 Volts at 100% load of the power module, a scaling operational