Rockwell Automation 23P PowerFlex DC Stand Alone Regulator and Gate Amplifier - Manual (Page 30)

30 Rockwell Automation Publication 23P-UM001D-EN-P - July 2012
Chapter 2 Stand Alone Regulator Installation, Wiring and Configuration
Current Feedback Calculations
Inputs to the CT Calculation:
• The variable “IdN_pm_rtg” used in the calculations is the value of
parameter 465 [Drive Size]
• Rb is the burden resistor which can be either 2.5 Ohms or 5.0 Ohms
• CT_pri is the current transformer primary rating
• CT_sec is the current transformer secondary rating
A range of CT values is initially calculated based on the system constraints of
IOC and the recommended gain range of the Binary Gain Amp. For the
constraint based on IOC, the CT minimum size would be at the IOC trip point
of 250%. The CT sizing based on the Binary Gain Amp is based on the error
encountered when using the DIP switch configuration to select the gain versus
exact resistor values. As the gain is increased, the gain becomes more granular and
the error is greater when compared with the exact gain. A gain value of 1.2 has
been chosen as a guideline to keep the error to 1% or less. Lower values of CT
ratio result in the improvement of “real system” resolution. The comparison
between the exact value of gain and the actual gain is shown at the end of the
calculation. There are trade offs between the value of the burden resistor and the
gain of the amplifier. With a larger value of burden resistor (5.0 Ohms), the
required gain is less and the binary value calculated is greater resulting in a more
accurate signal. However, the feedback level at IOC needs to be considered as
discussed later in this section.
Again, a value of 0.612 Volts represents 100% power module current. The
required gain of the Binary Gain Amp is calculated with this in mind. The value
of 0.612 Volts is divided by the actual voltage across the burden resistor at 100%
power module current. This provides the value of gain required to ensure that
0.612 Volts is fed back to the regulator at 100% power module current.
There is a maximum limit of 2.5 Volts that can be accepted by the regulator
current feedback. Beyond 2.5 Volts the circuitry will saturate. To ensure that this
does not occur, the maximum value of current feedback is calculated at the IOC
level.
The binary switch setting is based on the gain desired. The actual binary switch
setting will be obtained by rounding the calculated value to the nearest integer. To
convert from decimal to the required binary value, a calculator or personal
computer can be used.